Image copyright David Aguilar Image caption Artist's impression: Kepler has identified many exoplanets but few are within their star's "Goldilocks zone" 
 
 One of eight new planets spied in distant solar systems has usurped the title of "most Earth-like alien world", astronomers have said. 
 
 All eight were picked out by Nasa's Kepler space telescope, taking its tally of such "exoplanets" past 1,000. 
 
 Three of the new crop sit within the "habitable zone" of their host stars - and one in particular is rocky, like Earth, as well as only slightly warmer. 
 
 The find was revealed at a meeting of the American Astronomical Society. 
 
 Red sky 
 
 The three potentially habitable planets join Kepler's "hall of fame", which now boasts eight fascinating planetary prospects. 
 
 And researchers say the most Earth-like of the new arrivals, known as Kepler 438b, is probably even more similar to our home than Kepler 186f - which previously looked to be our most likely twin. 
 
 At 12% larger than Earth, the new claimant is bigger than 186f but it is closer to our temperature, probably receiving just 40% more heat from its sun than we do from ours. 
 
 So if we could stand on the surface of 438b it may well be warmer than here, according to Dr Doug Caldwell from the Seti (Search for Extra-Terrestrial Intelligence) Institute in California. 
 
 "And it's around a cooler [red dwarf] star... so your sky would look redder than ours does to us," Dr Caldwell said. 
 
 Image copyright NASA Image caption With the three new arrivals, Kepler's 'hall of fame' expands to eight small, habitable-zone exoplanets 
 
 That first-person encounter, however, is unlikely - both because the planet is 475 light-years away and because we still have essentially no idea what it's made of. 
 
 Images from the Kepler telescope, which trails behind the Earth and peers far into the distance as we orbit our own sun, are used to identify far-off planets by observing "transits". 
 
 This refers to the dimming of a star's light when a planet passes in front of it. 
 
 Kepler space telescope mission Image copyright NASA/Kepler mission/Wendy Stenzel Launched in 2009, the Kepler space telescope sought to find Earth-like worlds orbiting distant stars in the Constellation Lyra 
 
 It used the so-called transit technique - looking for the periodic dips in light as exoplanets pass in front of their host stars 
 
 Last year, a single release of Kepler results added 715 exoplanets to the tally How does the Kepler telescope work? How rare is our blue planet? 
 
 A large team of researchers then uses additional data from Earth-bound telescopes to further explore these unfamiliar solar systems. 
 
 They try to calculate how big the planets are, and how closely they orbit their host stars. 
 
 Not everything that causes such a dimming eventually turns out to be a planet, however. 
 
 At the same time as the eight confirmed new exoplanets were announced by a 26-strong team spanning Nasa and multiple US institutions, the Kepler mission's own scientists released another tranche of more than 500 "candidate" planets. 
 
 "With further observation, some of these candidates may turn out not to be planets," said Kepler science officer, Fergal Mullally. 
 
 "Or as we understand their properties better, they may move around in, or even outside, the habitable zone." 
 
 'Star Trek' scenario 
 
 Even once scientists have anointed a candidate as a confirmed exoplanet, the question of whether or not it is "Earth-like" is a fraught one, with fuzzy boundaries. 
 
 The size of the habitable, or "Goldilocks" zone, where a planet is far enough from its sun to hold water but not so distant that it freezes, depends on how confident scientists want to be with their guess-work. 
 
 According to Dr Cardwell, just three of the eight new exoplanets can be confidently placed in that zone - and only two of those are probably rocky like the Earth. 
 
 More detailed description is very difficult. 
 
 Image copyright SPL Image caption An artist's view of Kepler 186f, which experts say has now been pipped as "most Earth-like" known exoplanet 
 
 "From the Kepler measurements and the other measurements we made, we don't know if these planets have oceans with fish and continents with trees," Dr Caldwell told BBC News. 
 
 "All we know is their size and the energy they're receiving from their star. 
 
 "So we can say: Well, they're of a size that they're likely to be rocky, and the energy they're getting is comparable to what the Earth is getting. 
 
