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Dwarf planet

The meaning of «dwarf planet»

A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to planetary geologists is that, being possibly differentiated and geologically active bodies, they may display planetary geology, an expectation borne out by the Dawn mission to Ceres and the New Horizons mission to Pluto in 2015.

Counts of the number of dwarf planets among known bodies of the Solar System range from 5-and-counting (the IAU)[1] to over 120 (Runyon et al).[2] Apart from Sedna, the largest of these candidates have either been visited by spacecraft (Pluto and Ceres) or have at least one known moon (Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, Salacia), which allows their masses and thus an estimate of their densities to be determined. Mass and density in turn can be fit into geophysical models in an attempt to determine the nature of these worlds.

The term dwarf planet was coined by planetary scientist Alan Stern as part of a three-way categorization of planetary-mass objects in the Solar System: classical planets, dwarf planets and satellite planets. Dwarf planets were thus conceived of as a category of planet. However, in 2006 the concept was adopted by the International Astronomical Union (IAU) as a category of sub-planetary objects, part of a three-way recategorization of bodies orbiting the Sun.[3] Thus Stern and other planetary geologists consider dwarf planets to be planets, but since 2006 the IAU and perhaps the majority of astronomers have excluded them from the roster of planets.

Starting in 1801, astronomers discovered Ceres and other bodies between Mars and Jupiter that for decades were considered to be planets. Between then and around 1851, when the number of planets had reached 23, astronomers started using the word asteroid for the smaller bodies and then stopped naming or classifying them as planets.[5]

With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine planets, along with thousands of significantly smaller bodies (asteroids and comets). For almost 50 years Pluto was thought to be larger than Mercury,[6][7] but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure Pluto's mass accurately and to determine that it was much smaller than initial estimates.[8] It was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was still more than ten times as massive as the largest object in the asteroid belt, Ceres, it had only one-fifth the mass of Earth's Moon.[9] Furthermore, having some unusual characteristics, such as large orbital eccentricity and a high orbital inclination, it became evident that it was a different kind of body from any of the other planets.[10]

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