A crew of astrophysicists and citizen scientists have discovered an exoplanet trio that may have been the last planets that were uncovered by NASA’s retired Kepler space telescope during its almost decade-long mission.

These exoplanets are all between the sizes of Earth, and Neptune and orbit their stars in close proximity.

A senior at the University of Wisconsin-Madison, Elyse Incha said, “They’re exciting because Kepler observed them during its last few days of operations. It showcases just how good Kepler was at planet hunting, even at the end of its life”.

The trio’s study was published in the journal Monthly Notices of the Royal Astronomical Society by Incha.

In collaboration with Visual Survey Group, an alliance between citizen scientists and professional astronomers working on data from several space missions (Kepler, K2, and TESS), Incha and her team searched for exoplanets in Kepler data from its launch in March 2009 to its retirement in October 2018.

K2 was Kepler’s new mission after it was repaired from a mechanical problem in 2014. It then resumed its operations, changing its range of vision every three months, a period known as a campaign.

K2’s final campaign, ‘number 19’, lasted only for a month. Astronomers only had roughly seven days of high-quality data from ‘Campaign 19’ at the end of this.

In this research, citizen scientists looked for evidence of transiting worlds, or planets passing in front of their stars, overall Campaign 19’s light curves, which documented how monitored stars brightened or diminished.

A former US Navy officer, and Visual Survey Group team member, Tom Jacobs said, “People doing visual surveys – looking over the data by eye – can spot novel patterns in the light curves and find single objects that are hard for automated searches to detect. And even we can’t catch them all”.

“I have visually surveyed the complete K2 observations three times, and there are still discoveries waiting to be found”, Jacobs continued.

In the high-quality dataset, Jacobs and colleagues discovered one transit for each of the three planet candidates, each orbiting a different star.

Following their first discovery, Incha and her team examined the lower-quality data from the rest of Campaign 19 and discovered one additional transit each from two of the three stars indicated in the visual search.

Andrew Vanderburg, an assistant professor of physics at the Massachusetts Institute of Technology (MIT) said, “The second transits for those two planet candidates helped us confirm their discovery”.

“No one had found planets in this dataset before, but our collaboration was able to find three. And we’re really pushing up against the last few days, the last few minutes, of observations Kepler collected”, the professor added.

Using the transit data, Incha and her colleagues computed the possible sizes and orbital periods world.

K2-416 b, the smallest planet discovered, is about 2.6 times the size of Earth and orbits its red dwarf star about every 13 days.

K2-417 b, which is somewhat larger than Earth, also orbits a red dwarf star but every 6.5 days.

EPIC 246251988 b, the final unconfirmed planet, is about four times the size of Earth and orbits its Sun-like star in around ten days.

Also read: Third Of Milky Way’s Most Common Planets Could Harbor Life