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Useful Notes / Dwarf Planets
aka: Pluto

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Dwarf Planets are those weird things that cannot be classified as planets due to their lower mass, but are too massive to be called planetoids or asteroids. The term is fairly new and was created in 2006 to classify Pluto and other objects whose gravity is large enough to compress them into a sphere (aka, being in so called hydrostatic equilibrium), but not large enough to achieve so-called "orbital dominance", where they are the only thing in their orbits.

Officially only two non-planetary objects (Pluto and Ceres) are confirmed to be in Hydrostatic equilibrium. But the IAU (whose opinions are probably the most universally accepted as things get in these matters) has also given the designation to three other objects (Eris, Haumea and Makemake) for various reasons and four others (Orcus, Quaoar, Gonggong and Sedna) tends to be generally accepted as dwarf planets even if the IAU has yet to officially give them that label. And based on our current knowledge of what's required for hydrostatic equilibrium, there could be as much as 50 more catalogued objects that also qualify.

The five officially accepted ones are:

Ceres as seen from the Dawn probe.
  • Designation: Asteroid Belt Object
  • Diameter: 946 km
  • Mass: 0.00015 of Earth
  • Density: 2.08 g/cm3
  • Surface Gravity: 0.03 g
  • Semi-major Axis: 2.76 AU
  • Orbital Period: 4 Years
  • Rotational Period: 9 Hours
  • Axial Tilt: 4°
  • Average Surface Temperature: -105° C
  • Notable Features: Occator Crater, Ahuna Mons
  • Number of Total Missions: 1

Named after a Roman goddess of agriculture, Ceres is the largest asteroid and only dwarf planet in the Asteroid Belt, making up about a third of the belt's mass. It is the smallest of all the dwarf planets, but was the first to be discovered due to its proximity to Earth relative to the other ones. It has gravity of less than 3% that of Earth and possesses a tenuous atmosphere consisting largely of water vapor.

It has been theorized that life might had once existed on Ceres due to its large quantities of ice (which could have once been a subterranean ocean) and the presence of organic compounds. Ceres is even one of the more plausible candidates for Panspermia in relation to Earth.

Due to its proximity within the Asteroid Belt, it's believed that Ceres is the original proto-planet that failed to form into a full-fledged planet because of Jupiter's gravity robbing it of most of its surrounding material during the Solar System's formation. Had Ceres had enough material to form, it would've likely became the fifth terrestrial planet in the inner Solar Systemnote 

Ceres has a complicated history of designations given to it since its discovery, even more so than Pluto. It was originally labeled as a planet when it was discovered on New Year's day 1801, and was quickly joined by the asteroids Pallas, Juno, and Vesta. Ceres' discovery was taken as proof of the now-defunct Titus-Bode law. William Herschel argued as early as 1802 that Ceres and Pallas were not planets and at the time coined the term "asteroid" to distinguish them from the other planets and labeled Ceres as such, but his opinion was in the minority until the discovery of the fifth asteroid, Astraea in 1845. By then, telescope technology had advanced enough that thousands of asteroids were discovered over the next few years, and astronomers accepted that Ceres was merely the first. Ceres, Pallas, Juno, and Vesta were formally demoted from planet status in 1851, the first to do so. Their demotion also notably occurred after the discovery of Neptune in 1846; few people remember that Neptune spent its first five years post-discovery as the solar system's twelfth planet. Ceres remained classified as an asteroid until 2006 with the reclassification of dwarf planets by the IAU; it was officially labeled as the sole dwarf planet in the inner solar system after being classified as the largest asteroid in the Asteroid Belt for over a century and a half.note 

So far, Ceres has been visited by one spacecraft named Dawn in March of 2015 after exploring most of the Asteroid Belt and Vesta for eight years. It found that Ceres' surface is similar to that of Mercury and The Moon, covered in countless impact craters, but due to being farther out from The Sun its composition is thought to be a combination of rock and water ices. Its most interesting features are bright white spots in the center of some craters, and for a short while, scientists were puzzled as to what they were. Later observations confirmed that these spots are patches of a type of salt brine coming from deep beneath Ceres' surface, revealing that Ceres has active cryovolcanoes, but for a brief moment, scientists were spooked over the possibility of alien civilizations based there.

