Since its launch a decade ago, NASA’s planet-hunting Kepler spacecraft has found thousands of worlds, revealing that across the galaxy, planets are more numerous than stars.
But even though Kepler is done collecting data, scientists are still finding treasures in its vaults, including 18 new, relatively small worlds. Many of these previously overlooked planets are similar to Earth in size, and one of them even lives in an orbit that could be life-friendly.
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“I’m excited, but not surprised,” says Caltech’s Jessie Christensen of the results, reported in two Astronomy and Astrophysics publications. “It was inevitable that improved searches of the data would uncover previously undetected small planets.”
I can show you the worlds
From 2009 to 2013, Kepler stared at a single patch of starry sky, watching for the footprints of planets marching across their stars’ faces. To Kepler, these planetary transits looked like a brief dimming of starlight, and from those blips in brightness, called light curves, scientists can calculate a planet’s size and orbit.
When an on-board malfunction crippled the spacecraft and it could no longer stare at that single patch of sky, Kepler scanned more of the heavens, rebirthed as the K2 mission, until the spacecraft ran out of fuel in late 2018.
At the mission’s conclusion, the Kepler team announced its official haul. For the primary Kepler mission, that was some 2,300 confirmed planets and 2,400 more candidates. The K2 mission added 500 or so planets and candidates.
It’s this second data set that attracted the attention of René Heller, of Germany’s Max Planck Institute for Solar System Research, and his colleagues.
New horizons to pursue
Heller re-analysed the K2 data using a program that would be more sensitive to finding Earth-size planets. These small worlds are harder to see because they blot out such a tiny proportion of their star’s light, and their transits can be partially obscured by other variations in their star’s brightness, including miniscule dips caused by starspots.
Astronomers look for subtle changes in a star's brightness to determine whether it has a planet in orbit. While the traditional method looked for abrupt dips, the new method is able to spot smaller worlds by looking for smoother curves in the light.
PHOTOGRAPH BY NASA/SDO (SUN), MPS/RENÉ HELLER
More generally, stars appear dimmer around their edges and brighter at their middles. So when a small planet begins to plod across a star’s face, its very first steps might not make an abrupt, noticeable dent in the already dim light at the edge.
With this in mind, Heller and his colleagues re-scanned the K2 data looking for potential planetary transits that began with a smooth dip in brightness. They focused on stars that already had at least one known planet, because additional detections in those systems are statistically much more likely to be real and not false positives.
They started with the K2 data because it would be a much faster search.
“The primary mission has more than 2,000 confirmed planets, and each light curve is 1,600 days long,” Heller says. “K2, on the other hand, has only about 500 confirmed planets, and these light curves are only 80 days long.”
In the end, the team uncovered 18 additional planets from 517 K2 light curves. All of them are small, with the largest being just a bit wider than two Earths. One of the worlds is among the tiniest Kepler has yet found; it’s just 70 percent of Earth’s width. Another orbits in the habitable zone of a red dwarf star, where the temperature might allow liquid water to remain on its surface.
Endless diamond sky
Given that 18 new planets tumbled out of the K2 data set on Heller’s initial runs, it seems likely that numerous small worlds are hiding in the rest of the Kepler data. Heller says they’ve only scanned a small fraction of the K2 data, and the team is planning to sift through primary Kepler data next.
The 18 newly discovered planets, seen in this illustration in orange and green, are all smaller than Neptune, with three even smaller than Earth. The green planet, dubbed EPIC 201238110.02, is the only one in the new haul that might be friendly to life.
PHOTOGRAPH BY NASA/JPL (NEPTUNE), NASA/NOAA/GSFC/SUOMI NPP/VIIRS/NORMAN KURING (EARTH), MPS/RENÉ HELLER
“I already can tell you that there will be many more Earth-size planets that people hadn’t seen before,” Heller says, estimating that at least a hundred small planets are hiding in the primary mission data.
Christiansen concurs, noting that the team’s planet-finding methods are solid.
“I’m sure there are even more that will be found, as well,” Christiansen says. “This is the power of well-archived and well-documented public data sets from NASA. People will keep discovering planets, even in the original Kepler data!”