If we wish to discover if there is intelligent life outside our solar system we must first look for planets like Earth that orbit around other stars. Now thats difficult. Planets don’t emit light like stars so are extremely faint, almost invisible to our most powerful telescopes.
But there an indirect method that we can use to infer if a star has planets orbiting it. The method does not allow us to see the planets but simply infer that they are there.
If you have studied celestial mechanics you would have learnt that planets do not circle their parent star. Actually the star and the planet both orbit the center of mass of the system. Here is an excellent animation taken from wikipedia that illustrates the concept. The cross is the center of mass of the system.
Check out this blog entry for more details. Part 2 of the blog entry.
As you can see, the star wobbles as the planet goes around it. Light emitted from this star will be blue shifted as the star is approaching Earth and red shifted as it recedes from Earth. This periodic Doppler shift will allow you infer the existence of planets around other stars. Of course, there are lots of caveats and complications but this is the essential concept.
Its is tremendously simple but amazing idea and I wanted to blog about it, even though umpteen websites talk about the same thing.
Here is a good article that talks about this method in greater detail
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