One Search To (Almost) Rule Them All: Hundreds of Hidden Planets Found in Kepler Data

Jonathan O’Callaghan, writing for Scientific American: Most of the more than 4,000 exoplanets astronomers have found across the past few decades come from NASA’s pioneering Kepler mission, which launched in 2009 and ended in late October 2018. But among Kepler’s cavalcade of data, more planets are still waiting to be found — and a new method just turned up the biggest haul yet from the mission’s second, concluding phase, called K2. The K2 run from 2014 to 2018 was notable for its unique use of the functionality, or lack thereof, of the Kepler space telescope. Essentially a large tube with a single camera, Kepler relied on four reaction wheels (spinning wheels to orient the spacecraft) to point at specific patches of the sky for days or even weeks on end. Such long stares were beneficial for its primary planet-finding technique, known as the transit method, which detects worlds by watching for dips in a star’s light caused by an orbiting planet’s passage in front of it. But when two of Kepler’s reaction wheels failed, one in 2012 and another in 2013, mission planners came up with an ingenious method of using the pressure of the solar wind to act as a makeshift third wheel, allowing observations to continue, albeit with some limitations.

“We had this issue because the K2 mission was working off of two reaction wheels; it rolled a little bit every six hours,” says Susan Mullally of the Space Telescope Science Institute. “And as a result, the light curves have these little arcs that run through them that you have to first remove.” Various efforts were subsequently made to extract planets from the K2 data. But none have been more successful than one reported in a new paper by Ethan Kruse of NASA’s Goddard Space Flight Center and his colleagues, which was posted on the preprint server last week and accepted for publication in the Astrophysical Journal Supplement Series. Kruse employed an algorithm known as as QATS (for Quasiperiodic Automated Transit Search) and a light-curve-analysis program called EVEREST (for EPIC Variability Extraction and Removal for Exoplanet Science Targets) to better account for the spacecraft’s rolling and other sources of instrumental and astrophysical “noise” in the K2 data. The result was a whopping total of 818 planet candidates — 374 of which had never been spotted before — from the first nine of K2’s 20 observation campaigns.

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