The NASA Kepler mission is, by far, the most successful exoplanet detector to date. Over 3000 confirmed exoplanets were found with this space-based transit satellite – this is more than half of the total exoplanets we know of today. Kepler’s primary mission was to stare at a small patch of sky for several years, looking for the periodic dip in brightness of a star which could be due to the presence of a planet orbiting in front of it. Unfortunately, a mechanical failure meant that the telescope could no longer keep observing this particular patch of sky but luckily, engineers were able to find a way to keep using the satellite to hunt for exoplanets, just at different locations in the sky. This new mission, which began in 2014, was named K2 – for Kepler Second Light.
At the very end of this mission, when the spacecraft was running out of fuel, it completed a final set of observations known as K2 campaign 19. Only one week’s worth of high quality data is available from this campaign (compared to the typical 80 days of observations), as after this the fuel reserves were further depleted and the pointing of the telescope began to suffer. Amazingly, a team has recently announced that they have detected 2 new planets within the campaign 19 data and a further planet candidate.
To begin the search for planets in this final campaign, the team, led by undergraduate student Elyse Incha of the University of Wisconsin-Madison, had to search by-eye the observations of all 33,000 stars that were observed. From this they found 3 potential planet candidates orbiting the stars referred to as EPIC 246191231, EPIC 245978988, and EPIC 246251988. However, because of the short window of good observations of only one week, only a single transit event (where the planet passes in front of the star and blocks some of the light) was detected for each, which is not enough to confirm a planet. The team then re-processed the data for these 3 stars and tried to search for additional transit events in the lower-quality data at the end of the campaign. In two cases, for EPIC 246191231 and EPIC 245978988, they managed to find additional transit signals which were consistent with the ones they had already seen.
The next step in confirming these was to rule out other possible sources of a dip in the brightness of the star, for example from other background stars. After ruling out these other sources by using further data from other telescopes on Earth, and then doing a statistical validation, the team were able to confirm two of these candidates are very likely to be planets. The two new planets, named K2-416 b and K2-417 b, join the growing population of small planets which are found orbiting small stars.
On the importance of this discovery, the authors wrote in the discovery paper:
Given the tremendous influence of Kepler on the field of exoplanets, and more broadly, on humanity’s understanding of our place in the cosmos, searching for more planets from Kepler’s final dataset is of interest to both fully exploit the telescope’s data and learn the identity of its last planet discoveries.
Unfortunately, the stars these planets orbit are very faint, meaning that additional follow-up observations are very challenging.
Whilst these new planets are unlikely to be studied in depth in the future, the fact that they were found and confirmed even in this challenging data set makes them an interesting addition to the legacy of the Kepler mission.