Note: Although you can read this page on multiple devices, I recommend viewing it on a computer in fullscreen.
See here for more information:
https://docs.google.com/document/d/1QjNGdx5wJLKFSCb3ivkjfBOuSvk1robD7-curnZlUVY/edit
Alternate Solar System
Version 0.7
This document will be discussing an alternate version of our solar system—that is, our solar system, but with several changes and alterations. These changes have been made largely to make the solar system a more intriguing place to study and explore.
The changes made here will have some caveats, however; they have to obey known science. For example, there would be no toroidal or cubic celestial bodies(gravity would pull it into a spheroid), no surface water on the Moon(it would gradually leak into space and be split by high-energy rays), and no gas giants with clouds that would probably be solid at their temperature(like chlorine clouds for Saturn).
This document will be written to focus more on the altered and added worlds than how they compare with our solar system’s counterparts. Though it’ll largely be written in-universe, there’ll also be certain moments viewed through an external viewpoint—which means that although I’ll sometimes mention attributes of celestial bodies humanity in this timeline wouldn’t have discovered yet, I’ll also bring up the nomenclature of “known” landforms and moons.
Although the effects this would have on Earth aren’t the main focus of this document, they’ll still sometimes be addressed. For one, the push for space exploration would be much greater, especially with the discovery of bodies of liquid on Venus, Mars, and planets beyond the habitable zone. These changes would also result in different names for several celestial bodies; for example, since there’s a planet named Rhea, the Saturnian moon named Rhea in our timeline would be named something else.(The name I chose was “Hyperion”, and I had to rename the moon called Hyperion in our timeline as well). A more detailed explanation of these changes can be found in the base document, which will be discussed shortly.
I’ve also made a “base document” for this document on Google Docs. It’s mainly focused on the differences between this solar system and our timeline’s, and has information on the masses of most objects(in Earth masses unless specified otherwise), and their semi-major axis(distance, more or less) from the celestial body they orbit. The writing, however, is much more fragmentary, and instead of full sentences, information is frequently given in fragments(eg. “Slightly richer in volatiles.”, “More cratered, smaller”). I highly recommend reading it-the link is here: https://docs.google.com/document/d/1QjNGdx5wJLKFSCb3ivkjfBOuSvk1robD7-curnZlUVY/edit
Some sections of this project may be revised, reworked, or scrapped altogether. You can see these sections in the “Scrapped and Revised Sections” page, linked at https://www.planetaryarchives.space/alternate-solar-systems/scrappedandrevised .
So, without further ado, let’s see the worlds of this alternate solar system!
The Sun
This yellow dwarf star is the center of our solar system, and where we start our journey. All objects in our solar system orbit the Sun(technically a barycenter, but one usually within the Sun), and have been ever since its formation around 4.6 billion years ago.
Although the Sun is the most massive body in(and namesake of)the solar system, it is not the only star in the system. In the solar system’s furthest reaches are a few star-like objects and a very dim red dwarf star.
The Sun was venerated by many cultures since antiquity, with solar deities being prevalent in most polytheistic religions, such as the Japanese Amaterasu, the Egyptian Ra, the Greek Apollo(the namesake of the dwarf planet), and the Roman Sol, from which the solar system gets its name.
The Sun, imaged in true color
(Credits: Matúš Motlo on Wikimedia)
The Episolar Belt
The Episolar (from Greek epi- “near” combined with solar “of the sun”) Belt is a belt of rocky and metallic asteroids, as well as many dwarf planets. The most well-known of them is the second largest dwarf planet, Apollo. However, there are also other major dwarf planets in similar orbits to Apollo, such as Orpheus; a dwarf planet around the tenth of our moon’s mass, whose day is significantly longer than its year as a result of being in a 3:2 spin:orbit resonance with the Sun.
Quite a few Episolar dwarf planets orbit even closer, such as the Icarus/Daedalus binary, heated up to red-hot temperatures from their sheer proximity to the indescribably hot Sun. Because of this, their surfaces have become significantly more malleable, smoothing out most differences in elevation. This has caused these worlds to be more spherical than would be expected of any object of their mass.
Due to their lack of volatile compounds like water and ammonia, and the abundance of metals and rare minerals, the asteroids of the Episolar Belt have been categorized as a separate category of minor planet, dubbed “Helioids” (a combination of Greek helio- “concerning the sun” and “asteroid”).
The abundance of minerals in the Episolar Belt has made it extremely lucrative for proposed asteroid mining operations. Some organizations even plan to turn the belt into a swarm of solar collectors, or mirrors directing light to a central solar collector, in order to produce massive amounts of energy.
Apollo and Diana
The dwarf planet Apollo, around the mass of Mars, is well-known for being part of a binary system with its moon, Diana. Although most celestial bodies are described as “going around” their parent body, they actually circle a common center of mass, which is called a barycenter. For most planets, stars, and moons, their barycenter is located within the body they orbit. However, some pairs of celestial bodies have a barycenter located outside of the parent body. This is because the mass of the orbiting body is larger in proportion to the parent body. For the Apollo/Diana binary system, Diana is approximately 6% Apollo’s mass.
Apollo, the larger body in this binary system, is a world of cratered, rocky plains and lava lakes. Diana’s gravity stimulates volcanism on the Diana-facing side of Apollo. These volcanoes produce gases that have formed a tenuous atmosphere. However, Apollo’s poles are not nearly as active, and they’ve accumulated significant ice deposits in cratered basins that never see sunlight.
Diana is less massive than Apollo, and more cratered. Diana’s lower mass means its interior is more inert than its twin, making the surface of Diana similar to our moon. However, Diana, like the Moon, also has vast plains of basalt and other igneous rocks that cover much of the landscape. Notably, these Dianan mares (Latin for “sea”; a term used to describe our Moon’s darker basins) are more concentrated on the side facing Apollo, indicating that Diana might have been similar to Apollo at one point; like a rocky eyeball with its “pupil” of lava pointed towards its parent world. Diana also has notable deposits of ice in its poles, like Apollo.
The binary system likely formed when a larger body(tentatively named “Leto”, after the mother of Apollo and Diana in classical mythology) was split, probably through a powerful impact. This shattered Leto into shards of molten rock, which then slowly re-condensed into the two worlds.
However, if the force of the impactor had been greater, or more material was ejected into space, the impact might have resulted in a single body, with its outer layers stripped. This iron-rich world would be a glorified core, with a cratered surface similar to our moon. There might still be activity within the interior of this world-perhaps enough to create a magnetic field-but it would likely be too low to sustain volcanism.
Despite the binary system’s great size compared to other dwarf planets in the Episolar Belt, it was not discovered until the late 18th century due to interference from both the Sun and Mercury.
Apollo’s sole large moon Diana, viewed from the Leto I probe
(Credits: Crescent moon over Sarıçam, Turkey, Zeynel Cebeci on Wikimedia)
Mercury
Mercury, the first planet from the sun, is a rocky world around 8 times Earth’s mass. Because Mercury was the first known planet of this type, massive rocky planets like Mercury are called “Hermian worlds”, from the Roman god Mercury’s Greek counterpart. Though Mercury’s thick atmosphere made up mostly of carbon dioxide, hydrogen, and helium may appear similar to a gas or ice giant from space, the surface is a completely different environment. While the intense pressure and extreme heat present in much of Mercury’s lower atmosphere is also present on the surface proper, the atmosphere above has blocked all sunlight from reaching the surface.
However, some light can be seen on the Mercurian surface—not from the nigh-invisible sun, but from the planet’s many lava lakes and volcanoes. These were created through Mercury’s intense internal activity, caused largely through the planet’s abundance of radioactive material in its core—a likely side effect of its great mass. Mercury’s surface also has various lakes of liquid metals, which often evaporate into vast expanses of vaporized metal fog, as well as clouds of metal vapor. These can, in cooler regions of the planet, precipitate metallic snow!
A collision with another, smaller world has halted Mercury’s rotation, leaving one side in eternal day and the other in eternal night. Though Mercury’s too hot for cloud formation, its upper atmosphere is stained with iodine gas, tinting it a brilliant violet.
As large as Mercury is now, there are signs it many have once been even greater in size. The most notable of these is the abundance of hydrogen and helium in the Mercurian atmosphere. This is uncommon for a terrestrial planet, but could have been the result of a gaseous Mercury entering from outside of the solar system’s “snow line”(a boundary roughly located along the asteroid belt, beyond which more volatile materials like water, methane, and hydrogen and helium can be found), and losing much of its atmosphere, leaving behind a rocky core.
The exact mechanism behind this is unclear, though; some have posited that millions of years being tidally locked to the Sun slowly stripping away the Mercurian atmosphere through the faint “tail” of gasses trailing behind the planet. Others have hypothesized Mercury lost much of its atmosphere through a close encounter with the sun; still others have suggested the collision that halted Mercury’s rotation was to blame.
Mercury’s immense mass in relation to the inner Core Solar System’s other major bodies has been frequently used in recent years to bolster the hypothetical concept of massive terrestrial “first-generation” planets that orbited close to the Sun. According to this hypothetical model, all of these planets save for Mercury were pushed into the Sun as gas drag pulled Jupiter inwards per the Grand Tack theory. Granted, the hypothesis’s assumption of Mercury having formed inwards of the snow line has lead some to question its connection with Mercury’s odd qualities, but the commonality such a hypothetical early solar system shares with most discovered extrasolar planetary systems means it still retains a major degree of credibility—or at least notability as a possible stepping-stone towards a more proper understanding of the early Solar System.
Due to being incredibly close to the Sun, Mercury lacks any known bodies within its Hill sphere(the area where a body is the dominant gravitational influence in attracting satellites); nothing so much as a lumpy asteroid moonlet. As such, Mercury, despite being the most massive of the terrestrial planets, is one of the two ringless terrestrials(the other clearly being Earth).
Since antiquity, Mercury has been visible to many peoples across the world. The Chinese knew the planet as 水星 (“shuǐ xīng”), which translates to “water star”, while the Norse named Mercury “Odin”, for the head of their pantheon-a deity of warfare and wisdom.
Mercury’s night side, from the MAIA probe
(Credits: Artistic depiction of WASP-39 b, NASA,ESA,CSA, J.Olmsted)
Venus
Second from the Sun is the terrestrial planet Venus, in many ways a sister world to Earth. The two planets share many similarities: they’re both composed mainly of minerals and metal, their masses are around the same, they both have active magnetospheres generated by magnetic fields from their cores, and they both have bodies of liquid water on their surface. However, Venus has more differences with Earth than attributes they share.
For one, Venus is a sweltering world, being around 27% closer to the Sun than Earth. Venus’s scalding temperatures are only exacerbated by its thick, smothering atmosphere 17 times the pressure of Earth’s. This atmosphere is composed of 91% nitrogen, with 6% CO2, 2% H2O, and 1% other gasses. However, the titanic pressure of this atmosphere actually keeps Venus’s water from evaporating, even despite the intense greenhouse effect.
Although Venus’s atmosphere is already thick now, it might have been significantly thicker. Many signs, such as Venus’s retrograde rotation, hint to a prior collision with another planet early in the solar system’s history. This likely knocked off much of what atmosphere Venus had, and gave it its week-long days and dramatic axial tilt.
However, if Venus had retained more of that atmosphere, it would be a radically different world. Greater levels of carbon dioxide might have evaporated all water away from Venus, and rendered the surface hot enough to melt lead!
Venus’s blend of greenish and white clouds is due to copper-rich dust from the planet’s surface frequently tinting the planet’s cloud tops. This gives an effect similar to a Martian duststorm, albeit with a much thicker atmosphere. The atmosphere of Venus also hosts droplets of sulfuric acid in its clouds, whose acidic rains mix with the shallow seas below. Though this makes the Venusian seas more hostile to organisms that lack a tolerance to sulfuric acid like us, extremophilic bacteria could survive in these caustic waters. Perhaps there could be lifeforms with biochemistries using sulfuric acid in place of water! However, given the lack of any concrete proof for Venusian life, much of this is pure speculation.
Aside from the low sea levels, Venusian geography differs in several other major ways from Earth’s, with the lack of mobile tectonic plates coming foremost. Without a crust broken up into distinct continental plates, Venus lacks any truly defined continental uplands. A side-effect of this is that Venus’s crust undergoing “resurfacing” events separated by geological timescales, in which massive volcanic events pave over much of the Venusian surface with newly cooled igneous rock. The gradual cooling of Venus’s core has—to the best of our knowledge—ceased such events, but Venus still maintains regular volcanic activity.
Being the brightest planet in the night sky, Venus has had cultural importance in many regions. In Japan, Venus was known as 金星(“Kinsei”), “gold star” for its brightness. The Sumerians associated Venus with their goddess Inanna, and the Akkadian name for Venus was “Ishtar”, the Akkadian counterpart to Inanna.
