An artist's illustration of the strange Earth-like rocky planet Kepler-78b, which orbits a sun-like star 400 light-years from Earth. The planet is 20 percent wider and 80 percent more massive than Earth. 
 
 Credit: David A. Aguilar (CfA) View full size image 
 
 A puzzling alien planet is the closest thing to an Earth twin in size and composition known beyond our solar system, though it's far too hot to support life, scientists say. 
 
 The exoplanet Kepler-78b, whose supertight orbit baffles astronomers, is just 20 percent wider and about 80 percent more massive than Earth, with a density nearly identical to that of our planet, two research teams report in separate papers published online today (Oct. 30) in the journal Nature. 
 
 "This is the planet that, in many respects, is the most like Earth that's been discovered outside our solar system," said Andrew Howard, of the University of Hawaii at Manoa's Institute for Astronomy and lead author of one of the studies. "It has approximately the same size. It has the same density, which means it's made out of the same stuff as Earth, in all likelihood." [The Strangest Alien Planets (Gallery)] 
 
 Studying a lava world 
 
 Kepler-78b, whose discovery was announced last month, orbits a sunlike star in the constellation Cygnus, about 400 light-years from Earth. 
 
 The alien world circles 900,000 miles (1.5 million kilometers) or so from its parent star — just 1 percent of the distance between Earth and the sun— and completes one lap every 8.5 hours. Surface temperatures on Kepler-78b likely top 3,680 degrees Fahrenheit (2,000 degrees Celsius), Howard said. 
 
 The planet was found by NASA's prolific Kepler space telescope, which has spotted nearly 3,600 potential exoplanets since its March 2009 launch. (Kepler was hobbled in May of this year when the second of its orientation-maintaining reaction wheels failed, but scientists are still sifting through the instrument's huge databases.) 
 
 Kepler flagged alien worlds by noting the telltale brightness dips they caused when passing in front of, or transiting, their parent stars from the spacecraft's perspective. Kepler's measurements allow researchers to estimate an exoplanet's size but not its mass, meaning that other strategies are required to get a handle on a world's density and composition. [Gallery: A World of Kepler Planets] 
 
 One such method is the radial velocity technique, which measures the wobble in a host star's light induced by the gravitational pull of an orbiting planet. Both new studies employed this method to investigate the Kepler-78 system, with Howard's group using the HIRES spectrograph at Hawaii's Keck Observatory and another team, led by Francesco Pepe of the University of Geneva, relying on the new HARPS-N instrument on the Telescopio Nazionale Galileo in the Canary Islands. 
 
 The two teams came to very similar conclusions. Howard's group determined Kepler-78b's mass to be 1.69 times greater than that of Earth, while Pepe's team calculated it to be 1.86 times higher than Earth's. The results of the Pepe-led study suggest a density of 5.57 grams per cubic centimeter for Kepler-78b, while those of Howard's team imply a density of 5.3 grams per cubic cm. 
 
 These numbers agree to within the error range independently estimated by both teams, suggesting that they are quite accurate, Howard said. 
 
 "The fact that we agree to within our errors — in science, that's basically as good as you can do," Howard told SPACE.com. 
 
 Earth's density is about 5.5 grams per cubic cm, so Kepler-78b probably has an Earth-like composition, complete with a rocky interior and an iron core, both studies suggest. 
 
 A mysterious origin 
 
 The extremely tight orbit of Kepler-78b puzzles astronomers. According to prevailing theory, the alien world shouldn't exist where it does, because its host star was significantly larger when the planet was taking shape. 
 
 "It couldn't have formed in place because you can't form a planet inside a star," Dimitar Sasselov, of the Harvard-Smithsonian Center for Astrophysics and a member of the Pepe-led team, said in a statement. "It couldn't have formed further out and migrated inward, because it would have migrated all the way into the star. This planet is an enigma." 
 
 What is clear, however, is that Kepler-78b's days are numbered. The planet will continue circling lower and lower until the immense gravity of its host star tears it apart, likely within 3 billion years or so. 
 
 "Kepler-78b is going to end up in the star very soon, astronomically speaking," Sasselov said. 
 
 The search for another Earth 
 
 The hellishly hot Kepler-78b is not a good place to hunt for alien life. But the determination of its density marks a milestone in the ongoing search for a true "Earth twin" — a planet very much like Earth in size, composition and surface temperature. 
 
 "The existence of Kepler-78b shows that, at the very least, extrasolar planets of Earth-like composition are not rare," astronomer Drake Deming, of the University of Maryland, writes in an accompanying commentary article today in the same issue of Nature. 
 
