A rocky planet located at the right distance from its star to be hospitable to life likely has an atmosphere, according to a new study.
The exoplanet, known as LHS 1140 b, is about 49 light-years from Earth, meaning it is a relatively close neighbor, given that the Milky Way is more than 100,000 light-years across. The world was find in 2017 and was already considered a prime candidate in the search for planets likely to support life. Researchers have now announced the discovery of helium leaking from LHS 1140 b into space, which only makes the possibility that life could exist on the planet more likely, says Collin Cherubim, a postdoctoral researcher at Harvard University.
Cherubim is the lead author of the study detailing the new findings, which was published in Science THURSDAY. He points out that LHS 1140 b meets the three main criteria for being habitable: it is rocky, it has the right temperature to support liquid water and it is now proven to have an atmosphere.
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“It’s the only planet we know of that has all three of these elements, and it happens to be relatively close,” he says. “Another huge thing, which is really, really cool, is that it’s orbiting a relatively quiet star,” a star that’s not prone to hitting the exoplanet with coronal mass ejections and other catastrophic flares.
Atmospheric signs were detected on other rocky exoplanets, but these discoveries have been relatively rare and not completely definitive, given that there are likely millions, if not billions, of such planets in the Milky Way.
The presence of the atmosphere is considered essential to a planet’s ability to support life, both because of what it can contain and what it can keep out. While protecting a planet’s surface from radiation, an atmosphere can also trap water on that surface. That makes detecting helium escaping from LHS 1140 b “an important milestone,” says Laura Kreidberg, an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany, who was not involved in the new study.
Cherubim had hypothesized that some small, rocky planets like LHS 1140 b might develop atmospheres containing large amounts of helium over time. To test this, he made a list of all known planets meeting certain criteria of mass, radius and temperature.
“LHS 1140 b was one of the relatively higher probability predictions, and it rightly received a lot of scrutiny because it’s a really exciting rocky planet that’s close to Earth, in the habitable zone,” he says. “I thought that was a good place to start.”
Using an infrared spectrograph mounted on the Magellan Clay Telescope at Las Campanas Observatory, Cherubim and his colleagues searched for wavelengths of light that would indicate the absorption of stellar radiation by helium. Helium is important. Although it is one of the most common elements in the universe, small planets with thin atmospheres, like Mars, have difficulty retaining it because it is so light that it drifts away, leaving only heavier gases like carbon dioxide. If LHS 1140 b had helium escaping into space, this would imply the presence of a thicker, multi-layered atmosphere, likely with water trapped lower down, near the surface.
As the data came in and was analyzed, it became apparent to the Cherubim that the helium was in fact being heated by the stellar x-rays and was escaping into space.
There was a surprise: while helium appeared in 2024, when the Cherubim used the telescope to examine the exoplanet again in 2025, it was gone.
“It was quite a shocking discovery,” he said. “That made me go back to the original 2024 data and think, ‘Oh my God, am I missing something?'”
The lack of helium in 2025 is the “only belly rumbling I have” regarding the study, Kreidberg says. “It would have been better if they confirmed it. But at the same time, that type of variability isn’t crazy. They’re going to follow up on it, and I hope they see it again, and I think they will. I think it would be hard to explain it any other way.”
There are several possible reasons for the divergent observations, including the reaction of the atmosphere to different temperature levels during its orbit or the change in magnetic activity in the exoplanet’s star. The most likely explanation, according to Cherubim, is that helium is only detectable at such distances when it is in an excited state.
“It might be possible that in 2025, this much helium was escaping, and we just didn’t see any of it because… the amount that was populated in that excited state was different,” he says.
The presence of escaping helium goes against some previous orthodoxies within the astronomy community, Kreidberg says. The community largely believed that a planet the size and age of LHS 1140 b would have long since lost any helium it might have had. The planet could be in a transition period in which, as it loses helium to space over time, it shrinks from its current sub-Neptune size to something that could be more Earth-like.
“I think it’s very possible that what we’re seeing is a snapshot of this evolution from the smallest gas giant to the largest rocky planet,” Kreidberg says.
The exoplanet is expected to be among those examined as part of the Rocky Worlds Director’s Discretionary Time Program, which uses the James Webb Space Telescope and the Hubble Space Telescope to search for evidence of the atmospheres of rocky planets. While this may answer many open questions about the atmosphere of LHS 1140 b, the Cherubim still have bragging rights.
“This planet is one of the targets of their small sample of rocky worlds, where they are looking for an atmosphere that they consider to be the most promising,” he explains. “Well, they thought about it. I kind of beat them to it and already answered the question.”
