Definition of planet

type of substellar object larger than a gas giant

in fluid mechanics, state in which compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction

history and evolution of the term planet

Mesoplanets are planetary-mass objects with sizes smaller than Mercury but larger than Ceres. The term was coined by Isaac Asimov. Assuming size is defined in relation to equatorial radius, mesoplanets should be approximately 500 km to 2,500 km in radius.

definition of a planet as a body orbiting the Sun, in hydrostatic equilibrium, having cleared the neighborhood around its orbit; ratified by the IAU in August 2006, thereby reclassifying Pluto as a dwarf planet instead

criterion for a celestial body to be considered a planet

nuclear fusion forming a helium-3 nucleus

astronomical object that achieves core fusion during its lifetime