Question

What forces keep a planet in orbit and how does each one work on the planet?

Answer 1

Gravity keeps a planet in orbit.

Explanation:

A planet's orbit is governed by gravity. Orbits were first described by Johannes Kepler. His three laws of planetary motion are:

1. The orbit of every planet is an ellipse with the Sun at one of the two foci.
2. A line joining a planet and the Sun sweeps out equal areas during equal intervals of time
3. The square of the orbital period is directly proportional to the cube of the semi-major axis of its orbit.

Isaac Newton described gravity as an attractive force between bodies using the equation:

`F=(GMm)/r^2`

Where `G` is the gravitation constant, `M` is the mass of the Sun, `m` is the mass of the planet and `r` is the radius of the orbit.

Albert Einstein showed that gravity is not in fact a force. It the effect of curvature of 4 dimensional spacetime caused by the masses of the Sun and planets. Each planet follows a geodesic which is the 4 dimensional equivalent of a straight line.

Newton's model of a force of gravity is a good approximation as long as the masses and speeds are not too big. In fact Newtonian gravity can't accurately predict the orbit of Mercury because there are relativistic effects.

So, there are no forces keeping a planet in orbit Each planet is simply following a geodesic in curved spacetime.