Were There Always Nine Planets?

So far, we know of nine planets in our solar system. Some astronomers think that there may be a tenth planet beyond the orbit of Pluto, the farthest known planet from the sun. And others think there was once a tenth planet that doesn't exist anymore!

The space between Mars, the fourth planet from the sun, and Jupiter, the fifth planet, is filled with thousands of asteroids — small rocky bodies that revolve around the sun. The biggest asteroid is about 600 miles across, but most are less than a mile in diameter.

Some astronomers believe that these asteroids may be all that's left of a planet that exploded millions or billions of years ago. But these asteroids may also be just pieces of space dust that never formed into a planet.

The orbit of one asteroid occasionally brings it "only" 485,000 miles away from earth!