Officially, Pluto is a planet. In reality, it's a member of numerous Kuiper Belt objects which orbit the Sun beyond Neptune. When it was found, 74 years ago, Clyde Tombaugh was searching for a planet and so assumed what he had found was one.
Another large Kuiper Belt object was found a few days ago, as has been reported by the BBC (http://news.bbc.co.uk/1/low/sci/tech/3506329.stm) - 2004 DW. Its diameter is estimated as between 1500km and 2600km; Quaoar, another Kuiper Belt object found in 2002, is about 1200km across; Pluto is 2320km across. The evidence is mounting that Pluto is simply a large example of an ordinary Kuiper Belt object. If 2004 DW turns out to be as large as, or larger than Pluto, will it be called a planet?