 Deming points to NASA's upcoming Transiting Exoplanet Survey Satellite mission, or TESS, which is slated to launch in 2017 to hunt for transiting planets around nearby stars (as contrasted with Kepler, whose gaze was more distant). 
 
 "By focusing particularly on small stars cooler than the sun, TESS should find exo-Earths whose mass can be measured by trading the close-in orbit of Kepler-78b for more distant orbits around low-mass stars, approaching orbital zones where life is possi­ble," Deming writes. "That trade-off probably cannot be pushed to the point of measuring an Earth twin orbiting once per year around a sun twin, but it will allow future scientific teams to probe habitable planets orbiting small stars." 
 
 Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on SPACE.com. ||||| LOS ANGELES (AP) — Astronomers have discovered what they say is the most Earth-like planet yet detected — a distant, rocky world that's similar in size to our own and exists in the Goldilocks zone where it's not too hot and not too cold for life. 
 
 This artist's rendering provided by NASA on Thursday, April 17, 2014 shows an Earth-sized planet dubbed Kepler-186f orbiting a star 500 light-years from Earth. Astronomers say the planet may hold water... (Associated Press) 
 
 The find, announced Thursday, excited planet hunters who have been scouring the Milky Way galaxy for years for potentially habitable places outside our solar system. 
 
 "This is the best case for a habitable planet yet found. The results are absolutely rock solid," University of California, Berkeley astronomer Geoff Marcy, who had no role in the discovery, said in an email. 
 
 The planet was detected by NASA's orbiting Kepler telescope, which studies the heavens for subtle changes in brightness that indicate an orbiting planet is crossing in front of a star. From those changes, scientists can calculate a planet's size and make certain inferences about its makeup. 
 
 The newfound object, dubbed Kepler-186f, circles a red dwarf star 500 light-years from Earth in the constellation Cygnus. A light-year is almost 6 trillion miles. 
 
 The planet is about 10 percent larger than Earth and may very well have liquid water — a key ingredient for life — on its surface, scientists said. That is because it resides at the outer edge of the habitable temperature zone around its star — the sweet spot where lakes, rivers or oceans can exist without freezing solid or boiling away. 
 
 The find "is special because we already know that a planet of this size and in the habitable zone is capable of supporting life as we know it," lead researcher Elisa Quintana of NASA's Ames Research Center said at a news conference. 
 
 The discovery was detailed in Friday's issue of the journal Science. It was based on observations that were made before the Kepler telescope was crippled by a mechanical failure last year. 
 
 The planet probably basks in an orange-red glow from its star and is most likely cooler than Earth, with an average temperature slightly above freezing, "similar to dawn or dusk on a spring day," Marcy said. 
 
 Quintana said she considers the planet to be more of an "Earth cousin" than a twin because it circles a star that is smaller and dimmer than our sun. While Earth revolves around the sun in 365 days, this planet completes an orbit of its star every 130 days. 
 
 Scientists cannot say for certain whether it has an atmosphere, but if it does, it probably contains a lot of carbon dioxide, outside experts said. 
 
 "Don't take off your breathing mask if you ever land there," said Lisa Kaltenegger, a Harvard and Max Planck Institute astronomer who had no connection to the research. 
 
 Despite the differences, "now we can point to a star and know that there really is a planet very similar to the Earth, at least in size and temperature," Harvard scientist David Charbonneau, who was not part of the team, said in an email. 
 
 If the planet is habitable, photosynthesis may be possible, said astronomer Victoria Meadows of the University of Washington, Seattle. 
 
 "There are Earth plants that would be quite happy with that," she said. 
 
 Since its launch in 2009, Kepler has confirmed 961 planets, but only a few dozen are in the habitable zone. Most are giant gas balls like Jupiter and Saturn, and not ideal places for life. Scientists in recent years have also found planets slightly larger than Earth in the Goldilocks zone called "super Earths," but it is unclear if they are rocky. 
 
 The latest discovery is the closest in size to Earth than any other known world in the habitable region. 
 
 Astronomers may never know for certain whether Kepler-186f can sustain life. The planet is too far away even for next-generation space telescopes like NASA's James Webb, set for launch in 2018, to study it in detail. 
 
 NASA has not yet decided whether to keep using the crippled Kepler telescope on a scaled-back basis. While the instrument may never detect another planet, scientists have a backlog of observations to wade through. 
 
