IAU definition of planet
The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System,[1] a planet is a celestial body that:
This article is about the formal definition established in 2006. For other uses, see Definition of planet.
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]
Closing issues[edit]
Substance[edit]
Alan Stern, the lead scientist on NASA's robotic mission to Pluto, contended that Earth, Mars, Jupiter, and Neptune have not fully cleared their orbital zones, just like Pluto. Earth orbits with 10,000 near-Earth asteroids. Jupiter, meanwhile, is accompanied by 100,000 Trojan asteroids on its orbital path. Stern has asserted: "If Neptune had cleared its zone, Pluto wouldn't be there."[50]
Some astronomers counter this opinion by saying that, far from not having cleared their orbits, the major planets completely control the orbits of the other bodies within their orbital zone. Although Jupiter does coexist with a large number of small bodies in its orbit (the Trojan asteroids), these bodies only exist in Jupiter's orbit because they are in the sway of the planet's huge gravity. Earth accretes or ejects near-Earth asteroids on million-year time scales, thereby clearing its orbit. Similarly, Pluto may cross the orbit of Neptune, but Neptune long ago locked Pluto and its attendant Kuiper belt objects, called plutinos, into a 3:2 resonance (i.e., they orbit the Sun twice for every three Neptune orbits). Since the orbits of these objects are entirely dictated by Neptune's gravity, Neptune is therefore gravitationally dominant.[51]
On June 11, 2008, the IAU announced that the subcategory of dwarf planets with trans-Neptunian orbits would be known as "plutoids". In an accompanying press release, the IAU said that:[52]