Narrator: This is Science Today. In the summer of 2006, the International Astronomical Union, or IAU, a group of scientists that decides astronomical rules, decided that Pluto – formerly the ninth planet in our solar system – was not a planet after all. Seran Gibbard, a physicist at the Lawrence Livermore National Laboratory, explains that based on new discoveries in recent years, planets are now defined using three criteria and Pluto no longer makes the cut.
Gibbard: There's been controversy for years about whether Pluto was a planet because it doesn't have a very circular orbit and it crosses the orbit of Neptune , actually. Which now is what essentially disqualifies it as a planet.
Narrator: Basically the IAU has ruled that a planet must be large enough that its gravity makes it round, it must have cleared its path of any objects and it must orbit the sun.
Gibbard: Because otherwise the moon – all the moons of all the giant planets would be planets, right, because they're round, many of them are round.
Narrator: Pluto is now defined as a dwarf planet, which is a new category of objects. For Science Today, I'm Larissa Branin.