Seeing Extrasolar Planets

Two groups announced today the first direct observations of light from extrasolar planets. In both cases, the telescope used was the Spitzer Space (Infrared) Telescope. Up until now we've been able to detect these objects only indirectly. Most have been found via their gravitational effects on their host stars by studying the changes in their star's radial velocity. A rare few have also been seen to cause a slight dimming in their host star when they pass in front of the star as seen from Earth. The two planets involved here are HD 209458 b and TrES-1 and were observed when they, in turn, pass behind their host star. In the infrared, the total light was seen to dip by a fraction of a percent while the planet was in behind the star. That difference is the light emitted by the planet itself.