Hi,

Well, this is a little complicated. The group of people that get to decide is the International Astronomical Union (IAU). Things became a problem when Mike Brown (an astronomer at Caltech) and co-workers found an object beyond Pluto which orbits our Sun and is 27% bigger than Pluto. It was eventually named Eris. So, if Pluto is a planet then Eris should definitely be one. So then the IAU started examining and debating the definition of 'planet'. The problem with Pluto is that it is very much unlike all the other planets. Beyond Mars all the planets are large, gaseous balls - except Pluto. All the planets roughly orbit the Sun in the same plane (that is, all the orbits are pretty much not tilted with respect to each other) - except Pluto. All the planets have the Sun at the center of their orbits - except Pluto (at closest point of its orbit it is actually closer to the Sun than Neptune is). All the planets have moons that are significantly smaller and orbit the planet - except Pluto (its moon Charon is almost as big as Pluto and they both orbit a spot in between them). So Pluto has always been a bit different.

At one point the IAU said everything round is a planet (which included Pluto and Eris = 10 planets). Then it thought about calling Pluto and Charon a double planet. Then it thought Pluto shouldn't be a planet (many people were particularly upset by that). I think the final word (since 2006) is that Pluto is a dwarf planet. So we have 8 (official) planets in our solar system and at least 3 (official) dwarf planets, Pluto, Eris, and Ceres (Ceres is actually in the asteroid belt between Mars and Jupiter). But Mike Brown has identified at least 44 dwarf planets (which are all waiting review and acceptance and naming by the IAU) and expects to eventually find hundreds.

Best,

Christina