     Appearances in fiction 

Pluto as seen from the New Horizons probe.
  • Designation: Plutino (Kuiper belt object in an 2:3 orbital resonance with Neptune)
  • Diameter: 2,376 km
  • Mass: 0.00218 of Earth
  • Density: 1.88 g/cm3
  • Surface Gravity: 0.06 g
  • Semi-major Axis: 39.48 AU
  • Orbital Period: 248 Years
  • Rotational Period: 6 Days (Tidally Locked to Charon)
  • Axial Tilt: 122.53°
  • Average Surface Temperature: -229° C
  • Notable Features: Tombaugh Regio, Cthulhu Macula
  • Number of Moons: 5
  • Number of Total Missions: 1

Named after the Roman god of underworld, Pluto is the most well-known of the dwarf planets, though it's mostly known for its career as the system's ninth planet. Discovered in 1930 by Clyde Tombaugh, for decades, it was classified as a full-fledged planet, but as more objects of similar size to Pluto were discovered in the outer Solar System, that method of classification became impractical and virtually impossible to be meaningfully utilized. Thus, in 2006 with a vote by the International Astronomical Union, the classification of dwarf planets was created.

Pluto's name was famously suggested by Venetia Burney, at the time an eleven year-old student at Oxford. Burney was well-versed in Classical Mythology and felt that the god of the underworld was a fitting name for such a cold and dark planet. Her grandfather was a librarian at Oxford who saw that Burney's suggestion was relayed to Tombaugh; that same grandfather was the brother of the man who suggested the names currently used for Mars's moons. Burney lived to see Pluto's demotion, and stated in an interview that while her personal preference was for it to remain a planet, she understood why it was demoted. Burney was reportedly not happy when people mistakenly claimed that she took the name Pluto from Mickey Mouse's Dog.

Pluto is the largest of the dwarf planets, though not quite the most massive. Its mass is less than a fifth of that of the Moon, its surface gravity is 6% of Earth's, and it has a surface area roughly similar to that of Russia. It consists of a silicone core covered by a thick layer of ice. Similar to Europa, it likely has a liquid ocean beneath its crust. It has no magnetic field to speak of.

Pluto's orbit is highly inclined to the rest of the ecliptic plane by 17°, making its motion appear tilted when viewed from a side angle. In addition, it has a moderately-high orbital eccentricity. At perihelion (closest approach), Pluto is at a distance of 29 AU and closer to the Sun than even Neptune, while at aphelion (farthest approach), it slingshots as far out as 49 AU. This unusual orbital motion is what clued astronomers and scientists about Pluto's characteristics and is one of the contributing factors to its demotion to dwarf planet status. Pluto takes 248 Earth years to complete one revolution around the Sun.

Pluto's most peculiar feature is its largest moon, Charon, which has one-eighth of Pluto's mass and one-half of its volume - an unusually big proportion for a planet and a moon. It lead to Pluto being occasionally labeled a dual planet, since calling Charon a satellite isn't fully accurate with how the objects revolve around one center of mass that lies in the empty space above Pluto's surface; this is in stark contrast to the Earth-Moon system, where the shared center of mass lies beneath Earth's surface. Pluto and Charon also have the distinction of being the only gravitationally round objects to have achieved a two-way tidal lock within the lifespan of the solar system; no matter what, Pluto and Charon always show each other the same face, like two dancers holding hands while spinning in a circle. In addition, Pluto has four other irregular moons named Styx, Nix, Kerberos, and Hydra, which compared to Charon, are likely captured small Kuiper Belt objects similar to Mars' asteroid moons; Phobos and Deimos. An alternate hypothesis is that they are leftover debris from the giant impact that created Charon that somehow failed to coalesce. Like Charon, these moons do no orbit Pluto directly, instead revolving around the system's shared center of gravity in between Pluto and Charon.