Neith, Magellan probe
(Credits: Image of Eros, NASA NEAR Shoemaker probe)
Vulcan
Vulcan is the largest Venusian moon, and was likely a captured dwarf planet. When the solar system was still forming, Vulcan entered the Hill Sphere of Venus—the area where Venus’s gravitational pull dominated over the gravity of all other celestial bodies. This resulted in Vulcan being captured as a moon—though, because of this unorthodox origin, Vulcan has an elongated orbit that goes in the opposite direction to the moonlets of the Venusian rings.
The stretched orbit of Vulcan exerts tidal forces on the moon that spur its tectonic activity(like the Jovian moons Semele and Io further out in the solar system). The high level of tectonic activity means that Vulcan’s surface is one of the newest rocky surfaces in the solar system, with the old crust constantly covered by layers of lava ejected from the many volcanoes. This incessant ejection of molten rock has built up many volcanoes of the Vulcanian surface, the most prominent being Pandora Mons on the Hephaestus Plateau. This mighty shield volcano(like those of Mars’s Tharsis plateau or Earth’s Hawaiian archipelago) is the largest volcano currently active in the solar system.
Though much of the ejected material falls back to Vulcan, some of the magma and gasses expelled through Vulcan’s eruptions ends up in a ring surrounding Venus. This “Vulcan Ring” is the outermost of the Venusian rings, and is wholly unlike any of the rest.
Though the valuable information that could be gathered from studying Vulcan’s intense tectonic activity is a key motivation behind many efforts to colonize the volcanic moon, there are several other factors that make colonizing Vulcan a prime objective in exploring the Venusian system. For one, the geothermal heat provided by Vulcan could be used as an important power source for colonies in the Venusian system, in case any power sources based on solar(or less likely, nuclear) energy could be rendered inoperable due to a lack of sunlight or fuel, or damage sustained to the infrastructure. However, Vulcan’s volcanic activity also brings many valuable metals and minerals from its core to the surface, cementing the importance of the Venusian system in extraterrestrial mining operations.
Despite the many benefits of having a colony on Vulcan, the moon’s frequent tectonic activity poses a grave danger for any outposts, which would have to face the constant threat of their infrastructure being demolished by a severe “Vulcanquake'“, or their habitats being incinerated by a lava flow from one of the moon’s many volcanoes. The multiple threats that come with establishing an outpost on Vulcan only make it more integral to find a suitable place to colonize the moon; a location distant enough from the volcanoes and fault lines to not be in imminent danger, but close enough that the research, energy, and mining opportunities presented by Vulcan can the easily utilized.
Venus in true color, MAIA probe
(Credits: Gilese 832 b artist’s impression, Radialvelocity on Wikimedia)
Neith and the Venusian Rings
Venus has a ring of debris formed when a prior Venusian moon(possibly from the great collision) fragmented. This ring has several moonlets similar to asteroids, with the most massive being Neith, a moonlet rich in metal. The Venusian rings have been condensing into these moonlets for millennia, and will probably continue to do so until the rings have disappeared.
Neith is the largest and most massive of these moonlets, weighing in at around 76 trillion kilograms! Neith is particularly rich in rare metals, indicating that it might have been from the core of the moon whose destruction created the Venusian rings. Neith has acted as a “shepherd” for the rings, clearing its orbital path by flinging particles away from its orbit or simply colliding with the ring particles, producing copious craters on Neith’s rocky surface.
Neith is a lucrative target for potential colonization of the Venusian system, largely due to its unique conditions. Venus proper has scalding temperatures and a crushing atmosphere, and even if a base were to be established in the upper cloud decks, the relatively high gravity(though still less than Earth) would require a copious amount of propellant. The ring particles have much lower gravity, but their proximity to each other may make navigation difficult-and even possibly make the particles prone to collide.
However, although Neith’s gravity is low, it still had enough mass to either attract or kick out most bodies in its orbital space, making the risk of collision lower than with other particles. This, combined with the abundance of rare minerals, makes Neith a promising location for a colony in the Venusian system. The importance of Neith in the colonization of the Venusian system has led to several planned missions to survey the surface of Neith for a suitable landing site for human exploration, as has been done for Phobos, Deimos, and Harmonia in the Martian system.
Vulcan, currently inactive, FORGE probe
(Credits: Made in PlanetMaker [ planetmaker.apoapsys.com ] )
Earth
Of course, when discussing our solar system, it’s important to also bring up the Earth—our lonely blue marble and the only place in the solar system where life has been found. Earth’s suitability for life comes down to a few key factors:
The presence of water as a liquid solvent
Ample amounts of energy(from hydrothermal vents, sunlight, and more!)
The basic elements that present in all known life: hydrogen, carbon, nitrogen, and oxygen
It is important to note, however, that many of these attributes of Earth conducive to life are present on many bodies in our solar system. For example, amino acids which contain the elements essential to life have been found across the solar system, including places like comets and the Venusian atmosphere. Liquid water is also extremely abundant in the solar system, along with energy—the Saturnian moon Enceladus and the Rhean moon Dione having hydrothermal vents supplying heat and nutrients to the oceans below their icy crusts. Despite this, there is no known instance of native life in the solar system outside of Earth.
Although Earth’s biosphere is one of the most important aspects of the planet, Earth remains intriguing even outside of this context. For instance, Earth is the only terrestrial planet to have plate tectonics—the movement of pieces of the Earth’s crust, perpetuated by mantle convection. This rare phenomenon has created Earth’s distinct continents and oceans. The only other worlds to have these defined oceans and landmasses are the frigid planets Vesta and Minerva, whose tectonic systems are driven by the convection of liquid water and solid methane(respectively) in their interiors.
Earth is also unusual for its large moon, the only natural satellite orbiting it. The Moon’s formation was rather unusual for a natural satellite; instead of being captured or accreting from spare dust and gas caught in its parent planet’s orbit, the Moon was the aftermath of a collision between early Earth and a protoplanet the mass of Mars(nicknamed Ouranos, after the husband of the goddess personifying the Earth in Greek mythology), which obliterated both planets and created a cloud of melted rock. From this disparate cloud, the Earth started to coalesce back into a sphere, while most of the orbiting detritus solidified into the Moon.
The full Moon
(Credits: Telescope photograph of the full Moon, Gregory H. Rivera on Wikimedia)
The famous “Blue Marble” image of Earth, taken by Apollo 17
(Credits: “Blue Marble” photo, Apollo 17 crew/NASA)
The Moon
Our moon is an anomaly among natural satellites, with a violent and chaotic formation. Billions of years ago, a Mars-sized protoplanet smashed into proto-Earth, erasing both worlds and creating the Earth we know today, with an abnormally large moon as a result. However, that isn’t where the Moon’s chaotic story ends; as one last collision would happen to create our modern moon. While our present-day moon’s near side is smothered with splotches of maria, vast, dark plains formed from cooled seas of lava, these basins are virtually nonexistent in the far side. However, this may be because of an impact of the Moon’s south pole before the maria even became active—an impact that also created the largest impact basin on the Moon, the South Pole-Atiken basin. This impact might have distributed materials in the mantle such that the near side would eventually be much more prone to volcanism than the far side—creating a paucity of maria on the lunar far side, and an abundance of them on the near side.
The Moon’s proximity to Earth has made it the target of several missions, manned and unmanned. The first of these missions was the probe Luna 1 in 1959, sent to observe the Moon by being sent to collide with it, gathering information as it got closer. Though the probe instead ended up extremely far away from its target, ending up orbiting the Sun, it still marked a pivotal moment in the history of space exploration—with Luna 1 being both the first manmade object to reach escape velocity and the first of mankind’s attempts to send a spacecraft to study its nearest celestial neighbor. This legacy is still present in the many accomplishments achieved in the quest to understand the Moon, from getting the first manned spacecraft into lunar orbit (Apollo 8, 1968) to landing the first man on the Moon proper in the legendary Apollo 11 mission. In keeping with this rich history of exploration, the first lunar colony is planned to be established in the next few decades, signaling a new step forward in humanity’s journey towards the stars.
Mars
Mars is the fourth planet from the sun; a desolate world of sand and frost. Although Mars’s size, mass, and density all vary greatly from Earth, they share a similar origin—as oceanic worlds prime for the development of life. Unfortunately, though, solar radiation and gradual leakage of Mars’s atmosphere reduced these once-mighty bodies of water to small, briny lakes only present in the deepest of basins. Only three major seas remain on Mars as relics of this bygone time: the Acidalia, Utopia, and Hellas Seas. The former two are widely speculated to be remnants of a much larger ocean that filled up the planet’s great northern basin, while the latter is an immense crater that contains the deepest point on Mars. Unlike the other two, the Hellas Sea is covered by a great glacier that obscures it from view; still, frequent activity and resurfacing within this glacier similar to that on icy moons like Europa and Enceladus has allowed the sea’s presence to be inferred.
However, there are other reserves of water left on Mars besides the hypersaline remnants of Mars’s ancient seas; the polar ice caps and patches of permafrost underground could hold enough water to enlarge the Martian oceans to their former size. The polar ice caps also hold deposits of dry ice-carbon dioxide frozen out from Mars’s thin atmosphere.
Though Mars’s glory days are long past, there are still many reminders of its active past. The long-dry rivers and oceans have left canyons and salty, mineral-rich seabeds to their name; with one of these canyons, Valles Marineris, being the width of the contiguous United States! In its past, Mars also sustained volcanic activity, with the massive volcanoes of the Tharsis plateau persisting to this day, albeit dormant. The highest of these, Olympus Mons, is more than twice as high than Earth’s highest mountain! Although Mars’s tectonic activity has waned over billions of years, igneous formations dating back to the last few millennia show that the massive shield volcanoes of Mars might still harbor some signs of activity.
While the gradual waning of Mars’s atmosphere, water, and geologic activity has left the planet a shadow of what it once was, if Mars’s mass was less than it is in our timeline, it could have suffered a far more terrible fate. In such a universe, Mars would have lost all of its water; even the vaunted triplet seas would be bone-dry basins. The only places where there would still be some trace of water would be frozen deposits in the similarly reduced ice caps and underground permafrost layer. A lower-mass Mars might also lose its magnetosphere, much more of its already thin atmosphere, and its geologic activity, making Mars a much harsher place than it is in our timeline.
Mars’s Earthlike past(and, to some extent, its present) has made the planet one of the most important places in the solar system for the ongoing quest to find extraterrestrial organisms. Though it’s likely most lifeforms that could have existed during Mars’s early “wet period” would’ve gone extinct by now, there could still be microbial organisms hiding in the saline, half-frozen lakes, or in underground pools kept warm by the planet’s geothermal heat.
Mars’s sanguine, ruddy hue(caused by iron oxide in the planet’s soil) has captivated the imagination of many peoples throughout history, giving it a connection with bloodshed, and by extension, warfare. In Hinduism, Mars was personified as Mangala, the god of aggression and anger. Its color gave it the moniker of '“Har Deshur” in Egyptian mythology, translating to '“Horus(a primary deity in Egyptian religion) the Red”.
Harmonia, true color, MRO probe
(Credits: Also made using Universe Sandbox)
A photograph taken by the UAE’s Hope probe of Mars’s Acidalia region, with the Acidalia Sea near the upper-right. The planet’s rings are faintly visible, as is Phobos (center-left) and Deimos(top-center).
(Credits: Made in Universe Sandbox )
Mars’s Moons and the Martian Rings
One particularly interesting feature of Mars is its close-in rings. These rings are theorized to have formed from the destruction of an asteroid moon like the three Martian moons. The rings will eventually dissipate through ring particles falling to the Martian surface. Various recent craters have been formed through the impact of some of the larger particles, whose sizes can vary drastically.
Outside of the Martian rings, Mars has three asteroid moons: Phobos, Deimos, and Harmonia. These satellites are of unknown origin; though they could have been captured, they also could have been the debris ejected from a collision with Mars. The three moons, like Neith, are crucial to travel in and out of Mars due to their low gravity, making them good “rest-points” for spacecrafts to refuel and resupply without needing excessive amount of propellant to continue traveling.
The Asteroid Belt
Between the rocky worlds of the inner solar system and the gas giants lies a belt of rocky, metallic, and icy material. This region of cosmic detritus is the Asteroid Belt, named after the non-rounded lumps of various substances that make up the belt.
Although the Asteroid Belt was the first place asteroids were found, the region’s namesake celestial body can be found all throughout the solar system: asteroids particularly rich in minerals and metals can be found around the Helioid Belt, bands of asteroids group together at the Lagrange points of several gas giants, and some particularly icy asteroids in the outer solar system are known as “centaurs”, because of their similar composition to the comets, minor planets similar to asteroids but composed mainly of volatile chemicals like water, ammonia, and methane.
