Scientists have discovered a planet not too much bigger than Earth that's circling a distant star that's much like our own sun. What's more, this planet is in the "Goldilocks zone" around that star — a region that's not too hot and not too cold. That's the kind of place that could be home to liquid water and maybe even life.

The planet, known as Kepler-22b, is the first near-Earth-sized planet to be found smack dab in the middle of the habitable zone of a twin to our Sun.

The planet is about 2 1/2 times the size of the Earth. It orbits a little closer to its star than our planet does to our sun, and goes around once every 290 days compared with our 365. But its star is a bit cooler than our sun, says William Borucki of NASA Ames Research Center, who heads NASA's Kepler space telescope mission, which detected this planet.

"That means that that planet, Kepler-22b, has a rather similar temperature to that of the Earth," Borucki says. "Its surface temperature would be something like 72 Fahrenheit."

It's not yet clear what kind of surface the planet might have — researchers don't know if the planet is made mostly of rock or water or something else. And don't expect astronauts to climb on a rocket and go there anytime soon.

"The star is some 600 light-years away," says Borucki, "so it's not terribly far away, but not terribly close either."

More 'Viable Candidates' Likely

Kepler-22b marks a significant first for NASA's Kepler space telescope, which launched in 2009 on a mission to find other Earths outside our solar system. The telescope has been staring at more than 150,000 stars in one patch of sky, watching and waiting for a slight dimming. That telltale dimming happens when an orbiting planet passes in front of a star, partly obscuring its light.

So far, Kepler has seen signs of 2,326 planets, ranging in size from Earth-sized to Jupiter-sized. The vast majority of these possible planets still need to be confirmed through more observations.

At least five of those candidates appear to be small planets in the habitable zones of their stars, suggesting that Kepler-22b is just the first of more to come. "We believe that we've got some very, very viable candidates here that are Earth-sized, near-Earth-sized, and in the habitable zones," says Natalie Batalha of San Jose State University in California, who is on the Kepler team.

Over the last decade or so, scientists have confirmed the presence of hundreds of planets around distant stars, but they're mostly gas giants. Small worlds that could potentially have a rocky surface and maybe even alien life crawling around have been much harder to find.

Just being small and in the habitable zone, however, does not mean a planet is actually habitable. Marc Kuchner, an astrophysicist at NASA Goddard Space Flight Center, says the habitable zone has been a useful concept for thinking about planets in the past, but it may be time to start asking more targeted questions.

"I think it may be time to discard the 'habitable zone,' " says Kuchner. "I think maybe what this discovery is telling us is that it is time to move past it and start asking the next question, which is, 'Is the planet moist and juicy, like our own planet Earth?' "

He says scientists are already working on powerful instruments that could someday detect signs of water on one of these alien worlds.