why do planets circle the sun? HeLp! Science project!

Let me fill in another part of this phenomenon. The sun, being much larger than the earth, keeps on tugging the earth towards itself. Then why doesn't the earth fall into the sun after all these years? The reason being that there is also gravity pulling on earth in the opposite direction from outer planets and distant stars. Overall, it's able to set up a balance to keep the earth in its present orbit. The shape of the orbit depends on the overall gravitational pull from all the heavenly bodies and matters within able distance in space. Gravitational forces' effect on stars orbits show binary stars existence, black hole locations etc over huge distance in space as well. It is a very important rule and tool in astronomy. Furthermore, gravity is the ultimate force that's able to start atomic fusion once the gaseous star is over the critical size. It's invisible but controls and rules everything in space. Birth of stars, black holes, solar system orbits, death of stars, star explosions, release of cosmic rays and electromagnetic radiation (i.e. light, the support of life on earth). Very fascinating indeed.

2008-12-08 11:20:56 補充：
If you insist why planets circle around the sun, there is no reason or answer. Planets are passive objects in space that are trained to follow external forces as they come. Those forces are tremendous coming in from all directions in space.

Why do the planets rotate around the sun?

The basic reason why the planets revolve around, or orbit the sun (rotate actually is used to describe their spin, for example, the Earth completes one rotation about its axis every 24 hours, but it completes one revolution around the Sun every 365 days), is that the gravity of the Sun keeps them in their orbits. Just as the Moon orbits the Earth because of the pull of Earth's gravity, the Earth orbits the Sun because of the pull of the Sun's gravity.

Why, then, does it travel in an elliptical orbit around the Sun, rather than just getting pulled in all the way? This happens because the Earth has a velocity in the direction perpendicular to the force of the Sun's pull. If the Sun weren't there, the Earth would travel in a straight line. But the gravity of the Sun alters its course, causing it to travel around the Sun, in a shape very near to a circle. This is a little hard to visualize, so let me give you an example of how to visualize an object in orbit around the Earth, and it's analogous to what happens with the Earth and the Sun.