Ceres
The dwarf planet Ceres is the second dwarf planet found(with Apollo being the first). Ceres is composed of a mixture between ice and rock, and at first glance seems merely like a more spherical asteroid. However, when looking closer at Ceres’s cratered surface, records of a more interesting side to the small world emerge. Large, circular basins are the remnants of once-mighty cryovolcanoes, whose eruptions created a tenuous, yet still present atmospheric haze. Deep below the bottom of these craters may lie unicellular organisms that utilize geothermal heat from Ceres’s core as an energy source.
Another intriguing feature of Ceres caused by past cryovolcanic eruptions is a thin ring of small ice particles, and an equatorial ridge directly below. This ridge was formed from the gradual descent of the ring’s ice particles, and as the ring waned, the ridge grew higher. The ring itself was created from the same cryovolcanism that made Ceres’s thin atmosphere.
However, some of this ring might also have gone into another important facet of Ceres—its lumpy moon, Kore. This moon, named after the daughter of Ceres in classical mythology, bears a similar composition to the dwarf planet it orbits. This, combined with Kore’s prograde orbit(revolution in the same direction as Ceres’s rotation), suggests that Kore might have formed from Ceres. The composition of Kore in particular, being slightly more icy than its parent, suggest an origin through the same cryovolcanic eruptions that created the rings of Ceres.
Jupiter
Jupiter is the fifth planet from the sun, and one of the most massive planets in the solar system, at 308 times Earth’s mass. Jupiter is a “gas giant”-a class of planets composed of chemicals gaseous at Earthlike pressures and temperatures, like hydrogen and helium, which make up the majority of Jupiter. Because of Jupiter’s prominence, this class of planet is also known as a “Jovian” planet, after an alternate name for the Roman deity Jupiter, who was the king of the gods and a deity of thunder and the sky. This name fits Jupiter’s nature as well-having multiple circular storms similar to Earth’s cyclones, one(the Great Red Spot) having a greater surface area than any continent on Earth!
Jupiter’s many cloud bands, of different shades and colors, are tinted with many varying gasses. For example, the brighter zones are rich in ammonia ices, while the Great Red Spot may owe its color to tholins-a warm-colored pigment common in many parts of the solar system beyond the asteroid belt. However, the composition of Jupiter’s clouds is not well understood at the moment, making it one of the main research targets for missions sent to the gas giant.
Although Jupiter’s cloud layers are certainly stunning, its system of moons is just as interesting. Jupiter has the most known moons for a planet, and the largest moon of a gas giant in the core solar system. Jupiter’s seven largest moons(Io, Semele, Europa, Ganymede, Callisto, Alcmene, and Themis) are known as the Galilean moons, and are all worlds rich with incredible features, from the sulfur plains of Semele and Io, to the magnetic field of Ganymede and the teal tundras of Themis.
Since Jupiter was one of the few planets visible to the naked eye for ancient peoples, the gas giant has gained a prominent role in the mythologies and cultures of many societies. Its slow orbital period led many cultures to associate it with their chief deity—for the Babylonians, the planet was named after the major deity Marduk, while the planet’s current name comes from the Roman god of storms, the sky, and light, who was the ruler of the pantheon. Jupiter was associated with major deities in other societies as well, such as Brihaspati, the religious teacher of the gods in Hindu mythology.
Io and Semele
Semele and Io are the inner two Galilean moons, and are both incredibly active worlds. Semele is the moon closer to Jupiter, having vast dark patches of semi-molten rock on the side facing Jupiter. These often burst with gashes of lava erupting out from the thin, rocky crust. Even when not active, these “lava plains” are still scorching hot from the magma beneath. The side of Semele facing away is not much better; the surface is stained yellow by vast plains of sulfur. This coating originates from eruptions from activity from the many volcanoes that dot this hemisphere, which also creates reddish rings around themselves from volcanic plumes. In fact, Semele’s entire geography is punctuated by vast remnants of titanic calderas!
Io is similar, although larger and lacking the lava plains; instead, the surface of Io is covered with many shades of sulfur, whose vibrant tones(which range from burnt black and rich ochre, to yellow and pale white) all give the moon a fiery appearance fitting of its active nature. The many volcanoes of Io spit out molten minerals extremely frequently, sometimes putting even Vulcan’s volcanism to shame! As a result, many of Io’s landforms are named after deities of fire due to the moon’s heightened volcanism. The volcanic eruptions have also formed a “donut” of charged particles, which is strong enough to warp Jupiter’s magnetic field!
The volcanism of Semele and Io may seem odd, even contradictory to their location in the frigid region of the solar system beyond the habitable zone. However, tidal forces like those experienced by their inner solar system cousin Vulcan stretch and deform these worlds, creating a geology completely alien when contrasted with the more placid, dormant moons of most other giant planets.
Europa, true color, Phaeton probe
(Credits: Photo of Europa from the Juno probe, NASA/JPL/Caltech/SwRI/MSSS/Kevin M. Gill)
Callisto and Alcmene
Though Callisto and Alcmene are both very similar to Ganymede, they still have several important characteristics that define them as distinct worlds in their own right. Callisto is the larger of the two, however it’s still smaller and less massive than Ganymede, with a surface pockmarked with a myriad of craters, large and small. Alcmene is only three-quarters Ganymede’s volume, yet almost as massive as Callisto. This may be because of its dense core rich in heavy metals.
An important difference between these two moons and Ganymede is their non-resonant orbits; Callisto’s orbital period isn’t half that of Alcmene. While this deprives Callisto of the tidal energy needed to sustain a directly visible subsurface ocean(though, of course, there still may be one very close to the core), Alcmene’s abundance of radioactive minerals sustains its geological activity, making it even more active than Ganymede!
The various ways Callisto and Alcmene diverge from Ganymede not only serve as a good exercise in comparative planetology, but also give insight on many different fields. Although there’s little geological activity on Callisto, the old terrain, disrupted only by the occasional impact, serves as a “time capsule” to extract information about the conditions of the early solar system. Conversely, the comparatively active Alcmene can be used to investigate how heavy metals and other rare elements may be distributed throughout the solar system.
Themis seen in true color, Galileo probe
(Credits: Made in PlanetMaker [ planetmaker.apoapsys.com ] )
Jupiter(with Ganymede’s shadow), New Horizons probe
(Credits: Jupiter, NASA/JHUAPL/SwRI )
The Galilean Moons
The Galilean moons of Jupiter get their names from the astronomer who discovered them, Galileo Galilei. Galileo, who would later become prominent for advocating for the Copernican heliocentric model(as opposed to the geocentric cosmology proposed by Ptolemy, and adapted upon by Aristotle), discovered the moons in 1609. However, this garnered some controversy, not only from the Catholic Church at the time(who maintained the belief that all celestial bodies revolved around the Earth), but from Galileo’s contemporary Simon Marius, who claimed to have discovered the moons at the same time.
Although the current names of the Galilean moons are based off of consorts of the Roman god Jupiter(the namesake of the planet they orbit), this naming scheme wasn’t the original iteration of the Galilean moons’ names. One of Galileo’s initial ideas was to name the moons the “Medicean Stars” after his patron, Cosimo de’ Medici, but many other people put forth differing naming ideas. Jacques Ozanam, a well-recieved mathematician, suggested naming simply naming the moons the “satellites”(after the Latin word for “escorts”), while Simon Marius intitially proposed naming the moons after the known planets at the time, based on their known characteristics(so Semele would have been “the Mars of Jupiter” due to its ruddy color, Ganymede would have been “the Jupiter of Jupiter” due to its size, et cetera). Though Marius’s next idea for the moons’ names would slowly gain traction, and would eventually be the naming scheme we use for the Jovian moons today, it still would be less preferred to another naming scheme used by some astronomers today: to combine the initial of the planet a moon orbits with its order of discovery(for example, since Amalthea was the eighth moon discovered around Jupiter, it’s occasionally referred to as “Jupiter VIII”).
Io, true color, Galileo orbiter
(Credits: Io, NASA/JPL/University of Arizona on WIkimedia)
Europa and Ganymede
Europa and Ganymede are two Jovian moons that, despite their many differences, share multiple things in common that make both moons intriguing places to study and explore. Europa, around the mass of our moon, is mostly covered in cracked plates of ice, while the icy crust of Ganymede, a moon around three times Europa’s mass, is much thicker and similar to silicate rock. Though the tidal stress from Jupiter creates enough motion in Europa’s icy crust to cause the ice sheets covering its surface to collide and rift apart, it seems to be insufficient to produce a magnetic field as strong as that of Ganymede(Europa’s magnetic field, in fact, being a mere sixth the strength). This crustal motion also regularly “refreshes” the Europan surface, wiping away most crater impacts—a phenomenon not present on the crater-pocked Ganymede.
However, as different as the two worlds may be, they share some similarities. For one, the two worlds have brackish oceans beneath their frozen surfaces(though Ganymede’s ocean has layered seabeds of exotic high-pressure ice, unlike Europa). They also have silicate mantles and metallic cores that, when combined, make up roughly half their mass. Finally, Ganymede and Europa(as well as Io) have resonant orbits—that is, one Ganymede orbital period is two Europa orbital periods, which is four Io orbital periods. This may be one of the causes of the tidal heating experienced by these three Galilean moons(albeit to radically different degrees).
Many of these similarities have resulted in these moons garnering scientific attention, but none may be as prominent as the subsurface oceans of Europa and Ganymede. Their discovery outside of the traditional “habitable zone”(where water can remain liquid on the surface, assuming an Earthlike atmospheric pressure)has led to a paradigm shift in the traditional notions of what can be considered a habitable world, which has broadened the scope of the search for life outside of Earth. This break from what some have called “habitable-zone chauvinism” has even led some to speculate life based upon exotic biochemistry; like methane-based worms digging in the riverbeds of Tethys, or fishlike creatures swimming in lakes of liquid nitrogen on Minerva!
Callisto in true color, from the Galileo probe
(Credits: Callisto, NASA/JPL on APOD)
Themis
Themis is by far one of the most interesting of the Jovian moons, being completely unlike anything in Jupiter’s Hill Sphere. For one, instead of being largely airless, Themis’s atmosphere is thick enough to give its days a noticeable blue-green hue-from the copper oxides present.
Themis is the largest(with a radius of around 2776.5 kilometers), most distant and most massive moon of Jupiter. This, combined with its retrograde orbit and other major differences it has with the other Jovian moons, suggests that Themis was a captured protoplanet.
Themis’s surface and internal composition seems to be similar to both Mars and many objects in this region of the solar system, indicating that it might have originated from around Jupiter’s orbit. The Themian surface is colored turquoise due to oxidized copper-containing minerals in the regolith(a term for extraterrestrial soil), in much the same way that oxidized iron-containing minerals tint Mars’s surface red. Though much of Themis’s surface is similar to Mars-mountainous peaks(including the highest known mountain on a terrestrial body Orthys Mons) great basins, sandy dunes-the polar regions are vastly different. Themis’s polar glaciers are titanic in size, and are likely part of a permafrost complex deep below the surface.
Beneath the surface of Themis, some scientists speculate that there may be bodies of liquid water, ranging in size from small pools to (possibly)entire subterranean seas! If this is true, then Themis, at least in this regard, is more similar to icy bodies like Ariadne(a moon of Bacchus) or the planet Vesta, which both harbor oceans of liquid water beneath their frozen exterior. Already, there have been pockets of liquid water discovered within the great polar glaciers, signaling that larger bodies of water might not be too implausible. Liquid water has also been seen to have been on Themis’s surface during its distant past; various lakebeds and river-formed valleys can be found, now dry and bereft of moisture.
The possible presence of liquid water on Themis makes it an intriguing world in the search for extraterrestrial life, bolstered by the detection of biogenic chemicals (chemicals typically produced by life, like molecular oxygen) in the ice caps of Themis’s northern hemisphere. This not only gives credence to the prospect of Themian organisms, but also could hint at similar chemicals being present on Mars—a world similar in many respects to Themis—and thus, the presence of Martian life!
Vesta
Vesta is the sixth planet from the sun, and while largely similar to Earth in many aspects, it differs drastically in others. One of the most significant differences between the two worlds is Vesta’s temperature—being around 7 times more distant from the sun than Earth, it’s so cold that methane, a gas on Earth, condenses to form a liquid! Being so cold, Vesta’s water is not only frozen into ice, but rendered rock-hard—being the bedrock of the Vestan landscape.
Despite the many ramifications of Vesta’s chilly temperatures, the Mars-mass world does have some things in common with Earth. For one, both worlds have relatively thick atmospheres with a high amount of nitrogen. Vesta also has a magnetic field like Earth, though due to the planet having accrued less iron during its formation(as compared to Earth), the Vestan magnetosphere is far weaker. Because of the presence of a magnetic field and an atmosphere, Vesta, like Earth, has also managed to support bodies of liquid on its surface. These range in size from shallow pools to immense oceans, but all of them are composed of liquid methane—something nigh-impossible on a planet with a temperature similar to Earth.
