Why are planetary orbits elliptical and not circular?
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Imagine you’ve got a pencil on a table. There are a lot of orientations it can be sat in – point to the left, point to the right, point facing you, a million different ways it can be laying flat on the table. But standing up on its point or on the eraser is also totally valid and doesn’t contradict any physical laws (it’s not very stable, but it doesn’t have to be).
In the same way, when you look at planetary orbits you check the physical laws and look at the possible solutions, and you see a huge range of ellipses and also circular. You would expect to see a million planets in elliptical orbits and only occasional circles, and if you look at cometary orbits that’s exactly what you see – lots and lots of very eccentric ellipses and few closer to circular.
So the real question you should be asking is given that, why are so many of the planetary orbits so very close to circular?
The main reason I can think of, and I’m happy to have more pointed out – the gas that formed the planets wasn’t a uniform cloud, it was thrown from the dust that became the sun as it was collapsing down, and it turns out if you model that spinning dust blob collapse, you get circular rings thrown off every so often from the equator of the big spinning dust blob. Those dust rings coalesced into most of the planets, so they were left with pretty circular orbits. You do get less circular orbits – Pluto and Mercury are pretty elliptical for planetish things.
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