If you are old enough, you may remember a time when we thought Pluto was a planet. In 2006 the International Astronomical Union (IAU) adopted a standardized definition of what is and isn't a planet. Unfortunately Pluto no longer made the cut. However, the IAU also adopted a new term: "dwarf planet." A dwarf planet is a planet that orbits the sun, has enough mass for its gravity to form it into a nearly round shape, has not cleared other large objects from the region it crosses during its orbit, and is not a satellite of another object in space. Pluto fits this definition perfectly.