What source keeps the planets in place?

Answer 1

first it is not a source it is a force. There are at least two equal and opposite forces that keep an object in an orbit. Gravity and centrifugal force. Gravity is an attraction which is proportional to the product of the two gravitating bodies divided by the square of the distance between them. Without a force to counteract this attraction, the bodies would move toward each other, the distance decreases, the force between them increase and the two bodies will eventually collide. Centrifugal force on the other hand is a force generated by the radial acceleration of a body moving on a curve. Thus as the planet revolved around a star, centrifugal force is generated which is opposite in direction to the gravitational attraction of the planet and the star. Balance is therefore attained and the planet stays in its orbit. This is the same thing that happens between a star and the core of the galaxy where it is located. The star and its planetary complement revolve around the core of the galaxy and the centrifugal force generated by the revolution counteracts the attraction of the galactic core and the star, hence balance is maintained.