18 Earth-sized planets found in our galaxy, all hidden from view of all

Since its launch ten years ago, the Kepler probe, in search of planets in the planet, has discovered thousands of worlds, revealing that, throughout the galaxy, the planets outnumber the stars.

But even if Kepler has finished collecting data, scientists still find treasures in their coffers, including 18 new, relatively small worlds. Many of these previously neglected planets are similar in size to Earth and one of them lives even in an orbit that could be conducive to life.

"I'm excited but not surprised," said Jessie Christensen of Caltech about the results, reported in two Astronomy and astrophysics publications. "It was inevitable that better research in the data would discover small planets hitherto undetected."

From 2009 to 2013, Kepler observed only one part of the starry sky, watching the footprints of planets parading on the faces of their stars. For Kepler, these planetary transits resembled a brief attenuation of starlight, and from these luminous flashes called light curves, scientists can calculate the size and orbit of a planet.

When a malfunction on board paralyzed the spacecraft and that he could no longer stare at that part of the sky, Kepler swept a larger part of the sky, reborn under the K2 mission name, until he reached the end of his flight. the spacecraft runs out of fuel by the end of 2018.

At the end of the mission, the Kepler team announced its official mission. For the Kepler main mission, there were 2,300 confirmed planets and 2,400 additional candidates. The K2 mission has added about 500 planets and candidates.

It was this second set of data that caught the attention of René Heller of the Max Planck Institute for Solar System Research and his colleagues.

Heller re-analyzed K2 data using a program that would be more sensitive to finding planets the size of the Earth. These small worlds are harder to see because they hide a tiny portion of their star's light, and their transits may be partially obscured by other variations in their star's brightness, including tiny hollows caused by starry spots.

More generally, stars appear paler on their edges and brighter in their middle. Thus, when a small planet begins to sneak past the face of a star, its very first steps do not necessarily lead to a sudden deterioration of the already dimmed light at the edge.

Keeping this in mind, Heller and his colleagues re-analyzed K2 data in search of potential planetary transits beginning with a gradual decline in brightness. They focused on the stars that already had at least one known planet, because additional detections in these systems are statistically much more likely to be real and not false.

They started with K2 data because the search would be much faster.

"The main mission has more than 2,000 confirmed planets and each light curve lasts 1,600 days," said Heller. "K2, on the other hand, has only 500 confirmed planets, and these curves of light only last 80 days."

In the end, the team discovered 18 additional planets out of 517 K2 light curves. All are small, the largest being just a little wider than two lands. One of the worlds is among the most tiny Kepler has yet found; it's only 70% of the width of the Earth. Another orbit in the habitable zone of a red dwarf star, where the temperature could allow liquid water to remain on its surface.

Given that 18 new planets fell from the K2 dataset during Heller's first trials, it seems likely that many small worlds are hiding in the rest of Kepler's data. Heller says they have scanned only a small fraction of the K2 data and that the team plans to sift through Kepler's key data.

"I can already tell you that there will be many more Earth-sized planets than people have ever seen before," says Heller, estimating that at least a hundred small planets are hiding in the data. of the main mission.

Christiansen agrees, noting that the team's research methods are solid.

"I'm sure there are even more to be found," Christiansen said. "It's the power of well-archived and well-researched public data sets from NASA. People will continue to discover planets, even in Kepler's original data! "