The Solar System
Earth is one of nine known planets that go around our star, the sun. Other objects go around the sun, too — large chunks of rocks called asteroids, balls of ice called comets, and dust particles. All of these belong to our solar system.
Our solar system is in the Milky Way galaxy. There are millions of other systems in this galaxy. The Milky Way galaxy is part of the universe.
From Earth, the other planets look flat. But they are really shaped more like balls.
Planets move in two ways. Every planet turns around and around like a spinning top. This movement is called rotating.
As each planet rotates, it also follows a path around the sun. The path is called an orbit. We say the planet orbits (revolves around) the sun.
Something else makes our solar system work the way it does. It is a force called gravity.
Gravity — the Greatest Force Of All
When you hold a pencil in your hand and let it go, it falls to the ground. Why doesn't it float in the air?
A great scientist, Albert Einstein, explained why. Everything is made up of matter. The amount of matter an object has is called its mass. Some objects have great mass. Some have little. In space objects with great mass pull other objects to them. That pull is a force called gravity.
Because Earth's mass is great, its pull of gravity — or gravitational [GRAV-it-TAY-shun-ul] force — is also great. It pulls the pencil toward Earth's center and keeps it from floating in air. Gravity keeps us from floating in air, too.
The sun, which is a million times larger than Earth, contains an enormous amount of matter. Its mass is far greater than Earth's.
The sun's gravitational force is very strong. If it were not, a planet would move in a straight line out into space forever. The sun's gravity pulls the planet toward the sun, which changes the straight line of direction into a curve. This keeps the planet moving in an orbit around the sun.
Because of the sun's gravitational pull, all the planets in our solar system orbit around it.
The sun is a huge ball of super-hot gas. Deep inside the sun, the temperature is hotter than the hottest furnace. The sun is so hot that it heats and lights all the planets in our solar system.
The Orbits Of The Planets
Some planets orbit closer to the sun than others. And some are far away from the sun. The planets that are farthest from the sun are called the outer planets.
The planets that are closer to the sun are called the inner planets. The inner planets are close to each other as well as close to the sun. There are four inner planets:
The inner planets have a hard, rocky surface. It is possible to land a spacecraft on planets that have a hard surface.
The five outer planets are not only farther from the sun, but they are also far apart from one another.
Four of them are much, much bigger than the inner planets. The four giant planets are:
Scientists believe that the surface of these huge planets is not solid. It is probably slushy.
The last planet discovered in our solar system is farthest away from the sun. It is Pluto. We don't know much about Pluto yet except that it is very, very cold.
Pluto is not like the other outer planets. And it is not like the inner planets, either. Pluto is a mystery.
Adapted from Scholastic's "A Book About Planets and Stars" by Betty Polisar Reigot.