Over 5,000 extrasolar planets are known. NASA’s Kepler Space Telescope, which outlived its primary mission, discovered almost half. The spacecraft followed Earth for nine and a half years, searching for brightness dips that may indicate a planet passing in front of its star.
As gasoline ran out, the telescope recorded star brightness. The spacecraft decommissioned on October 30, 2018, with its fuel tanks empty.
MIT and UW-Madison astronomers and citizen scientists may have found Kepler’s final planets.
The scientists found three dimmed stars in the same area of the sky in the telescope’s penultimate week of high-quality data. Two stars have planets, while the third has a “candidate” planet.
The two confirmed planets are K2-416 b, which is roughly 2.6 times the size of Earth and orbits its star every 13 days, and K2-417 b, which is somewhat bigger and orbits its star every 6.5 days. Both are “hot mini-Neptunes” due to their size and closeness to their stars. 400 light years from Earth.
EPIC 246251988 b, the biggest of the three candidates, is approximately four times the size of Earth. This Neptune-sized contender is 1,200 light years from Earth and circles its star in 10 days.
Data crunch
NASA launched the Kepler telescope in 2009 to track millions of stars in the northern sky. Astronomers discovered thousands of extrasolar planets by recording the brightness of over 150,000 stars over four years.
Kepler observed until May 2013, when its second reaction wheel broke. The telescope’s gyroscopes were the wheels. Scientists paused Kepler’s observations.
One year later, Kepler relaunched as “K2,” a redesigned mission that employed the sun’s wind to balance the shaky spacecraft and keep the telescope stable for campaigns of a few months. K2 observed approximately half a million stars for four more years until running out of fuel during its 19th campaign. This last campaign had a week of high-quality observations and 10 days of noisier results as the spacecraft quickly depleted fuel.
Vanderburg explains, “We were curious to see if this short dataset could yield anything useful.” “We tried to squeeze out the last information.”
By eye
Vanderburg and Incha challenged the Visual Survey Group, a group of amateur and professional astronomers that search satellite data for exoplanets. They visually analyze millions of recorded light curves of each star for typical dips in brightness that indicate a “transit,” or a planet crossing in front of its star.
Citizen scientists excel at analyzing brief datasets like K2’s final campaign.
Vanderburg claims they can detect transits from other oddities like instrument glitches. “That’s helpful especially when your data quality begins to suffer, like K2’s last bit of data.”
Astronomers spent many days systematically reviewing Kepler’s light curves from 33,000 stars. After a week of high-quality telescope data, the crew lost fuel and attention. The team found one transit in three stars in this brief data frame.
Incha and Vanderburg then examined the telescope’s final 11 days of lower-quality data to see whether they could find any more transits in the same three stars, indicating a planet was orbiting its star.
The telescope’s vision drifted throughout this 11-day period as the spacecraft’s thrusters fired irregularly as it lost fuel. To find other transits at less data-noisy moments, the researchers examined each star’s light curves between thruster activity.
This search found a second transit for K2-416 b and K2-417 b, confirming their planets. MIT’s Transiting Exoplanet Survey Satellite (TESS) data showed a comparable brightness fall for K2-417 b. TESS confirmed this star’s planet candidate.
“Those two are definitely planets,” Incha remarks. Ground-based observations ruled out background star interference and close-in stellar binaries.
“These are Kepler’s last chronologically observed planets, but every bit of the telescope’s data is incredibly useful,” Incha explains. “We want to make sure none of that data goes to waste because there are still a lot of discoveries.”