However, though the surface of Vesta is cold enough to freeze any drop of water rock-solid, the interior of Vesta is another story. The decay of radioactive minerals in the rocky mantle of Vesta has created a subsurface ocean beneath its frozen landscape. This ocean, in turn, creates convection currents (a form of heat transfer where hotter materials rise and cooler materials fall) that, in a fashion similar to the convection of Earth’s mantle, moves around the plates that make up the Vestan crust, with some plates colliding to create great mountain ranges, while others diverge, leaving deep trenches behind. This tectonic activity has given Vesta a geography surprisingly similar to Earth’s—and made Vesta just as active, if not even more so. “Vestaquakes” frequently occur at plate boundaries, and many cryovolcanoes(similar to a volcano, but instead of shooting out molten rock, they shoot out molten ice—that is, water) can be found dotting the Vestan landscape. There are even calderas large enough to create small seas!
Cold worlds with methane seas and subsurface oceans like Vesta are intriguing places to search for life, especially because of the two radically different environments present of these “Vestan worlds”. While the subsurface oceans of many moons and planets in the solar system are indeed interesting, their coexistence with a surface also bearing bodies of liquid provides an important opportunity to both study two drastically different potential biospheres and to compare them—in the niches present, the transfer of energy, and even how they could interact with one another. Though a lot of this is largely postulation, the prospect of life based on liquid methane provided by worlds like Vesta is still a captivating one.
Despite Vesta’s key importance in modern astronomy, the world wasn’t discovered until the early 18th century, largely due to its small size and distance from Earth. A consequence of this, however, is that Vesta was the earliest discovered planet with a known date of discovery. Even with the influx of attention Vesta has gained in the following decades within the scientific community, however, no moons have been detected orbiting Vesta, leading many to conclude that Vesta simply hadn’t captured or otherwise gained any natural satellites. In spite of this lack of companion moons, Vesta still has an important position in astronomy—especially considering it was the first of the three known celestial bodies known to have liquid methane on their surfaces.
A telescope image of Vesta, true color
(Credits: Titan from Cassini, NASA/JPL/UoA/DLR)
Saturn
Though Saturn, the seventh planet, is a gas giant made mostly of hydrogen and helium like Jupiter, it’s less dense and less massive than its cousin. In fact, Saturn’s density is so low that something as dense as it would float in water! Though the cloud bands of the world are more clement and muted in tone(as compared to Jupiter), there are hexagon-shaped storms at its poles even larger than Jupiter’s Great Red Spot(which is already much wider than Earth)!
However, the most notable feature—and the attribute of Saturn most know it for—is the planet’s big, bright rings. These rings, mostly made of icy material, likely formed from the destruction of a moon, and are split into several sub-rings by “shepherd moons” that fling out or accrete(that is, collide with) any debris in their way—in a manner similar to Venus’s moon Neith. Saturn also has an outer E-ring formed from material ejected by the Saturnian moon Enceladus.
Another interesting feature of Saturn is its abundance of moons, with only Jupiter having more known natural satellites. These moons formed from material that was attracted into Saturn’s orbit during its formation but never accreted into the planet itself. However, in addition to these moons, Saturn has Rhea, a “trojan planet” of Saturn caught in a Saturnian Lagrange point—and thus orbiting on the same orbit as Saturn. As far as we know, Rhea is the only non-moon celestial body to be in such a situation.
Though Saturn is further from the sun than Vesta, its large size allowed it to be discovered in antiquity. As such, it has had an important presence in many cultures and mythologies. The planet was named “土星”(“Tuxing”) in Chinese—a name translating to “Soil Star”, because its color resembled yellow earth. Its slow orbital period also lead to it being associated with primary deities in many mythologies, like Jupiter. This included deities such as Cronus, the Greek titan of time and agriculture, and the leader of the Titans(a group of mythological figures similar to, but preceding, the gods)—whose Roman counterpart would lend the planet its name.
Saturn’s largest moons to scale, with Earth for reference. From top to bottom: Mimas, Enceladus, Koios, Krios, Hyperion(without rings), Tethys, Potamoi, Iapetus, Phoebe, and Theia
(Credits: Edited using Pixilart
Theia: NASA / JPL / Space Science Institute, with edits from J. Major.
Other Moons: “File:Moons of solar system v7.jpg” on Wikimedia, uploaded from NASA )
Rhea
Rhea is a Vesta-like dwarf planet at a Saturnian Lagrange point on Saturn’s orbit(where the gravitational pull from Saturn and the Sun reaches an equilibrium). Because the asteroids at Jupiter’s L4 and L5 Lagrange points were named after figures from the Trojan War in Greek mythology, planet-like celestial bodies located at Lagrange points(like Rhea) are known as “Trojan planets”. Rhea’s location on one of the Lagrange points located on the orbit of Saturn means that the orbit of Rhea is shared with Saturn; wherever Saturn is on its orbit, Rhea is close by its side(though not close enough to be captured as a moon). It is unknown how Rhea ended up in this position, which has contributed to its intrigue when studying Saturn’s orbital space.
Although Rhea’s unusual orbit is certainly interesting, the dwarf planet itself is captivating as well. Despite being a dwarf planet, Rhea’s more than twice as massive as Mars! This has helped it hold onto an atmosphere much thicker than Earth’s, and the high quantity of greenhouse gasses in this smothering blanket of air heats up the surface to nearly the boiling point of methane(though still not being enough to melt ice). However, like on Venus, the pressure of the atmosphere keeps the methane liquid, allowing the planet to retain a global methane ocean, with the highest point on Rhea being miles below the surface!
Rhea has two moons: the Enceladus-like Dione and the Koios-like Cybele. Though Cybele is the more massive of the two, its distance from Rhea has meant that it’s less active than Dione.
Dione, viewed in true color from the RSE(Rhea System Explorer) probe
(Credits: Infrared view of Ganymede from JIRAM, NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM on Wikimedia)
A true-color image of Saturn, taken by the Cassini probe
(Credits: Saturn from Cassini, NASA/JPL/SSI)
Saturnian Moons
Though Saturn has many moons, there are a few of particular interest. Moving out from Saturn proper, we reach the twin moons of Mimas and Enceladus. While Mimas is very heavily cratered, with a particularly large crater(Herschel Crater) making the moon look like an eye, Enceladus is much more active, due to the tidal heating it experiences from both an eccentric orbit and forces from the other bodies in the Saturnian system. As a result, Enceladus has a relatively pristine surface with few craters, since the more pliable surface is easier for gravity to deform, and thus eventually smoothen. Beneath this exterior of bright-white ice—an exterior that makes it the brightest object in the core Solar System—Enceladus harbors an ocean of water, which spurts out of its many geysers. The presence of water on Enceladus, along with other chemicals integral to the formation of life, makes it a key focus in finding life in the Saturnian system.
Though less active, the larger moon Hyperion is still an interesting world to study, chiefly due to its system of bright rings, which have partially condensed onto the surface to from a ridge like its sibling Iapetus. It is unknown the source of the rings—although they could’ve been formed from cryovolcanism like those of Ceres, the relative paucity of signs of tectonic activity on Hyperion makes this hypothesis less likely. Some propose instead that the rings were formed by a smaller moon getting too close to Hyperion and being ripped apart—in a fashion analogous to the formation of its parent planet’s rings.
By far the largest and most prominent moon in the Saturnian system is Tethys. This amber-hued world is an oddball among the moons of Saturn, mainly due to both its mass and its atmosphere, composed mainly of nitrogen and hazed over with a mist of tholins—organic chemicals that pigment many worlds in the outer solar system, from the Hypnos-Thanatos binary dwarf planet to the moons of the ice giant Hecate. If Vesta was a methane-based twin of Earth, Tethys is the methane counterpart to Mars—Tethys is smaller and less massive than Vesta, and although the moon does harbor bodies of liquid methane, they’re much smaller and fewer than initially thought, as several dark patches were initially though to be large methane seas(another aspect of the moon similar to Mars). Another similarity Tethys and Mars share is being relatively geologically quiescent—with Tethys being even more so than Mars, lacking any major confirmed volcanic features. Even still, Tethys remains a captivating world, especially in the field of comparative planetology, where the moon’s unique conditions can provide clues to how and why the climates of Tethys and Vesta diverged from one another.
After Tethys is the asteroid-like Potamoi, whose terrain’s heavy cratering has made the moon appear like a sponge. This moon is one of the largest non-spherical objects in the Saturnian system, and may have gained its shape from being a piece of a larger body blown apart by an impact. Still, Potamoi has retained much of its parent body’s mass, being around 10% of Mimas’s mass. The moon gets its name from the sons of the Titan Tethys in Greek mythology, largely due to its orbital resonance with the much larger Tethys. These river spirits that give Potamoi its name also are the namesakes of many of the moon’s myriad craters.
Moving on from Potamoi, we find Iapetus, sometimes known as the “two-toned moon”. The strange appearance of Iapetus—one side covered with dark organic chemicals, the other with bright ice—may have stemmed from the originally icy moon heating up due to its slow revolution speed, melting away the ice at the moon’s darkest parts, which would expand the dark-pigmented area of the moon(since darker material absorbs more heat than light-colored material) until Iapetus gained its present appearance: one hemisphere dark, the other light. The moon also has a prominent equatorial ridge, which hints at a former ring system like its sibling Hyperion—that eventually fell onto Iapetus, forming the ridge we know today. In place of this ring system, however, Iapetus has gained the tiny, irregular submoon(a moon orbiting a moon) Clymene, whose icy composition may indicate an origin from the Saturnian rings. As far as we know, Clymene is the only submoon in the entire Saturnian system(as Rhea, though moving in tandem with Saturn, isn’t directly orbiting the gas giant, making it more akin to a dwarf planet than a moon—which means the Rhean moons aren’t submoons).
The most distant moon of Saturn still capable of maintaining a spherical shape is Theia—whose albedo, almost as high as Enceladus, gave it the name of a mythological Titan of light. (Specifically, the consort of the Titan Hyperion, as the two worlds have very similar compositions) This bright appearance seems to contradict its position as Saturn’s most distant rounded moon, as Iapetus, being closer to Saturn, has had its long orbital period result in much of its ice being melted off—a phenomenon still present on Theia, but much subdued. This, combined with the moon’s signs of prior geologic activity, has led some scientists to postulate that Theia had a much closer and more eccentric orbit, but was ejected into the outer Saturnian system. This ambiguity of Theia’s origin has made it an intriguing place for research concerning the Saturnian system, and has resulted in several proposals for a probe to be sent to Theia.
Rhea, with its largest moon Cybele peeking out from behind.
(Credits: Titan and Tethys from Cassini, NASA/JPL/SSI)
Dione and Cybele
Dione is the smaller and closer of Rhea’s two moons. It’s similar to Enceladus in many aspects, but has important differences that distinguish Dione as an interesting world to explore in its own right. For starters, Dione is larger and more massive than Enceladus, a trait that, when compounded with its tight yet eccentric orbit of Rhea, has made it significantly more active. Just like Enceladus, Dione has cryovolcanoes, but the increased influx of energy(relative to Enceladus) from both Rhea’s tidal forces and the Dionean core has made the cryovolcanoes of Dione much greater in number and significantly more powerful. This, in fact, has led to the formation of a hazy atmosphere of outgassed material from Dione’s subsurface ocean, mostly made of water vapor. The cryovolcanic activity of Dione has also led to the formation of a faint ring of ice particles around Rhea, similar to how Saturn’s E Ring formed from Enceladus’s cryovolcanoes.
However, the surface of Dione outside of the many cryovolcanoes is also highly divergent from that of Enceladus. The most notable of these deviations is the presence of large patches of thin ice, which act like windows to the deep, dark ocean below. The Dionean ocean is also rich in organic chemicals and minerals, which can tint parts of Dione’s icy crust at times. The thinner and less opaque crust of Dione, compared to other similar bodies(like the Jovian moon Europa and the Bacchian moon Ariadne), has made it a fascinating opportunity to explore the watery interior of an icy world—an opportunity rare to find, as most icy bodies have kilometer-thick crusts of ice above their subsurface oceans(if they haven’t frozen solid).
