A planetary body or planetary object is any secondary body in a star system that has a planet-like geology due to being geologically differentiated or in hydrostatic equilibrium: a planet, dwarf planet, or the larger moons and asteroids.
A celestial object can be categorized as a planetary body if it meets the criteria proposed by Alan Stern and Harold Levison:
- The body must be low enough in mass that at no time (past or present) can it generate energy in its interior due to any self-sustaining nuclear fusion chain reaction.
- The body must be large enough that its shape becomes determined primarily by gravity rather than mechanical strength or other factors (such as surface tension or rotation rate) in less than a Hubble time, so that the body would on this timescale or shorter reach a state of hydrostatic equilibrium in its interior.
- Brown dwarfs, stars and small bodies such as planetesimals are excluded.
- ↑ Stern, S. Alan; Levison, Harold F. (2002), Rickman, H., ed., "Regarding the criteria for planethood and proposed planetary classification schemes", Highlights of Astronomy, San Francisco, CA: Astronomical Society of the Pacific, 12, pp. 205–213, Bibcode:2002HiA....12..205S, ISBN 1-58381-086-2. See p. 208.