Dwarf Candidates

Astronomers are continually finding new objects orbiting the sun with suggestions that some of them are dwarf planets. I have a list here of almost 20 candidates. Some of them have crazy names. Some of them have no names at all, relying on the Minor Planet Center (MPC) for their interim nomenclature. So when you see the likes of 90568 or 225088 you will know it is an identified object somewhere in the ether and catalogued by the MPC.

Sedna and the Outer Solar System
From Nasa Jet Propulsion Laboratory. Image credit: Nasa/Caltech

The following is a roundup of the more notable characters awaiting classification. For the time being let’s call them “gnome planets”. To avoid long repetitive phrases I am using the abbreviations AB (Asteroid Belt), KB and KBO (Kuiper Belt/Object), SDO (Scattered disc Object), TNO (Trans-Neptunian Object). But first some definitions.

The asteroid belt contains a host of small rocky bodies ranging in size up to 1 000 kilometres in diameter. They are variously called asteroids, minor planets, or planetoids, depending on who you talk to, and what they are talking about. Asteroids sit primarily between the orbits of Mars and Jupiter. The vast majority of asteroids— main-belt asteroids—have orbital periods between three and six years.

Centaurs are Small Solar System bodies that revolve around the sun in the outer solar system, mainly between the orbits of Jupiter and Neptune. Because they usually cross the orbits of the planets their own orbits tend to be unstable. They are similar in size to asteroids, but to comets in composition. Centaurs include Chiron (the first centaur to be discovered- see below), Pholus, discovered in 1992, Chariko in 1997. Evidently Hidalgo, discovered in 1920, was classified as a Centaur only after Chiron was identified. Centaur objects originate from the Kuiper Belt, before being lured into their unstable orbits by the gravity of Neptune. Because of this they can expect a short life.

The Kuiper Belt is a flat ring of small icy bodies that orbit the Sun—beyond the orbit of Neptune—at a distance of between 4.5 billion to 14.9 billion kilometres. The KB is thought to be the source of most of the observed short-period comets, particularly those that orbit the Sun in less than 20 years, and for the icy Centaur objects, which have orbits in the region of the giant planets.

Scattered disc is a disc of planetesimals, dust and stuff in the outer solar system beyond the orbit of Neptune, and beyond the edge of the Kuiper Belt. Whereas KBOs generally enjoy stable orbits, those in the scattered disc are wild and random. SDOs are typically very small and made up of water ice or frozen methane. It is thought that when Neptune was bumped out from the inner solar system by the gravitational forces of Jupiter and Saturn the planet crashed through the KB, scattering KBOs far and wide, some creating the Scattered Disc as we know it today. The motions of the objects within the disc are still affected by the gravity of Neptune

Trans-Neptunian objects (also called Plutinos) are dwarf planets that orbit the sun beyond Neptune, or at a greater average distance than Neptune. TNOs include KBOs, SDOs, and Centaurs, and cover an outer solar system area as far as the Oort cloud.

Dwarves in waiting

Chiron. Chiron was discovered as long ago as 1977 when it was initially classified as an asteroid. If there was any doubt about the confusion over the classification of bodies that are neither stars nor planets, then this candidate will put that to rest. Confusion abounds! Chiron was later reclassified as a “minor planet”. Later still it was found to exhibit the typical behaviour of a comet (and it had a dusty envelope to prove it). So, guess what, they then classified it as a comet. It is named after the Centaur in Greek mythology. Other objects in the region have also been classified as “Centaurs” for exhibiting similar behaviours as Chiron—which incidentally is still designated a Centaur, a minor planet, and a comet!

Chiron orbits the sun approximately every 50 years. It has a very eccentric orbit: 1.25 billion kilometres at perihelion and 2.82 billion kilometres at aphelion.

Gonggong. An SDO, Gonggong has a highly eccentric orbit ranging from 5.1 billion kilometres (nearest the sun) to 15.1 billion kilometres (farthest point from the sun).

Hygiea. Currently an asteroid, and living in the AB, Hygiea may well qualify to become a dwarf planet, albeit it would be the smallest in the solar system. For the moment it is the fourth-largest asteroid, commanding 2.9% of the mass of the AB. It has a diameter estimated at 430 kilometres.

