NASA's Kepler mission is searching for Earth-like planets by looking for them to cross the face of alien stars.

NASA's prolific Kepler Space Telescope may get to extend its search for alien planets by a few years.

Funding for Kepler — which has identified 1,235 candidate alien planets to date and recently discovered the first exoplanet with two suns in its sky — is due to run out in November 2012. But mission managers are writing up a proposal for a mission extension, and they should know by next spring whether it's approved.

"I think the discoveries we're making are showing what could be done if we continue to extend it," said Charlie Sobeck, Kepler deputy project manager at NASA's Ames Research Center in Moffett Field, Calif. "So we're hopeful, but there's no guarantee." [Gallery: The Strangest Alien Planets]

It would cost about $20 million per year to keep the Kepler mission running at its current level of activity beyond November 2012, Sobeck added.

A wealth of alien planets

The $600 milllion Kepler observatory launched in March 2009. Its mission is to find roughly Earth-size planets in or near the habitable zones of their parent stars — a just-right range of distances that could support liquid water and, perhaps, life as we know it on the alien worlds.

Kepler's overall goal is to help scientists determine just how common such planets may be throughout our galaxy.

Kepler finds alien planets using what's called the transit method. The telescope detects the telltale dips in brightness caused when an alien planet crosses in front of, or transits, its star from Kepler's perspective. Kepler needs to witness three of these transits to firmly identify a planet candidate.

This technique has been extremely effective. In just its first four months of operation, Kepler discovered 1,235 exoplanet candidates. So far, two dozen of them have been confirmed by follow-up observations — including Kepler-16b, a world with two suns that was announced recently. [Video: "Tatooine" Alien Planet With 2 Suns Discovered]

Kepler team members have estimated that 80 percent or so of the telescope's candidates will probably end up being the real deal. If that's the case, Kepler's finds to date would more than double the number of known alien planets, which currently stands at about 685.

NASA's Kepler mission has discovered a world where two suns set over the horizon instead of just one. The planet, called Kepler-16b, is not thought to be habitable. It is a cold world, with a gaseous surface, and it circles two stars, just like "Star Wars" Tatooine. (Image credit: NASA/JPL-Caltech)

Looking for alien Earths

The Kepler discoveries are about much more than just adding to the total exoplanet count. The telescope's main task, after all, is to help scientists determine just how many potentially habitable, Earth-size alien planets may be out there.

Of those first 1,235 planet candidates, 68 are roughly Earth-size and 54 appear to orbit in their stars' habitable zones. And five candidates meet both of those criteria.

"What we're seeing is this trend — the smaller the planet, the more of them there are," Sobeck told SPACE.com. "That's great news for the idea of finding Earth-like planets, or Earth-size planets. Once you have Earth-size planets, all it has to do is be in the right orbit, and it's habitable."

Kepler's discoveries have also opened astronomers' eyes to the incredible diversity of alien planets and solar systems. [A Field Guide to Alien Planets]

For example, the mission has detected one world as dense as iron (Kepler-10b) and another as light and airy as styrofoam (Kepler-7b). And it found one solar system (Kepler-11) whose six known planets all orbit closer to their star than Venus does to the sun.

Keeping the lights on

When Kepler launched in 2009, the telescope's science mission was set to run through November 2012 — a lifetime of 3.5 years. But the instrument could operate for six years, or perhaps longer, if it receives more funding, team members have said.

Giving Kepler more time to search for extrasolar planets could yield big dividends for several reasons, researchers said. Because of the three-transit requirement, most of the worlds Kepler has found to date zip around their stars relatively quickly, in close-in orbits.

But keeping Kepler going for another few years would give it a chance to look for planets in more distant orbits, allowing the telescope to survey the habitable zones of warmer stars. (It would take a hypothetical faraway Kepler a minimum of three years, after all, to see Earth transit the sun three times.)

Seeing more transits would also increase the signal-to-noise ratio for closer-in planets, allowing more of them to be detected, researchers said.

This illustration shows all 1,235 of the potential alien planet candidates NASA's Kepler mission has found to date. The planets are pictured crossing front of their host stars, which are all represented to scale. (Image credit: Jason Rowe and Kepler team)

$20 million per year?

The Kepler team plans to submit its extension proposal to NASA headquarters in January, Sobeck said. He expects an answer back by April or May.

If Kepler is extended, exactly how much money the mission would get depends on what NASA wants it to do, Sobeck added. The mission currently costs about $20 million per year to operate, so that's how much it would cost to keep the mission running the way it does today.

That may not seem like much, especially since the space agency has already sunk about $600 million into the planet-hunting mission. But Sobeck realizes that every dollar is precious in these tough economic times, so he and the Kepler team aren't taking anything for granted.

"The budget is simply very tight everywhere," Sobeck said. "Kepler is a wonderful mission. It has a lot of support at headquarters as well as in the community, but so do other missions."

But some researchers stress that the tough budget environment strengthens the case for re-upping Kepler.

"In budget-tightening times, you want to get the very best value for your money," said astronomer Greg Laughlin of the University of California, Santa Cruz. "And a great way to get value for your money is to extend missions that are working beautifully and which have a team in place, and which have basically zero risk."

Kepler is just such a mission, Laughlin said.

"It would seem to me just nuts to have it out there and turn it off," he told SPACE.com. "If we as a nation can't manage to keep the lights on on Kepler, it's a little disconcerting and disheartening."

You can follow SPACE.com senior writer Mike Wall on Twitter: @michaeldwall. Follow SPACE.com for the latest in space science and exploration news on Twitter @Spacedotcom and on Facebook