NASA's set to launch an extraordinary satellite today that has a particularly exciting mission. Kepler is being called the first spacecraft with the ability to peer at distant stars and distinguish if a particular type of planet is orbiting them: Planets like Earth.
Whether you know it or not a key moment in human history happened in early 1992 with a discovery by two radio astronomers named Aleksander Wolszczan and Dale Frail, who discovered the first "extra-solar" planets around the pulsar 1257+12. It's generally regarded as the first definitive detection of a planet orbiting a star other from our own Sun.
Ever since then, the detection rate of extra-solar planets, or exoplanets, has increased every year—due to improved search techniques and more sensitive equipment. The planets that are most often found tend to be in the Jupiter-like gas giant class, simply because it's easier to see a larger planet, or detect its gravitational influence on its parent sun. Finding Earth-sized planets is particularly tricky, since we have to look closer into a star and then it's a hunt for an even smaller target at a range of trillions of miles.
The French satellite CoRot has the record for the tiniest earth-like exoplanet yet found with COROT-exo7b found in February—it's less than twice Earth's diameter, and is orbiting a sun-like star, albeit much closer than we orbit the Sun, since it's "year" is just 21 of our hours.
But Kepler should easily be able to surpass that discovery. It's roughly a one meter-across telescope that has a very large field of view of 105 square degrees (large for an astronomical telescope) and it's coupled to a hyper-sensitive photometer to measure light intensity. For its entire mission it will stare at the same star field, that contains around 100,000 stars and monitors them for a period of 3.5 years—the data it collects will discover if any of the stars have Earth-like planets.
It's designed particularly to search the so-called "Goldilocks zone" near stars like the sun—a region of space which could contain an Earth-sized planet. The Goldilocks zone has been deemed the most likely conditions in which an extra-solar planet could not only be Earth-sized, but Earth-like: with potentially liquid water, comfortable temperatures and, just possibly, life.
Can Kepler tell us that "we're not alone"? No. But it can tell us where to look more closely for life, perhaps, and it's a mission in the best traditions of human exploration. It's due to go aloft at 10:48PM tonight aboard a Delta II rocket, and you can follow the NASA Kepler Twitter feedhere.