Here's a
page on methods used to detect many extrasolar planets, including the radial velocity, transit and gravitational microlensing methods.
The radial velocity method uses the Doppler shift of the starlight caused by the star going around the center of mass of the star and planet. That center is known as the
barycenter. Here's an
animation showing how the star moves. When the star moves towards us, the light is blue shifted and when it is moving away, it's red shifted. Watching the shifts of the absorption lines of the star's spectrum can reveal this.
I made a spreadsheet of how much the sun is displaced by each planet. The barycenter of the Sun-Jupiter system is 710 to 780 thousand km from the center of the Sun, depending on where Jupiter is in in its non circular orbit. The radius of the sun is 696000 km, putting the barycenter above its surface.
The displacement due to Saturn is around 0.6 times the radius of the Sun. For Earth, it is about 450 km.
The displacements due to each planet add up, making the Sun move in a complex path around the solar system's barycenter.
If you have a recent version of the free Celestia program, you can select the object named SSB and see how the Sun moves around it and follow its reaction to Jupiter and Saturn by splitting the window into two views to also show those two planets in their orbits from a distance.
We see this effect with other stars by looking at the Doppler shift of the absorption lines in the star's spectrum and then doing frequency analysis to extract the frequencies of the planets' orbits. Someone out there could easily find Jupiter and Saturn around our star if they watched for long enough to cover enough of Jupiter and Saturn's orbital periods.
The three super earths orbiting one star that were recently in the news were detected with this method.
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The transit method involves watching the star periodically dim and then brighten again as the planet passes in front of it. When the planet is behind the star, then we get just the starlight. Then we could subtract that from the combination of the starlight and light from the planet to get what's emitted by the planet itself. It should be possible to use the equipment at the
RIT observatory to use the transit method. There had been people doing projects to detect known extrasolar planets with this method at RIT.
Actually, it had been
done by the professor who taught my University Astronomy class.
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The gravitational lensing method looks for stars that happen to have another star pass in between it and us. The gravity of the star in between would bend the light from the more distant star a bit like a lens would. If there is a planet orbiting the star in between, it would make the light be bent differently from how a star with no planets would do it. The alignment between the star with the planet and the more distant star doesn't last because those stars and we are moving through space, breaking the alignment, so we can't detect the planet again unless it was close enough to use another method on. This method can still give us a statistic picture of how common planets are.
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Other possible methods include using a coronagraph to block out the starlight to get light from the planets. The
Kepler Mission will watch 100 thousand stars and use the transit method as well as watching for variation in the light due to the phases of large planets, like we can see different amounts of light from the Moon as it goes through its phases. Kepler should be able to find Earth sized planets. Another way that had worked is directly imaging a planet, like
this one. It worked because it's a large, hot planet far from the brown dwarf it orbits and brown dwarfs are not so bright.
