Why do planets orbit stars?
There are two ways of explaining this phenomena :-
A Planet revolve around a Star because of the mutual gravitational interaction between them and this force is given by
This theory tells us that the Space-Time is like a piece of Fabric and the mass of an object causes to bend the curvature of Space-Time around it. And more massive the object is , the more space it bends around it.
So, in case of Star and a planet , since the star is more massive ; so it bends the space around it much more than the planet does ; which causes the planet to be in a state of falling towards the star ( just like gravitational interaction) which causes it to orbit the star.