Why do the planets stay in their orbit around the sun?

The Sun's gravity constantly pulls on the planets, preventing them from leaving their orbit and the solar system. The forward motion of the planets prevents the Sun's gravity from pulling planets straight into it.

Explanation:

when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.