Is Pluto a planet?

Yes, Pluto is a planet. There has been some confusion over this issue recently because a number of large icy bodies have been discovered in orbits beyond Pluto, in an area known as the Kuiper Belt. These bodies are believed to be hundreds of kilometers in diameter, leading to speculation that still larger bodies (essentially giant comets) exist in this region, including some larger than Pluto. Recent studies of Pluto's composition have also led to the idea that Pluto was once one of these bodies. However, Pluto's chemical make-up and origin don't affect Pluto's designation as a planet (any more than the Earth's make-up and composition would cause us to reclassify it as a giant asteroid.) Whether these other bodies in the Kuiper Belt will ever be designated as planets remains to be seen. As a rule of thumb, there are three properties of a planet: 1) It must (directly) orbit a star 2) It must be small enough that it has not been undergoing internal nuclear fusion (i.e. it is not a star or star-like object) 3) It must be large enough that its self-gravity gives it the general shape of a sphere. You might notice that under these rules, the asteroid Ceres could qualify as a planet - I guess we're hedging our bets by calling it a "minor planet".

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