Due to its small size and massive distance from the Sun, for two-thirds of a century, the best images of Pluto were just a blurry dot of reflected light. Even when the Hubble Space Telescope observed Pluto in 2002 and 2003, it could barely make out its blurry surface features despite being the most powerful space telescope launched by at that point (this video explains in further detail why Hubble could barely resolve Pluto's surface features despite seeing galaxies in fine detail millions of light years away). The mysteries surrounding Pluto were why NASA launched the New Horizons spacecraft in 2006. It arrived at Pluto in July of 2015, taking the first high resolution images of the dwarf planet in unprecedented detail.

Pluto's surface was revealed to be geologically young with a mixture of mountains, plains, and craters, hinting at cryovolcanic activity renewing its surface. Its most striking surface feature is the large heart-shaped terrain around the equator officially named Tombaugh Regio (after Pluto's founder), but more commonly known as just "Pluto's Heart". The west side of the heart is a relatively flat plain of ice deposits officially named Sputnik Planitia, while the east side of the heart is an area of rugged mountainous terrain. Several other features have been identified and named after explorers (Hillary Montes), spacecraft (Voyager Terra), and other hell deities from various mythologies. One particular feature close to the heart is an area of dark terrain named the Cthulhu Macula, after H. P. Lovecraft's iconic Eldritch Abomination.

The official IAU definition for fictional names of features on Charon is items and milestones of "fictional space".note  With that one, the New Horizons team is really going to town. Vulcan Planum (with Spock Crater), Gallifrey Macula, Ripley Crater and Nostromo Chasma, Macross and Serenity Chasmae, Organa, Skywalker and Vader Craters...

Lastly, it was revealed that Pluto has a thick (by minor planet standards) atmosphere consisting of mainly nitrogen, methane, and carbon monoxide. Originally, it was thought that its atmosphere was only present when Pluto approached its perihelion due to warming up from closer proximity to the Sun, and then freezing entirely and leaving the dwarf planet bare when it approached aphelion. Later analysis from New Horizons suggests that it isn't the case, and Pluto's atmosphere is actually a constant feature for the entirety of its orbit. Despite lacking a magnetic field, due to its far distance from the Sun, it experiences less atmospheric loss than some of the planets in the inner Solar System.

Like Ceres before it, Pluto's demotion was largely a result of improvements in telescope technology. When first discovered, Pluto was thought to be the solar system's largest planet, surpassing even Jupiter. As telescope resolution improved, Pluto gradually "shrunk", going from similar in size to Uranus and Neptune, to similar to Earth and Venus, to similar to Mars. A running joke among astronomers at the time was that Pluto was going to disappear entirely at the rate they were going. The beginning of the end came with the discovery of Charon in 1978. With a natural satellite in play, astronomers could finally apply Kepler's Third Law to get a reasonably accurate measurement of Pluto's mass, discovering that Pluto was dwarfed even by our own Moon. Charon's discovery also occurred shortly after the introduction of the Giant Impact Hypothesis that explained the origin of Earth's moon. The similarities between the two systems was quickly noticed, and Charon was suggested to have formed the same way. This raised the obvious question of where the impactor that caused Charon's creation came from; probability alone indicated that Pluto wasn't alone in its region of space, and astronomers began actively searching for other members of the hypothetical Kuiper Belt.

The search bore fruit with the discovery of Albion in 1992. Initially declared the first member of the Kuiper Belt, it didn't take long to realize that the honor actually belonged to Pluto. The debate over Pluto's classification began in earnest, especially as hundreds more Kuiper Belt Objects and Scattered Disc Objects were discovered over the next decade. Public pressure however led to the issue being held off until it was finally forced with the discovery of Eris in 2005. Once a Trans-Neptunian Object more massive than Pluto was confirmed, the classification issue could no longer be pushed down the line, and Pluto was formally demoted in 2006.

Since its demotion in 2006, Pluto has become a Butt-Monkey of sorts in the Solar System to the point of being expendable in some forms of media. Nonetheless, it is still considered an Ensemble Dark Horse to many people because of its legendary status as the system's former ninth planet. Some space books and Solar System models acknowledge this by including Pluto in the package.

