Space News Deep Dive: The Atmosphere of LHS 1140b

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Well isn’t this an interesting development? At long last, over thirty years after the discovery of the first exoplanets, we have finally reached a critical discovery milestone: confirmation of an atmosphere around a rocky habitable zone planet. Interesting.

I will admit that the fact that it’s this particular planet, LHS 1140b, that became the first to gift us with such a discovery is not necessarily a shock. There’s a lot of reasons why we suspected this planet could, in fact, have an atmosphere, and reasons why this planet was in such an advantageous position to be the one to show its atmosphere off to us. 

So let’s get to know this planet and its star, what we just found, and why it’s all so very interesting.

The Basics

Okay first some basic background about the planet and its solar system. The system is located just under 49 light years away from us in the direction of the constellation Cetus. In astronomical terms, that makes it basically a next-door neighbor. The star, LHS 1140, is a wee red dwarf with less than a fifth of the Sun’s mass.

We currently know of two planets orbiting this tiny star. The one we’re particularly interested in is LHS 1140b, which was discovered in 2017, but there’s a second world, LHS 1140c. This second planet is a super-Earth just under 2 Earth masses tucked in very close to the star, orbiting in less than four Earth days. It was discovered in 2018 and thus far appears to be an airless rock.

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The planet we’re focusing on, LHS 1140b, is also a super-Earth, a rocky planet estimated to weigh in at about 5.6 Earth masses. It’s also pretty close to the star, orbiting once every 25 or so Earth days. That may not sound like it’s in the habitable zone, but you have to remember that the star is a red dwarf, significantly cooler than the Sun. For things to be warm enough to be considered habitable with such a star, you actually have to get pretty close in (not as close as LHS 1140c though. There’s such a thing as too close even to a red dwarf). 

Newsmaker

Being less than 50 light years away from us makes the LHS 1140 system an easy one for us to study, and we’ve had our eye on it for several years now. And there’s been plenty of evidence along the way to suggest that LHS 1140b was a world worth watching.

It has actually been featured in the Spacing Out newsletter twice before (well, three times, but one of those was my top space stories of 2024 edition, so it doesn’t really count). The first time was in January 2024, when an analysis of all available observations of this world proved that LHS 1140b was not dense enough to be pure rock.

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That left two possibilities. Either this planet has a very extensive atmosphere (making it essentially a min-Neptune) that’s full of extremely light elements like hydrogen or helium, or it has a lot of water on its surface, much more than Earth. At the time it was hoped that observations by the Webb Telescope might help narrow things down. 

And observing this world with Webb was far too great a temptation to pass up. Webb has something called “director’s discretionary time” which is telescope time unassigned to any particular project or group, which can be allotted to things like transients that need a quick response or high priority targets worth jumping the queue for. LHS 1140b was deemed just such a target and by July 2024 I was reporting on the results of Webb’s analysis of this planet, and things remained very interesting.

The first thing Webb found was that this world is not a mini-Neptune but a super-Earth—a rocky world. It also found a hint (only a hint, mind you, nothing confirmed) of a nitrogen atmosphere. Based on the previous estimates of the planet’s density, that suggested a rocky world rich in water of some form. It could, after all, be an ice planet.

Or.

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Or it could be a tidally locked world, one side always facing its sun. And that side, warmed by the red dwarf’s rays, could host a gigantic ocean surrounded on all sides by ice. And if there’s liquid water, well. Who knows what could possibly be swimming around in that water. Of course, for it to be able to sustain liquid on its surface, as in this model that would make the planet look like a watery eyeball, it would have to have a long-term stable atmosphere. And red dwarf stars can make that tricky.

Red dwarfs are noted for flaring dramatically in such a way that they can strip any nearby worlds of their atmospheres. TRAPPIST-1, for instance, famous for its system of seven planets, is very active and at least the first several of its planets have been confirmed to be airless rocks. LHS 1140, intriguingly, does not display quite such hostile behavior and seems surprisingly chill for a red dwarf. Still, such a star could make keeping an atmosphere difficult at best.

Squeaky Voice World

But apparently LHS 1140b did just that. Because now we know it definitely has an atmosphere. The team behind this latest discovery actually adapted a technique used for observing atmospheres around gas giant planets and applied it to LHS 1140b using data from a spectrograph on the Magellan Telescope in Chile. And hey presto! The data showed a distinct, direct, undeniable gas detection—helium.

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Wait, helium?

Yeah, it turns out that at least the upper atmosphere of LHS 1140b is rich in, of all things, helium. That’s an odd thing for a rocky world’s atmosphere to be made of given that helium atoms are light and tend to be easily lost to space. And this planet is definitely losing its helium, at an estimated rate of 220 tons (which turns out to be the weight of an adult blue whale, who knew) per second. But there’s still enough there to register loud and clear in the spectrographic data.

And that represents only the uppermost part of the atmosphere. That potential nitrogen signature that Webb may have found could represent the lower levels of the air. And combined these two results pose some interesting possibilities.

Spying on the Neighbors

The helium signature proves that this planet has an atmosphere. It proves that rocky habitable zone planets around red dwarf stars can have sustained atmospheres, if the red dwarf is feeling friendly enough. That right there is a history-making discovery.

It could also be a sign that this planet is still holding on to part of a primordial atmosphere, the first gas envelope the planet accreted when it was young. The idea that planets might have a first atmosphere of light elements such as hydrogen and helium that eventually gets lost to the ether and replaced by heavier gases is a theory, and perhaps this planet and its helium-rich upper atmosphere could help prove it. That’s also a pretty big discovery.

But if the Webb signatures also hold up…if this planet has a lower atmosphere rich in things like nitrogen (which, if I may remind you, makes up nearly 80% of Earth’s atmosphere)…well, that along with the density measurements suggesting a water-laden world make the possibility of that giant ocean much, much more likely.

Nothing has been proven (yet) obviously. And even if we did find that this atmospheric rocky habitable zone planet housed an enormous ocean (which would be yet another history-making find, the first confirmed liquid water on a planet in another solar system), it doesn’t necessarily lead to the next thing, which is the planet’s ability to support life.

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We know of several places in our own solar system that have water. The outer solar system is riddled with ocean-bearing moons, most of which we think are incapable of supporting life because life needs more than liquid water. But liquid water is an awfully good first step.

So what we have is a neighboring solar system with a habitable zone world holding on to a proven atmosphere that has already hinted at the possibility of an enormous, liquid water ocean on its surface. If that doesn’t intrigue you, then my friend, you are reading the wrong kind of blog right now.

We have not heard the last of this system, not by a long shot. And who knows what we’ll be hearing about it next??