 ___ 
 
 AP Science Writer Seth Borenstein contributed to this report. 
 
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 Follow Alicia Chang at http://twitter.com/SciWriAlicia ||||| MOFFET FIELD, CALIFORNIA—For the first time, astronomers have found a planet smack in the middle of the habitable zone of its sunlike star, where temperatures are good for life. "If this planet has a surface, it would have a very nice temperature of some 70° Fahrenheit [21°C]," says William Borucki of NASA's Ames Research Center here, who is the principal investigator of NASA's Kepler space telescope. "[It's] another milestone on the journey of discovering Earth's twin," adds Ames director Simon "Pete" Worden. 
 
 Unfortunately, the true nature of the planet, named Kepler-22b, remains unknown. It is 2.4 times the size of Earth, but its mass, and hence its composition, has not yet been determined. "There's a good chance it could be rocky," Borucki says, although he adds that the planet would probably contain huge amounts of compressed ice, too. It might even have a global ocean. "We have no planets like this in our own solar system." 
 
 Kepler-22b is 600 light-years away. Every 290 days, it orbits a star that is just a bit smaller and cooler than our own sun. The Kepler telescope, launched in 2009 to scan the skies for Earth-like worlds, found the planet because it sees the orbit edge on. That means that every 290 days, the world transits the surface of the star, blocking out a minute fraction of its light. 
 
 Borucki likes to call the new discovery the Christmas planet. "It's a great gift," he said at a press conference here this morning. "We were very fortunate to find it." The first of the three observed transits occurred only days after Kepler started observing. The third one was seen just before Christmas 2010, shortly before the spacecraft was unable to carry out any observations because of a technical glitch. Says Borucki: "We could've easily missed it altogether." 
 
 "There are two things really exciting about this planet," adds Natalie Batalha, Kepler's deputy science team leader. "It's right in the middle of the habitable zone [the region around a star where temperatures are neither too high or too low for liquid water to exist], and it orbits a star very similar to our sun." Previously discovered "habitable" planets orbited dim, red dwarf stars, or they were located at the edge of the habitable zone, with more extreme temperatures. 
 
 At the press conference, which marked the start of the 5-day First Kepler Science Conference here at NASA Ames, Batalha also announced 1094 new planet candidates found by Kepler since February 2011, bringing the total to a whopping 2326. So far, only 29 of these (including Kepler-22b) have been confirmed to be genuine planets, but Kepler scientists have good reasons to expect that at least 90% of all candidates will turn out to be real. 
 
 Forty-eight of these planet candidates orbit in the habitable zone of their parent stars. Most are substantially larger than Earth, but 10 are about the same size as our home planet. Some of these are in multiplanet systems. "It's conceivable that any-or many-of these 48 habitable zone candidates, or their moons, could have life," Borucki says. 
 
 Jill Tarter of the SETI Institute in Mountain View, California, says the habitable Kepler planets are prime targets for the Search for Extra-Terrestrial Intelligence (SETI), carried out with the dedicated 42-dish Allen Telescope Array in northern California. "We're taking everything we can get from our Kepler colleagues to look for techno-signatures" that might betray the existence of an alien civilization, she says. 
 
 So far, the question about extraterrestrial life is very much open. "We don't know whether Earth as it is, and life as we know it, is very unusual or very common," Tarter says. However, if scientists find a second place in the universe where life once got started, it will be obvious that life must be widespread. Says Tarter: "In this field, the number two is important. We count one, two, infinity."
– Astronomers have discovered a planet where a certain red guy with horns could make himself right at home. It's a planet much like our own Earth—about the same size, with the same mixture of rock and iron, and it orbits a star like our sun—except that Kepler 78b is an infernal ball of fire. Located less than a million miles from its host star—that's 1% of the distance between Earth and our sun—its temperature ranges between 3,500 and 5,000 degrees Fahrenheit. That's "well above the temperature where rock melts," an astronomer tells the New York Times. "This is probably one of the most hellish planets that have been discovered yet." At 400 light-years from Earth, the exoplanet was first spotted by the Kepler spacecraft but astronomers on Earth took a closer look at its light frequency to discover its orbit and mass. The findings, published in two studies in Nature, show a planet that tightly orbits its sun—which from the surface would look 80 times larger than our sun—in just eight and a half hours. Adds an expert, per LiveScience: "This planet is an enigma. It couldn't have formed in place because you can't form a planet inside a star." But while astronomers are baffled as to how the planet got there, they're fairly certain it shows friendlier Earthlike planets are still out there.