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Launched in March 2009, the Kepler space-based telescope completed its initial mission in late 2012, after which NASA officials quickly approved a three-year extension. By aiming a 95-megapixel camera at a small patch of sky, the telescope monitored the light from 160,000 stars (out of some 4.5 million detectable), seeking the slight dimming that occurs when a planet passes in front of a star. Using this technique–known as the transit method–the Kepler mission has so far discovered more than 950 confirmed planets.

The latest findings using Kepler data, reported this week in a scientific paper published in the journal Science, are some of the most remarkable yet. Out of five planets circling Kepler 186, a red dwarf star about half the size of the Sun located in the constellation Cygnus, four are orbiting extremely close to the parent star, making temperatures too hot for liquid water to flow at their surfaces. The outermost planet, however, is inside the so-called “Goldilocks zone,” where temperatures are moderate enough to sustain liquid water rather than ice or vapor at the surface–a necessary requirement for supporting life as we know it.

Not only that, but Kepler 186f (as it is known) has a diameter of 8,700 miles, or only 10 percent wider than Earth, making it the closest match to our planet yet found. According to lead author Elisa V. Quintana of the SETI Institute and NASA’s Ames Research Center, “Kepler 186f is the first validated, Earth-size planet in the habitable zone of another star….It has the right size and is at the right distance to have properties similar to our home planet.”

The smaller size of Kepler 186f means that the planet is more likely to have a rocky surface, similar to that of Earth, rather than a gaseous one. Along with supporting liquid water at the surface, a rocky surface is a basic requirement for sustaining life. Last year, two planets were found to be orbiting within the Goldilocks zone of another star, Kepler 62, but both had masses several times that of Earth. As a result, the gravity of these planets was likely powerful enough to pull in hydrogen and helium gases, making them more comparable to mini Neptunes rather than larger Earths.

Scientists believe that the new planet may be made up of the same materials as Earth (iron, rock, ice and liquid water) and that it may have roughly the same gravity, but since it is so far away, they can only speculate. Significant differences also exist between Kepler 186f and Earth. Kepler 186f is much closer to its star than we are to the Sun, and completes one orbit in 130 days versus 365. It’s also much colder than Earth, as it receives only a third of the stellar energy that Earth gets from the Sun. As a result, more of its surface might freeze. According to Thomas S. Barclay of the Bay Area Environmental Research Institute, who was involved in the study, the new planet might be “more of an Earth cousin than an Earth twin.”

Earth-like exoplanets, or planets that orbit stars outside of the solar system, are not uncommon, though this is the first validated detection of one of them orbiting in the habitable zone of its parent star. Scientists are hoping to find more similar exoplanets closer to us in order to conduct more in-depth analysis. In 2017, NASA will launch the Transiting Exoplanet Survey Satellite (TESS), which aims to identify nearby exoplanets that can be analyzed to learn more about their mass, composition and atmosphere. Scientists plan to use the James Webb Space Telescope (scheduled for launch in 2018) to measure the physical and chemical properties of such exoplanets to assess their ability to support life.

Like the Kepler telescope, TESS will also use the transit method, though some scientists see drawbacks to this way of locating exoplanets. Using the transit method, a planet has to be correctly aligned in order for the observer to see the dip in light values; a tighter orbit and a larger planet-to-star size ratio increases the likelihood that the method will work. Such restrictions undoubtedly leave many exoplanets undetected, critics say. For example, Earth itself would be hard to spot using the transit method, due to its distance from the sun and its relatively small size.

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