3 min read
Forty-eight light-years away, a world a little bigger than Earth is clinging onto its air, and for the first time ever, astronomers have caught a rocky planet doing it in the one place that actually matters. LHS 1140 b has become the first rocky planet in a star’s habitable zone with a confirmed atmosphere. If you have been waiting for the search for life beyond Earth to produce something that is not a shrug and a maybe, this is the closest it has come.
Why “the habitable zone” is the whole game
The habitable zone is the band of orbits around a star where it is neither so hot that water boils away nor so cold that it freezes solid: the Goldilocks region where liquid water, and therefore life as we know it, is at least possible. We have found thousands of planets. We have found atmospheres on some of them. But a confirmed atmosphere on a rocky world sitting in the habitable zone, the exact combination that could in principle host life, has never once been nailed down. Until now.
How they know
The proof is a little counter-intuitive: they found the atmosphere by watching part of it leak. A Harvard-led team detected helium escaping from the upper reaches of LHS 1140 b, and that escaping gas is the smoking gun that the planet holds a real, persistent atmosphere rather than being a bare airless rock. They caught it using the Magellan Telescope in Chile during a rare alignment where LHS 1140 b and a sibling planet crossed their star on the very same night. The result is published in Science. Crucially, the atmosphere appears to have survived for potentially billions of years, which is no small feat around a red dwarf, stars notorious for blasting their planets with radiation.
The part where I temper your excitement
Before anyone starts sketching little green men: an atmosphere is not life, and helium is not a biosignature. Detecting escaping helium tells you there is gas held by the planet’s gravity. It does not tell you what most of that atmosphere is made of, whether there is water, or whether anything is swimming in it. Those are the next, much harder questions. The team’s plan is to aim the James Webb Space Telescope at LHS 1140 b in 2027 to try to read the composition of the lower atmosphere directly. That is when it gets genuinely interesting.
But do not let the caveats flatten the moment. For decades, “a rocky planet in the habitable zone with an atmosphere” has been a hypothetical, the thing every exoplanet hunter was working toward without being able to point at a single one. As of this month, it is a real place with a name and coordinates. The search just moved from “does this kind of world even exist” to “let’s go read what its air is made of”. That is a very good week for anyone who has ever looked up.
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Did you know: red dwarfs like LHS 1140 are the most common stars in the galaxy, making up roughly three-quarters of all stars. If rocky planets can hang onto atmospheres around them despite their harsh radiation, it hugely widens the number of places worth searching for life, because there are simply so many red dwarfs to check.