It seems as though we’ve been finding exoplanets left and right—from Proxima b to the TRAPPIST-1 system’s multiple Earth-sized planets—but what we’ve really been looking for is a planet with an atmosphere. It’s likely that Proxima b’s atmosphere has been stripped away by its host star, and we’re not quite sure of what’s happening with the TRAPPIST-1 planets yet.
But now, it looks as though we’ve discovered a planet with an atmosphere. Just 40 light years away, the rocky planet LHS 1140b orbits the red giant star LHS 1140, and it appears to retain its atmosphere.
Our sun, a yellow dwarf, is much hotter and smaller than red dwarf LHS 1140, which makes it okay that LHS 1140b is much close to its star than Earth is to the Sun. In fact, it’s better than okay—LHS 1140b is smack dab in the middle of the habitable zone, which is the distance from the star a planet needs to be in order to potentially have liquid water on its surface. Too close, and the water will boil off; too far, and it will freeze.
The planet’s discovery was initially made by Harvard’s MEarth facility, and was confirmed by ESO’s HARPS instrument (High Accuracy Radial velocity Planet Searcher)—observations with HARPS also helped determine LHS1140b’s orbital period, as well as the exoplanet’s mass and density. Planned observations with the NASA/ESA Hubble Space Telescope and studies further into the future with new telescopes (like ESO’s Extremely Large Telescope, planned to operate in 2024) may make more detailed observations of the planet’s atmosphere, if indeed one exists.
An atmosphere, or the layer of gases surrounding a planet, is important in our exoplanet search because it’s crucial for finding extraterrestrial life. We’re not going to find recognizable alien life on a planet that doesn’t have an atmosphere. But the presence of an atmosphere doesn’t automatically mean there will be either liquid water or alien life. Venus has an incredibly thick atmosphere that traps poisonous and corrosive gases with it. Mars also has an atmosphere, albeit a thin one. The key with LHS 1140b is the combination of a possible atmosphere and the planet’s size—a “super-Earth”, slightly larger and more massive than our own—plus the distance from its star. This makes it one of the likeliest exoplanets we’ve found so far to potentially support life.
It’s important to note that LHS 1140b has not always been this distance from its star; LHS 1140 is a red giant in the later stages of its life. Once a main sequence star has consumed all of its hydrogen fuel, it begins to contract, which brings an additional infusion of hydrogen. This sudden addition of more hydrogen increases the reaction rate, making the star more luminous and causing it to expand greatly into a red giant. The planet LHS 1140b was once much further from its star; there’s no telling what surface conditions were like before the star’s expansion, nor what the violent transformation of the star into a red giant did to the planet. It’s possible that the red giant stripped away any water that was once on the surface of this planet.
Whatever the state of LHS 1140b, one thing is clear: it’s a really exciting time to be a planet hunter.
Swapna Krishna is a freelance writer, editor, and giant space and sci-fi geek. You can find her on Twitter at @skrishna.