Do all satellites orbit in the same direction?

17 January 2010

Question

Do all satellites around a planet, star, etc orbit in the same direction? If so, why?

Answer

Well the answer is, in most cases, yes. This is because they condense out of what's called a proplid, or proto-planetary disc. This is a big spinning disc of material which initially starts off as an envelope around a star that forms the centre of the solar system or in a particular galaxy area you're looking at. This disc of matter then condenses or accretes into planetesimals, little objects that accrete more matter to become much bigger objects. And since angular momentum is conserved, this means that they will all be spinning. Then, once everything has collided with everything else that it's going to, and when you look at the equations, you end up with a cluster of bodies that are usually all turning in one net direction.

But that's not to say that big collisions can't come in and then turn things around a bit. Uranus, for example, is turning on it's side, and we think that this is possibly due to a big collision with something else way back in history.

So the answer is yes. We think things do go around in one direction and in the same plane, but there are exceptions at the level of individual bodies, which can spin in funny directions.

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