An exoplanet orbiting a red dwarf star 40 light-years from Earth may be the new holder of the title “best place to look for signs of life beyond the Solar System”.
Using ESO’s HARPS instrument at La Silla, and other telescopes around the world, an international team of astronomers discovered the “super-Earth” orbiting in the habitable zone around the faint star LHS 1140. The exoplanet, named LHS 1140b, has likely retained most of its atmosphere and this, along with the fact that it passes in front of its parent star as it orbits, makes it one of the most exciting future targets for atmospheric studies.
“This is the most exciting exoplanet I’ve seen in the past decade,” said Jason Dittmann of the Harvard-Smithsonian Center for Astrophysics (Cambridge, USA), lead author of the study that will appear in the journal Nature today. “We could hardly hope for a better target to perform one of the biggest quests in science — searching for evidence of life beyond Earth.”
The astronomers estimate the age of the planet to be at least five billion years. They also deduced that it has a diameter 1.4 times larger than the Earth — almost 18 000 kilometres. But with a mass around seven times greater than the Earth, and hence a much higher density, it implies that the alien world is probably made of rock with a dense iron core.
The newly discovered super-Earth orbits around a faint red dwarf star, in the constellation of Cetus (The Whale), every 25 days. Red dwarfs are much smaller and cooler than the Sun and, although LHS 1140b is ten times closer to its star than the Earth is to the Sun, it only receives about half as much sunlight from its star as the Earth and lies in the middle of the habitable zone.
For life as we know it to exist, a planet must have liquid surface water and retain an atmosphere. When red dwarf stars are young, they are known to emit radiation that can be damaging for the atmospheres of the planets that orbit them. In this case, the planet's large size means that a magma ocean could have existed on its surface for millions of years. This seething ocean of lava could feed steam into the atmosphere long after the star has calmed to its current, steady glow, replenishing the planet with water.
Two of the European members of the team, Xavier Delfosse and Xavier Bonfils both at the CNRS and IPAG in Grenoble, France, conclude: “The LHS 1140 system might prove to be an even more important target for the future characterisation of planets in the habitable zone than Proxima b or TRAPPIST-1. This has been a remarkable year for exoplanet discoveries!”
Still, Dittmann thinks that the TRAPPIST-1 system, also located roughly 40 light years away, stands out as particularly intriguing.
“I really want to emphasize that both our system and TRAPPIST-1 are exciting and both worthy of intense future study,” he says. “LHS 1140 is brighter at optical wavelengths because it’s slightly bigger than the TRAPPIST-1 star. So, when the future 30 meter optical telescopes are built (the Giant Magellan Telecopes and European Extremely Large Telescope), LHS 1140 can feasibly be studied by these telescopes.”
Indeed, we won’t know much more about any of these planets’ habitability until we can observe their atmospheres, which will require more powerful telescopes than today’s state-of-the-art. There are already concerns about the habitability of Proxima b, which, like LHS 1140b, orbits a red dwarf. Some scientists worry that the frequent solar storms from Proxima b’s host star could strip it of its atmosphere, dashing our chances of finding biosignatures there. It’s possible this could be a concern with LHS 1140b, too.
“There’s definitely a concern that high energy radiation from M dwarfs might ‘spoil’ the habitability of their planets,” Dittmann said. “In the case of Proxima b, the star seems to be very active, flaring quite often. This is also true in the case of TRAPPIST-1. In contrast, LHS 1140 is slowly rotating (130 days), and we haven’t seen any flares from the star. We also expect—and hope to check with future data—that the star is very quiet at high energies as well. So, at least in the present day, LHS 1140b finds itself orbiting a very nice, quiet host star.”
Ultimately, time will tell. NASA’s James Webb Telescope (JWT), which is set to launch in October 2018, could provide some of the answers alien hunters astronomers are desperately seeking. Once it’s completed, JWT will be the most powerful space-based telescope ever deployed—it’ll be used to peer into the atmospheres of all of these planets and more.
Image: Constellation of Ceta