For a planet to be in a stable circular orbit it must have a velocity which is

For a planet to be in a stable circular orbit, it must have a velocity that is constant. This is because in a stable orbit, the gravitational pull of the star (or other body) that the planet is orbiting is balanced by the centrifugal force experienced by the planet due to its motion. This balance of forces ensures that the planet remains in its orbit without being pulled into the star or flung out into space.

Here are the steps to understand this:

1. The gravitational force between the planet and the star pulls the planet towards the star. This force is always directed towards the center of the star.

2. At the same time, the planet is moving. This motion creates a centrifugal force that pushes the planet away from the star. This force is always directed away from the center of the star.

3. For the planet to be in a stable orbit, these two forces must be balanced. This means that the gravitational force pulling the planet in must be equal to the centrifugal force pushing it out.

4. The speed at which the planet moves determines the strength of the centrifugal force. If the planet moves too slowly, the gravitational force will be stronger and the planet will be pulled into the star. If the planet moves too quickly, the centrifugal force will be stronger and the planet will be flung out into space.

5. Therefore, for the orbit to be stable, the planet must be moving at just the right speed for the centrifugal force to balance the gravitational force. This speed is the constant velocity required for a stable circular orbit.