Dwarf planet

Of these ten, two have been visited by spacecraft (Pluto and Ceres) and seven others have at least one known moon (Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Salacia), which allows their masses and thus an estimate of their densities to be determined.

[5] With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine major planets, along with thousands of significantly smaller bodies (asteroids and comets).

[15] Eris (then known as 2003 UB313), a trans-Neptunian object, was discovered in January 2005;[16] it was thought to be slightly larger than Pluto, and some reports informally referred to it as the tenth planet.

[19] Although concerns were raised about the classification of planets orbiting other stars,[20] the issue was not resolved; it was proposed instead to decide this only when dwarf-planet-size objects start to be observed.

[28] Names for large subplanetary bodies include dwarf planet, planetoid (more general term), meso-planet (narrowly used for sizes between Mercury and Ceres), quasi-planet, and (in the transneptunian region) plutoid.

[31] Indeed, the draft of Resolution 5A had called these median bodies planetoids,[32][33] but the plenary session voted unanimously to change the name to dwarf planet.

[34] In most languages equivalent terms have been created by translating dwarf planet more-or-less literally: French planète naine, Spanish planeta enano, German Zwergplanet, Russian karlikovaya planeta (карликовая планета), Arabic kaukab qazm (كوكب قزم), Chinese ǎixíngxīng (矮行星), Korean waesohangseong (왜소행성 / 矮小行星) or waehangseong (왜행성 / 矮行星), but in Japanese they are called junwakusei (準惑星), meaning "quasi-planets" or "peneplanets" (pene- meaning "almost").

[36] Other departments of the IAU have rejected the term: ...in part because of an email miscommunication, the WG-PSN [Working Group for Planetary System Nomenclature] was not involved in choosing the word plutoid.

In terms of the dynamics of the Solar System, the major distinction is between bodies that gravitationally dominate their neighbourhood (Mercury through Neptune) and those that do not (such as the asteroids and Kuiper belt objects).

[46] Alan Stern and Harold F. Levison introduced a parameter Λ (upper case lambda) in 2000, expressing the likelihood of an encounter resulting in a given deflection of orbit.

A gap of five orders of magnitude in Λ was found between the smallest terrestrial planets and the largest asteroids and Kuiper belt objects.

[43] Jean-Luc Margot refined Stern and Levison's concept to produce a similar parameter Π (upper case Pi).

The extreme example of a body that may be scalene due to rapid rotation is Haumea, which is twice as long on its major axis as it is at the poles.

Though the definition of a dwarf planet is clear, evidence about whether a given trans-Neptunian object is large and malleable enough to be shaped by its own gravitational field is often inconclusive.

In order of discovery, these three bodies are: The IAU only established guidelines for which committee would oversee the naming of likely dwarf planets: any unnamed trans-Neptunian object with an absolute magnitude brighter than +1 (and hence a minimum diameter of 838 km at the maximum geometric albedo of 1)[56] was to be named by a joint committee consisting of the Minor Planet Center and the planetary working group of the IAU.

Research since then has cast doubt on the idea that bodies that small could have achieved or maintained equilibrium under the typical conditions of the Kuiper belt and beyond.

[68] Since 2011, Brown has maintained a list of hundreds of candidate objects, ranging from "nearly certain" to "possible" dwarf planets, based solely on estimated size.

But the lower albedos and densities of Gǃkúnǁʼhòmdímà, 55637, Varda, and Salacia suggest that they never did differentiate, or if they did, it was only in their deep interiors, not a complete melting and overturning that involved the surface.

[72] In 2023, Emery et al. wrote that near-infrared spectroscopy by the James Webb Space Telescope (JWST) in 2022 suggests that Sedna, Gonggong, and Quaoar underwent internal melting, differentiation, and chemical evolution, like the larger dwarf planets Pluto, Eris, Haumea, and Makemake, but unlike "all smaller KBOs".

On the other hand, the surfaces of Sedna, Gonggong, and Quaoar have low abundances of CO and CO2, similar to Pluto, Eris, and Makemake, but in contrast to smaller bodies.

They hypothesised that Quaoar originally had a rapid rotation and was in hydrostatic equilibrium, but that its shape became "frozen in" and did not change as it spun down due to tidal forces from its moon Weywot.

[78] The trans-Neptunian objects in the following tables, except Salacia, are agreed by Brown, Tancredi et al., Grundy et al., and Emery et al. to be probable dwarf planets, or close to it.

Salacia has been included as the largest TNO not generally agreed to be a dwarf planet; it is a borderline body by many criteria, and is therefore italicized.

Unicode includes symbols for Quaoar , Sedna , Orcus , Haumea , Eris , Makemake , and Gonggong that are primarily used by astrologers: they were devised by Denis Moskowitz, a software engineer in Massachusetts.

Ceres displays such evidence of an active geology as salt deposits and cryovolcanos, while Pluto has water-ice mountains drifting in nitrogen-ice glaciers, as well as a significant atmosphere.

New Horizons has captured distant images of Triton, Quaoar, Haumea, Eris, and Makemake, as well as the smaller candidates Ixion, 2002 MS4, and 2014 OE394.

Vesta, the next-most-massive body in the asteroid belt after Ceres, was once in hydrostatic equilibrium and is roughly spheroidal, deviating mainly due to massive impacts that formed the Rheasilvia and Veneneia craters after it solidified.

[91][92] Triton is more massive than Eris or Pluto, has an equilibrium shape, and is thought to be a captured dwarf planet (likely a member of a binary system), but no longer directly orbits the sun.

[93] Phoebe is a captured centaur that, like Vesta, is no longer in hydrostatic equilibrium, but is thought to have been so early in its history due to radiogenic heating.

These larger moons are not physically distinct from the dwarf planets, but do not fit the IAU definition because they do not directly orbit the Sun.