Science Questions

What causes elliptical planetary orbits?

Sun, 1st Apr 2012

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Question

Benjamin Lunsky asked:

 

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 itís orbiting around.  But the gravitational pull of the star in a particular direction is pulling it back, so itís 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 wouldnít 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 itís gaining kinetic energy.  But itís 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, itís wobbling in its distance from the central star.

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