Yesterday, NASA’s Kepler spacecraft began its 3 and a half year mission to hunt for planets among over 100,000 stars. It has been in an Earth-trailing heliocentric orbit for over 2 months since its flawless launch from Cape Canaveral aboard a Delta II rocket, and now that it has finished commissioning and has been focussed optimally, it is ready to start… staring.

Kepler will stare continuously at a single patch of sky throughout its mission. This is known as a fixed field of view. By looking at exactly the same stars with great sensitivity, the Kepler science team hope to measure very slight periodic dips in their apparent brightness. This will signal the transit of an orbiting planet from our perspective. The Kepler mission will achieve an unprecedented level of precision, by using a collection of techniques known as differential ensemble photometry. Variations in seeing and extinction are accounted for, and shot noise is dealt with onboard the spacecraft.

So high is Kepler’s sensitivity, that it will be able to detect transits from planets comparable in size to Earth orbiting in the habitable zones of their stars. Of course, the mission was designed this way from the outset and in fact, in the case of K- and M-type stars, it will be able to detect planets as small as Mercury! To begin with though, we can expect the first findings to be large gas giants, as these can be confirmed relatively quickly. NASA states that such discoveries may be announced early in 2010.

You can keep up with the Kepler mission at NASA’s mission news page.