 "As we fill in these gaps in our solar system that we don't have, we learn more about what it means to be Earth-like, in some sense." 
 
 Speaking at a related event at the conference, Prof Debra Fischer from Yale University said she remembered a time before the first exoplanet was discovered, more than two decades ago. 
 
 "I remember astronomers before that point being very worried," she said. 
 
 "We really had to step back and say: Maybe the Star Trek picture is wrong. That filled me with despair." 
 
 Prof Fischer said that sensitive telescopes like Kepler had ushered in an era of "amazing and impressive work". 
 
 "We're talking about a planet - and we can only see its star with a powerful telescope. 
 
 "And we can draw graphs and sketch its composition and have serious scientific discussions. This is incredible." 
 
 Follow Jonathan on Twitter. ||||| New discovery is likely to be a rocky world in the ‘Goldilocks’ zone of its parent star where the temperature is just right for liquid water to flow 
 
 An alien world that orbits a distant star in the constellation of Lyra may be the most Earth-like planet ever found outside the solar system. 
 
 The planet, named Kepler 438b, is slightly larger than Earth and circles an orange dwarf star that bathes it in 40% more heat than our home planet receives from the sun. 
 
 The small size of Kepler 438b makes it likely to be a rocky world, while its proximity to its star puts it in the “Goldilocks” or habitable zone where the temperature is just right for liquid water to flow. 
 
 A rocky surface and flowing water are two of the most important factors scientists look for when assessing a planet’s chances of being hospitable to life. 
 
 Kepler 438b, which is 470 light years away, completes an orbit around its star every 35 days, making a year on the planet pass 10 times as fast as on Earth. Small planets are more likely to be rocky than huge ones, and at only 12% larger than our home planet, the odds of Kepler 438b being rocky are about 70%, researchers said. 
 
 Scientists at the Harvard-Smithsonian Center for Astrophysics announced the discovery at a meeting of the American Astronomical Society in Seattle on Tuesday, along with seven other planets that also lie in the habitable zones of their stars. The haul doubles the number of small planets – those less than twice the size of Earth – believed to be orbiting in their parent stars’ habitable zones. 
 
 All were spotted with Nasa’s Kepler space telescope which detects planets as they move across the faces of their stars, causing the light picked up by the telescope to dim periodically by a minuscule amount. 
 
 One of the other planets, Kepler 442b, lies in the same constellation 1,100 light years away. It is about a third larger than Earth, receives about two thirds as much starlight, and has a 60% chance of being rocky, according to a report to be published in The Astrophysical Journal. 
 
 Kepler-186f is part of five-planet system 795 lights years away. The find is described in the journal Science as ‘a landmark on the road to discovering habitable planets’. Photograph: Nasa Ames/SETI Institute/JPL-Cal/PA 
 
 Guillermo Torres, lead author on the study, said the size and amount of light falling on the planets made them the most Earth-like planets yet found beyond our solar system. Before their discovery, the exoplanets most similar to our own were Kepler 186f, which is 10% larger than Earth and receives a third as much light, and Kepler 62f, which is 40% larger and gets about 41% as much light. 
 
 The scientists do not know if the planets have atmospheres, but if they are cloaked in insulating layers of gas, the mean temperatures of Kepler 438b and 442b are expected to be about 60 and zero degrees Celsius respectively. 
 
 The Harvard-Smithsonian team used a computer program called Blender to confirm that the planets originally spotted by the Kepler space telescope were real. False sightings can happen when pairs of stars that lie behind the one being studied eclipse each other, causing the background light to dim slightly. In some cases, this can be mistaken for a planet moving in front of its star. 
 
 “The pair of stars can be way behind the target star, but if they are in the same line of sight, the result is a very tiny dimming that can look like a planet,” said Torres. 
 
 The Blender program gives a statistical probability that the planet is real and not an effect of background stars eclipsing one another. Of 12 suspected planets Torres and his colleagues assessed with the program, 11 came out at more than 99.7% likely to be real. 
 
 David Kipping, a co-author on the study, said that Kepler 438b and 442b were “as close to Earth analogues as we’re going to find in the Kepler data”. 
 
