What is a planet?

Amazingly, until this week, there was no scientific agreement on what constitutes a planet. So, since 1930, Pluto has been called one, even though there has been increasing challenges to this classification.
At the Interneraional Astronomical Union meeting in Prague this week, a definition was finally agreeed: a planet is a body that orbits the sun, is so large its own gravity makes it roughly spherical, and, crucially, also dominates its region of the solar system.
On this basis of this definition, Pluto is not a planet, so all our textbooks will have to be revised. However, there is now a new term: dwarf planet. Apparently there are three of these: Pluto, Ceres (an asteriod between Mars and Jupiter) and Xena (officially known as 2003 UB313). A third category covers all other objects except satellites.
So now you know. More information here.

One Comment

  • Nick

    Late breaking news: The Plutonian Astronomers’ Union has declared
    Jupiter, Saturn, Uranus, Neptune, Pluto, Sedna, and Xena to be the
    only planets in the solar system. Bodies closer to the sun are now classified as Near Sun
    Objects, not planets.
    With thanks to “Chuck”!