How exactly is this done? Is it done in the visible wavelengths or in the radio? Also, how small can the (planet size)/(sun size) ratio be. I'd expect that given noise especially, there's a definite lower limit.
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Just do a Google search on the relevant terms (extrasolar, planet, transit, etc) and you will get all the information you need. It's possible to discover extrasolar planets through visible-light photometry with small amateur telescopes. The trick is that the plane of the planets' orbits have to be pointed pretty much at us, so we can see the planet superimposed over its star.