 Astronomers are keenly waiting on the next generation of telescopes, including Hubble’s replacement, the James Webb Space Telescope, and the European Extremely Large Telescope, which is being built in the Atacama desert in Chile, to help them examine the atmospheres of distant planets for signs of life. 
 
 In the meantime, scientists plan to look for other, indirect signs, that a planet may be well-suited for life. Kipping is searching through the Kepler data for hints that some planets have moons, which can improve their odds of being habitable. Our own moon stabilises Earth’s tilt, making the temperatures far less erratic than they would be otherwise. Alien planets that share a solar system with a gas giant like Jupiter are also interesting, because the vast size of the planet acts as a shield against devastating asteroid and comet impacts. ||||| After five years of searching, researchers using data from NASA's exoplanet-hunting Kepler spacecraft have discovered what look to be two of the most Earth-like worlds yet. Dubbed Kepler 438 b and Kepler 442 b, both planets appear to be rocky and orbit in the not-too-hot, not-too-cold habitable zones of their stars where liquid water can exist in abundance. Astronomers announced the planets along with six other newfound small, temperate worlds today at a meeting of the American Astronomical Society in Seattle. Their findings will be published in The Astrophysical Journal. The discoveries double the number of known potentially habitable exoplanets. They also push Kepler's tally of vetted, confirmed worlds to just over 1,000, marking a milestone in the mission's epochal search for alien Earths. 
 
 
 
 Both planets are many hundreds of light-years away and orbit stars smaller and dimmer than our sun. Like most of Kepler's finds, they were discovered via transits—the shadows they cast toward our solar system as they cross the blazing faces of their stars. Transits allow astronomers to measure a planet's size, orbit and exposure to starlight. Kepler 438 b is only about 12 percent larger than Earth, and basks in 40 percent more starlight; Kepler 442 b is 30 percent larger and receives about 30 percent less light. Both spheres may be somewhat warmer than Kepler's two previous premier rocky worlds, Kepler 186 f and Kepler 62 f, each of which gets significantly less starlight—similar to that received by Mars. “We can't say for sure whether these planets are truly habitable—only that they are promising candidates for habitability," says study co-author David Kipping, an astronomer at the Harvard–Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass. 
 
 
 
 When Kepler launched into orbit in 2009 to survey a patch of sky containing some 150,000 stars, one of its primary goals was to find mirror Earths, worlds about the same size as our own in approximately 365-day orbits around sunlike stars. The task was expected to take just over three years because many things can cause stars to dim besides transiting planets, and astronomers would need to glimpse the periodic recurrence of any mirror Earth's transit not once or twice but three times to be convinced that any particular dimming was due to a planet. 
 
 
 
 The sunlike stars had other plans, however, proving to be more variable in brightness than mission planners had anticipated, muddying Kepler's search for the faint shadows. Kepler's scientists consequently focused on smaller, dimmer, quieter stars and asked for more time to gather additional data. The mission was extended in 2012 but was struck by stabilizer equipment failures in spring of 2013 that sent Kepler's once-steady gaze drifting askew, bringing its survey to a premature close. Last year the spacecraft was resurrected as the "K2" mission after researchers devised a new method to aim the telescope. K2 is a more limited transit survey and has scant hope of finding a mirror Earth. The last, best option for the mission to succeed in discovering Earth twins was to sift through Kepler's archival data from 2013 and earlier, which is filled with thousands of unconfirmed candidate planets. "I think these latest planets are about as good as we're going to get from the Kepler data," Kipping says. "I would like to be surprised—and I'm hopeful—but I'm not sure we'll find planets closer to Earth twins than the objects we present in our paper." 
 
 
 
 Other researchers might disagree. At the same meeting, Fergal Mullally, a staff scientist and Kepler team member at NASA's Ames Research Center at Moffett Field, Calif., announced an additional 554 yet-to-be-confirmed candidate planets added to Kepler's catalogue, bringing its total candidate count to 4,175 possible worlds. Eight of those newly announced worlds are less than twice the size of Earth and orbit in their stars' habitable zones. Six of those eight orbit sunlike stars, and could potentially fulfill Kepler's elusive goal of finding an Earth 2.0—but only if follow-up studies can reveal more about them. 
 
