Scientists studying the planets dancing around distant stars have found big planets and small ones, burning-hot planets and icy-cold planets. But in their search of other solar systems, astronomers had never found an Earth-size planet that could boast liquid water until now.

NASA's Kepler space telescope has detected a faraway planet almost exactly the size of Earth and close enough to its star to be in the so-called habitable or Goldilocks zone, where conditions are not too hot or too cold for liquid water essential for life to persist. Many of the planets spotted outside our solar system sport thick layers of gas, but this new planet could very well be formed of rock. In that case, a planet with a family resemblance to our own is orbiting a star 500 light years away.

'We now know that these Earth-size planets in the habitable zone of other stars exist,' says Elisa Quintana of NASA's Ames Research Center and the SETI Institute, co-author of the study reported in the journalScience. 'We thought they did, everybody on the Kepler team hoped they did ... but knowing that they exist gives us motivation to find more.'

The find 'looks very exciting,' says astronomer Gregory Laughlin of the University of California, Santa Cruz, who was not affiliated with the new study. 'It's exactly the kind of stuff the (astronomical) community was hoping to see when the Kepler mission was funded. This is a great step in our understanding of the galactic planetary census.'

The new planet was detected by the faint dimming of its star as the planet blocks some of its light. Compared to the Earth, the new planet is only 10% bigger in diameter, a size difference 'too little to quibble about,' says Quintana's colleague Jason Rowe, also of Ames and the SETI Institute. The scientists don't know what this Earth-size world is made of, but Quintana says it's 'highly likely' to be rocky given planetary theory and the traits of other planets outside our solar system. That would make it the odd planet out compared to other similarly placed planets, which are mostly huge balls of gas.

The planet, unglamorously known as Kepler-186f, occupies the outer, chillier edge of the habitable zone around a faint, cool star. If the planet were a little bit farther from its star, any water on it would freeze; if it were a lot closer, any water on it would boil away. But the researchers think it sits nicely positioned to harbor liquid water, assuming it has an atmosphere to keep its surface toasty and that it collected water earlier in life.

'If you were to live on Kepler-186f, with an atmosphere like Earth's, it would be like living in San Francisco on a cold day,' Quintana says. All the same, 'we don't consider it Earth's twin. We consider it more of Earth's cousin.'

That's because Kepler-186f circles a feeble type of star known as an M-dwarf. This particular star, which lies in the constellation Cygnus, is just barely visible from Earth with powerful binoculars, Rowe says. But M-dwarfs make up for dimness with ferocity, unleashing massive radiation spikes that could make it difficult for any life to survive.

'Anyone that's there will need a hard hat,' jokes Don Pollacco, a planetary scientist at Britain's University of Warwick and the head of the science team for a future European planet-hunting mission. 'The radiation field will be pretty intense. ... If there's life there, it's going to be quite different.'

There are other reasons why Kepler-186f may not have life, or only microscopic life. A planet orbiting an M-dwarf is vulnerable to losing its water, says planetary scientist James Kasting of Penn State University. The new planet's perch in the cooler part of the habitable zone means to accommodate liquid water, it would have to have a very thick, warming atmosphere of a substance such as carbon dioxide, says Abel Mendez of the University of Puerto Rico at Arecibo. But 'even if the temperature is right for life, we don't know of any complex life (plants and animals) that can survive such toxic levels of CO2,' Mendez says via e-mail.

The discoverers of the new planet say it could have overcome any early water loss, and they say it's too early to know whether the planet needs a super-thick atmosphere. But they agree with other scientists that we won't know for a long time, if ever, whether Kepler-186f is home to life.

'To go any further than they've done in this paper is effectively impossible,' Pollacco says. 'There is no instrument to do that,' and building one would take many billions of dollars that are more likely to be spent on looking at stars closer to our solar system.

Rowe acknowledges that it would take a costly new telescope to reveal more details about this Earth-size world. But the find is worthwhile even without more data, he says.

'It tells us our solar system and our universe are very good at producing other Earth-size planets, so you start to think about big, lofty questions,' he says. 'Are we alone in the universe? I really think we're going to be able to answer that sooner rather than later, because of discoveries like 186f.'

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