For a body in the solar system:
- is in orbit around the Sun
- has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
- has cleared the neighbourhood around its orbit
For a body within another planetary system:
- is in orbit around a star or the remnants of one
- has a mass below the limiting mass for thermonuclear fusion of deuterium
- is above the minimum mass/size requirement for planetary status in the Solar System.
Therefore, the solar system counts 8 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. In addition to these eight, three dwarf planets are also known: Ceres, Pluto and Eris. As for extrasolar planets, 241 are known as of September 2007.