NASA's planet-hunting Kepler Space Telescope has begun its extended mission, which should keep the prolific instrument searching for alien worlds for another four years, agency officials announced today (Nov. 14, 2012).

Kepler officially embarked upon the extended mission after completing its 3 1/2-year prime mission, which aimed to determine how common Earth-like planets are throughout the galaxy. The extended phase, which NASA announced this past April, funds the instrument through at least fiscal year 2016.

Kepler is staring at more than 150,000 stars continuously. It detects exoplanets by noticing the tiny brightness dips caused when they transit — or cross the face of — these stars from the telescope's perspective.

The instrument generally needs to observe three such transits to spot a planet. So the extra hunting time is vital, scientists say, allowing Kepler to discover smaller planets and worlds that orbit relatively far from their stars. (A hypothetical alien Kepler, after all, would need to observe our solar system for several years to witness three transits by Earth.)

Kepler has already made a series of impressive discoveries. Since its March 2009 launch, the telescope has detected more than 2,300 exoplanet candidates. Just 105 of them have been confirmed by follow-up observations to date, but the Kepler team estimates that at least 80 percent will end up being the real deal.

Confirmed discoveries include Kepler-10b, the first unquestionably rocky planet ever found beyond our solar system, and Kepler-22b, a world 2.4 times larger than Earth that orbits in its star's habitable zone — that just-right range of distances where liquid water could exist.

And the Kepler candidates include hundreds of Earth-size worlds, as well as a number of habitable-zone planets.

Note - I know this is two months old, but better late than never. Also, this is a prelude to the latest discoveries announced this past week. More coming soon.

Credit: Space.com