NASA's Transiting Exoplanet Survey Satellite (TESS) mission has been very busy lately. Back in April, the satellite discovered its first Earth-sized planet, in early July it found three more in one system, and just this week another system of two sub-Neptunes and an Earth-like turned up. Now, the productive little satellite has made another important discovery – the closest super-Earth that's potentially habitable.
As astronomers stepped up the search for planets around other stars over the past decade, they noticed that our solar system is missing the most common type of planet – the so-called super-Earth. These planets are roughly defined as being up to 10 times more massive than our Earth, but are smaller than ice giants like Uranus and Neptune. And they could be perfect breeding grounds for extraterrestrial life.
And now, TESS has turned up the closest of these worlds that could be habitable. Known as GJ 357 d, the planet has at least six times the mass of Earth, and while its exact size and composition are still unknown, if it's rocky it would measure between one and two times the size of Earth.
But most importantly, the planet orbits its host star within the habitable zone. In this case that's about five times closer to the star than Earth is to the Sun, but because this star is so much dimmer than the Sun, the planet actually receives less energy than Earth does. But if GJ 357 d is rocky and has just the right kind of atmosphere, liquid water and maybe even life could exist on its surface.
"GJ 357 d is located within the outer edge of its star's habitable zone, where it receives about the same amount of stellar energy from its star as Mars does from the Sun," says Diana Kossakowski co-author of a study describing the find. "If the planet has a dense atmosphere, which will take future studies to determine, it could trap enough heat to warm the planet and allow liquid water on its surface."
GJ 357 d isn't alone in its star system – it has two sibling planets. GJ 357 b is about 22 percent bigger than Earth and orbits 11 times closer to the star than Mercury does to the Sun. That makes it a scorched world broiling away at an estimated 254° C (490° F). The middle planet, GJ 357 c, orbits at about twice the distance of the innermost world, and is thought to have an equilibrium temperature of around 127° C (260° F).
But what makes GJ 357 d particularly special is its proximity to us. At just 31 light-years away, it's the closest super-Earth that could also be habitable. It's not the closest that's ever been found – that honor belongs to one orbiting Barnard's Star, just six light-years away. But that planet is a cold, desolate world starved of energy from its dim host star, and seems pretty inhospitable to life as we know it.
GJ 357 d is a great candidate for further observation by future telescopes to confirm if it has an atmosphere and whether it can – or maybe even does – support life.
The research was published in the journal Astronomy & Astrophysics. The video below gives a tour of the system.
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