Story highlights Researchers find an Earth-sized planet that may be habitable

It "proves the existence of worlds that might be similar to our own," institute says

Scientists are looking for emissions that could suggest extraterrestrial life

Previously discovered worlds were larger than Earth

It's like finding a needle in a universe-wide haystack. Researchers have located a planet roughly the size of Earth that could be habitable.

Designated Kepler-186f, the planet is 490 light-years away. But in the search for worlds similar to ours, nothing has come closer.

"This is the first definitive Earth-sized planet found in the habitable zone around another star," said Elisa Quintana of the Search for Extraterrestrial Intelligence (SETI) Institute at NASA. "Finding such planets is a primary goal of the Kepler space telescope."

"This discovery not only proves the existence of worlds that might be similar to our own but will undoubtedly shape future investigations of exoplanets that could have terrestrial surface environments," the institute said in an announcement Thursday.

Photos: Where life might live beyond Earth Photos: Where life might live beyond Earth The Kepler mission has discovered 1,284 new planets. Of these newly discovered planets, nine orbit in the habitable zone of their star and nearly 550 are possibly rocky planets roughly around the same size as Earth. Hide Caption 1 of 14 Photos: Where life might live beyond Earth This artist's impression shows an imagined view from the surface one of the three planets orbiting an ultracool dwarf star just 40 light-years from Earth that were discovered using the TRAPPIST telescope at ESO's La Silla Observatory. Given the proximity of the dwarf star, the rosy sun would appear very large in the sky. Hide Caption 2 of 14 Photos: Where life might live beyond Earth On Thursday, July 23, NASA announced the discovery of Kepler-452b, "Earth's bigger, older cousin." This artistic concept shows what the planet might look like. Scientists can't tell yet whether Kepler-452b has oceans and continents like Earth. Hide Caption 3 of 14 Photos: Where life might live beyond Earth Kepler-452b is about 60% larger than Earth, left. It's about 1,400 light-years from Earth in the constellation Cygnus. Hide Caption 4 of 14 Photos: Where life might live beyond Earth A team of astronomers announced April 17, 2014, that they discovered the first Earth-size planet orbiting a star in the "habitable zone": the distance from a star where liquid water might pool on the surface. That doesn't mean this planet has life on it, says Thomas Barclay, a scientist at the Bay Area Environmental Research Institute at Ames and a co-author of a paper on the planet, called Kepler-186f. He says the planet can be thought of as an "Earth-cousin rather than an Earth-twin. It has many properties that resemble Earth." The planet was discovered by NASA's Kepler Space Telescope. It's about 500 light-years from Earth in the constellation Cygnus. The picture above is an artist's concept of what it might look like. Hide Caption 5 of 14 Photos: Where life might live beyond Earth Scientists announced in June 2013 that three planets orbiting star Gliese 667C could be habitable. This is an artist's impression of the view from one of those planets, looking toward the parent star in the center. The other two stars in the system are visible to the right. Hide Caption 6 of 14 Photos: Where life might live beyond Earth This diagram shows the planets thought to orbit star Gliese 667C, where c, f and e appear to be capable of having liquid water. The relative sizes, but not relative separations, are shown to scale. Hide Caption 7 of 14 Photos: Where life might live beyond Earth This diagram lines up planets recently discovered by Kepler in terms of their sizes, compared with Earth. Kepler-22b was announced in December 2011; the three Super-Earths were announced April 18, 2013. All of them could potentially host life, but we do not know anything definitive about their compositions or atmosphere. Hide Caption 8 of 14 Photos: Where life might live beyond Earth This illustration depicts Kepler-62e, a planet in the habitable zone of a star smaller and cooler than the sun. It is about 1,200 light-years from Earth in the constellation Lyra. Hide Caption 9 of 14 Photos: Where life might live beyond Earth This illustration depicts Kepler-62f, a planet in the habitable zone of a star smaller and cooler than the sun, in the same system as Kepler-62e. Hide Caption 10 of 14 Photos: Where life might live beyond Earth This diagram compares the planets of our own inner solar system to Kepler-62, a five-planet system about 1,200 light-years from Earth. Kepler-62e and Kepler-62f are thought capable of hosting life. Hide Caption 11 of 14 Photos: Where life might live beyond Earth The planet Kepler-69c is about 2,700 light-years from Earth in the constellation Cygnus. Hide Caption 12 of 14 Photos: Where life might live beyond Earth This diagram compares the planets of our own inner solar system to Kepler-69, which hosts a planet Kepler-69c that appears to be capable of hosting life, in addition to planet Kepler-69b. Hide Caption 13 of 14 Photos: Where life might live beyond Earth This artist's illustration represents the variety of planets being detected by NASA's Kepler spacecraft. Hide Caption 14 of 14

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After spotting it, the institute wasted no time searching for emissions that could indicate the presence of ETs. So far, no emissions have been found.

The size -- estimated to be 10% larger than Earth -- and distance from its star don't just make for interesting factoids. They give scientists hope that Kepler-186f might sustain life as we know it.

Of nearly 1,800 "confirmed exoplanets" that have been found, approximately 20 orbit their host stars within habitable zones, where it's believed surface water would not freeze or boil. In 2011, NASA announced that Kepler had observed five planets approximately the size of Earth and in the habitable zone.

But the "previously discovered worlds are larger than Earth, and consequently their true nature -- rocky or gaseous -- is unknown," the SETI Institute said in a written announcement on Thursday. "On the basis of the observed dimming of starlight from Kepler-186, the authors estimate that this newly discovered planet is roughly the same size as the Earth."

Theoretical models and observations tell scientists that planets the size of Kepler-186f likely have a composition of iron, rock and ice, like Earth, Quintana told reporters Thursday.

Even if Kepler-186f is rocky, it's not necessarily habitable, scientists warned Thursday. First, a lot would depend on the atmosphere, if it has one, Thomas Barclay of NASA's Kepler mission said. And scientists right now don't have the technology to know what the atmosphere is.

The star's size -- it's an M-dwarf star, smaller and less hot than our sun -- also could come into play. Because it is smaller, the habitable zone is closer, so radiation could prevent life if the atmosphere isn't dense enough, said Victoria Meadows of the University of Washington Virtual Planetary Laboratory.

But the Webb space-based telescope, now under construction, should be able to gather images of planets around closer dwarf stars and study their atmospheres.

Scientists are especially keen about checking dwarf stars because their habitable planets are more easily detectable, and because they are the most abundant type of star in our galaxy, Barclay said.

For researchers, the discovery of Kepler-186f is like a beginning. It's a first but "not a record we wish to keep," Quintana said. "We want to find more of these."

It's likely they will. Astronomers at the University of California, Berkeley, and the University of Hawaii, using data from Kepler, estimate there are tens of billions of Earth-size, possibly habitable planets in our Milky Way galaxy.