2 super-Earths found in habitable zone of star

Two newly discovered planets in a five-planet system are the right distance from their star to support liquid water and are potentially habitable to life as we know it.

"They are the best candidates found to date for habitable planets," saidWilliam Borucki, an astrophysicist atthe NASA Ames Research Center, at a news conference Thursday.Boruckiled the study that discoveredthe new planets.

Details of the findwere publishedThursday online in Science Express.

The planetswere detected by NASA's Kepler mission, a planet-hunting space telescope that looks for the telltale dimming of stars caused by planets crossing in front of them.

They are the two outermost of five planets orbiting a dwarf star, called Kepler-62, that is a little bit smaller and one-fifth as bright as the sun. It islocated about 1,200 light years away from Earth in the constellation Lyra.

The inner of the two planets, dubbed Kepler-62e,receives an amount of energy from their star comparable to the solar energy received on Venus and completes its orbit once every 122 Earth days. The outer one, Kepler-62f,gets about the same energy as Mars and orbits once every 267 Earth days.

The pair are the smallest planets ever detected by the Kepler mission within the habitable zone of a starthe region around the star that could have the right conditions for liquid water, and therefore living things,to exist. In particular, the outer of the two is right in the middle of the habitable zone. The inner planet, like many other habitable zone planets previously discovered, is more towards the edge of the zone.

"From what we can tell, from their radius and orbital period, these are the most similar objects to Earth that we have found yet," said University of Notre Dame astrophysicist Justin Crepp,who co-authored the study.

The planets have a radius 1.61 times that of the Earth and 1.41 times that of the Earth, respectively, putting them in a class of planets called "super-Earths" that are thought to be potentially capable of supporting life.

• Another potentially habitable super-Earth was found in a 6-planet system last fall

The paper cautioned that the researchers don't yet know whether the planets are rocky, whether they have atmospheres or whether they actually host any liquid water.

Until scientists are able to take chemical fingerprints of the planets' atmospheres, "we cannot determine whether they are in fact habitable," the paper said.

• Scientists recently detected in detail the atmosphere of another planet

However, Borucki saidKepler-62f has a good chance of being rocky like Earth, given that other distant planets its size have been confirmed to be rocky.

As for Kepler-62e, he added, it may be rocky or it may bea water world, a new type of planetdiscovered last year.

"It seems to me both those planets have a real chance of habitability," Borucki said. "But we really don't know what's required for life to get started."

The other three known planets in the system eachhave a radius ofbetween 0.54 and 1.97 times that of the Earth. Borucki said there may be additional planets in the system that have not yet been discovered.

Super-Earth found near sun-like star

In addition to Kepler-62e and Kepler-62f, NASA announced Thursday that it had also found another super-Earth on the edge of the habitable zone of a different star. The planet, known as Kepler-69c, has a radius about 1.7 times that of Earth and orbits a star 2,700 light years away in the constellation Cygnusthat is much more sun-like than Kepler-62.

Thomas Barclay,a scientist at theBay Area Environmental Research Institute in Sonoma, Calif.,who led that study, said that planet is right on the inner edge of the habitable zone of its star.

"We consider this more to be a super-Venus than a super Earth," he said at the NASA news conference.

Barclayadded that none of the planets found by Kepler to date, including the three announced Thursday,are small enough to be truly Earth-like: "We're still progressing toward finding the first truly Earth-like planets."

Kepler has previously discovered otherrelatively small exoplanets in the habitable zonesof their stars:

• Kepler 22b, which had a radius 2.4 times that of Earth.
• Kepler 47c, which has a radius about five times that of Earth.

As of Thursday morning, Kepler had discovered 2,740 planet candidates and 115 confirmed planets.