Ixion. Ixion is a large TNO with a diameter of 617 kilometres, and the fourth largest known Plutino. It was discovered in 2001. It is named after the king in Greek mythology sentenced to roll a wheel through the underworld. In his wayward ways he became the father of the Centaurs.

Niku. Niku is the weirdest object in our list. First, it orbits the sun backwards (clockwise), the only solar system body to do so. And it orbits way beyond Neptune at 110° to the plane of all the planets. It has a diameter of 200 kilometres. Niku is the Chinese word for rebellious.

Orcus. Orcus was one of the Roman gods of the underworld. Also a TNO, Orcus has a moon called Vanth, which is named after the winged female demon of the Etruscan underworld.

Pallas. Pallas lives in the AB and one of the largest asteroids in the solar system. Pallas accounts for 7% of the mass of the AB.

Quaoar. Quaoar, like Pluto, has a surface rich in methane Nitrogen and Carbon monoxide. It has a diameter of 1 121 kilometres and is the second-largest known member of the Kuiper Belt. It is composed largely of ices mixed with rock, not unlike the makeup of a comet. Quaoar was discovered in 2002, and even has a moon called Weywot. Its average distance from the sun is 6.4 billion kilometres. Unlike Pluto, its orbit around the Sun is circular, even more so than most of the planetary-class bodies in the solar system. There is a theory that Quaoar was a fragment formed from a collision with Haumea.

Quaoar is named after a creation god of the Native American Tongva tribe, who were the original inhabitants of the Los Angeles basin. According to legend, Quaoar “came down from heaven; and, after reducing chaos to order, laid out the world on the back of seven giants. He then created the lower animals, and then mankind.”

Salacia. Salacia is a TNO in the KB with a diameter of 850 kilometres. It is named after the Roman mythological female deity of the sea, and the goddess of saltwater. It was discovered in 2004. It has an orbital period of 274 years. Again like most of these objects in this section it has a very elliptical (elongated) orbit, ranging from 5.64 billion kilometres at the nearest approach to the sun to 6.98 billion kilometres at its farthest orbital point.

Sedna. Sedna is the furthest known object in the solar system, and way outside the Kuiper Belt. It is a  large planetoid and has an orbital period of—wait for it— 10 663 years in an extremely elongated path ranging from 11.37 billion kilometres at perihelion (next there in CE2077) to 148 billion kilometres at aphelion over 6 000 years later in around CE8207. It’s a wonder astronomers could even track it. Its composition is similar to many TNOs, a mix of water, methane, nitrogen ices and tholins. Tholins are muddy gunk-like residues found on the surface of icy bodies. Their key elements are carbon, nitrogen and hydrogen, and it appears the action of ultra-violet radiation has some role to play in their formation.

Sedna is named after the Inuit goddess of the sea and marine animals. It was discovered in November 2003.

Varuna. Varuna was discovered in 2000. It is a TNO in the KB. It is about 680 kilometres in diameter.

Vesta. Vesta lives in the AB. It has a diameter of 525 kilometres.

And now the no-name brands:

AZ84. AZ84 is a TNO with a very elongated shape. It has a diameter of 940 kilometres along its long axis. Maybe that’s why it has not been named, unlike the smaller round bodies.

55565. 55565 is a TNO in the KB with a diameter of at least 700 kilometres and the largest unnamed object in the solar system.

55636. 55636 is a KBO with a diameter of a minimum of 286 kilometres but may be up to 800 kilometres. No one really knows.

90568. 90568 is a TNO with a diameter of approximately 680 kilometres.

Table of selected dwarf planet candidates  
DiscoveredDiameterOrbital PeriodDistance from the Sun
Orcus2004946 kms246 years4.53 billion - 7.19 billion kms
AZ842003686 kms247 years4.8 billion - 6.96 billion kms
Ixion2001617 kms250 years4.5 billion - 7.42 billion kms
905682004677 kms273 years5.79 billion - 6.82 billion kms
556362002<800 kms†285 years5.66 billion - 7.24 billion kms
Quaoar20021 121 kms288 years6.27 billion - 6.8 billion kms
555652002735 kms323 years6.12 billion - 7.95 billion kms
Sedna2003<1 600 kms‡10 663 years11.4 billion - 148 billion kms
†55636 could be as small as 284 kms in diameter
‡Sedna probably has a diameter of about 1 000 kms

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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar

Last updated: Tuesday, 31 March 2020