Kepler, NASA’s remarkable space telescope, exceeded expectations by confirming over 5,000 planets beyond our solar system. Although the mission was designed for a limited duration, Kepler persevered for more than nine years, continuously observing the skies. By detecting periodic dips in starlight caused by planet transits, it contributed significantly to the field of exoplanet research.
In its final days, as its fuel supplies dwindled, Kepler diligently recorded star brightness. On October 30, 2018, the telescope was officially retired, but not before capturing valuable data that may reveal its last planetary discoveries.
Thanks to the collaborative efforts of astronomers from MIT, the University of Wisconsin at Madison, and citizen scientists, the team meticulously analyzed Kepler’s last week of high-quality data. Within a specific region of the sky, they identified three stars that exhibited brief dimming events, indicating potential planetary transits. After thorough examination, two stars were confirmed to possess planets, while the third star hosted a planet candidate awaiting verification.
The first validated planet, K2-416 b, boasts a size approximately 2.6 times that of Earth. It completes an orbit around its star every 13 days. The second confirmed planet, K2-417 b, is slightly larger than Earth, with a size just over three times that of our home planet. Its orbital period is shorter, revolving around its star every 6.5 days. Both planets, dubbed “hot mini-Neptunes,” reside approximately 400 light years away from Earth.
The third discovery, EPIC 246251988 b, represents the largest of the three worlds, nearly four times the size of Earth. This Neptune-sized planet candidate completes an orbit around its star in approximately 10 days and is located at a greater distance, around 1,200 light years away from us.
Andrew Vanderburg, assistant professor of physics at MIT’s Kavli Institute for Astrophysics and Space Research, reflects on the significance of these findings: “We have found what are probably the last planets ever discovered by Kepler, in data taken while the spacecraft was literally running on fumes. The planets themselves are not particularly unusual, but their atypical discovery and historical importance make them interesting.”
Today, the team publishes their remarkable discovery in the scientific journal Monthly Notices of the Royal Astronomical Society. Elyse Incha from the University of Wisconsin at Madison leads the research alongside amateur astronomers Tom Jacobs and Daryll LaCourse, as well as scientists from NASA, the Center for Astrophysics of Harvard and the Smithsonian, and the University of North Carolina at Chapel Hill.
In 2009, NASA initiated the launch of the Kepler telescope, sending it into space to trail the Earth’s orbit. Its primary objective was to survey a section of the northern sky, meticulously monitoring the brightness of millions of stars. Over a span of four years, Kepler’s vigilant observations led astronomers to uncover thousands of potential exoplanets outside our solar system.
However, in May 2013, Kepler encountered a setback when its second reaction wheel malfunctioned. These wheels were vital gyroscopes that enabled the spacecraft to maintain a steady focus on specific celestial targets. Scientists diligently sought a solution to resume Kepler’s observations.
One year later, a transformed mission called “K2” emerged. Kepler was reengineered to utilize the pressure of the sun’s rays, which helped to stabilize the spacecraft intermittently for several months at a time. Each stabilized phase was referred to as a “campaign.” K2 continued for an additional four years, scrutinizing an additional 500,000 stars. Eventually, during its 19th campaign, Kepler depleted its fuel reserves, bringing its mission to an end. The last campaign’s data encompassed only a week of high-quality observations, accompanied by ten days of noisier measurements as the spacecraft’s fuel rapidly diminished.
Despite the limited duration of the final dataset, scientists remained determined to extract any valuable information it contained. Andrew Vanderburg expressed their curiosity, stating, “We were curious to see whether we could get anything useful out of this short dataset. We tried to see what last information we could squeeze out of it.”
Kepler’s extraordinary mission, spanning over a decade, revolutionized our understanding of exoplanets and opened new frontiers in the field of astronomy.
Vanderburg and Incha enlisted the assistance of the Visual Survey Group, a dedicated team of both amateur and professional astronomers skilled in exoplanet detection from satellite data. These citizen scientists possess a unique ability to visually examine thousands of recorded light curves for each star, seeking distinct dips in brightness that signify a possible transit—an indication of a planet passing in front of its host star.
The expertise of these citizen scientists becomes particularly valuable when sifting through concise datasets like the final campaign of K2.
“They possess the capacity to differentiate transits from other irregularities, such as instrumental glitches,” explains Vanderburg. “This becomes especially helpful when the data quality diminishes, as it did during K2’s concluding phase.”
The astronomers embarked on a meticulous analysis, spending several days scrutinizing the light curves obtained by Kepler from approximately 33,000 stars. Despite having access to only a week’s worth of high-quality data before the telescope’s focus began to degrade, the team managed to identify a single transit event for each of the three stars.
In their subsequent investigation, Incha and Vanderburg turned their attention to the telescope’s final 11 days of operation, which encompassed lower-quality observations. The spacecraft’s thrusters exhibited erratic behavior during this period, leading to drift in the telescope’s viewpoint. Consequently, the team concentrated their analysis on the segments of the light curves captured between thruster activity, hoping to identify additional transit events during these moments of less noisy data.
This meticulous search yielded positive outcomes. The team identified a second transit event for both K2-416 b and K2-417 b, confirming the existence of planets orbiting these stars. Furthermore, they observed a similar decrease in brightness for K2-417 b in data obtained from NASA’s Transiting Exoplanet Survey Satellite (TESS), an MIT-led and operated mission. The TESS data served as additional evidence to validate the planet candidate associated with this particular star.
Incha expresses confidence in the findings: “There is no doubt that these two planets exist. We further conducted ground-based observations to eliminate any potential false positive scenarios, such as interference from background stars or nearby stellar binaries.”
While these discoveries mark the last planets chronologically observed by Kepler, Incha emphasizes the ongoing value of every piece of data collected by the telescope. “We want to ensure that none of this data goes to waste because there are still numerous discoveries waiting to be made,” she asserts.