Name: Alice E Packard
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
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
Jasjeet S Bagla
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Update: June 2012