How does a planet revolves on it's axis?

Answer 1

For the purposes of this question we will consider a planet as a solid body - which it pretty much is anyways. As a result of a planet's formation it is left with some rotational energy. This is merely a result of chaotic systems and it would be extremely difficult to have a body with exactly zero rotational energy. Since the planet is a solid body it has to turn uniformly- an as it turns uniformly there has to be an axis of rotation, a line through the body around which it turns. If a planet is a long ways away from it centre of orbit the tidal forces are negligable and there is no impediment to it rotation. All the planets Earth and beyond rotate with a period different from their period of orbit. Venus and Mercury are tidally locked with their solar orbital period in some resonance ratio. Earth's moon and most planetary moons are like-wise tidally locked with a period similar to their orbital period. It has been proposed that Uranus was hit by some massive object that alterned its axis of rotation to an extreme degree. If this is true the mass and energy of the object must have been considerable.