How can you tell between different types of stellar wobble?

30 May 2010


Hi Chris and all of you Naked Scientists!

This is Carol from California here.  My entire family listens regularly to your show and we just love it.

I have a question for the astrophysists:

When they discover an exosolar planet by the wobble it emparts on its star, how do they distinguish between the wobble from the center of mass of the planetary system and that of a single planet?

Thanks and keep up the good work!

Carol Hatcher


We put this question to Dominic Ford from Naked Astronomy

Dominic - Carol is absolutely right. Whenever a planet orbits around the star, it exerts some gravitational force on that star and that force pulls the start back and forth and that produces a red shift in the spectrum that you can then observe. You can observe it quite easily because the spectra of stars have quite narrow, well defined lines in them that you can see wobbling back and forth as the star moves. By looking at the wobble of those lines, you can deduce both the radius of the orbit of the planet and also its mass. The mass determines how far the features wobble and the radius determines the frequency of which they wobble. If you've got several planets, they will orbit with different periods and that will produce a different period of oscillation in those features. So for example, you might have an oscillation on a 10-day time scale superimposed on oscillation with 100-day time scale, just like a chord in music. By taking apart those frequency components, you can work out how many planets there are and what radius orbits they're in.

Chris - You basically have to build a model and come up with the only solution that satisfies the wobble that you're seeing.

Dominic - Yes, that's right.

Chris - It's interesting because we interviewed someone on this program called Dr. Christophe Lovis who was from the Geneva Observatory back in 2006 and he did precisely what you've just said for a star about 40 light years away from earth. It was called HD69830 and in fact, there's a very interesting interview with him on the Naked Scientists website, where you can hear a slightly different explanation for basically what you described Dominic.

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