Earth’s twin could be waiting for us hundreds of light-years away. In fact, thousands of Earth doppelgängers may be lurking in the cosmic distance, orbiting stars just like our sun and maybe, just maybe, harboring life of their own. Although telescopes have identified more than 300 planets outside our solar system, most of them are too harsh to host life. One notable exception to the typical “hot Jupiter” model is a rocky Earth-like planet discovered in 2007, dubbed Gliese 581 c. This April, NASA’s orbiting Kepler telescope will begin a three-year effort to scour distant space for more planets like Gliese 581 c.

Kepler won’t look for them directly—Earth-like planets are too small to see even with the best telescopes. Instead it will hunt for them based on how they affect the stars they orbit. The Kepler telescope will focus on one small slice of our galaxy, the wing of the swan-shaped constellation Cygnus, observing how the region’s 170,000-odd stars change over time. If a star dims once, it could be because a planet is crossing in front of it, or it could just be a sunspot. But if a star dims several times, and the same amount of time passes between each dimming event, a planet must be orbiting it.

Disappointingly, the mission won’t tell us whether any planets are teeming with little green men. Kepler is a stepping-stone, “a foundation upon which mankind will find its place in the universe,” says principal investigator William Borucki. But, he adds,”if we find, as we expect to, lots and lots of ‘Earths’ in habitable zones, then there is probably lots and lots of life in space.” With evidence in hand, NASA will no doubt set out to find it.

Meanwhile, closer to home, look for these notable launches to Earth-orbit this year:

The first flight of Virgin Galactic’s SpaceShipTwo could take six customers to 360,000 feet and offer them weightlessness and 1,000-mile views in all directions.

In June, NASA’s Glory satellite will launch to help predict future climate change by gauging the magnitude of the sun’s energy and studying atmospheric aerosols.

Wide-field Infrared Survey Explorer
The infrared telescope of the WISE satellite, launching in November, will scan space for brown dwarfs and super-luminous galaxies.

Starting in November, the European Space Agency’s CryoSat-2 interferometric-radar satellite will circle the planet, measuring the thickness of polar ice caps.

Read more of Popular Science’s predictions for 2009.