If we couldn’t figure out what to call CD-35 2722B b, which I assume is the correct way to refer to the planet-sized object in this red dwarf / brown dwarf/ gas giant system, another planet has a bit of a definitional problem as well. LHS 1140 b is an interesting super-Earth orbiting a red dwarf in Cetus in its habitable zone. Some 49 light years out, this planet is almost 6 times Earth’s mass and boasts a radius 1.7 times larger. Receiving 42 percent of the stellar radiation that Earth does, its surface temperatures allow the presence of liquid water.
But exactly what kind of planet is this? Is it an airless, rocky world, an ocean planet, or even a mini-Neptune? We can probably rule out the latter because its mass would be low for that category, and we can also, contrary to some press reports, not consider it in any way, shape or form ‘Earth-like.’ But thanks to new work out of Harvard and the Carnegie Institution for Science, we can now declare that it does have an atmosphere.
This is a useful finding because rocky planet atmosphere detection has been conspicuously difficult. What you gain with studying M-dwarf planets is the low contrast between a relatively dim star and a planet’s light, but you’re also dealing with a category of star prone to violent flare activity. Every M-dwarf in the galaxy is a young star compared to its likely lifespan, given that these stars, perhaps 85 percent of the stellar population, have lifetimes that can reach well beyond a trillion years.
Pulling a signal from water or carbon dioxide out of spectrographic data is problematic because these volatiles species would be expected in the lower levels of a planet’s atmosphere, a tough catch for any observatory. But helium escape is another matter. In detecting helium at LHS 1140 b, the new work, demonstrates that an atmosphere is indeed present, whether this is a water world or not.
Young M-dwarfs are notorious for violent flare activity, a problem not only because flares can obscure the signal of a transiting planet, but also because they can potentially scrub nearby planets of their atmospheres entirely. In this case the flares have helped us: The escaping helium is thought to be driven by bombardment of the upper atmosphere from the star’s X-ray and extreme ultraviolet (XUV) activity.
Helium escape is especially valuable because it is observable from the ground, unlike the classic hydrogen (Lyman-alpha) escape signature. We’re seeing the residue of a process that can play out over gigayears: lighter hydrogen escapes preferentially over time — a loss we can’t easily observe directly — leaving behind a helium-enriched upper atmosphere that we can detect.
The paper may even tell us something about the ‘cosmic shoreline,’ the dividing line between an orbit where an atmosphere can be sustained and one where it cannot. Indeed, this work, conducted using the WINERED spectrograph at Las Campanas Observatory in Chile, shows that the inner world at LHS 1140 is evidently airless, with the ‘shoreline’ located between the two worlds.
From the paper:
The cosmic shoreline is a proposed boundary that separates airless rocky planets from those that retain atmospheres for billions of years. The two planets in the LHS 1140 system are on either side of the proposed cosmic shoreline. Our non-detection of helium absorption by LHS 1140 c is consistent with the previously measured dayside emission, which indicates that the planet has little to no atmosphere. Therefore, this system is consistent with the proposed position of the cosmic shoreline.
Moreover, we seem to be tracking a phenomenon that changes in short order. Co-author Shreyas Vissapragada (Carnegie Science Observatories) comments:
“After much careful analysis and consideration of the spectra, we determined that helium was escaping from LHS 1140 b’s atmosphere in 2024 due to heating from stellar X-rays and extreme ultraviolet radiation. However, our 2025 observations revealed no escaping helium, so the atmospheric escape appears to be variable. It is a rare privilege to witness the atmosphere of an extrasolar planet change on such short, human timescales!”
Image: In this artist’s rendering, the exoplanet LHS 1140 b is shown in the foreground, surrounded by a helium-rich atmosphere. Another nearby rocky planet orbits the same cool red dwarf star in the distance. This new study provides the strongest evidence yet that LHS 1140 b has retained an atmosphere. Credit: Carnegie Science.
Colin Cherubim (Harvard University), lead author of the paper in Science, points out that this is the first detection of an atmosphere on a rocky planet in the habitable zone of any star. Nice work, as Cherubim’s team had predicted precisely this mechanism in the LHS 1140 system. Escaping gases may indeed be a useful tool as we press on with more detailed investigations of planetary atmospheres on smaller, more Earth-like worlds.
The paper is Cherubim et al., “Helium escaping from the atmosphere of a nearby rocky exoplanet orbiting in a habitable zone,” Science 16 July 2026. Full text.
While “cosmic shoreline” is an evocative phrase, and rather Sagan-esque, I think it conveys teh wrong impression of a location. Presumably its location is going to depend on not just the orbit, but the surface gravity of the planet, and the amount of flaring by its star. As we have seen with the temporal change in the HZ with the term CHZ and the known position of the HZ dependent on a star’s luminosity on the main sequence, as well as the state of its atmosphere – composition and amount, the cosmic shoreline is a fuzzy location.
Shoreline is a rather fuzzy location, even on Earth, due to tides and overall sea-level due to climate and land mass distributions, and even possibly external additions of water from asteroids and comets. Some shorelines can move a km due to its location, such as the famous French island of Mont-Saint-Michel. Current sea-level rise is already changing the shoreline location in a number of low-lying places, such as on the Gulf or Mexico coasts of the USA. A relatively small amount of sea-level rise will put much of Florida below sea level. A total melting of the Antarctic and Greenland ice sheets will dramatically change the shorelines in much of the world, erasing some nations entirely.