How do gravitational forces ensure planets' circular motion around stars?

Gravitational forces ensure planets' circular motion around stars by constantly pulling them towards the star's centre.

In more detail, the concept of gravitational forces and circular motion is a fundamental aspect of astrophysics. The force of gravity is a mutual force of attraction that exists between any two masses. In the context of a planet orbiting a star, the star's massive size creates a strong gravitational pull on the planet. This force of gravity pulls the planet towards the star.

However, the planet doesn't fall into the star because it's also moving sideways. This is due to the planet's initial velocity when it was formed or captured by the star's gravity. This sideways motion is called tangential velocity. The combination of the planet's tangential velocity and the star's gravitational pull creates a balance, resulting in the planet's circular or elliptical orbit.

This balance is a result of the gravitational force acting as the centripetal force, which is the force that keeps an object moving in a circular path. The gravitational force pulls the planet towards the centre of the star, while the planet's inertia, its tendency to move in a straight line, tries to pull it away. The balance of these two forces results in the planet moving in a circular or elliptical path around the star.

In essence, the gravitational force acts as a tether, keeping the planet from flying off into space, while the planet's inertia keeps it from falling into the star. This delicate balance, governed by the laws of physics, ensures the stable orbits of planets around stars.

It's also worth noting that the speed and distance of the planet's orbit depend on the mass of both the star and the planet, as well as the distance between them. This is described by Kepler's laws of planetary motion and Newton's law of universal gravitation. These laws provide the mathematical framework for understanding the motions of planets and other celestial bodies.