June 2005

Scott Mair

If you want to make a kid angry, tell them Pluto is not a planet. For kids there is no controversy - Pluto is definitely a planet.
There's some argument among astronomers though. The Adler Planetarium at the American Museum of Natural History in New York City created a lot of controversy a few years ago by excluding Pluto from the planets hanging in their lobby.

The reason some astronomers want to exclude Pluto is because its not like the other planets. First, doesn't orbit the Sun in the same plane as the rest of the planets. In the night sky it's way off the ecliptic, the line the sun, moon and all the other planets cross the sky. Its orbit is also highly eccentric - that's astronomer speak for very oval in shape. All the other planets are pretty close to a circle. In fact, Pluto's orbit is so elliptical that for part of its journey around the sun its not the farthest 'planet' out there. Only after 1997 did Pluto get out past Neptune.

Pluto also looks weird; all of the other planets beyond the asteroid belt are gas giant planets, but Pluto is a little ice ball with a moon half as big as it is!

Its clear that Pluto didn't' form like all the other planets; from a disk of spinning gas that also created the sun. Astronomers consider Pluto to be the largest (so far) member of the Kuiper Belt - a zone of very distant, icy rocks that formed on the edge of the solar system - hundreds have been discovered so far and there is probably millions more out there.

It depends what you call a planet - if its something that circles a sun then Pluto's a planet and so are all the asteroids and comets and ... If a planet had to form from the primordial spinning gas of the young sun, that might leave Pluto out.

I don't care. Its fun to look for Pluto and this month we have the best opportunity to see it in my lifetime. It won't be easy though. Pluto is a magnitude 13 object - that's dim. We can only see up to magnitude 6 with our eyes. You'll need at least a 6 inch telescope to find it. If you're up to the task the serpent's tail is the constellation to point your telescope. If you're lucky you'll see what Clyde Tombaugh first saw 75 years ago.