Cybele, on the other hand, is much less active and bears a thicker crust of ice. Even so, various ridges and rifts in the surface hint at a subsurface ocean(or at least the remnants of one)like its more active neighbor, albeit one far further below the moon’s frozen surface. However, Cybele’s main source of interest is its retrograde orbit, which, along with the moon’s composition being closer to a Kuiper Belt object, may indicate Cybele was captured from outside the Saturnian system. Although this would be expected of a planet like Saturn, which has many captured moons, it wouldn’t be probable for a dwarf planet like Rhea, which is not only low in mass, but would probably have lost such a captured moon to Saturn by now due to its proximity. This capture might, in fact, have also pushed Dione into the elongated orbit it has today. Regardless of however it happened, Cybele’s capture remains an intriguing part of the history of both it and the Rhean system as a whole.
Minerva
Minerva, the 8th planet from the Sun, is an icy world with a composition similar to the dwarf planet Rhea, and might have had a global ocean of methane at its surface(another similarity the planet may have shared with Rhea). However, as the planet’s primordial heat waned, this ocean froze solid to create a thick crust of solid methane. Although such a crust made of water ice would be able to develop a subsurface ocean, since solid methane is denser than liquid methane, any pockets of liquid methane warmed by geothermal heat would eventually rise to the surface—and freeze. Because of this, the only methane left liquid on Minerva was in small caverns and pockets of more porous ice near the planet’s center. Still, though the geothermal heat of Minerva couldn’t fully melt the methane crust, it was able to cause convective activity through the aforementioned process of hotter liquid methane rising and colder solid methane sinking. This Minervan cycle of convection has created a system of plate tectonics that has produced a geography somewhat like that of Earth or Vesta.
Despite the frigid temperatures, though, Minerva is still able to sustain a hydrosphere—one of liquid nitrogen. The atmosphere of Minerva is predominantly nitrogen, like that of Earth, Tethys, or Rhea, but the planet’s cold temperatures have caused liquid nitrogen droplets to accumulate, resulting in the Minervan atmosphere being perpetually hazy. As the planet slowly cooled over billions of years, much of this haze precipitated into a global ocean similar in extent to the previous hydrosphere of methane. The haze of the Minervan atmosphere also gives it an appearance similar to Tethys or Rhea, though the tholin compounds that give those worlds their amber-hued hazes instead condense onto Minerva’s surface, giving it an appearance somewhat similar to the dwarf planet Hypnos.
Minerva has a few small moons and one larger one, Pallas, a cratered moon around half the mass of our moon. Pallas’s heavy cratering may indicate that the moon formed early in the history of the solar system—likely earlier than the other, non-rounded moons. The mass of Pallas is around 29% that of Minerva, which has led some to refer to them as a “double planet”.
Minerva in visible light, from the Aegis I probe.
(Credits: Made in Universe Sandbox)
Juno
Juno, the ninth planet from the sun, is a gas giant, whose composition, mass, and size differentiate it heavily from the two prior gas giants(Jupiter and Saturn). Being around 9% the mass of Jupiter, Juno is much smaller and less massive than the other gas giants, and—largely due to its distance from the sun—much colder as well. In fact, the planet is cold enough for some of the ammonia cloud bands that give the planet its whitish color(like the bright zones on Jupiter) to sink lower into the atmosphere, making those bands darker. Juno also has a slightly blue tinge, caused by both an abundance of atmospheric methane and Rayleigh scattering(the same process that causes Earth’s sky to be blue).
However, these differences aren’t just skin deep; the interior of Juno is much richer in “ices” like water and ammonia, making it denser than the other gas giants as well. Despite this, though, the planet still has enough hydrogen and helium to be considered a gas giant, instead of a more hydrogen/helium-poor “ice giant” like the next few giant planets(Bacchus, Neptune, Pluto, and Hecate).
Like all giant planets in the outer solar system, Juno has rings—and quite prominent ones at that! They’re similar in composition and makeup to the rings of Saturn; chunks of ice of varying sizes. In addition to the rings, Juno’s orbital space also has 12 rounded moons named after deities and figures in mythology associated with the goddess Juno, and 19 non-rounded moons named after various goddesses of motherhood or royalty. By far the largest and most recognizable of these moons is Bellona, a world similar to Themis or Mars—but much, much colder!
Though there are many possible insights that could be gleaned from Juno’s atmosphere, its rings, or its moons, the considerable amount of time and resources needed to get a spacecraft to a planet as distant as Juno has been argued to be better spent sending probes to more exotic worlds like the liquid nitrogen-rich Minerva or the Hypnos/Thanatos binary dwarf planet. As such, only one mission(the Regina probe) has been sent to Juno. However, after considerable lobbying, the Nephele probe is set to launch to explore the atmosphere of Juno in this decade, potentially giving valuable information on the formation, composition, and other vital attributes of both gas and ice giants—something that can only be gained from Juno, the planet toeing the line between either category.
A photo of Bellona from the Regina probe, which explored Juno and its surrounding moons.
(Credits: Webcam image of Mars taken from Mars Express, ESA)
Juno in true color, from the Regina probe
(Credits: Made in PlanetMaker [ planetmaker.apoapsys.com ] )
Bellona
Bellona is the closest, largest, and most massive moon of Juno—and the most intriguing one as well. Bellona’s composition is largely rocky, with a minute iron core—unlike the other moons of Juno, which are less rich in silicates and more icy. Though this may at first seem to imply Bellona was ejected from the inner solar system, such a captured moon would be unlikely to settle into a close-in, prograde and circular orbit like that of Bellona.
Because of this, it may be that Bellona was once like the other moons of Juno in the distant past, but had much of its ice boil away due to heat from the still-forming Juno, in a manner similar to Semele and Io’s loss of volatiles. However, unlike these two volcanic moons, Bellona would’ve been distant enough to not experience the immense tidal forces that made Io and Semele as active as they are now. Despite this, though, the weaker tidal forces from Juno and the other moons still cause the surface of Bellona to experience geologic activity; the Bellonan atmosphere, while meager, isn’t completely absent due to prior volcanic outgassing, and some highland formations and depressions may hint at plate tectonics being present at some point in the moon’s past.
Bellona’s surface resembles that of Mars—various impact craters are scattered throughout the barren landscape, while iron oxides tint the soil a ruddy hue. However, Bellona also has titanic glaciers of frozen carbon dioxide, which slowly creep across the landscape over millennia, a slow march perpetuated by Bellona’s residual heat causing convection currents. These vast sheets of dry ice were likely the result of Bellona’s atmosphere slowly freezing—as although carbon dioxide(which makes up most of Bellona’s atmosphere)is a greenhouse gas, the thinness of Bellona’s atmosphere means that it’s much less effective at trapping heat(as compared to worlds like Earth and Venus).
Neptune
Neptune is the tenth planet from the sun, and is an ice giant mainly composed of hydrogen, helium, and volatile compounds like ammonia and water. The composition, mass, and density of Neptune and its colder ice giant sibling Pluto are somewhere between the large, hydrogen-rich Juno and the small, hydrogen-poor Bacchus.
Neptune, when viewed from space, is a blue-green color, from high quantities of methane in the planet’s hydrogen-helium atmosphere. Like Jupiter, the Neptunian atmosphere is split into multiple bands, with the lighter areas having more methane clouds than the darker areas. Below this atmosphere may lie an ocean of slushy volatile chemicals similar to that of Bacchus, though the intense pressure and lack of internal heat may crush it into high-pressure ices. Though the same pressure might be enough to condense parts of the local atmosphere into high-pressure supercritical fluid, a lack of thorough exploration of Neptune’s interior means that any theories—especially high-pressure oceans—about the interior of Neptune are uncertain at best.
One of Neptune’s major features is its heavily tilted axis, which results in extreme seasons. This tilt was likely caused by a collision with another planet in the early solar system. The collision also produced both the Neptunian rings and its system of moons. These moons are named after Greco-Roman mythological figures associated with the Roman Neptune(or his Greek equivalent, Poseidon), god of the sea and freshwater. The four largest of the Neptunian moons are Salacia, Triton, Venilia, and Nereus.
Compared to Pluto, the activity of Neptune’s interior is much lower, which could be due to a barrier trapping the planet’s heat within the Neptunian core, or the result of the collision that tilted Neptune’s axis. However, Neptune still has enough activity to form the aforementioned bands of clouds(as opposed to having none), and several large storm systems the size of countries. If the body that collided with Neptune had been larger, it could’ve disrupted the flow of heat from the interior to a greater degree, potentially rendering the ice giant inactive. This alternate collision would have also tilted Neptune much more, possibly even giving it an axis perpendicular to its orbital plane—a 90 degree tilt!
Neptune, as imaged in true color by the Voyager 2 probe
(Credits: Uranus, from the Hubble Space Telescope, NASA, ESA, L. Sromovsky and P. Fry (University of Wisconsin), H. Hammel (Space Science Institute), and K. Rages (SETI Institute) on Wikimedia)
Bacchus
Bacchus is the eleventh planet from the Sun, and is largely composed of hydrogen, methane, and water. Below the thick, crushing atmosphere that shrouds the planet, Bacchus has a world ocean made of slush that serves as a transition layer to the thick sheath of ice around the planet’s rocky core. However, despite Bacchus being ridiculously far from the sun, this frosty interior is scorching hot! This is mainly due to heat from Bacchus’s formation being trapped beneath the many layers of its thick atmosphere. In fact, the main reason Bacchus’s layers of slush and ice aren’t gaseous isn’t because of the planet’s cold temperatures—it’s because of the intense pressure!
Though Bacchus’s primordial heat is the main factor that keeps the planet’s slushy oceans from solidifying, there are other factors contributing to this. For one, the planet’s thick atmosphere is rich with heat-trapping greenhouse gases like carbon dioxide and methane. Tidal forces from Bacchus’s gigantic moon Ariadne and from the planet’s elliptical orbit also contribute. Though this ocean(mostly water and ammonia) may seem promising to life, the thick layer of high-pressure ice beneath blocks out any possible thermal vents from the core. This eliminates a key source of nutrients and energy, making it substantially more difficult for life to arise on Bacchus. Light energy from the Sun isn’t available either, not only due to Bacchus being around 12 times further from the Sun than Earth, but also because of the smothering atmosphere blocking all sunlight from ever reaching the surface. This also means that the oceans of Bacchus are always in perpetual night—whether at the “surface”(which is more akin to a dense fog) or at the very bottom, everything would be pitch-black!
Above the inky depths of the planet, Bacchus’s atmosphere is replete with storms, a maelstrom hidden beneath the outermost region of the planet—a layer of hazes that obscure the chaos below. However, one feature of Bacchus is visible even when looking from space—the auroras, created by magnetic currents in the oceans below.
Bacchus hosts a handful of moons, but one dwarfs them all: the Europa-like Ariadne. This frozen world, around seven times the mass of our moon, exerts tidal forces that spur much of Bacchus’s activity. Bits of material left over from Bacchus’s formation and ejecta from Ariadne’s cryovolcanoes have also created a tenuous ring—albeit one far thinner than those of most other giant planets, owing to Ariadne’s accretion of much of these ring particles.
Bacchus’s largest moon, Ariadne, imaged in true color by the Maenad 2 probe.
(Credits: Enceladus from Cassini, NASA/JPL-Caltech/SSI/CICLOPS/Kevin M. Gill)
Bacchus with its largest moon Ariadne, from the Maenad I probe
(Credits: Made in PlanetMaker [ planetmaker.apoapsys.com ] )
Ariadne
Ariadne is the largest of Bacchus’s 11 moons, and the only one with enough mass to retain a spherical shape. Though Ariadne’s icy surface and subsurface ocean(caused by tidal forces from Bacchus) make the moon similar to Europa, it also has a magnetic field, caused by a larger iron core. This may make colonizing the surface of the moon easier, as some of the harmful high-energy solar radiation that plagues many of the solar system’s moons would be deflected. The surface of Ariadne is also more active than Europa, likely due to an eccentric orbit. This orbit is also retrograde, which may indicate Ariadne was captured by Bacchus.
Pluto
Pluto is the 12th planet from the Sun, and is similar to Neptune in many aspects. Like Neptune, Pluto has an atmosphere largely composed of hydrogen and helium, with a layer of volatile compounds over a dense(likely rocky) core. Because of this and its mass, Pluto is an ice giant like Neptune.
However, no significant collisions have been known to disrupt the interior of Pluto. As a result, the planet has a fairly conventional axial tilt and is slightly more active than Neptune. The lower amounts of methane in the Plutonian atmosphere has also made the planet a darker, deeper shade of blue.
Pluto has a few faint rings and a system of moons, with the largest and most notable being Persephone. Persephone’s retrograde orbit indicates that it was likely captured by Pluto, and seems to be similar to dwarf planets like Eris and Hypnos, based on its composition and surface. The surface of Persephone is largely dominated by large glaciers of nitrogen ice, which emit plumes of nitrogen gas occasionally. However, these nitrogen geysers are tiny compared to the various cryovolcanoes that can be found dotting the landscape. Material from these cryovolcanoes has covered older parts of Persephone’s surface, “resurfacing” the moon. This activity has meant that Persephone’s surface might be one of the youngest in the solar system! The presence of cryovolcanoes also lends credence to the possibility of a subsurface ocean of liquid water beneath the frozen surface of Persephone. Though the tidal forces exerted by Pluto are largely the cause of these captivating features, they’ll likely push the moon closer to its parent planet, until Persephone’s torn apart into a ring of debris.
