This article is about the formal definition established in 2006. For other uses, see Definition of planet.
Euler diagram showing the IAU Executive Committee conception of the types of bodies in the Solar System.
The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System,[1] a planet is a celestial body that:
is in orbit around the Sun,
has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
has "cleared the neighbourhood" around its orbit.
A non-satellite body fulfilling only the first two of these criteria (such as Pluto, which had hitherto been considered a planet) is classified as a dwarf planet. According to the IAU, "planets and dwarf planets are two distinct classes of objects" – in other words, "dwarf planets" are not planets. A non-satellite body fulfilling only the first criterion is termed a small Solar System body (SSSB). An alternate proposal included dwarf planets as a subcategory of planets, but IAU members voted against this proposal. The decision was a controversial one, and has drawn both support and criticism from astronomers.
The IAU has stated that there are eight known planets in the Solar System. It has been argued that the definition is problematic because it depends on the location of the body: if a Mars-sized body were discovered in the inner Oort cloud, it would not have enough mass to clear out a neighbourhood that size and meet criterion 3.[2] The requirement for hydrostatic equilibrium (criterion 2) is also universally treated loosely as simply a requirement for roundedness;[3] Mercury is not actually in hydrostatic equilibrium,[4] but is explicitly included by the IAU definition as a planet.
The working definition of an exoplanet is as follows:[5][6]
Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability (M/Mcentral < 2/(25+√621)) are "planets" (no matter how they formed).
The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
^Exoplanets are addressed in a 2003 position statement issued by a now-defunct IAU Working Group on Extrasolar Planets. However, this position statement was never proposed as an official IAU resolution and was never voted on by IAU members.
^Cite error: The named reference plutoplanet was invoked but never defined (see the help page).
^Brown, Mike [@plutokiller] (February 10, 2023). "The real answer here is to not get too hung up on definitions, which I admit is hard when the IAU tries to make them sound official and clear, but, really, we all understand the intent of the hydrostatic equilibrium point, and the intent is clearly to include Mercury & the moon" (Tweet) – via Twitter.
^Sean Solomon, Larry Nittler & Brian Anderson, eds. (2018) Mercury: The View after MESSENGER. Cambridge Planetary Science series no. 21, Cambridge University Press, pp. 72–73.
^"Official Working Definition of an Exoplanet". IAU position statement. Retrieved 29 November 2020.
^Lecavelier des Etangs, A.; Lissauer, Jack J. (June 2022). "The IAU working definition of an exoplanet". New Astronomy Reviews. 94: 101641. arXiv:2203.09520. Bibcode:2022NewAR..9401641L. doi:10.1016/j.newar.2022.101641. S2CID 247065421.
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