The Dwarf Planet's: There's a lot of them
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| Pluto & Charon. Credit: NASA/JHUAPL/SwRI |
In 2006, the International Astronomical Union "demoted" Pluto. There was a lot of fuss about it, and many people say it deserves to be a planet. Some have science to back up their arguments, but others are just sentimental. The IAU decided that Pluto belonged to a new catagory: dwarf planets. The IAU did a poor jub of defining exactly what a dwarf planet is however, so the exact number of dwarf planets in the solar system is indeterminate. This article will only focus on the named objects that may count as dwarf planets, and list the rest of the unnamed ones at the end (possibly including some named ones with limited information). It is hard to stay up-to-date on the matter, and informaton about some of the obscure dwarf planets here may be incorrect.
There are lots of objects on this list, so be prepared for a long read. With that out of the way, lets start with everyone's favourite dwarf planet: Pluto.
Pluto
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| Richly detailed colour view of Pluto from New Horizons. Credit: NASA/JHUAPL/SwRI |
- Date Discovered: January 23, 1930
- Designation: 13434 Pluto (1930 BM)
- Mass: 1.309x10^22 Kilograms
- Radius: 1151 Kilometres
- Density: 2.05 g/cm^3
- Surface Gravity: 0.66 m/sec^2
- Rotational Period: -6.387 Days*
- Orbital Period: 90487.28 Days (247.74 Years)
- Moons: 4 (Charon is a binary partner, not a moon)
*Negative time indicates retrograde (clockwise) rotation. In otherwords: it spins backwards (and on its side, like Uranus)
Charon
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| A beautiful image of Charon from New Horizons. Credit: NASA/JHUAPL/SwRI |
- Date Discovered: June 22, 1978
- Designation: I Charon
- Mass: 1.546625823x10^21 Kilograms
- Radius: 1151 Kilometres
- Density: 1.678 g/cm^3
- Surface Gravity: 0.283 m.sec^2
- Rotational Period: -6.387 Days
- Orbital Period: -6.387 Days
- Moons: 4 (Charon is a binary partner of Pluto, and the minor moons orbit both of them equally)
Charon is Pluto's Binary partner, but many sources say it is the largest moon. It isn't. Both Charon and Pluto orbit each other, and the smaller moons orbit the common centre of mass between them. Charon has a string of cliffs and canyons that wrap around it in this image. It also possesses an unusual orange-brown splotch on the top. This might be contamination from Pluto, or due to geological activity within Charon. Charon's surface features are named after fictional adventurers and explorers from science fiction, as well as science fiction writers and directors.
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| First round of names for Charon's surface features, released in 2018. Credit: NASA/JHUAPL/SwRI |
Ceres
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| Ceres in True Colour from Dawn. Credit: NASA/JPL/Caltech |
- Date Discovered: January 1, 1801
- Designation: 1 Ceres (A801 AA)
- Mass: 9.47x10^20 Kilograms
- Radius: 469.7 Kilometres
- Density: 2.09 g/cm^3
- Surface Gravity: 0.28 m/sec^2
- Rotational Period: 9.07 Hours
- Orbital Period: 1681.4 Days (4.6 Years)
- Moons: None
Ceres was the first asteroid discovered, and it now possesses a dual classification of asteroid, and dwarf planet. Ceres is officially the only dwarf planet in the inner solar system, but Vesta, Pallas, and Hygiea could count too. Ceres was visited by the Dawn spacecraft in 2015, and many fascinating discoveries have been made since. Ceres possesses many craters, but not nearly as much as other asteroids do. This is due to large amounts of ice present. Some scientist think Ceres could even have a liquid ocean under the surface.
Eris
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| Eris and its moon Dysnomia. Credit: NASA, ESA, and M. Brown (California Institute of Technology) |
- Date Discovered: October 21, 2003
- Designation: 136199 Eris (2003 UB313)
- Mass: 1.672216x10^22 Kilograms
- Radius: 1163 Kilometres
- Density: 2.09 g/cm^3
- Surface Gravity: 0.8246 m/sec^2
- Rotational Period: 25.9 Hours
- Orbital Period: 2546169.11 Days (562.652 Years)
- Moons: 1 (Dysnomia)
Eris is partially the reason that dwarf planets are even a thing. Many objects similar to Pluto were being discovered in the late 90s, and early 2000s, but they were all smaller than Pluto. But Eris, then known as "Xena" by researchers, changed all that. Eris was though to be bigger, and more massive than Pluto, this led the IAU to reclassify these objects as dwarf planets. However, newer measurements have put Eris at slightly smaller than Pluto, but it's still more massive. Eris is the Greek goddess of discord, appropriate for the commotion it caused in the astronomical community. Dysnomia, Eris's moon, was discovered in 2005.
