I don't know the exact definition Noss, but it is something like a body that has it's own gravity but not enough to clear it's orbit of other objects. Pluto was down graded to dwarf status in 2008 and they have discovered one in the asteroid belt (Ceres) and three more beyoud Pluto (Eris, Makemake, and Haumea). Until I read about them to my son in his library books I had never even heard of them. He has really taken the bit firmly in his teeth with this whole astronomy thing so we're trying to encourage it. One can only take so much Spongebob!