When I was in third year Physics of the Earth, I remember reading a paper about the discovery of extrasolar planets (i.e., planets that are outside of our solar system). From what I recall, there are two primary ways in which extra-solar planets are discovered: through their transitory passes in front of their stars (causing the star to dim very slightly from our perspective on earth, just enough to be detected by telescopes) and through their gravitational effects on their stars (again, just barely detectable by our telescopes). A major problem with both of these methods is that it only detects large planets, such as those the size of Jupiter or Saturn. Most of the extrasolar planets found to date are of the large gas giant type because we didn't have the power to discover smaller terrestrial planets like ours. At the time, they were very hopeful about the Kepler mission, which went up in 2009.
The Kepler mission telescope has a lot of advantages compared to the earth-bound telescopes that are most often used for discovery of extrasolar planets. Most importantly, it's in space, so there is less interference by our atmosphere. The Kepler telescope can detect a 1/10 000th dip in brightness in stars, which is the approximate dip in brightness that would be caused by a planet of Earth's size transiting a star about the size of the Sun. Over 3.5 years, the goal is to assess the frequency of occurrence of possibly inhabitable planets. We haven't detected any of these yet, mostly because we need a long observance period first, but the Kepler mission is already proving adept at finding other types of extrasolar planets.
A number of new extrasolar planets have already been reported, mainly large planets that have very short orbital periods close to their stars. In October, a system of three planets around the star Kepler-9, currently named Kepler 9a, Kepler 9b and Kepler 9c. Keplers 9b and 9c are the larger planets, with orbital periods of 19.24 and 38.92 days respectively. Kepler 9a has the mass of about 0.25*(Mass of Jupiter), while Kepler 9c has the mass of about 0.17*(Mass of Jupiter). After the signals from these two planets were removed, a third smaller candidate planet was detected. Further measurements will need to be taken to determine if it is a planet for certain, but if it is then it has a very short orbital period of only 1.6 days and is about 1.5*(Mass of Earth).
What's interesting about this system is that all three planets appear to rotate at a distance from their star smaller than the distance between our own Sun and Mercury. It's thought that they formed farther away from their star and migrated to this position. The process by which this occurs could give us many indications as to the development of solar systems.
References
McLaughlin, G., Science, 330, 47, 2010.
Holman et al., Science, 330, 51, 2010
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