Joni Lynn Bahro, age 10, of Ottawa, Ont., Canada, for her question:
Why are all planets round?
A planet is around ball surrounded on all sides by vast oceans of space. However, it is not out of touch or alone. It obeys certain strict laws of the entire universe that make it move in certain ways, and certain forces mold it into a certain shape.
A round global shape is called a sphere, and in a perfect sphere every line from the center to the surface is exactly the same length. The planets of the solar system are spheres. Most of them are somewhat wider around the waistline, and not one of them is a perfect sphere.
They are, of course, ruled and molded by the great, invisible laws of the universe. Certain rl11es force them into round, globe shapes, and other rules make them into imperfect spheres that are a litt1e fatter around the lines of their equators.
Every planet has its proper share of gravity. This gravitation is a built in force that pulls things down to the very center of the globe. It is gravity that gives weight to objects, making some things heavier than others. Heavy stones sink down through water because the water is made of a lighter substance. A light feather falls through the air because the air is made of even lighter material.
The force of gravitation pulls harder at heavy objects. The rocks and minerals of the earth are of different weights and masses. The earth pulls the hesv1est materials down to the bottom and the bottom is at the center of the sphere. The lighter materials rest in layers on top of the heavy core and upon them rest lighter and still lighter layers. The force of gravitation makes a planet round. The planet's gravitation arranges the materials in round layers like the skins of an onion, and this builds the solids into a sphere.
Meantime, the planet spins around on its axis like a top. All of its solid materials spin with it and spinning materials tend to pull away from the center of the spin. This causes the solids around the equator to bu1ge outward and pull the perfect sphere a little out of shape.
These forces seem to work on all sizable heavenly bodies. The planets are round and so is the sun and so also are the huge, blazing stars. Between mars and Jupiter are swarms of small planetoids called asteroids. Some are sizable chunks of matter and some are mere pebbles. So far as we know, even asteroids a few miles wide are molded into spheres.