Kepler lifts off tonight at 10:48 pm EST. Everything you could possibly want to know about it can be found at Kepler.NASA.gov. The basic idea behind Kepler is that it will look at that » same spot of Milky Way (Cygnus Region) for the next 3 years, taking an observation every half hour. If any of the stars in its field of vision dime during that time as a result of exoplanets crossing between the star and Kepler, they'll use those measurements to determine the size of the planet and its distance from the star it orbits. Using that data they can possibly discover extrasolar earth-size planets in the Goldilocks Zone.
Another consideration to keep in mind is that for Kepler to find a planet, the planet has to be orbiting its star along the axis of Kepler's observation. If not, the planet will never transit its sun from our field of view.
Personally, I'm a huge fan of space-based telescopes. Worth far more than the weight of their delivery vesicles in gold as far as I'm concerned.
To date, we have discovered over 300 extra-solar planets, but they've all been of the massive gas giant type due to the limitations of the instruments used to find them. Kepler changes that.
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