Haumea
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| Illustration of Haumea. Credit: Instituto de Astrofisica de Andalucia |
- Date Discovered: October 21, 2003
- Designation: 136108 Haumea (2003 EL61)
- Mass: 3.94652x10^21 Kilograms
- Radius: 869.5 Kilometres (Average)
- Density: 1.438 g/cm^3
- Surface Gravity: 0.3477 m/sec^2
- Rotational Period: 3.915 Hours
- Orbital Period: 2546169.11 Days (562.652 Years)
- Moons: 2 ( Hi'iaka and Namaka)
Haumea is notable for its oblong shape. While it's large enough to reach hydrostatic equilibrium, it isn't a sphere because it rotates really fast. With a day only 3.9 hours long, Haumea gets pulled into an oblong egg-like shape. Surprisingly, Haumea was found to possess a small ring system that was discovered in 2017. Haumea is also believed to have a red splotch on it's surface, similar to the one on Charon. Haumea's moons, Hi'iaka and Namaka, were named after the daughters of Haumea in Hawaiian mythology.
Makemake
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| Makemake and its moon. Credit: NASA, ESA, and A. Parker, and M. Buie (SwRI) |
Makemake (pronounced: "mah-kee-mah-kee") is a reddish dwarf planet that is officially recognized by the IAU. Along with Eris, the discovery of Makemake lead to the "demotion" of Pluto. Hubble spotted a small and dark moon around Makemake in 2016. This moon has yet to be named (the IAU likes to take its sweet time with some things), and is usually refered to as "MK 2." The moon is expected to be around 160 Kilometres in diameter.
Gonggong
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| Gonggong and its moon Xiangliu. Credit: NASA, ESA, C. Kiss, and J. Stansberry |
Gonggong was relatively famous as the largest unnamed body in the solar system, back when it was called 2007 OR10. In February of 2020, 2007 OR10 recieved the name Gonggong, after a chinese water god. It's moon is named Xiangliu, after Gonggong's minister, who is depicted as nine-headed serpent monster. Gonggong is probably reddish, like other bodies in the kuiper belt and scattered disk.
Quaoar
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| Quaoar and its moon Weywot. Credit: HST and Michael E. Brown |
Quaoar is a small reddish dwarf planet discovered in 2002. In late 2022, a ring system was discovered around Quaoar. These rings were unusual, because they were beyond the roche limit. The roche limit is the limit were a moon can hold itself together. If it gets too close, it'll get ripped apart by tidal forces and form a ring. Quaoar's ring may be stable due to gravitational resonances with its moon Weywot, but research is ongoing.
Sedna
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| Hubble image of Sedna. Credit: NASA |
Sedna is the largest Oort cloud object discovered so far. It has an insanely long orbit that takes over 12,000 years to complete. Surprisingly, Sedna will be at perihelion, the closest point to the sun in its orbit, around 2070. This might be a great time to send a probe there, because there won't be another chance for 12,000 years. According to wikipedia, a flyby mission with a gravity assist of Jupiter could be done in 24.5 years. Sedna is believed to be extremely red, due to tholin ices. Sedna's orbit is highly elongated, and could be due to gravitational disturbances caused by a ninth planet in the outer solar system.
Orcus
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| GIF of Orcus being orbited by its moon Vanth. Credit: HST/Michael E. Brown, and Nrco0e |
Orcus is around the size of Ceres, and has a moon named Vanth. Orcus likely has methane and ammonia ices on its surface, and probably look's an icy grey, rather than reddish like other dwarf planets. Orcus is locked in a 2:3 resonance with Neptune. This means that Neptune makes three orbits for every two orbits of Orcus. Orcus is sometimes called "anti-Pluto," because it orbits away from Pluto, and has a name with similar meaning (Orcus is the Roman equivalent of Pluto or Hades, the god of the underworld).
