TESS Discovers Its First Nearby Super-Earth That Could Be Habitable

Artist's impression of exoplanet GJ 357 d and its sibling planets orbiting GJ 357. Jack Madden/Cornell University

Early this year, NASA’s Transiting Exoplanet Survey Satellite (TESS) discovered its first nearby super-Earth and researchers have evidence suggesting that two more similar-sized planets are orbiting the same star, and one of them might have the conditions for life.

As reported in Astronomy & Astrophysics, the system, called GJ 357, is located 31 light-years away, just down the road in galactic terms. It's composed of an M-dwarf star, roughly one-third the size of our Sun, and three planets. In order of both size and distance from their star, these planets are known as GJ 357 b, GJ 357 c, and GJ 357 d. The latter could be up to seven times heavier than Earth.

Planets c and d were not observed directly by TESS but their presence was gleaned by means of other properties of the system. GJ 357 d is particularly interesting because it receives roughly 40 percent of the stellar radiation Earth gets from the Sun, so it is the so-called habitable zone around the star.

In a second paper, accepted for publication in The Astrophysical Journal Letters, part of the team investigated the habitability of GJ 357 d and what its climate might be like. Based on their model, if it is analogous to Earth, this world could be a frozen rocky planet. But if the planet has more carbon dioxide available, then it could be temperate.

This is a big if but it is not an unlikely one. If follow-up research can observe the planet transiting, GJ 357 d would become the closest potentially habitable exoplanet to Earth. And, obviously, astronomers are very interested in this object.

"This is exciting, as this is humanity's first nearby super-Earth that could harbor life – uncovered with help from TESS, our small, mighty mission with a huge reach," lead author of the second study Professor Lisa Kaltenegger, director of Cornell's Carl Sagan Institute and a member of the TESS science team, said in a statement.

The closeness of the planet is key for future detailed observations. We may be able to characterize its atmosphere with high precision and that might mean seeing signatures that hint at the presence of life.

While the discovery of the system is certainly thanks to TESS, the researchers also looked at about two decades' worth of observations on the star. Using that data, they found subtle gravitational tugs on the star that are caused by all three planets, and couldn’t have been produced by GJ 357 b alone.

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