Exoplanets: Definition, Detection, and Habitability

Definition of exoplanet and IAU criteria

An exoplanet (extrasolar planet) is a planet outside the Solar System. While many definitions exist in practice, the International Astronomical Union (IAU) provides an official working definition for what counts as a planet in the exoplanet context. The IAU’s working definition (amended in August 2018) uses mass and orbital stability criteria. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (about 13 Jupiter masses for solar metallicity) that orbit stars, brown dwarfs, or stellar remnants are classified as “planets” (regardless of how they formed), provided the object’s mass ratio with the central body is below the L4/L5 instability threshold. Objects above the deuterium-fusion limit are classified as “brown dwarfs,” and free-floating objects in young clusters below the deuterium-fusion limit are not considered planets (instead termed “sub-brown dwarfs” or by an appropriate name).

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Alternative planet versus brown dwarf criteria

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