Planetary termsEdit

  • Albedo—the reflecting power of a planet, satellite, or asteroid, expressed as a ratio of reflected light to the total amount falling on the surface.[1]
  • Antipodal point—the point that is on the direct opposite side of a planet.[2]

There are eight planets in the Solar System.

IAU definitionEdit

The 2006 resolution of the IAU's General Assembly defines planets within the Solar System as follows:[3]

A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Under this definition, the Solar System is considered to have eight planets. Bodies that fulfill the first two conditions but not the third (such as Ceres, Pluto, and Eris) are classified as dwarf planets, provided they are not also natural satellites of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion.[4] After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.[5]


  3. "IAU 2006 General Assembly: Result of the IAU Resolution votes". International Astronomical Union. 2006. Retrieved 2009-12-30. 
  4. Rincon, Paul (2006-08-16). "Planets plan boosts tally 12". BBC. Retrieved 2008-08-23. 
  5. "Pluto loses status as a planet". BBC. 2006-08-24. Retrieved 2008-08-23.