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," said William Borucki, an astrophysicist at the NASA Ames Research Center, at a news conference Thursday. Borucki led the study that discovered the new planets.

Details of the find were published Thursday online in Science Express.

The planets  were 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 is located about 1,200 light years away from Earth in the constellation Lyra.

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The newly discovered planets are 'the most similar objects to Earth that we have found yet,' said Justin Crepp, and astrophysicist at the University of Notre Dame who co-authored the paper. The planets were discovered by the Kepler mission, which detects planets by looking for the telltale dimming of stars caused by planets passing front of them. (Matthew Cashore and Keith Davis/University of Notre Dame)

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 star — the 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.

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.

However, Borucki said Kepler-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 be a water world, a new type of planet discovered 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 each have a radius of between 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 Cygnus that is much more sun-like than Kepler-62.

Thomas Barclay, a scientist at the Bay 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.

Barclay added 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 other relatively small exoplanets in the habitable zones of their stars:

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

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The diagram compares the planets of the inner solar system to Kepler-62, a five-planet system about 1,200 light-years from Earth. The habitable zones are shown in green. (NASA Ames/JPL-Caltech)