Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Spinning planets
Name: Alice E Packard
Status: Other
Age: N/A
Location: N/A
Country: N/A
Date: N/A 

My three-year-old niece asked "Why do planets spin?" and had us all stumped. Why? Why was the matter that formed the Earth spinning? Why does every- thing in the universe, from galaxies down to atoms, spin?

Wow, what a neat question. I think it is related to the effect that we have seen when a skater who has arms extended and is slowly spinning suddenly pulls the arms into the body surface. The result is that spin rate increas- es. To even spin faster the skater will be standing straight with the arms up over the head. This makes the distributions of mass as close toe the spin axis as possible and makes the spin as large as possible. The main idea is that angular momentum is conserved, i.e., cannot change. Angular momentum is the product of the spin rate and the moment of inertia (a measure of the distribution of the mass about the axis of rotation). The product is constant. If mass is widely dispersed as before condensing into a planet, and is very slowly rotating, then when it condenses into a planet (making a small moment of inertia) the rate of rotation could be large. So condensing amplifies whatever rotation there was initially. Now, what are the chances of something not rotating exactly before it condenses? Appar- ently, very small since everything seems to have some rotation. I am not sure this gets to your question of why, but it has been a great question to think about. Thanks.

Samuel P Bowen

Good answer Dr. Bowen!

I considered responding to this question but did not know how to address the central issue: "since everything seems to have some rotation" Why? I think it is like asking why does everything have mass, or charge, or linear momentum... I am not sure how to answer except to say that these are all fundamental properties of all matter.


One possible answer to your question is this: Consider objects condensing out of a gas cloud, as they collapse the tidal force of one lump can induce rotation in a nearby lump. As angular momentum is conserved, there is no overall rotation in the system ( Universe ) but matter at all scales will have some rotation. Total angular momentum must add to zero. Tidal force is a short range force that essentially arises from variation of some force (here gravitation) over the size of the object we are interested in. To see how this can lead to transfer of angular momentum, consider the example of Earth-Moon system. Here, tidal force of Earth has slowed down rotation of the Moon and now the Moon is doing the same thing to the Earth. Such a transfer requires nonspherical distribution of mass - even a small departure from sphericity can lead to a large transfer of angular momentum over a long enough time.

As far as spinning stars in spiral galaxies are concerned, there is a simpler answer: Differential rotation of the galaxy makes gas clouds rotate at a slow rate. This can also explain the definite sense of rotation in the solar system.

Jasjeet S Bagla

Click here to return to the Astronomy Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory