Kepler’s field of view (the rectangles) is aimed along the length of the star-rich Orion Spur, covers 100 square degrees, and includes an estimated 4.5 million stars. Clever pointing maximizes the number of the brightest stars that fall into the gaps between the 42 CCD blocks. The bright starlight would saturate the nearby pixels, blinding the telescope to potential faint flickerings from planets circling dimmer stars nearby.

Other Suns, Other Earths

The quest to discover Earth-sized planets orbiting stars at distances such that water could pool on the planet’s surface—another step in the search for life on other worlds—kicked into high gear when NASA’s Kepler spacecraft roared off the pad at Cape Canaveral aboard a Delta II rocket at 10:49 p.m. EST on Friday, March 6.

The spacecraft has entered a sun-centered orbit, and is drifting away from Earth at about one kilometer per second. (The Spitzer Space Telescope, which was launched into the same orbit more than five years ago, is now some 100 million kilometers away.)

From this vantage point, Kepler will watch upwards of 100,000 stars simultaneously, 24/7, for at least three and a half years, looking for a periodic dimming of their light that would be caused by a planet passing between them and Earth.

Engineers have now turned on and focused Kepler’s single instrument, an 0.95-meter-diameter telescope with a wide-field CCD camera. This 95-megapixel camera, the largest ever launched into space, can detect a change in a star’s brightness of 20 parts in a million. As soon as the camera is properly calibrated, the search will begin in earnest.

The first planets to catch Kepler’s eye are expected to be portly “hot Jupiters”—gas giants that circle close and fast around their stars. Neptune-sized planets will most likely be found next, followed by rocky ones as small as Earth. The true Earth analogs—ones orbiting stars like our sun at distances where surface water, and possibly life, could exist—will take at least three years to discover, as such planets will have orbital periods similar to Earth. Each planet will have to pass in front of its star at least three times to be discovered—if the time interval between successive pairs of dimmings repeats with high precision, then we can conclude that a planet is responsible. Observations by the Hubble and Spitzer space telescopes will be used to confirm some of Kepler’s discoveries and analyze the planets’ atmospheres. Ground-based telescopes will verify some of the finds as well.

In the end, Kepler will give us our first look at the frequency of Earth-sized planets in our corner of the Milky Way galaxy, and an idea of the fraction of them that could theoretically be habitable. “Everything about Kepler has been optimized to find Earth-size planets,” says James Fanson (MS ’82, PhD ’87), Kepler’s project manager at the Jet Propulsion Lab. “Our images are road maps that will allow us, in a few years, to point to a star and say a world like ours is there.”

Kepler is a NASA Discovery mission. The science principal investigator is William Borucki of NASA’s Ames Research Center at Moffett Field, California, which is responsible for the ground-system development, mission operations, and science data analysis. JPL manages the Kepler mission development. Ball Aerospace & Technologies of Boulder, Colorado, built the spacecraft. For more information, click here. —WC