 
 
 Like many of Kepler's most tantalizing finds, Kepler 438 b and 442 b were too small and far away to be confirmed in the traditional way, which involves independently measuring the gravitational wobble an orbiting planet induces on its star. Rather than being "confirmed," they were "validated" through a series of simpler follow-up observations and complex statistical tests designed to rule out false positives. According to lead author Guillermo Torres, another astronomer at the CfA, this validation process took more than a year and a half and relied on some of the world's largest telescopes as well as intensive simulation runs on a NASA supercomputer. As impressive as this validation process is, it cannot provide the crucial information available when a planet is confirmed by measuring its associated wobble. Such a measurement allows astronomers to estimate a world's mass, which, paired with the size estimate provided by a transit, yields a density and composition estimate. Planets revealed to be small and heavy must be made mostly of rock or metal whereas worlds seen to be large and light must possess much more gas. 
 
 
 
 Without confirmatory wobbles, determining that Kepler 438 b and 442 b are Earth-like worlds of rock and metal relies on statistics for now. "Lots of hard work over the past few years suggests the cutoff—the transition between rocky and gaseous planets—occurs at about 1.5 times the size of Earth," Kipping says. "Everyone is now converging on that number. Anything smaller than that is most likely rocky"—and therefore more likely to be habitable. 
 
 
 
 The latest study to bolster this argument was presented earlier in the meeting by lead author Courtney Dressing, another CfA astronomer, who measured the masses and sizes of a handful of small transiting planets to estimate the rocky-to-gaseous transition zone. Dressing pegs the general upper limit for rocky planets as 1.6 times Earth size. "We now know most planets in the galaxy are intermediate in size between Earth and Neptune but we don't have any examples of these worlds in our own solar system," Dressing says. "In order to figure out whether these planets, the most common in our galaxy, are mostly rocky and potentially habitable or mostly gaseous and probably not very habitable, we have to perform these measurements to learn where exactly this transition occurs and how broad it is." 
 
 
 
 Most planet hunters now pin their immediate hopes for discovering Earth-like planets on NASA's upcoming Transiting Exoplanet Survey Satellite, or TESS, which is slated to launch in 2017. TESS is expected to perform an all-sky survey focused on finding transiting rocky planets around nearby stars, planets that could then be studied in further detail by NASA's James Webb Space Telescope, which would launch no sooner than 2018. Even so, the quest for finding truly habitable—maybe even inhabited—planets beyond the solar system may require more ambitious, next-generation observatories. 
 
 
 
 "The impact of this work is to prove that these sorts of worlds are out there," Kipping says. "This is a journey where the end point is investigating planetary atmospheres to look for signs of life, and this result shows it's justifiable to build a space telescope to do exactly that."
– If you're sick of Earth and have a spacecraft capable of traveling hundreds of light-years, astronomers have spotted the most promising destination yet. Kepler-438b is a newly-confirmed potential "Earth twin" detected by the Kepler Space Telescope, the BBC reports, one of eight confirmed new exoplanets. The planet, about 470 light-years away in the Lyra constellation, is the most Earth-like ever spotted outside our solar system. It's only slightly larger than our planet, sits in the "Goldilocks zone" where the temperature is right for liquid water to flow, and probably has a rocky surface, researchers say. Its dwarf star gives it around 40% more heat than our planet receives from the sun, and its years are just 35 days long. The latest find brings the number of confirmed worlds spotted by Kepler—including Kepler-186f, the previous most Earth-like find—to just over 1,000, with more than 3,000 still to be vetted, reports Scientific American. To search for clues that life may exist on Kepler-438b and other planets, researchers plan to examine Kepler data for signs of moons, which help stabilize orbits and temperatures, and for gas giants in the same solar system, which reduce the odds of catastrophic asteroid hits, the Guardian reports. (Last month, Kepler data revealed a possible water world, or "mini-Neptune," 180 light-years away.)