Harry Tilis, age 10, of Pittsford, New York, for his question:

Was there ever a planet between Mars and Jupiter?

Astronomers cannot say for certain. But there is a lot of evidence to suggest that perhaps another planet once orbited the sun between little Mars and giant Jupiter. The evidence looks logical, but astronomers do not have enough facts to prove it. Hence, we must call the grand idea a theory, which is an educated guess. Perhaps our educated space probes will relay the proveable evidence needed to settle the question:

A model of the Solar System reveals that the orbiting planets form an orderly pattern    with one exception. The distances between the orbits farther from the sun grow gradually wider except for the extra long jump between the orbits of Mars and Jupiter. The space between their two orbits averages about 342 million miles. This is almost four times wider than our distance from the sun.

Certainly there is room for another planet between Mars and Jupiter and it looks as if there should be one there to complete the planetary picture. This gives us a logical reason to suspect that there once was such a planet, but a logical guess is not enough to prove a scientific fact. However, there is some other evidence to back it up. That gaping space between the orbits of Mars and Jupiter is not exactly vacant. It is occupied by the assorted asteroids, orbiting the sun like swarms of golden bees.

Some astronomers call them the planetoids, meaning small planets. Moons orbit in curlicue orbits around their parent planets as they go around the sun. The asteroids travel around in smooth, planetary type orbits. Most, but not all of them stay within a wide belt, more or less where we would expect to find that missing planet.

This is enough evidence to give us another logical idea to support our theory. Perhaps the asteroids are the fragments of a whole planet that once shattered asunder. There is more evidence, but not yet enough to prove it. Some of the runaway asteroids cross the orbit of Mars and loop close enough to strike the earth. They come blazing through our atmosphere and land as meteorites    and these fallen space travelers bring mysterious evidence.

A solid planet is arranged in shells with its heaviest materials concentrated in the center and the lightest ones at the surface. Fallen meteorites come in concentrated chunks of either metals, light rocks and liphter clays. This would be so if they are really fragments from the different layers of a shattered planet.

There is, as you see, a lot of evidence that a missing planet once orbited the sun between Mars and Jupiter. On the other hand, nobody knows enough about the past history of the Solar System to prove this was so, or how such a whole planet could be shattered. Perhaps after all, it was never there and the asteroids are merely bits of debris left over when the other planets were created.