A week after jettisoning its protective dust cover, NASA's Kepler telescope has released the first image of the patch of sky that it will scrutinise for the next three-and-a-half years in search of Earth-like planets.
The $600 million probe will stare at this area, looking for periodic dips in stellar brightness that could signal the passage of a planet in front of its host.
Kepler's target is a patch of sky that covers 100 square degrees, or roughly the same area of sky that a hand would when seen at arm's length. The patch sits above the plane of our galaxy in the constellations Cygnus and Lyra and contains an estimated 4.5 million Milky Way stars.
More than 100,000 stars in the patch were selected as ideal candidates for planet hunting. Among those, researchers are hoping to find Earth's twin – a similarly-sized planet orbiting close enough to its host star for liquid water, a requirement for life as we know it, to survive on its surface.
"For the first time, we can look for Earth-size planets in the habitable zones around other stars like the sun," Kelper's principal investigator William Borucki of NASA's Ames Research Center in California said in a statement.
At the heart of the search is the probe's 95-megapixel camera, the largest ever launched into space. It can detect tiny changes in a star's brightness of only 20 parts per million.
That should allow Kepler to detect planets with orbits and sizes similar to our own. Viewed from a distance, the Earth would dim the sun's light by some 80 parts per million as it passed in front of the star, Kepler's deputy principal investigator, David Koch, told New Scientist.
Since Kepler will stare at the same patch of sky for more than three years, it could catch three transits of Earth twins. That is the minimum needed to determine the interval between dimming episodes and rule out other effects, like fluctuations in the star's brightness.
By contrast, the orbiting French-built Corot telescope, which is also looking for the transits of planets, can only watch the same patch of stars for 150 days, Koch says. That limit means it can only detect planets on tight, star-scorched orbits of 50 days or less.
Kepler's first star pictures illustrate the telescope's set-up (see image). Stars appear in 21 squares, each of which boasts a pair of light-detecting charge-coupled devices (CCDs).
The structures that hold the camera together form a grid that blocks out light. When the telescope was deployed, the team oriented the telescope so that this grid blocked out the brightest stars, to prevent saturation of the camera pixels.
At the corners of the image are four black squares, where the telescope's fine-guidance sensors are located. They measure where the telescope is pointing on the sky 10 times every second. This information is fed back to the system that controls the spacecraft's orientation, helping to hold the telescope's gaze steady.
Kepler launched on 6 March. The probe trails behind Earth on its own orbit of the sun – a path designed to maximise its viewing time.
The planet hunt is set to begin in early May after some final adjustments and calibrations.