![]() “That’s helpful especially when your data quality begins to suffer, like it did in K2’s last bit of data.” “They can distinguish transits from other wacky things like a glitch in the instrument,” Vanderburg says. The citizen scientists are especially suited to combing through short datasets such as K2’s very last campaign. They search by eye through thousands of recorded light curves of each star, looking for characteristic dips in brightness that signal a “transit,” or the possible crossing of a planet in front of its star. Vanderburg and Incha presented the challenge to the Visual Survey Group, a team of amateur and professional astronomers who hunt for exoplanets in satellite data. “We tried to see what last information we could squeeze out of it.” “We were curious to see whether we could get anything useful out of this short dataset,” Vanderburg says. The data from this last campaign comprised only a week of high-quality observations and another 10 days of noisier measurements as the spacecraft rapidly lost fuel. K2 went on for another four years, observing over half a million more stars before the spacecraft finally ran out of fuel during its 19th campaign. One year later, Kepler restarted as “K2,” a reworked mission that used the sun’s wind to balance the unsteady spacecraft in a way that kept the telescope relatively stable for a few months at a time - a period called a campaign. Kepler’s observations were put on pause while scientists searched for a fix. The wheels served as the spacecraft’s gyroscopes, helping to keep the telescope pointed at a particular point in the sky. ![]() Kepler kept observing beyond its original three-and-a-half-year mission, until May 2013, when the second of four reaction wheels failed. Over four years, the telescope recorded the brightness of over 150,000 stars, which astronomers used to discover thousands of possible planets beyond our solar system. In 2009, NASA launched the Kepler telescope into space, where it followed the Earth’s orbit and continuously monitored millions of stars in a patch of the northern sky. Vanderburg’s co-authors are lead author Elyse Incha, at the University of Wisconsin at Madison, and amateur astronomers Tom Jacobs and Daryll LaCourse, along with scientists at NASA, the Center for Astrophysics of Harvard and the Smithsonian, and the University of North Carolina at Chapel Hill. The team has published their discovery today in the journal Monthly Notices of the Royal Astronomical Society. “The planets themselves are not particularly unusual, but their atypical discovery and historical importance makes them interesting.” “We have found what are probably the last planets ever discovered by Kepler, in data taken while the spacecraft was literally running on fumes,” says Andrew Vanderburg, assistant professor of physics in MIT’s Kavli Institute for Astrophysics and Space Research. This Neptune-sized candidate orbits its star in around 10 days, and is slightly farther away, 1,200 light years from Earth. ![]() The planet candidate is EPIC 246251988 b - the largest of the three worlds at almost four times the size of the Earth. The two validated planets are K2-416 b, a planet that is about 2.6 times the size of the Earth and that orbits its star about every 13 days, and K2-417 b, a slightly larger planet that is just over three times Earth’s size and that circles its star every 6.5 days. For their size and proximity to their stars, both planets are considered “hot mini-Neptunes.” They are located about 400 light years from Earth. The scientists determined that two of the stars each host a planet, while the third hosts a planet “candidate” that has yet to be verified. The team combed through the telescope’s last week of high-quality data and spotted three stars, in the same part of the sky, that appeared to dim briefly. Now, astronomers at MIT and the University of Wisconsin at Madison, with the help of citizen scientists, have discovered what may be the last planets that Kepler gazed upon before going dark. 30, 2018, its fuel tanks depleted, the spacecraft was officially retired. In its last days, the telescope kept recording the brightness of stars as it was running out of fuel. Over nine and a half years, the spacecraft trailed the Earth, scanning the skies for periodic dips in starlight that could signal the presence of a planet crossing in front of its star. Over half were discovered by NASA’s Kepler Space Telescope, a resilient observatory that far outlasted its original planned mission. More than 5,000 planets are confirmed to exist beyond our solar system.
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