PLANETS

How many planets are there?

The International Astronomical Union has recently found a need to create a definition for planets. Because of this, Pluto was said to be eliminated from the list of planets. But as a result of the new definition, we now have 12 planets in our solar system.

• The asteroid Ceres, which is round, would be recast as a dwarf planet in the new scheme.

• Pluto would remain a planet and its moon Charon would be reclassified as a planet. Both would be called "plutons," however, to distinguish them from the eight "classical" planets.

• A far-out Pluto-sized object known as 2003 UB313 would also be called a pluton.

Ceres, Charon, 2003 UB313, and Pluto have been defined as dwarf planets.
Mercury, Venus, Earth, Mars, Saturn, Jupiter, Neptune, and Uranus however, will retain their position as planets.


The definition for a planet as defined by the IAU is as follows:

"A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."


SOURCE: http://www.space.com/scienceastronomy/060816_planet_definition.html