Dec. 6, 2011  -- NASA's Kepler space telescope has found a new planet -- the most Earth-like yet -- circling a yellow star similar to Earth's sun and 600 light-years away, according to the space agency.

The most promising thing about this world, called Kepler-22b for now, is that it's in the so-called Goldilocks zone around its host star. Its surface temperature is estimated at an average of 72 degrees, which means liquid water -- considered essential for life as we know it -- would be possible there.

"We are certain that it is in the habitable zone and if it has a surface, it ought to have a nice temperature," said Bill Borucki, the Kepler principal investigator at NASA's Ames Research Center.

But just how realistic are the prospects for life on that distant world? Even in their excitement, the researchers caution that they have found no proof that we are not alone.

The Kepler team has done a prodigious job of detection and mathematical calculation, but Kepler has not actually seen the planet or taken any chemical measurements. They know its host star is slightly smaller and cooler than the sun, and they found that its light dims ever so slightly once every 290 Earth days. That means the dot of the planet is passing in front of it. It's a little closer to its sun than we are to our sun.

From there, they can extrapolate. For the planet to be in a nice, nearly circular orbit, not too hot and not too cold, they figured out that it's probably 2.4 times the diameter of Earth.

That makes it among the smallest planets yet found orbiting other stars, but it's a smidgen larger than an ideal candidate for extraterrestrial life would be.

"That smidgen makes all the difference," said Geoff Marcy of the University of California, Berkeley, who is one of the pioneers of planet-hunting outside Earth's solar system, and a member of the Kepler team.

Scientists know, from looking at Earth's solar system, that rocky worlds like the Earth's are a precious commodity. If a world is too small (think of Mercury or Earth's moon), any atmosphere will escape into space before life could possibly form. If a world is too large (think of Jupiter or Neptune) it's likely to be all atmosphere, a giant ball of gas or slush that thickens quickly as you plunge beneath its cloud tops, but probably has no solid surface where living things could thrive.

Kepler-22b might be the right temperature, but it is probably closer in mass to icy Neptune than to Earth. "I would bet my telescope that there is no hard, rocky surface to walk on," Marcy told the Associated Press.

Alan Boss of the Carnegie Institution for Science, a colleague of Marcy's, wrote in an email to ABC News, "We know the star is sun-like, and we know the orbit is Earth-like, but the size is super-Earth-like. As Meatloaf sings, two out of three ain't bad."

Still, the discovery sets scientists' minds racing.

"In less than 20 years, we have gone from not knowing if any other planets exist in the universe, to being able to look out at the night sky and realize that essentially any star we can see has at least one planet, and a good number of those are likely to be habitable," said Alan Boss. "That is a revelation that has not yet dawned on the general public, and even astronomers are having their minds blown when they think about it."

"This discovery shows that we Homo sapiens are straining our reach into the universe to find planets that remind us of home," Marcy said. "We are almost there."

The Associated Press contributed to this report.