Our universe is full of mysterious objects and amazing phenomena. All the objects of universe rotate in one or other way. For example planets spin on their axis and revolve around the sun. The sun also spins and revolves around the center of the galaxy. Even the galaxy itself rotates. Have you ever thought why do these bodies rotate?
Conservation of angular momentum
The phenomenon of rotation can be explained by the concept of angular momentum. Everybody which rotates, also possess angular momentum. According to the conservation of angular momentum, the angular momentum of any rotating or spinning object always remains constant when it is not influenced by any external twist. The object will keep on spinning forever unless an external torque, twist or rotating force is applied to change the angular momentum. The direction of angular momentum also remains conserved along the axis of rotation.
The planets will spin on their axis forever unless they are hit by some other huge body or influenced by external force. As the planets are spinning in the vacuum so the rate of slowing down of spin is negligible.
Origin of rotation of planets
Now it is clear that once a body starts rotating it is not going to stop itself. But then the question arises how the planets started spinning?
The initialization of spin can also be explained by the concept of angular momentum. For that we have to look at the mathematical form of angular momentum:
Angular momentum = Mass x Angular Velocity x (Radius)2
When there is no external force (torque) on the body, the angular momentum should be constant. Here the mass of the body is not going to change. So the product of angular velocity and radius should always be constant. Angular velocity and radiusare inversely proportional. It means if the radius decreases then the angular velocity will increase. The increase in radius will result in the decrease of angular velocity.
The above concept can be observed in the dancing of skater. The dancer starts with very small rotational speed with his/her hands stretched outwards. As he/she brings his/her arms closer to its body the size (radius) decreases which results in the increase of angular velocity.
According to the same idea we can say that any object, whose size or radius decreases, will rotate with faster angular velocity. Now if we think about the origin of planets and star we can get the clue. Our sun and the planets were formed form a big cloud of gases called solar nebula.
Every star and planetary system forms from a very huge cloud of gas molecules. The gaseous cloud collapses due to its own gravity which will result in the decrease in size. The density of collapsing cloud keeps on increasing and ultimately results in the star and the planets. But while collapsing, the radius decreases which will lead to increase in angular velocity. So a small amount of rotation will result in huge angular velocity as the size of molecular gas cloud decreases.
Every molecule has angular momentum and when we add the angular momentum of all the molecules, the net angular momentum is usually not zero. Let us consider the angular momentums of all the molecules are cancelling each other and net result is exactly zero. Even then there are other ways which can introduce small amount of angular momentum. Suppose there are two gas clouds, it might be possible that the one gas cloud is gravitationally attracting one end of the other gas cloud more than the other end. So this will result in small amount of rotation in the second gas cloud.
Direction of rotation of planets
As all the planets and sun have formed form the same gas cloud. They all rotate in same direction. I mean that the direction of rotation of sun and revolution of planets around the sun is in the same direction. The same pattern is followed by the satellite of the planet because planet and satellite are also formed from the same cloud. Thus the direction of rotation of planet and revolution of satellites are same.
Now you must be thinking about the exceptions to above pattern. For example the planet Venus rotates very slowly and in the opposite direction and the axis of rotation of Uranus is tilted by 97.77 degree from the axis of revolution. This clearly indicates that they have suffered a huge collision in the past. The collision with some huge body has changed their angular momentum.
Even some satellite revolve in the opposite direction to the direction of spin of planet which indicates that the satellite was not formed at the time of origin of planet but it was captured by the planet much later.
Now we can conclude that the planets spin due to conservation of angular momentum. The same direction of revolution and rotation of planets indicates their common origin from the gas cloud. I hope you got answer to question, “ Why do planets spin?” If you have suggestion and ideas you are invited to comment. If want to know more about astronomy then you may subscribe us for email notification and follow us on facebook, twitter and google plus.