Charon in true color, from the Plutonian System Explorer 2.
(Credits: Processed version of images of Tethys taken by Cassini, NASA/JPL-Caltech/SSI/Kevin M. Gill)
Pluto, as imaged in true color by the Voyager 2 probe
(Credits: Processed version of an image of Neptune taken by Voyager 2, NASA/JPL-Caltech/Kevin M. Gill)
Charon
Charon is an icy trojan planet of Pluto around 3/5 the mass of Mars. It has a low density, largely due to the significant quantities of water ice in its interior. Though Charon may have been active in the past, the lack of an influx of energy into its interior meant that Charon eventually ceased activity. Even so, one product of this period of activity still remains: the thin, wispy atmosphere, comparable in composition and pressure to that of the dwarf planet Hypnos. Charon is covered with massive canyons, which were probably formed from the expansion of its cooling mantle(though some hypothesize that prior tectonic activity could have been the cause).
Though Charon’s relative lack of intriguing features meant that only one probe has visited it so far(the Plutonian System Explorer 2), the spacecraft has managed to detect three tiny moons, likely too small to maintain a spherical shape.
Kuiper Belt
The Kuiper Belt is a belt of inert comets and icy dwarf planets, roughly between the orbits of Pluto and Hecate. Though both it and the Asteroid Belt further in are largely comprised by small, non-rounded objects, the Kuiper Belt is much denser and larger than the Asteroid Belt, spanning the orbital region of around 34 AU to 50 AU from the Sun. The composition of the objects in the Kuiper Belt is also quite different to that of the Asteroid Belt, with much less dense materials like metals or silicates, and much more lighter materials like nitrogen, water, and ammonia—all in solid form due to the belt’s distance from the Sun. The Kuiper Belt is also much richer in dwarf planets, like Haumea, Quaoar, and others.
The name of the Kuiper Belt comes from the Dutch astronomer Gerard Kuiper, who was the first to suggest a belt of small, icy bodies between the orbits of Pluto and Hecate. However, a similar concept was suggested earlier by various other astronomers, including Frederick C. Leonard, Armin O.Leuschner, and Kenneth Edgeworth(the origin of the Kuiper Belt’s alternative alias, as the “Kuiper-Edgeworth Belt”). Despite this, the existence of a cometary belt like the Kuiper Belt was not confirmed until the early 2000s.
Pluto and Hecate exert a considerable influence on the orbits of many KBOs(a common shorthand for Kuiper Belt Objects), with quite a few settling into resonant orbits with the two ice giants. The resonant objects for Pluto(which include dwarf planets like Hypnos and Somnus) are known as “plutinos”(and sometimes “hypnoids” for their most famous member), but the resonant objects for Hecate lack a common name, namely due to the lack of exploration of these enigmatic bodies.
Hypnos in true color, viewed from New Horizons
(Credits: True-color image of Pluto taken by New Horizons, NASA/JHUAPL/SWRI/Alex Parker)
Other transplutonian Dwarf Planets
Though Hypnos is the most prominent dwarf planet beyond the orbit of Pluto, there are many other interesting dwarf planets in this “trans-Plutonian” region. Most of these dwarf planets have a surface and composition similar to Hypnos or Thanatos—covered in rock-hard ices, pockmarked with craters and bearing a few smears of organic compounds formed from reactions created by high-energy solar radiation.
Though Pluto has many non-rounded objects in resonant orbits to its own, dwarf planets in resonance with Pluto can also be found. Besides the archetypical Hypnos, there’s also Ixion, a third of Hypnos’s diameter and colored red by tholin compounds on its surface, and Somnus, a Ceres-sized world with a similar orbit to Hypnos—which has earned it the moniker of the “anti-Hypnos” and resulted it it being named after the Roman counterpart to the Greek god Hypnos. Haumea, a heavily prolate(egg-shaped) ringed dwarf planet might also belong in this group.
Beyond the resonant plutino dwarf planets are the worlds of the scattered disk—a region populated by objects whose orbits are perturbed by both Pluto and Hecate. Depending on some definitions, the range of the scattered disk can refer to objects beyond the orbit of Hecate, but most define the region as being between the orbits of Pluto and Hecate. The most prominent of these objects is Eris, the second-largest dwarf planet beyond Pluto’s orbit. Eris’s large mass makes it an intriguing target for research, as it could result in an active geology similar to that of Hypnos.
However, many of the objects in the Kuiper Belt don’t have any resonant orbits with the two ice giants on either side of the belt. The bodies in this group are called classical Kuiper Belt objects, and are sometimes called “cubewanos”. There are quite a few dwarf planets in this region, including the ruddy tholin-covered Makemake, the active Quaoar, the elongated Varuna, and many more!
Hypnos and Thanatos
Hypnos is the largest, most massive, and most well-known dwarf planet in the Kuiper Belt. The dwarf planet, which is mostly made of ice with a possible rocky center, is around 44% the mass of the Moon, and has Thanatos, a satellite 12% the mass of Hypnos. As a result of Thanatos’s mass, the two bodies are a binary pair analogous to the Apollo/Diana binary, orbiting around a common point well outside Hypnos’s surface.
This similarity between the two binary pairs can be extended to their levels of geological activity—Hypnos, like Apollo, is much more active than Thanatos, with activity from Hypnosian cryovolcanoes creating a tenuous atmosphere mainly consisting of nitrogen(though this thin atmosphere eventually condenses onto the surface of the dwarf planet). Though the tidal energy from Hypnos(even combined with primordial heat from the dwarf planet’s core) isn’t enough to create bodies of surface liquid like the lava lakes of Apollo, it’s still enough to resurface vast nitrogen glaciers, with the older layers being slowly replaced by newer strata of nitrogen ice, like the glaciers of Bellona. This activity also does allow for some liquid bodies to form on Hypnos—albeit underground, closer to the dwarf planet’s warmer core. On the other hand, Thanatos is an inactive and dead world, like its counterpart, Diana. However, like Diana, there are some signs of past activity on Thanatos, like the moon’s vast chasms likely caused by the expansion of a cooling mantle(making the moon similar to Charon in this regard).
Hypnos’s active surface has piqued interest for exploring the many other dwarf planets of the Kuiper Belt to find similar features. Many known dwarf planets have similar compositions and masses to Hypnos, like Eris and Makemake. This search for Hypnos-like dwarf planets is mainly driven by the probable presence of subsurface oceans on Hypnos, which could be a potential habitat for life even in the dark depths of the Kuiper Belt.
An image of Eris and its largest moon, Dysnomia(at the left) taken by the Hubble Space Telescope.
(Credits: Eris and Dysnomia from the Hubble Space Telescope, NASA/ESA/M.Brown(Caltech))
Hecate
The Kuiper Belt’s heavily defined edge spurred many questions about what might be lurking beyond. One of the most popular hypotheses was the presence of a planet whose gravity “shepherded” the edge of the Kuiper Belt, with objects close it its orbit either colliding with the planet or getting flung towards the sun or even further away. This hypothesis started to gain traction, and was confirmed with the discovery of Hecate—the planet marking the Kuiper Belt’s edge, named after the Greek goddess of crossroads, magic, and night.
Like Bacchus, Neptune, and Pluto before it, Hecate is an ice giant—a world composed of hydrogen and helium gas and a mantle made of “ices” like water and ammonia. However, Hecate is much larger and more massive than these other ice giants, being around 23 times the mass of Earth! Since Hecate is an ice giant, it lacks a solid surface, instead having a turbulent atmosphere with meteorological activity spurred by primordial heat in the planet’s dense core. Cloud bands of liquid nitrogen streak across the planet’s blue atmosphere(tinted as such due to Rayleigh scattering).
Hecate has a system of several moons, with 7 being confirmed as of now. Six of these moons are between the masses of Hypnos and the Moon, with one being approximately Apollo’s mass. Because of this, these moons are likely able to maintain a spherical shape. The largest of the seven moons may host an active geology due to tidal forces from its proximity to Hecate and a larger amount of stored heat in its interior(relative to the smaller moons). As of now, these moons haven’t been named, though there are several petitions and polls to assign them proper names. Hecate also has a ring system of unknown origin.
Hecate’s position and lunar system could make it an important junction between the core solar system and the secondary planetary systems of the Nemesis/Tyche binary, the Wotan system, and the Caelus system. Outposts on the moons of Hecate would be closer to these faraway worlds. Hecate’s mass means it could also be used to fling spacecraft further, via gravity assists. However, despite all these potential benefits of exploring Hecate being only applicable in the distant future, the impact Hecate’s orbit has on the Kuiper Belt and the potential research opportunities of both the ice giant and its moons are important motives for further exploration of Hecate.
The ice giant Hecate, seen in visible light from the Terminus probe.
(Credits: Made in PlanetMaker [ planetmaker.apoapsys.com ] )
The Sednoid Region
The region of our solar system between the orbits of Hecate and the Fortuna binary system lacks any major planetary bodies(at least, to the best of our knowledge), instead being filled with comets and a few dwarf planets on highly elliptical orbits.
Sedna, a body smaller than Ceres but larger than the Saturnian moon Koios, is the namesake of this region, as it was the first dwarf planet discovered there. Though Sedna’s great distance from the Sun has made observation extremely difficult, many measurements have shown the dwarf planet likely bears a ruddy hue from organic compounds staining its icy surface. Like most known dwarf planets outside Pluto’s orbit(barring outliers like Haumea), Sedna and the other dwarf planets in the Sednoid region are composed of an ice-rock mixture.
The most captivating aspect of Sedna and its fellow Sednoid dwarf planets is their heavily elongated orbits. Gravitational perturbations from the Fortuna binary are likely to blame. However, according to some models, the elliptical orbits of the Sednoids could have been even more stretched. If the Fortuna binary was captured into the Solar System on a different trajectory that would slingshot it back into interstellar space, their gravitational tug would pull the Sednoids’ orbits outwards, instead of hemming them in.
Though the Sednoids and their orbits are indeed fascinating opportunities to investigate the processes that formed our solar system, the avenues for research present in the Solar System’s sub-stellar systems(like that of the Fortuna binary, or the system of planets orbiting the red dwarf Wotan) are lacking for many of these dwarf planets. As such, none of the Sednoids has received a single probe mission.
Chione
The distant world of Chione(named after the Greek goddess of snow) is a Bacchus-like planet on an extremely eccentric orbit, between the edge of the Kuiper Belt and the orbit of the Fortuna binary system. Chione’s gravity shepherds the orbits of the Sednoids, while the Chionean orbit is in turn affected by the pull of the Fortuna binary. In fact, the gravitational influence exerted by Chione on the Sednoids was key to the planet’s discovery, just as the influence the Fortuna binary had on Chione and the Sednoids would help astronomers discover Nemesis and Tyche(the primary bodies of the Fortuna binary) as well.
Like the Sednoids, Chione’s orbit is likely truncated by the influence of the Fortuna binary, and would’ve likely been more eccentric and distant if the Fortuna binary was never fully captured by the Sun. This would have severly inhibited the discovery of the planet, and the concept of a Chione-like planet shepherding the Sednoids’ orbits might have remained pure conjecture.
Chione proper shares many similarities to Bacchus, as stated before. Both have thick envelopes of gas, but are significantly smaller and lighter than the other gaseous planets. Their great distance from the sun also means they’re freezing cold—but Chione in particular is notable for this, being more than 20 times further from the Sun than Bacchus. It’s so cold on Chione that the nitrogen in its atmosphere not only condenses into clouds of liquid droplets, but frequently freezes into nitrogen snow! Both planets also have a few moons(though Chione hasn’t been found to have any large moons like Ariadne).
While Chione’s immense distance from the sun has meant no probe has visited the planet as of now, a collaboratory network of several space agencies has planned to launch a probe to Chione and its moons, in an effort they’ve dubbed the “Boreas Project”. Despite the many other missions and ventures planned by many of these space agencies, the organizers of the Boreas Project have faith that their probe will manage to be realized by the end of the century.
Nemesis and Tyche
After the discovery of Terminus and Hecate, many raced to find a new planet. However, instead of discovering one world, a team of scientists managed to find a new planetary system just outside the Sun’s doorstep: the Fortuna system, centered around the binary gas giant-brown dwarf pair Tyche and Nemesis.
