Question

What is the centripetal force that holds planets in orbit?

Answer 1

Answer: Gravity force

If we approximate the orbit of the planets around the Sun to circular orbits with a uniform circular motion, where the velocity
`\vec{V}` is a vector, whose direction is perpendicular to the radius
`r` of the trajectory; the acceleration
`\vec{a}` is directed towards the center of the circumference (that's why it's called centripetal acceleration).

Now, according to Newton's 2nd law, the force
`\vec{F}` is directly proportional and in the same direction as the acceleration:

`\vec{F}=m.\vec{a}`

Therefore the net force resulting from the movement of a planet orbiting the Sun points towards the center of the circle, this is called Centripetal Force which is a central force that in this case is equal to the gravity force.