The Kepler space telescope is NASA's most ambitious effort yet to find Earth-like planets orbiting other stars in our galaxy. This three-foot-diameter telescope was launched in 2009 and has already produced tantalizing evidence of planets which were previously too small for our most sensitive instruments to detect.
The Kepler spacecraft, named after Johannes Kepler, blasted off in March 2009. It has a state-of-the-art photometer designed to detect planets with much greater sensitivity than any other instrument or telescope yet used in the search for planets. This is particularly useful because our current methods are heavily biased towards very large planets, orbiting very close to their stars. Essentially, the bigger and heavier the target, the easier it is to spot - and, conversely, the smaller or more distant an extrasolar planet, the harder it is to find.
For this reason, since the first extrasolar planets were discovered in the 1990s, our catalogue of known planets has been heavily skewed towards very large planets, too large - and usually far too close to their Sun, within the orbit of Mercury in our own solar system - to support life. This holds serious implications for the search for life: if Earth-sized planets in Earth-like locations are exceedingly rare in our galaxy, then it follows that recognizable life is probably exceedingly rare as well. However, scientists have long suspected that the disappointing nature of our planetary catalogue is actually a reflection of our poor technology, rather than an accurate reflection of what's actually out there.
The Kepler Mission is intended to address this problem. The Kepler Mission is currently scheduled to last for three and a half years, although so long as the spacecraft survives in an operational state, there is no reason not to continue operating it beyond that date. Using its state-of-the-art equipment, the Kepler space telescope is formally chartered to study the development of different types of planetary systems, but the central and most high-profile part of its mission is to search for small, Earth-like planets. Millions of potential targets have been identified as part of the mission and will be studied over the coming years.
After just one year in operation, the Kepler Mission is already producing the desired results. In summer 2009, NASA held a press conference to triumphantly announce that the first results from the Kepler space telescope were already coming in. In 2010, they confirmed that Kepler was already tracking multiple star systems with numerous planets in them, and that the first year's results confirmed exactly what so many had expected: that smaller planets are very numerous, and we simply lacked the capacity to find them until now.
The past decade of extrasolar planet discoveries led to a modest growth in pessimism, as many feared that Earth-like planets would turn out to be extraordinarily rare. However, the Kepler space telescope has resulted in a return to a more optimistic viewpoint, with some scientists even making well-publicized claims that there are as many as 100 million planets in the Milky Way capable of supporting life. It should be noted that this estimate does not necessarily mean there are that many planets with life on them: much more goes into determining theoretical habitability than solely planetary size and orbital location. Nevertheless, the Kepler space telescope is proving an invaluable window onto the galaxy around us, and will continue to provide this window for some years to come.