by Ethan Siegel, Forbes
Astronomically, bodies within the Solar System must achieve three criteria in order to gain the much-vaunted status of planet:
- Gravitationally pull themselves into a spheroidal shape, where they obtain hydrostatic equilibrium,
- Orbit the Sun in an ellipse and no other smaller, parent body,
and clear their orbit of any substantially-massed objects.
In our Solar System, only eight worlds make the cut given those criteria. The four rocky planets (Mercury, Venus, Earth, Mars) and the four gas giant worlds (Jupiter, Saturn, Uranus, Neptune) are the only ones that can be called planets under these definitions. Everything else, no matter how large or massive, fails on one of the latter two criteria.