Andree Schryburt, age 10, of Ottawa, Ontario, Canada, for his question:

WHAT EXACTLY IS A PLANET?

It is, of course, a heavenly body, in the great ocean of space. But in order to qualify as a genuine planet, a heavenly body must pass several tests. It must orbit around a star, just as the planet earth orbits around our sun. What's more, it may not orbit around another planet, as the moon orbits around the earth.

The main qualification is an orbit of its own. But a planet also is expected to be a reasonable size. For example, the asteroids between Mars and Jupiter each have orbits of their own. But they are too small to qualify as real planets, though we may call them planetoids  which means baby planets. Our moon may be as big as a small planet, but it cannot qualify because it belongs to the earth  orbiting around us as we orbit around the sun.