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The IAU working definition classifies exoplanets using a deuterium-fusion mass limit (≈13 Jupiter masses for solar metallicity) plus an orbital stability condition related to the L4/L5 instability threshold.
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).
The IAU working definition classifies exoplanets using a deuterium-fusion mass limit (≈13 Jupiter masses for solar metallicity) plus an orbital stability condition related to the L4/L5 instability threshold.
Objects below the deuterium-fusion limit that orbit stars, brown dwarfs, or stellar remnants are “planets” regardless of formation mechanism.
Objects above the deuterium-fusion limit are “brown dwarfs,” and free-floating low-mass objects in young clusters are not “planets.”
The IAU definition was modified by its Commission F2 in August 2018.
A planet located outside the Solar System.
The IAU’s criteria for classifying planetary-mass objects in exoplanet studies, based primarily on a deuterium-fusion mass limit and an L4/L5 stability condition.
The mass threshold above which an object can undergo thermonuclear fusion of deuterium; the IAU uses it as the dividing line between planets and brown dwarfs (about 13 Jupiter masses for solar metallicity).
A stability condition involving the mass ratio between an orbiting object and its central body, used by the IAU to help determine whether an object is classified as a planet.
An object with a true mass above the deuterium-fusion limiting mass, classified separately from planets under the IAU criteria.
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