Kepler photometer and spacecraft.

"[The] Kepler [mission] is the first step in finding life in our galaxy, life in the universe," explains Kepler mission Principal Investigator Bill Borucki of the NASA Ames Research Center in Moffett Field, California. "If Earths turn out to be rare, we might be alone. Kepler marks a crucial step for humans to find their place in the universe."

"Kepler's goal is to detect planets roughly the size of Earth, with orbits far enough from their stars that they might conceivably be habitable," adds Co-Investigator Ted Dunham of the Lowell Observatory in Flagstaff, Arizona.

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Alan Gould is a co-investigator on the Education and Public Outreach team and planetarium director at the Lawrence Hall of Science, University of California, Berkeley. According to Gould, "Its like seeing a gnat flying across a cars headlights on high beam from a distance of half a mile away."

Photometers provide neither pictures nor spectra. Instead, they send back a light curve representing intensity as a function of time, "like the light meter in your camera," explains Deputy Principal Investigator David Koch of NASA Ames, where Kepler is based. He adds, "Think of an EKG at the hospital, which typically has regular dips. With a photometer, you get a dip in the stars brightness every time a planet passes by."

"This is a really simple experiment," adds Co-Investigator Gibor Basri, a stellar activity expert and astronomy professor at the University of California, Berkeley. "Youre watching an eclipse, measuring the brightness of the star to two parts in 100,000 and noting every time it takes a dip."

At the heart of the instrument is a new detector package being developed by a team led by Dunham, along with John Geary of the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts and Rob Philbrick of Ball Aerospace. Building upon Dunham's expertise with occultations, Kepler's detector is a set of 46 charge-coupled devices (CCDs), each one a separate silicon chip roughly 1 by 2 inches (2.5 by 5 centimeters).

CCDs can be found in today's conventional TV cameras, camcorders and digital cameras, but, as Dunham explains, "Kepler's are much more sensitive and much larger format."

The result, as Borucki puts it: "Kepler is your camcorder with a really big lens."

Transits and photometers are foreign to the techniques that have been used so far to detect planets around other stars. In the bulk of those cases, astronomers have used what is known as radio velocity, involving Earth-based observations of minute, regular changes in the visible spectrum of normal stars caused by the orbits of close-in, Jupiter-sized planets.

Planets of the solar system, which the Discovery -class projects would explore.

A key limitation of the traditional method is that it can only detect large planets. Also, it cannot determine their size, only their mass. Keplers transit photometry can fill in the information gaps regarding these gigantic planets because it reveals size. Furthermore, when scientists combine the data sets, they can additionally derive planets density.

Kepler can find intermediate-sized terrestrial planets, larger than Earth, but much smaller than Jupiter. "It is reasonable to speculate that many of these could be covered by water or ice and, if so, they could constitute one of the primary habitats in the universe," says Basri.