In June 2005, the discovery was announced of the smallest extrasolar planet yet found around an average star. Prior to the discovery of this planet, most of the 155 known extrasolar planets have been massive -- Uranus-sized or bigger. At about 7.5 times the mass of Earth, this new planet marked an important milestone towards finding Earth-like planets outside of our solar system.

Because of their small size and mass, Earth-like planets are difficult to detect. To date, the most successful planet-finding technique has been the radial velocity method, which looks for a periodic shift in a star's spectra due to the gravitational influence of orbiting planets. In a binary orbiting system, both objects are actually orbiting around a common center of mass. In the case of a star and a planet, the star is so much more massive than the planet that the center of mass is significantly closer to the star. While it is accepted to say that the planet orbits the star, the slight distance to the actual center of mass is enough that the star will periodically wobble back and forth, causing a Doppler shift in its spectra. Small planets produce such a tiny wobble that observational instruments have only recently become advanced enough to detect them.

As technology continues to advance, the improved sensitivity of instruments will allow for more effective planet-hunting. The Kepler Mission, scheduled to launch in 2008, will make use of the transit method of planet detection. The Kepler telescope will continuously monitor the brightness of stars, looking for a periodic dimming of a star's light that would indicate that a planet is passing in front of it. To detect the subtle effects from small planets, Kepler will have specially designed instruments that can measure brightness with great precision. The findings from Kepler will inform future missions, such as the Space Interferometry Mission, now called SIM PlanetQuest, and the two Terrestrial Planet Finder (TPF) observatories.

SIM PlanetQuest, scheduled to launch in 2011, will use very precise astrometry -- the study of the positions and movements of stars -- to detect planets slightly larger than Earth and to learn more about the formation and evolution of planetary systems in general. TPF, scheduled to launch in 2016 and 2019, will build on the information that SIM PlanetQuest provides and allow even further insight into the atmospheric character and composition of extrasolar terrestrial planets. Because the presence of life affects the composition of the atmosphere -- for example, oxygen in Earth's atmosphere is a result of photosynthesis -- signature gases found in the right abundances in an alien atmosphere could indicate extraterrestrial life.

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