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Launched in 2009, the Kepler spacecraft is a specialized<br />
telescope that acts like a very precise light meter, a<br />
photometer. By carefully measuring changes in a star’s<br />
brightness, the Kepler team discovers planets as they<br />
cross in front <strong>of</strong> their stars. The crossing is called a transit,<br />
and these are seen as a dip in a graph <strong>of</strong> the star’s<br />
brightness. Of course, only a fraction <strong>of</strong> planetary sys-<br />
18 Photograph seti.org by Seth Shostak<br />
Closing in on E.T.’s Home<br />
By Edna DeVore<br />
Is Earth unique in the universe? How many Earth-size planets<br />
might exist? NASA’s Kepler Mission seeks to answer these questions<br />
by searching for Earth-size planets orbiting in the habitable<br />
zone <strong>of</strong> Sun-size stars.<br />
tems will be correctly aligned with Earth to be seen in<br />
transit, but since Kepler is continuously staring at more<br />
than 150,000 stars, many will – by chance – have this desired<br />
alignment. By measuring the depth <strong>of</strong> the dip and<br />
knowing the size <strong>of</strong> the star (using conventional astronomical<br />
methods), Kepler scientists can determine the<br />
size <strong>of</strong> the planet. From measuring the time elapsed be-