Pluto lost its planet status in August 2006 after the International Astronomical Union (IAU) decided it did not meet three major criteria for being a full size planet. The criteria are:
- It must be an object which independently orbits the Sun (this means moons can't be considered planets, since they orbit planets)
- It must have enough mass that its own gravity pulls it into a roughly spheroidal shape
- It must be large enough to "dominate" its orbit (i.e. its mass must be much larger than anything else which crosses its orbit)
Pluto, being so small, was not able to dominate its own orbit.
A new study by University of Central Florida alumnus Philip Metzger, however, is calling the IAU's assessment of Pluto into question.