Joseph Koziarz, age 9, of Rome, New York, for his question:

Why are the planets spheres?

Molding a round planet is an enormous job. But the same forces also do smaller jobs. When we watch them at work in a small way, we can understand why planets are round and also how they were molded. These big projects are related to faking stones and floating feathers    and also to what happens when you swing a pail of water in a fast whirl.

When you drop a stone, it falls to the ground. If you drop it in a pond, it sinks to the bottom. When you drop a cork, it also falls to the ground    but it floats on top of the water. As we know, all these everyday events are ruled by gravity. The mighty force hugs us to the surface of the earth and pulls us dean when we fall. It pulls at all the material in the whole planet. And it pulls all of this material toward a central point down in the very middle of the earth.

This most important point is called the center of gravity. From there it reaches out in all directions, pulling every bit of material in the planet toward it. Its force is equal on all sides, which means it pulls equally hard on all sides. If the earth were shaped like a square box, the force of gravity would pull down the high corners. It strives to keep all the material evenly arranged around that point in the middle. This is how gravity molded the earth and the other planets into round spheres. However, this is only part of the story.

The earth is made from many different substances, some heavier and some lighter. Gravity pulls heavier material down through lighter materials. This is why stones sink through water and even feathers sink through the filmy air. Corks bob on the waves because their material is lighter than water. When gravity molded the earth into a sphere, it pulled the heaviest materials into the center. The others were arranged like onion skins around the heavy core, with the lightest layer on the top.

Something else also helped to do this enormous job. It goes to work in a small way when you swing a pail of water over your head very fast. The water does not spill, even when the pail swoops upside down. This happens when things spin around fast. And the earth has been spinning around fast for billions of years. This helped to mold the planet into a sphere and also to arrange its materials in layers around the center.

A mighty helping of gravity is a built in part of every star and planet. Even most of the moons are massive enough to own a sizeable helping of gravity. And from the very beginning, this built in force worked to mold these heavenly bodies into spheres. The little moons of Mars are not very massive, so they have only a small quota of gravity. It is not strong enough to pull down the corners and mold them into round balls.