In 2002, planetary scientists Alan Stern and Harold Levison proposed the following algorithm to determine whether an object in space satisfies the definition for a planetary body. 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.
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.
^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: 205−213, Bibcode:2002HiA....12..205S, ISBN1-58381-086-2. See p. 208.
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