The binary has an eccentric orbit around the Sun, with an average distance of about 600 AU(short for an astronomical unit, the distance between the Earth and Sun). This oblong orbit causes many objects in the vicinity of the binary to either be ejected from the solar system or flung towards the Sun. This activity might have added material to the still-forming planets, and without the Fortuna binary, many planets might not exist. So far, there haven’t been any recorded impacts of any of these ejected objects on Earth, though some hypothesize that they could be the cause of a few lunar craters. Other ejected objects have settled into more stable orbits around the Sun, and may make up the bulk of a few ice-rich asteroid clusters in the Asteroid Belt.
Both Tyche and Nemesis are made up of predominantly hydrogen and helium, and are many times the mass of Jupiter. Nemesis, at around 30 thousand Earth masses, is near the highest mass a brown dwarf(a star-like object incapable of ordinary nuclear fusion, instead fusing the hydrogen isotope deuterium) can be before being able to sustain regular hydrogen fusion. This means that a more massive Nemesis could have become a star! By contrast, though Tyche is ridiculously massive, it’s not massive enough to sustain any form of fusion; so if brown dwarves are “failed stars”, Tyche is a failed failed star.
Though the internal heat of Tyche and Nemesis creates activity in their atmosphere, this activity would be much less intense without the extreme tidal forces they exert on each other. Though the solar system has many binary systems, few have partner bodies as close to each other as Tyche and Nemesis, pulled into prolate spheroids—a term referring to spheres stretched at their poles, like eggs. Because of this, both bodies of the binary are much hotter than they would be independently, with Nemesis glowing a bright red, and Tyche bearing water vapor clouds—just like on Earth! Because of this extra heat, the planets and other bodies of the Fortuna binary system are quite warm, compared to other bodies this far from the Sun, with Fortuna I even being able to sustain seas of water on its surface!
Fortuna II
The second major planetary-mass body orbiting the binary of Nemesis and Tyche is Fortuna II, an icy Hermian(a solid body more massive than Earth) world that shares similarities with the planet Minerva and the Plutonian moon Persephone. Tidal stress caused by its eccentric orbit, along with a large amount of radioactive material in its core(as a result of its high mass), has allowed Fortuna II to sustain frequent tectonic activity. This activity has resulted in the formation of many cryovolcanic complexes that spew out gasses that form the planet’s hazy atmosphere.
Similarly to Minerva and Persephone, the tectonic activity of Fortuna II is largely due to convection in the semi-solid layers of water ice and frozen methane below its surface. Below this is a thick rocky mantle—as while the planet certainly has a large amount of volatile material, it’s still mainly terrestrial in composition. Between the layers of convecting ice and the mantle may lie subsurface oceans—though, since the intense pressure exerted upon them would be enough to create exotic ices, this would be unlikely.
A major similarity Fortuna II shares with Minerva, however, is the presence of (semi-)liquid bodies of nitrogen on its surface. These evaporate during Fortuna II’s closest approach to the binary it orbits to create its atmospheric hazes.
Fortuna I
The closest world to the Fortuna binary is the tightly-orbiting world provisionally named Fortuna I. Though it hasn’t been yet detected by humans, Fortuna I bears sizable seas—of water, like Earth! A number of extremely unlikely factors maintain liquid water on this frigid world. First of all, the planet is, as previously stated, extremely close—which both means that it has significantly more solar energy(perhaps Fortunian energy?) than much of the rest of the system, and, when coupled with its elliptical orbit, creates heavy tidal activity that spurs the mantle of the “super-earth” planet into action.
Both of these factors make the planet warm enough to maintain its hyper-saline seas—though just barely, and only with its thick atmosphere rich in heat-trapping greenhouse gasses. However, Fortuna I’s close distance to its parent bodies has also heavily deformed its surface due to intense tidal forces, so much so that, like the gas giant and brown dwarf it orbits, the world is (slightly) egg-shaped! Because of this, almost all the surface liquid water on the planet sits on the band between the side of the planet facing the Fortuna binary and the side facing away. The same applies with the heavily clouded atmosphere, making the planet appear to have a belt of clouds along this band.
With a climate similar to Earth’s hottest deserts, the side of Fortuna I facing the binary is hot enough to sustain freshwater(instead of the just-above-freezing brine of the planet’s oceans)—but because of the planet’s deformed shape and the resulting lack of atmosphere on the poles, there are little to no bodies of water on this part of the planet. This landscape is instead covered in towering volcanoes(both larger shield and smaller conical) and their remnants; rugged mountain ranges eroded by the winds of the meager atmosphere, frequent lava flows from the nearby active neighbors, and the crushing pull of the planet’s gravity.
Fortuna I’s other, colder pole is not much better. While the binary-facing side of Fortuna I was a landscape of lava and chaos, this region is a wasteland of icy monotony. The entire pole is covered in a glacier the size of Asia, with only a few small volcanoes providing heat for the region. Pockets of water beneath this icy crust are present, but are few and far between.
Below the hostile surface, Fortuna I’s interior is largely similar in composition to that of Earth or other terrestrial planets, though its convective current system in its mantle(a system of currents carrying hotter material up, and colder material down) varies significantly due to the heavy tidal forces the planet experiences. Some models, however, predict an early Fortuna I as having a thick gaseous envelope that later evaporated, similarly to many major models of Mercury’s formation.
While Fortuna I may be a hostile and alien world from a human perspective, its inter-polar ocean could still harbor life, and the presence of surface water and a thick atmosphere make the odd world the most Earthlike object in Nemesis’s gravity well.
Fortuna III (and Fortuna III-b)
The largest and most massive body detected by humans orbiting the Fortuna binary is Fortuna III, a Hermian world with a thick, hazy atmosphere. This world’s gravity is crushing, due to it being both more massive than Earth and made of dense materials like minerals and metals.
This has allowed Fortuna II to retain much of its internal heat—warming it up enough for ammonia—crystal clouds tinted with organic tholin compounds to form in its atmosphere. Because of this, Fortuna III’s atmosphere heavily resembles that of the gas giant Jupiter in the core solar system(at least from space).
Below this atmosphere—thicker than even that of Mercury—Fortuna III’s surface is covered in many volcanoes spurred by the primordial heat retained within the planet’s interior. The intense volcanism of Fortuna III is also spurred by tidal forces from its massive, dense metal-rich moon.
Fortuna III’s only known moon(named Fortuna III-b as a placeholder) is an oddity, to say the least. For the other major planets in the Fortuna binary system, the gravitational preturbations caused by the Fortuna binary have made it impossible for a moon to be captured, or a moon formed in sync or through a collision with its parent body to remain in orbit. However, Fortuna III’s distance and mass have allowed it to capture a moon around 15% the mass of ours. This moon has had its mantle and crust stripped by a collision with another body(which would collide with Fortuna III). Because of this, Fortuna III-b is predominantly metallic, with more than half of its mass coming from its core.
Wotan
The red dwarf star Wotan is a truly special celestial body, as it is the closest star to Earth that isn’t the Sun. Though Wotan orbits much farther from the Sun, with an orbit that can reach around 4 thousand times the Earth-Sun distance, it’s still nearby in a stellar scale; the closest star not bound by the Sun’s gravitational pull is the somewhat similar Proxima Centauri in the Alpha Centauri quarternary star system—and it’s hundreds of times further away! Wotan was captured by the Sun very early in its formation. The effects of this capture can be seen in the “Ginnungagap”—a gap left by Wotan in much of the Solar System’s Oort Cloud(the outer spherical shell of comets that surrounds our Solar System). This area is notable for having even less bodies than the rest of the rather sparse Oort Cloud, as all the comets that may have been there were slingshotted inwards or flung even further towards interstellar space.
Wotan’s starhood has had many important effects on its local region of space, but perhaps the most crucial is its formation of a “mini-planetary system” within the Solar System, in a similar fashion to the Fortuna binary—but on a much larger scale. This “Wotan subsystem” is full of diverse worlds as interesting and captivating as those in the Core Solar System—and may hold potentially habitable worlds.
As such, several probes have been sent to Wotan in spite of its vast distance. These probes are part of the Sleipnir program for the exploration of the Wotan subsystem, and the special technologies used to get them billions of miles away from Earth may be used in future exploration programs to other auxilary planetary systems of our Solar System, such as the Fortuna binary and the Caelus system. The first probe missions intended to explore Wotan’s planetary system, however, were not part of the Sleipnir program, instead being the Huginn-Muninn spacecraft sent to gauge the orbits and masses of the Wotan system’s many bodies for future attempts.
Since the planetary system of Wotan is similar in magnitude to the Core Solar System, yet possessing several key idiosyncrasies that differentiate it as a distinct locale, its celestial bodies have been named after deites and elements of Germanic(largely Norse) religion, as it possessed a polytheist pantheon often parallel to that of the Roman pantheon from which many bodies of the Core Solar System are named.
Wotan from the Sleipnir 2 probe, true color
(Credits: Made in PlanetMaker [ planetmaker.apoapsys.com ] )
The Baldr Belt
The Baldr Belt is a region of the Wotan subsystem largely similar to the Helios Belt in the Core Solar System. However, the lower amount of material Wotan was able to attract when its planetary system formed, combined with frequent gravitational perturbations from Wotan and its namesake planet, has made the Baldr Belt far scarcer in material. Still, like the Helios Belt, this region does have a few dwarf planets, including Hermod and Hod, both of which are similar to an inactive Apollo.
In particular, Hod is a captivating world—because of its potential stores of radioactive material. The heat emitted by these radioisotope-rich metals and minerals creates frequent resurfacing events on Hod, where much of the surface is covered by lava gushing out from under the dwarf planet’s thin crust. A particularly violent resurfacing event, in fact, could’ve flung enough of Hod’s mass into space to create its thin, dark rings, and may have possibly even pelted Baldr with numerous asteroid strikes.
The airless dwarf planet Hod, seen from the Sleipnir 2 probe.
(Credits: Dione’s Aeneas crater from Cassini, NASA/JPL/SSI)
Baldr
Baldr is the closest major planet to Wotan, and is a terrestrial world around four-fifths Earth’s mass. With its thick atmosphere largely composed of carbon dioxide, Baldr is similar to a more hostile version of Venus, or a more placid Mercury. Though Baldr has several major volcanic complexes like Mercury, none have yet been observed as active.
Baldr’s atmosphere is also thinner than Mercury’s, but thicker than that of Venus. The entire world is blanketed with a thick layer of acidic clouds tinted orange by iron oxide dust and Wotan’s ruddy light. This orange tint, though, is only really prominent in the lower atmospheric layers, and from space, Baldr appears nearly white. Because of this, Baldr has the highest albedo in the entire Wotan subsystem. Baldr’s brightness is the primary reason for its name; the planet’s namesake being a highly venerated Norse god of light(among other things)
Like both Venus and Mercury, though, the surface of Baldr is nearly pitch-black from the sheer volume of obscuring clouds. This surface is, curiously enough, pockmarked with several large craters. As most crater formations erode quickly on planets with thick atmospheres like Baldr, these craters are likely from a very recent event. Heavy deposits of radioisotopes in similar ratios to those found on Hod may implicate the dwarf planet as the origin of Baldr’s mysterious craters.
Baldr, given its proximity to Wotan, was only able to hold onto a single moon: Nanna, a moon similar to our own but a tenth the mass. However, given that practically all known planets that orbit around Baldr’s distance from their parent star(s) are moonless, this is intriguing by itself.
An image of Baldr created with both radar and visual data from the Vali subprobe of the Sleipnir 2 mission. The former is used to illustrate parts of Baldr’s topography concealed by its thick cloud cover.
(Credits: NASA artist’s impression of the super-earth exoplanet L98-59 c on Wikimedia, NASA)
Frigg
Frigg is the second closest major planetary body to Wotan, and is a Hermian world with a thick, crushing atmosphere like Venus. However, Frigg’s hostile conditions make Venus seem hospitable by comparison, with its significantly thicker and hotter atmosphere made largely of carbon dioxide, and its immense mass that has resulted in the planet having the heaviest surface gravity of all bodies in the system(with the “surfaces” of the gaseous planets being where the pressure is 1 atm). Because of the planet’s mass, Frigg has been named after the queen of the gods in the Norse pantheon. In the former, Frigg is similar to its neighbor Baldr—though marginally cooler. Frigg, like nearly every celestial body in the Wotan system, is tidally locked—one side always faces Wotan, the other always faces away. Though this means Frigg’s split into a hotter and cooler half, the effect is heavily dampened by the heat circulation provided by the planet’s thick atmosphere.
Still, the cooler regions of Frigg can get cold enough to sustain liquid—sulfuric acid, that is. This has meant that Frigg doesn’t have as clouded an atmosphere as expected. Counteracting this is the positioning of most of the cooler zones on high-altitude plateaus and mountains, which results in much of the sulfuric acid trickling down to the scorching lowlands, where it evaporates once again. Despite this, a few small pools and lakes have survived near the poles and at the nightside. Larger bodies of sulfuric acid on Frigg were once likely present, but inward planetary migration resulted in them evaporating away, leaving behind the boiling basins most rain trickles into.
