What causes elliptical planetary orbits?

01 April 2012

Question

Hi Dr Chris,

What a fantastic radio show you put on!

Something I've been wondering for a very long time - I have understood your explanation of why planets revolve around stars - the fact that they have energy and are constantly 'falling' but because they have forward motion and want to keep moving in a straight line, they keep 'missing' the thing they are falling towards and therefore move around it. What I wanted to ask is why the orbits are often elliptical and not circular. I know that some of the plans have elliptical orbits, and some comets' orbits are very elliptical. How does this work and what causes it?

Thanks very much!
Benjamin

Cape Town
South Africa

Answer

Dominic - The simplest kind of orbit is a circle, where the planet is trying to travel in a straight line which is carrying it further away from the star it's orbiting around.

But the gravitational pull of the star in a particular direction is pulling it back, so it's staying at a constant distance from the star as it goes all the way around that central star.

Now, if you imagine that planet had slightly less speed, then it wouldn't have enough speed to keep at the same distance from the star, so it would begin to fall in towards the star. As it begins to fall in, it will start to move much more quickly, because the star is pulling it in and it's gaining kinetic energy.

But it's then moving too fast to be in a stable circular orbit that much closer to the star. So, it then has enough energy to move back out again.

So, it's "wobbling" in its distance from the central star, which makes the orbit elliptical

Comments

How do elliptical orbits actually work?

The answer is stated clearly above. The orbiting body accelerates towards the centre of gravity (whatever it is orbiting); the increase in kinetic energy means that it then travels further before being deflected from its course again; the result is a squashed circular orbit, otherwise known as an ellipse.

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