Why do planets orbit stars?

#1) # Newtonian Gravitational theory :-

A Planet revolve around a Star because of the mutual gravitational interaction between them and this force is given by #F#

#{F=(GMm)/r^2}#

where #M# is the mass of the star

#m# is the mass of the planet

#r# is the distance between their centres

#G# is the Universal Gravitational constant

#2)# Einstein's General theory of Relativity :-

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.