Criteria for a Solar System Body to Be a Planet


My name's Eric Loberg, the director of the Taylor Planetarium at the Museum of the Rockies, and I was going to discuss the criteria for a solar system body to be a planet. Many people are wondering now, Pluto's not a planet so what is it and what does make a planet. Well three things decided the IAU which is the International Astronomical Union. In 2006 they took a vote and have three rules for things to be a planet. And rule number one is it has to go around the sun, it has to orbit the sun. This eliminates all your moons which are going around the planets. Otherwise you have very large moons. Some of the moons are larger than Pluto, some of the moons are actually larger than Mercury. And so if you just say anything that's going around anything you're going to have a lot of planets. This eliminates the moon. Number two is the object has to be basically round, it has to be spherical and this gets rid of lots of asteroids. Otherwise you have an asteroid belt going round the sun. And you have all these potato shaped moons going out there. Also this somewhat tells you the size. If something's hemispherically stable, and if it's just about round it's going to be a big object. And then so that kind of says it's probably a planet, not just a moon or a comet or an asteroid. It's round, it's big enough to make that round shape. And number three is it has to have cleared out its local orbit. It can't actually be in a band of asteroids. And that's kind of what dooms Pluto and that's what also dooms some of our, there's one round asteroid called Sirius. It has to have cleared out of its local orbit.. It means it has to have more mass, more stuff in it than all the surrounding things in it. Jupiter is like that. Jupiter has 50 moons and lots of little asteroids that cross its belt. But Jupiter is so much bigger than the mass of all those other things combined. Earth is like that. We're so much bigger than our moon. The problem is Pluto isn't like that. Pluto and Charon its moon are about the same size and there's lots more other objects out there in the kuiper belt. And if we named Pluto a planet you're going to have to start memorizing more planets like Makemake and Arius. These other large objects that are out there that are going around the sun, they're spherical but they're not quite planets. They're just a little too small and there's a lot of them. It's basically another asteroid belt out there called the Kuiper Belt. I'm Eric Loberg with The Museum of the Rockies Taylor Planetarium.

J. Eric Loberg is the manager, lecturer, and program planner at the Taylor Planetarium at the World, a renowned museum of the Rockies in Bozeman, Montana.