ROBERT SIEGEL, HOST:
Now, some news from outside our solar system. Scientists have discovered a planet that shares key characteristics with Earth. It circles a distant star that is much like our sun and this planet is in the so-called Goldilocks zone around that star, not too hot and not too cold.
As we hear from NPR's Nell Greenfieldboyce, it is the sort of planet that could be home to liquid water and possibly to life.
NELL GREENFIELDBOYCE, BYLINE: In 2009, NASA launched the Kepler space telescope on a mission to find other earths outside our solar system. To find them, the telescope has stared at over 100,000 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, partially obscuring its light.
Today, at a press briefing, researchers announced Kepler's latest discoveries, including one planet that shares key features with our home.
WILLIAM BORUCKI: Today, I have the privilege of announcing the discovery of Kepler's first planet, an inhabitable zone of a sun-like star.
GREENFIELDBOYCE: NASA's William Borucki is the head of the Kepler mission. He says the new planet is called Kepler-22b. It's about two and a half times the size of the Earth. It's a little closer to its star than our planet is to our sun. Kepler-22b goes around once every 290 days compared to our 365, but its star is a bit cooler than our sun.
BORUCKI: That means that that planet has a rather similar temperature to that of the Earth.
GREENFIELDBOYCE: Borucki says it's not yet clear what kind of surface the planet might have. Researchers don't know if it's made mostly of rock or water or what, but at least, temperature-wise, it would be very pleasant.
BORUCKI: Its surface temperature would something like 72 Fahrenheit.
GREENFIELDBOYCE: Still, don't expect astronauts to climb on a rocket and go there anytime soon.
BORUCKI: The star is some 600 light years away, so it's not terribly far away, but not terribly close, either.
GREENFIELDBOYCE: 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.
Kepler-22b is the first one to be found smack dab in the middle of the habitable zone of a star like our own, but there's hints of more to come. Scientists are looking at five in particular of the more than 2,000 different sized planets Kepler has detected that need to be confirmed.
Natalie Batalha is a researcher on the Kepler team.
NATALIE BATALHA: We believe that we've got some very, very viable candidates here that are Earth-sized, near Earth-sized and in the habitable zones.
GREENFIELDBOYCE: Just being in the habitable zone, of course, does not mean a planet is actually habitable. Marc Kuchner is an astrophysicist at NASA Goddard Space Flight Center. He says the habitable zone has been a useful concept for thinking about planets in the past, but...
MARC KUCHNER: I think it may be time to discard the habitable zone. I think maybe what this discovery is telling us is it's time to move past it and start asking the next question, which is, is the planet moist and juicy like our own planet Earth?
GREENFIELDBOYCE: He says scientists are already working on powerful instruments that could someday detect signs of water on one of these alien worlds. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright National Public Radio.