Salacia
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| Salacia and its moon Actaea. Credit: Keck Observatory |
Salacia's status as a dwarf planet is hotly debated due to its small size, but it is large enough for me to include it here (not that I have any strict requirements or anything). A spectrum of Salacia was taken by the James Webb Space Telescope in 2022, and it was found to possess water ice, but no methane ices.
Varda & Ilmarë
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GIF of Varda and Ilmarë. Credit: HST, Will Grundy, and D. Bamberger |
Varda and Ilmarë are a binary system that orbits far out in the scattered disk of the solar system. Varda was discovered in 2003, and Ilmarë 4 years later in 2009. Varda and Ilmarë likely contain methanol ices, but little to no water ice.
Ixion
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| Hubble image of Ixion from 2006. Credit: HST/Michael E. Brown |
Ixion is very similar to Orcus, and also has a 2:3 resonance with Neptune. Unlike Orcus, Ixion likely has a reddish-brown surface due to organic compounds. The mass and density of Ixion are extremely rough estimates, because there are no known moons around Ixion. Using some special equations, the mass of an object can be accurately measured if you know the orbit of a moon around it.
Chaos
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| Chaos is in the centre of this image. Credit: HST/Michael E. Brown |
Chaos is a cubewano. What is a cubewano you may ask? It's any large Kuiper belt object that isn't in an orbital resonance with any planet. Chaos is very dark, but little is known about its surface composition. It takes its name from the primeval, unorganized state of the universe, in which the Greek gods appeared.
Gǃkúnǁʼhòmdímà
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Gǃkúnǁʼhòmdímà and its moon. Credit: NASA |
The first thing your thinking about when seeing the name for this is: "how in the world do I pronounce that?" Gǃkúnǁʼhòmdímà is pronounced something like: "nng-'tungue-click'-hmm-dima." If you couldn't follow that, check the pronounciation here. In English, the name can be pronounced: "gon-holm-dee-ma." The name comes from a girl in Jul'hoan mythology. Gǃkúnǁʼhòmdímà's moon is one of the reddest objects known in the Kuiper belt.
Varuna
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| Hubble image of Varuna. Credit: HST/Michael E. Brown |
Varuna's mass is unknown, because it lacks a moon for mass measurements to be made. Its designation, 20000, was to commemorate the 200th anniversary of the discovery of Ceres. This was appropriate at the time, because Varuna was thought to be about the same size as Ceres. Varuna is believed to be slightly oblong, similar to Haumea.
Dziewana
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| Hubble image of Dziewanna. Credit: HST/Keith Noll |
Dziewanna is a large member of the scattered disk. This region of icy bodies lies beyond the Kuiper belt, at a high inclination.
Huya
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| Hubble image of Huya and its moon. Credit: HST/Keith Noll, and Renerpho |
Huya is a reddish plutino in the Kuiper belt. Plutinos like Huya and Orcus have a 2:3 resonance with Neptune that was mention previously in the article. Huya has an unnamed moon, so we know the mass and density for it. Huya was at perihelion back in 2015, and is currently on its way to the farther end of its orbit.
There are lots more of these transNeptunian objects, so I only covered the majority of the named ones. Many more await naming, and further study. These are listed below. (I also included the object known as "Biden," because I wasn't able to find its official designation.)
- 2002 MS4
- 2002 AW197
- 2003 AZ84
- 2004 GV9
- 2005 UQ513
- 2005 RN43
- 2002 UX25
- 2006 QH181
- 2007 JJ43
- 2010 KZ39
- 2004 XA192
- 2010 RF43
- 2002 TC302
- 2005 RM43
- 2004 XR190
- 2004 NT33
- 2001 UR163
- 2003 UZ413
- 2004 TY364
- 2010 VK201
- 2008 ST291
- 2010 RE64
- 2010 FX86
- 2004 PR107 = 2012 SQ31
- 2003 VS2
- 2003 QX113
- 2004 PF115
- 2010 TY53
- 2011 GM27
- 2010 TJ
- 2010 VZ98
- 2011 FW62 = 2015 AJ281
- 2005 TB190
- 1999 DE9
- 2003 FY128
- 1998 SN165
- 2002 KX14
- 2000 YW134
- "Biden"
- 2002 VR128
- 2002 WC19
- 1999 CD158
- 2005 QU182
- 1996 GQ21
- 2002 CY248
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