It looks like the International Astronomical Union will vote next week on a new definition of what can be classed as a planet. The new definition will be any body that has enough mass to form a near spherical shape, due to gravity, and that orbits a star. Under this definition the Solar System will have 12 planets. In addition to the classical 9 at present there will also be Ceres (in the asteroid belt), Charon (which was formally a satellite of Pluto) and UB313 (which was discovered last year). The Charon on is interesting. As Pluto and Charon orbit a common centre of gravity, and this centre of gravity is not within the body of either of them, then this is now known as a double-planet. The Earth and Moon also orbit a common centre of gravity but this is inside the body of the Earth so the Moon is a satellite of Earth.
The new definition introduces a new subclass for the objects out beyond Neptune (mostly – Pluto/Charon do come closer than Neptune in part of it’s orbit) with orbits longer than 200 years. These will be known as Plutons. Ceres can be called a planet in the asteroid belt it seems.
I don’t like this definition. I’m not sure we can can come up with anything better though. As we discover more and more bodies, both in the Solar System and in other star systems, we need to have a definition of planet that is based on a scientific definition rather than historical discovery. My gut feeling is that we should have 8 planets in the Solar System. This would exclude Pluto, Charon and all the other small spherical bodies that are being discovered and that are predicted to be discovered in the future.
I’ve not seen any mention of Sedna. Using this new definition then it should be a planet. I wonder how the discoverer whose page is linked to above will react to that. He seems to be in the historical 8 planet camp.