    Appearances in fiction 
  • Batman: The Brave and the Bold: Honorable mention with the Planet Master, a supervillain who has powers based on all the planets in the Solar system (you don't want to know what power Uranus has), including Pluto. He gets upset when Blue Beetle informs him it's no longer considered a planet.
  • Cthulhu Mythos: The "ninth planet" is a significant location, but that designation might not mean Pluto.
  • Doctor Who. In "The Sun Makers", humanity has settled on Pluto which is lit by artificial suns and has a breathable atmosphere, after every other planet in the solar system has been mined of its resources.
  • Guardians of the Galaxy: By the 30th century, mankind has settled Pluto, with the inhabitants being genetically engineered to survive there. On the plus side, they get superpowers. Downside, they're all wiped out by the Badoon, save for one guy, Martinex, who becomes a founding member of the Guardians. Since the Guardians first appeared in the 60s, descriptions and depictions of Pluto have had to move with the times.
  • "In Plutonian Depths": This is the first work with Pluto as the setting of the work, and was published in 1931.
  • Mass Effect: In this multimedia work, Charon is actually an ancient structure made by precursors to enable interstellar travel. Once it's defrosted, Pluto's orbit experiences a change.
  • The Secret of the Ninth Planet, by Donald Allen Wollheim. Presumably the secret is that Pluto isn't actually a planet (though it does hold a race of evil alien abductors in this case).
  • Wonder Woman (1942): Pluto has a small population of robed humanoids who live within the crust and who are ruled by the mythological Pluto, who uses his ties to earth to abduct humans and divide them into their "color selves" which somehow keeps them just alive in multiple ethereal "bodies", and allows Pluto and his followers to subsist on their energy until they are used up. It also has dangerous unpredictable volcanic activity.
  • Pluto is a late-game area in Warframe. It's a snow-blanketed world dotted with Corpus outposts used as testing sites for Ambulas, Frohd Bek's pet project which involves using adaptive AI for war machines. Charon on the other hand appears to have been shattered, with most of its remains forming a ring around Pluto.
  • World of Ptavvs: A Short Story in 1965, and expanded into a Novel in 1966, explains Pluto as a moon of Neptune's that had been dislodged millions of years ago.
  • In the Captain Future novels by Allen Steele, Pluto is a penal planet.

Haumea, Hi'iaka and Namaka as seen from the Hubble space telescope
  • Designation: Cubewano (Kuiper belt object)
  • Diameter: 1,632 km
  • Mass: 0.00066 of Earth
  • Density: 2.6 g/cm3
  • Surface Gravity: 0.04 g at Equator
  • Semi-major Axis: 43.21 AU
  • Orbital Period: 284 Years
  • Rotational Period: 4 Hours
  • Axial Tilt: Unknown
  • Average Surface Temperature: -223° C
  • Number of Moons: 2

Named after a patron deity of Hawai'i, Haumea (pronounced "how me uh") was the last of the dwarf planets to be officially recognized as such. Its mass is about three times less than Pluto's and twenty times less than the Moon's. It has two natural satellites - Hi'iaka and Namaka, named after the daughters of its namesake.

Haumea itself is a very unique and interesting to astronomers for many different reasons. For starters it has one of the fastest rotational periods of any body larger than 100km. Completing a rotation in less than 4 hours (in fact, were it to go any faster it would break in half) and giving it the shape of an elongated ellipsoid rather than a sphere. It's also one of the smallest bodies confirmed to have its own ring system which if scaled up would rival that of Saturn's and the center of a large cloud of smaller bodies known as the Haumeids (ranging from half its size to a few km) which are gravitationally interacting with it. It also has a big red spot on its surface which might be from the impact of a large meteorite.

All these things suggests that Haumea is actually the largest fragment of a once much larger object. And based on the dispersal pattern of the Haumeids the event must have happened at least one billion years ago somewhere in the Scattered disc (The Kuiper belt is too sparingly populated for such an event) with the debris from it having since migrated closer to the sun. But why it hasn't slowed down or how the ring system hasn't dispersed yet is unknown. Its possibly that rings are also a recent (astronomically speaking) development created by whatever impact also made the big red spot. And which might also have sped up its rotation speed.

