Any of the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus,
or Neptune. In 2006, astronomers agreed to distinguish between two types
of planets: classical planets and dwarf
planets. According to the new definitions, Pluto
would be demoted to dwarf status.
A classical planet satisfies the following criteria:
This definition was adopted by astronomers at a meeting of the International
Astronomical Union (IAU) in Prague on Aug. 24, 2006. It has, however, come
in for some criticism on a number of grounds. For example, Neptune has not
"cleared the neighborhood around its orbit," otherwise Pluto would
not be where it is. The same can be said of Earth, Mars, and Jupiter, all
of which have Trojans in similar orbits.
- It is in orbit around the Sun.
- It has sufficient mass for its self-gravity to overcome rigid body
forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
- It has cleared the neighborhood around its orbit.