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Only one of them – Pluto – has been observed in enough detail to verify that its current shape fits what would be expected from hydrostatic equilibrium. [53] Ceres is close to equilibrium, but some gravitational anomalies remain unexplained. [54] Eris is generally assumed to be a dwarf planet because it is more massive than Pluto.
On the other hand, the surfaces of Sedna, Gonggong, and Quaoar have low abundances of CO and CO 2, similar to Pluto, Eris, and Makemake, but in contrast to smaller bodies. This suggests that the threshold for dwarf planethood in the trans-Neptunian region is around 500 km radius.
Eris has been measured at 2,326 ± 12 kilometres (1,445 ± 7 mi) in diameter; [12] its mass is 0.28% that of the Earth and 27% greater than that of Pluto, [23] [24] although Pluto is slightly larger by volume. [25] Both Eris and Pluto have a surface area that is comparable to that of Russia or South America. Eris has one large known moon, Dysnomia.
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest known trans-Neptunian object by volume, by a small margin, but is less massive than Eris.
However, acceptance of Eris as the tenth planet implicitly demanded a definition of planet that set Pluto as an arbitrary minimum size. Many astronomers, claiming that the definition of planet was of little scientific importance, preferred to recognise Pluto's historical identity as a planet by " grandfathering " it into the planet list.
International Astronomical Union (IAU) adopted a new definition of planet. A new distinct class of objects called dwarf planets was also decided. Pluto was redefined as a dwarf planet along with Ceres and Eris, formerly known as (2003) UB 313. Eris was named after the IAU General Assembly in 2006. [25] [26]
1st century sculpture of Pluto in the Getty Villa. In ancient Greek religion and mythology, Pluto (Greek: Πλούτων, Ploutōn) was the ruler of the Greek underworld. The earlier name for the god was Hades, which became more common as the name of the underworld itself.
With the development of the telescope, the meaning of planet broadened to include objects only visible with assistance: the moons of the planets beyond Earth; the ice giants Uranus and Neptune; Ceres and other bodies later recognized to be part of the asteroid belt; and Pluto, later found to be the largest member of the collection of icy bodies ...