NASA officials have announced that the Kepler mission will be extended through September 30, 2016.

This decision was a no-brainer. Now Kepler can continue doing what it was designed to do: find planets encircling other stars like the Sun.

In case you've missed it, the spacecraft has already discovered more than 2,000 exoplanet candidates since its launch in March 2009. It's been staring at more than 145,000 stars near the Cygnus-Lyra border, watching for little dips in each star's light caused by a planet passing in front of it. This transit technique is especially good at finding companions close to their host stars, and Kepler's main objective is to find as many small ones — sized like Earth — as possible.

Finding Earths requires extremely careful measurements. A planet like ours, passing in front of a star like ours, yields a brightness dip of just 85 parts per million (0.0085%). If the planet is orbiting far enough from a Sun-like star to be in the star's "habitable zone," its orbital period will be roughly one year and, consequently, its transits would only happen this often. So the mission was designed to last 3½ years, until the end of September 2012, in order to record three transits of Earthlike exoplanets in Earthlike orbits.

But Kepler's dilemma, as I detailed last July, is that star brightnesses are twice as "noisy" on average as astronomers expected. The up-and-down churning of gas in 12th-magnitude dwarf stars (considered best bets for detecting Earth clones) causes their brightness to vary by about 20 ppm, on average, over time scales of a few hours. This noisier background means that Kepler needs to record seven or eight successive transits, not just three, to coax out believable signatures for small planets.

So NASA's decision to let Kepler keep watching all those stars for four more years was logical — but not a sure thing. The spacecraft is in great shape, but an extended mission would require about $60 million, a lot of money given the space agency's lean budget. A few weeks ago Kepler managers made their pitch before a high-level review committee, which fortunately agreed that the mission is a gold mine of scientific discovery and should get the needed funding.

Remember, Kepler is doing more than discovering boatloads of exoplanets. Its continuous, extremely precise brightness measurements are revealing all kinds of things about the 145,000 target stars themselves. For instance, astronomers are now able to deduce accurate masses for many of those stars — a key characteristic that's difficult to obtain by other means. And Kepler's contributions to asteroseismology (how stars vibrate and jiggle by tiny amounts, revealing their interior structure) has been off the charts.

I've said this before, but it bears repeating: when its mission is finally over, whether due to a cutoff of extended-mission funding or some fatal malfunction, Kepler will be recognized as one of the most successful spacecraft that NASA ever launched.

By the way, the same review panel also gave a green light to mission extensions for the infrared Spitzer Space Telescope (to 2014) and the U.S.-funded portion of the European Space Agency's Planck microwave-background observatory (for one more year).