For a planet to be in a stable circular orbit it must have a velocity which is *1 pointPerpendicular to its orbitTangential to its orbitGreater

To determine the conditions necessary for a planet to be in a stable circular orbit, we must understand the relationship between gravitational force, centripetal force, and orbital velocity.

1. Understanding Perpendicular Motion: In a circular orbit, the gravitational force acts as the centripetal force that keeps the planet in its path. The gravitational force pulls the planet towards the center of the orbit, which is typically the star or body it is orbiting.

2. Tangential Velocity: For a stable circular orbit, the planet must travel with a velocity that is tangential to its orbit. This means that the velocity is directed along the tangent of the circular path at any point in the orbit. If the velocity were perpendicular to the orbit, the planet would not follow a circular path; instead, it would go off in a straight line due to inertia.

3. Greater Velocity: If the velocity is greater than the required tangential velocity for that distance from the central body, the planet would not stay in a circular orbit. Instead, it would enter an elliptical orbit or, if the velocity is high enough, escape the gravitational pull of the body it is orbiting.

Conclusion:

Thus, the answer is Tangential to its orbit. The planet must have a velocity that is directed along the tangent of its circular orbit in order to maintain a stable orbit.