Last week, NASA launched the Kepler spacecraft, which is designed to look for other habitable planets in our galaxy. Since water, it's believed, is necessary to support primitive life, the mission will look for Earth-size planets in the "habitable zones"—regions at distances from stars (like our sun) where temperatures allow water to stay liquid.
Are there lots of Earthlike planets in the galaxy or is ours unique? It's too early to speculate, and it will take the mission at least three years to find and confirm the existence of planets like our own. "Even if we find no planets like Earth, that by itself would be profound," says William Borucki, the mission's science principal investigator. "It would indicate that we are probably alone in the galaxy."
You can follow Kepler's progress with its updates on Twitter. —Heather Wax