Plutoids are celestial bodies similar to Pluto in configuration, including up to 70 other celestial bodies. They are dwarf planets that have not cleared their neighborhood and have a trans-Neptunian orbit. The term was adopted in 2008, but controversy over Pluto’s planet status remains.
A plutoid is a celestial body similar to Pluto in configuration. The term “plutoid” was chosen as a consolation nod to Pluto’s former glory as the ninth planet, though it hasn’t resolved the controversy over the precise definition of a planet, let alone whether or not Pluto should be considered a planet. Along with Pluto’s official redefinition as a plutoid came the discovery that up to 70 celestial bodies can qualify as plutoids, meaning Pluto is far from alone.
Several features define a plutoid. Plutoids are dwarf planets, which means that while they have enough mass to be rounded and achieve a specific orbit around the Sun, they have failed to “clean up the neighborhood,” as astronomers put it. In this case, clearing the neighborhood refers to a celestial body that has become gravitationally dominant, either displacing similarly sized objects from its orbit or pulling large objects into orbit around it, as the Earth has done with the Moon. In other words, dwarf planets have many rocky neighbors that are not under the influence of its gravitational pull.
To be considered a plutoid, a dwarf planet must also be a so-called “trans-Neptunian object,” which is a fancy way of saying that its orbit falls beyond that of Neptune. Objects that would otherwise meet the definition of a plutoid with orbits closest to the Sun are simply known as dwarf planets. Along with Pluto, another notable plutoid is Eris, a dwarf planet that is actually larger than Pluto, but much further away, making it difficult to see.
The term “plutoid” was adopted in 2008 and appears to be far from the last word on the Pluto controversy. Questions about Pluto’s status as a planet were brought to the fore in 2006, when the International Astronomical Union (IAU) abruptly downgraded Pluto to dwarf planet status, because it believed it did not meet the criteria to be considered a planet. This aroused great ire from both the layman and the scientific community.
One could reasonably quibble over the distinction between a plutoid and a dwarf planet, given that a plutoid is just a dwarf planet with a trans-Neptunian orbit. However, the distinction is important, as trans-Neptunian objects have a number of unique traits that make them interesting to astronomers. The discovery of additional objects beyond the orbit of Neptune each year illustrates the advances made in astronomy and the potential for further discoveries of scientific interest in the future.
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