Despite the same factors making Baldr’s moon improbable applying to Frigg, Frigg’s significantly greater mass(at around 10 times that of Earth!) and greater distance from Wotan has allowed it to hold three small asteroid-like moons. These are named Lofn, Hiln, and Gna after three major Norse goddesses of similar stature to Frigg.
Frigg in visible light, Sleipnir 5 probe
(Credits: NASA artist’s impression of the earth-mass exoplanet TRAPPIST-1 c on Wikimedia, NASA)
Loki
Loki is a volcanic world similar to the Jovian moons Semele and Io, albeit much more massive—twice as much as Mars, in fact. Like these moons, Loki has a surface covered in various shades of cooled sulfur spewed from its many volcanoes, and it has a few patches of molten rock on its surface like Semele. Unlike these two moons, Loki has also managed to accrue a tenuous atmosphere made primarily from expelled volcanic gasses.
The orbit of Loki is extremely eccentric, going from near that of Frigg to scraping by the inner boundaries of Wotan’s habitable zone. For this reason and its intense volcanism, Loki was named after a Norse god of trickery and fire. However, many models of the planet’s orbit imply it to have had a far more erratic orbit, sometimes even getting as far as the gas giant Thonar before settling down in its current position. This may be the cause for heavy amounts of meteorites with similar compositions to Loki being found nearly everywhere in the Wotan system.
While radioactive decay in its interior and its elongated orbit provide Loki with much of the energy it needs to sustain its volcanic activity, Loki’s single gigantic moon Hela contributes significantly to this as well. Hela also has a volcanic, sulfur-coated surface, though its level of activity is similar to that of Io; heavy, but not enough to form the super-calderas and lava lakes seen on Loki. There’s also a lower amount of activity in Hela’s farside(the side of Hela facing away from Loki), resulting in that half being largely predominated by smoother plains of lighter-colored sulfur, only infrequently punctuated by a few craters or a minor volcanic complex.
Nerthus
Nerthus is a terrestrial planet larger than Earth and more massive(at around 3.76 times Earth’s mass). Still, Nerthus is remarkably Earthlike, even despite its many alien qualities. Despite its position right at the habitable zone’s edge, Nerthus is able to sustain a nearly global ocean of liquid water due to a thick atmosphere with plenty of greenhouse gasses.
Nerthus seems to have formed unlike most conceptions of an Earth-like planet, with many models suggesting it having started out in the furthest regions of the Wotan system. Planetary migration eventually pushed the planet into a closer orbit, but it could have become an ice giant like Njord, a Neptune-like planet whose orbit was likely close to that of proto-Nerthus.
Though Nerthus’s static day/night cycle(caused by tidal locking with Wotan) means that half of the planet is significantly more prone to glaciation than the other, the heat-distributing capacities of both the planet’s atmosphere and oceans means that most ice sheets on the nightside are seasonal, though there is still a few country-sized glaciers near the main ice cap. This mitigation of the heat difference caused by a lack of a day/night cycle can also be found on Frigg, though more due to the planet’s atmosphere. Nerthus also has thick cloud layers that can make the glaring brightness of Wotan on the dayside more tolerable.
While Nerthus’s geology is rather Earthlike, with continental and oceanic plates, much of this is underwater, with only volcanic island chains and mountainous plateaus poking out of the world-ocean. These landmasses have a climate similar to polar and sub-polar climates on Earth.
Nerthus’s abundance in liquid water and multiple possible energy sources(radiation from Wotan, interior heat) make the world extremely promising in the search for life outside Earth, as well as putting it as a prime target for colonization beyond the Core Solar System. Because of this, it has been a primary focus of many proposed missions to Wotan, and was the main focus of the Sleipnir 3 and Sleipnir 4 probes. Nerthus’s similarities to Earth have also given it the name of a Germanic goddess who was associated with both the earth and sea—and with Njord, the Norse god of the sea that lends Nerthus’s distant gaseous cousin its name.
Nerthus in visible light, Sleipnir 3 probe
(Credits: NASA artist’s impression of the earth-mass exoplanet TRAPPIST-1 e on Wikimedia, NASA/JPL-Caltech)
Thonar
The fifth major planetary body in the Wotan system, Thonar is a gas giant similar to Jupiter, except smaller and less massive. Like Jupiter, Thonar lacks a solid surface, instead having thick layers of stormy cloud bands in a hydrogen-helium atmosphere extending down to its high-pressure mantle of liquid hydrogen. Thonar’s storms and similarities to Jupiter are the main reasons for it being named after an alternate name for the Norse god Thor, who ruled over the storms and lightning, and was in many ways analogous to the Roman god Jupiter.
Like many gas giants, Thonar has several moons orbiting it, though due to Wotan’s proximity and gravitational disruptions from its captured largest moon Sif, it has less than would be expected of a gas giant its mass.
The Thonarian moon Sif, Sleipnir 6 probe
(Credits: Another NASA artist’s impression of the earth-mass exoplanet TRAPPIST-1 e on Wikimedia, NASA/JPL-Caltech)
The gas giant Thonar, Sleipnir 5 probe
(Credits: Computer rendering of a Jupiter-like gas giant on the astronomy software program Celestia made by user CubicApocalypse on the Celestia forums, uploaded onto Wikimedia, Celestia)
Sif
Sif is a moon of Thonar that was likely once a terrestrial planet, but later captured by the gas giant, something probable from its great mass compared to Thonar’s other moons(being the most massive, at around twice Mars’s mass!) and its retrograde orbit(orbiting in the opposite direction to the rest of Thonar’s lunar system). This capture likely flung many of Thonar’s moons out of its gravity well or into the planet itself, giving an justification for why the gas giant has so few satellites.
Due to tidal heating from its parent and a greenhouse gas-rich atmosphere, Sif is warm enough for major bodies of liquid water to form on its surface, though these are largely in the form of small muddy lakes and ponds, with the largest seas still being rather small. Sif’s surface also has a few impact craters with meteorite remains oddly similar to Loki in composition.
Sif’s atmosphere is thin, but still enough to make sure its water hasn’t boiled away due to low pressures(something that can’t be said for much of Mars’s surface). This atmosphere is two-third nitrogen and around 30% carbon dioxide, with the rest being largely inert noble gasses.
Because of the presence of liquid water on its surface, it’s highly probable that Sif may be close to habitability, and the moon may be very likely to host some level of life. This potential habitability, along with it being Thonar’s most prominent moon, has led to it being named after a Norse goddess of the earth and fertility—who was also the wife of the god Thor, Thonar’s namesake.
Tiw
Tiw is a terrestrial planet with a mass and surface similar to Mars—albeit at a further state in its loss of its atmosphere and water. Unlike Mars, whose day/night cycle is (remarkably) similar to that of Earth’s, the day/night cycle of Tiw is nigh-absent due to the planet being tidally locked to Wotan. This has meant that while one half of Tiw is covered in deserts and mountains, the other is hidden beneath a kilometer-thick glacier made of water ice and dry ice frozen from the planet’s carbon dioxide atmosphere.
Tiw also has an asteroid moon—but, oddly enough, based on the moon’s composition, it may have once been a satellite of Loki. This moon would’ve been ejected on one of Loki’s eccentric orbits, being flung outwards until it would be captured by Tiw. From there, this moon would settle into a relatively close-in orbit—close enough, in fact, that it blocks Wotan at times, making Tiw the only planet in the Wotan system known to have “solar” eclipses(“solar” in quotation marks, as the star being eclipsed is Wotan)! Because of this, Tiw’s asteroid moon has been named “Fenrir”, a wolf in Norse mythology who was one of Loki’s children and is supposedly destined to swallow the sun and moon.
Frey and Freyja
Frey is a terrestrial planet around the mass of Earth, and has a fluid system like our “blue marble”. However, Frey’s oceans, rivers, and lakes are made of liquid ammonia instead of water—which is instead frozen rock-solid, making up the outer layer of the planet’s crust. Frey also has a thick atmosphere, rich in nitrogen like that of Earth.
Perhaps the most important factor impacting Frey’s geology and climate, though, is its gigantic moon, Freyja—one of the largest moons in the solar system, and so massive it forms a binary pair with Frey. Because of this, Frey’s coastlines and cliffs are battered with intense tides caused by Freyja’s intense pull(and the same holds through with Freyja’s bodies of liquid). Freyja’s great mass has also caused Frey and Frejya to be tidally locked to each other; with one side of Frey always facing Freyja, and one side of Freyja always facing Frey.
This has made it so that the two worlds have an equal day/night cycle—one that’s synced up with the orbital period of Freyja. Freyja and Frey’s presence of a day/night cycle has prevented either world from suffering the eternal day/night divide many other worlds in the Wotan system face due to being tidally locked to Wotan.
Meanwhile, Freyja’s great mass has allowed it to hold onto an atmosphere and sustain an ammonia “hydrosphere” like its parent world. Freyja’s geography, though, is different; much of Freyja’s ammonia is mixed with its soils(called “regolith” when referring to extraterrestrial locales) to create vast muddy regions; making Freyja an “ammonia swamp moon”. Freyja also has several major volcanic belts near its Frey-facing side stimulated by tidal energy.
The unique ammonia fluid systems of Frey and Freyja and the potential for exotic biochemistry—and ammonia-based life—they present make the two worlds prime targets for exploration in future planned Sleipnir missions. In fact, various plans have been made for this binary system to be visited by the first of the “Vanaheim Missions”, expeditions made to the outer planets of the Wotan system.
Freyja from space, Sleipnir 7 probe
(Credits: A photo of Mercury from a homemade telescope, InternetArchiveAndroid on Wikimedia)
Njord
Njord is an ice giant similar to Neptune and Pluto in the core Solar System. However, it’s significantly warmer than its core Solar System counterparts, and is able to sustain clouds of ammonia ice in its atmosphere as a result. The atmosphere of Njord is also tinted a cerulean blue due to the heavy presence of methane gas, like on Neptune.
Similarly to Neptune and Pluto, Njord also has quite a few moons, with five major ones having became rounded through their great mass(relative to the other, asteroid-like moons).
Though Njord stands alone as the only ice giant in the entire Wotan system, there’s a chance it could have turned out differently. Since most models predict Nerthus being near Njord’s orbit during its formation, what is now an Earthlike planet could have built up a gaseous envelope like Njord if it had never been preturbed into the inner system. As such, Nerthus and Njord are considered at times in planetology as “sister worlds”, despite their extremely divergent present conditions.
Njord in true color, Sleipnir 1 probe
(Credits: Neptune from the ESO(European Southern Observatory)’s VLT,ESO/P. Weilbacher (AIP))
Jormungand Belt
The farthest reaches of the Wotan system are largely devoid of any major planetary-mass objects, instead being populated by many smaller cometary and asteroid-like celestial bodies. This region is largely similar to the Kuiper Belt in the Core Solar System, though significantly more concentrated and with fewer dwarf planets.
Heimdall from Sleipnir 1
(Credits: True-color image of Pluto taken by New Horizons, NASA/JHUAPL/SWRI)
Heimdall
By far the most massive body in the Jormungand Belt is Heimdall, a dwarf planet similar to Hypnos. However, Heimdall is significantly more massive, at around 60% Earth’s mass! This allows Heimdall to retain more of its interior heat, allowing it to maintain an active surface like that of Hypnos.
Another major effect of Heimdall’s mass is its substantial(yet still extremely thin by Earth standards) atmosphere, which contains noble gasses like argon and helium, as well as nitrogen that vaporized from its surface on its closest approaches to Wotan.
Heimdall’s mass is also a major reason why the Jormungandr Belt is largely lacking in bodies massive enough to sustain a rounded shape, putting into question its initial designation as a “dwarf planet”. Whether Heimdall should be promoted to full planet status is a matter unsolvable until further information is gathered—giving credence to proposals for another Sleipnir mission to explore Heimdall in more depth. What is known, though, is that Heimdall has several clusters of asteroids and comets around its Lagrange points; these trojans(a term derived from similar clusters of asteroids around Jupiter’s orbit) revolve on the same orbit as Heimdall, just 60 degrees ahead of or behind the dwarf planet. These asteroid clusters have been called the “Bifrost Groups”, after the rainbow bridge in Norse mythology watched over by the god Heimdall.
A major similarity Heimdall also shares with Hypnos is its heavily elongated orbit; one that has allowed it to be the first celestial body besides Wotan directly photographed by a probe mission(Sleipnir 1). This is because the far end of Heimdall’s orbit is the closest end to Earth, making the distance needed to reach the dwarf planet shorter.