By Dr. Marc Rayman
Asked by our friends at the Gheens Science Hall & Rauch Planetarium at the University of Louisville in Kentucky.
Most people take this fact for granted, but the answer involves many interesting ideas and important concepts. I'll touch on a few.
First of all, saying the planets go around the Sun is just another way of saying the planets are in orbit around the Sun. A planet orbiting the Sun is like the moon or a NASA satellite orbiting Earth. Now why does a planet orbit the Sun and not the Sun orbit the planet? The lighter object orbits the heavier one, and the Sun is, by far, the heaviest object in the solar system. The Sun is 1000 times heavier than the largest planet, Jupiter (which also happens to be my favorite planet), and it is more than 300,000 times heavier than Earth (another planet I am very fond of). In the same way, the moon and satellites we launch orbit Earth because they are so much lighter than our planet.
But now we still have the question of why anything orbits something else. The reasons are complicated but the first good explanation was provided by one of the greatest scientists ever, Isaac Newton, who lived in England about 300 years ago. He was very well known when he was alive, being admired by many people for answering some of the most difficult and fascinating scientific questions of his day, but I'm sorry to say his long life generally was not a happy one. I wonder how he would have felt to know that even hundreds of years after his death, he is widely considered to be one of the most brilliant, important, and productive scientists ever to have lived.
Newton realized that the reason the planets orbit the Sun is related to why objects fall to Earth when we drop them. The Sun's gravity pulls on the planets, just as Earth's gravity pulls down anything that is not held up by some other force and keeps you and me on the ground. Heavier objects (really, more massive ones) produce a bigger gravitational pull than lighter ones, so as the heavyweight in our solar system, the Sun exerts the strongest gravitational pull.
Now if the Sun is pulling the planets, why don't they just fall in and burn up? Well, in addition to falling toward the Sun, the planets are moving sideways. This is the same as if you have a weight on the end of a string. If you swing it around, you are constantly pulling it toward your hand, just as the gravity of the Sun pulls the planet in, but the motion sideways keeps the ball swinging around. Without that sideways motion, it would fall to the center; and without the pull toward the center, it would go flying off in a straight line, which is, of course, exactly what happens if you let go of the string.
Here are some other Space Place pages to explore:
"Shoot a cannonball into orbit" demonstrates how objects fall and move sideways at the same time as they orbit.