What causes planets to stay in orbit around the sun?

Planets stay in orbit around the sun due to the balance between the sun's gravitational pull and the planet's forward momentum. Gravity constantly pulls the planet toward the sun, while the planet's velocity carries it forward. These two forces combine to produce a curved path around the sun rather than a straight line or a collision course. This is essentially what Newton described: the planet is always falling toward the sun, but moving fast enough sideways that it keeps missing it. Einstein later refined this by explaining gravity as the curvature of spacetime, where the sun's mass warps space and planets follow the natural curved paths, called geodesics, through that warped geometry.