What makes planets rotate

It depends on what you mean by rotate. It would have to rotate in the opposite direction to its star orbit with a day period exactly equal to its year.

There are a lot of planets in the universe, so maybe one fits this bill. With such long days and nights, temperature extremes probably preclude life on such a planet, but it would be fun to speculate on what astronomy any inhabitants might develop, with a sun and other planets that move very slowly across their sky while the stars at night are stationary. Planets normally spin but, given that the universe contains billions of stars, a few must stop spinning. A star or solar system is formed from a collapsing cloud of gas.

In the highly unlikely event that this cloud has no angular momentum and therefore no spin, the result would be a non-spinning star without any orbiting planets. Even if almost imperceptible, a molecular cloud normally has some angular momentum. Conservation of angular momentum means that this cloud spins faster as it collapses, much in the same way that ice skaters spin faster as they pull in their arms.

Some of this gas collapses into a protostellar disc, a ring of material that acts as the nursery for planets, which then naturally spin and orbit.

Kepler's laws are a direct consequence of gravity. The only thing that has to be kept in mind in rotation is that it results in a centrifugal acceleration that points radially from the center of motion. Hence, there has to be some force that conteracts this acceleration; otherwise the body will fly away in case of orbital motion or will disintegrate in case of spinning. In the case of orbital motion, the counteracting force is gravity; gravity causes the body to continually fall towards the center, and this exactly conteracts the force resulting from the centripetal acceleration.

In the case of a spinning object, it is the self-adhesion of the body itself that keeps it together. This results in a limit for how fast an object can rotate and still keep itself together. If it rotates too fast, the outward acceleration felt by the elements in the body may be more than the force that keeps them bonded together, and if this happens, the body breaks up. Other than this, there is no real law concerning rotations.

Note that rotational motion involves conservation of angular momentum just like linear motion conserves linear momentum. Jagadheep built a new receiver for the Arecibo radio telescope that works between 6 and 8 GHz. He studies 6. These masers occur at sites where massive stars are being born. He got his Ph. Why do planets rotate? Intermediate Why do the planets rotate? What force cause them to rotate? Are there some laws also in the case of rotations? This page updated on July 18, About the Author Jagadheep D.

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