Haumea is a good candidate for our next trans-Neptunian mission as it's much closer than Eris (a probe would "only" take around 14 years to reach it) and much more interesting than Makemake. And preliminary studies for an hypothetical mission are being done as of 2022.

Makemake and its moon as seen from the Hubble space telescope
  • Designation: Cubewano (Kuiper belt object)
  • Diameter: 1,430 km
  • Mass: 0.00073 of Earth
  • Density: 3.05 g/cm3
  • Surface Gravity: 0.06 g
  • Semi-major Axis: 45.56 AU
  • Orbital Period: 307 Years
  • Rotational Period: 22 Hours
  • Axial Tilt: Unknown
  • Average Surface Temperature: -231° C Estimate
  • Number of Moons: 1

Makemake (pronounced "mackie mackie") is the last dwarf planet to be discovered. It was named after the creator deity in the myths of Papa Nui, the native people of Easter Island. Prior to that, it was called an "Easterbunny" due to the date of discovery falling shortly after Easter. Its mass is about 0.07% that of Earth, making it slightly more massive than Haumea. It has one natural satellite (officially designated "S/2015 (136472) 1" or nicknamed "MK2"), which sadly doesn't have a creative name as of yet and there isn't really that much interesting going on with the dwarf planet itself.

Eris and Dysnomia as seen from the Hubble space telescope
  • Designation: Scattered disc object
  • Diameter: 2,326 km
  • Mass: 0.0028 of Earth
  • Density: 2.52 g/cm3
  • Surface Gravity: 0.08 g
  • Semi-major Axis: 67.78 AU
  • Orbital Period: 558 Years
  • Rotational Period: 25 Hours
  • Axial Tilt: Unknown
  • Average Surface Temperature: -230° C
  • Number of Moons: 1

The most massive of the dwarf planets and the reason Pluto isn't considered a planet anymore. It is named after the Greek goddess of discord, which is fitting considering it was the prime reason for the chaos which necessitated the creation of dwarf planet category — the only alternative was making Eris the tenth planet, but with dozens of potential dwarf planets awaiting confirmation that would have opened a can of worms that astronomers did not want to deal with.

Eris is the most notable member of the Scattered Disk, former members of the Kuiper Belt that were forced into heavily inclined and eccentric orbits after getting too close to Neptune's gravity well. Eris's orbit is tilted a whopping 44° and it has a difference of 59.455 AU between it's perihelion and aphelion. The fact that Eris was completely at the mercy of Neptune's gravity while Neptune itself was completely unaffected by Eris is considered one of the key distinctions between dwarf planets and true planets.

Eris is slightly smaller than Pluto, but its mass is higher, making it to 23% the mass of the Moon. The resulting higher density indicates that Eris has proportionally more rock content than the other outer dwarf planets. It has one known moon named Dysnomia — daughter of mythological Eris. Aside from being the ninth most massive object orbiting the Sun, it is also the next largest object in the Kuiper Belt that has not yet been visited by man-made objects. Given its massive distance from the Earth, this is unlikely to change any time soon.

Despite its size and importance to astronomy, Eris hasn't appeared in many works, largely because not much time has passed since its discovery. Ironically a work of fiction inspired the popular nickname for Eris prior to it receiving an official name, which was "Xena" (said series having recently gone off the air at the time). A relic of this connection remains in that the moon Dysnomia refers to the Greek goddess of lawlessness, Lawless being the surname of the actress who played Xena.

Eris has been the target for many hypothetical missions. Including sending New Horizon after it when it was done with its Pluto study. The problem though is that Eris elongated orbit and the fact that it recently (1977) hit its perihelion means that probes will essentially have to chase after it. And with our current technology the journey will take around 25 years.

     Appearances in fiction 

Alternative Title(s): Ceres, Pluto, Haumea, Eris