For the first time, astronomers have detected an atmosphere on a rocky, temperate world beyond our solar system. It is the strongest evidence yet for an exoplanet that could be able to support alien life.
The planet in question, called LHS 1140b, orbits a small, cool type of star called an M dwarf, about 50 light years from Earth. Researchers have been searching for atmosphere-shrouded planets around M dwarfs for decades because they are the most common type of star in the galaxy, but with no luck until now – some researchers had even postulated that planets around these small stars might not be able to hold onto atmospheres.
But when at Harvard University and his colleagues observed LHS 1140b in 2024 at the Las Campanas Observatory in Chile, they found a promising signal: a line in the spectrum of the planet’s light indicating helium atoms in an excited energy state. There are several possible explanations for this, but the shape and strength of the signal led them to conclude that the helium was part of an outflow, leaking away from a planetary atmosphere. “By far the most simple explanation is that this helium that we see is from the planet itself,” says Cherubim.
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This would put LHS 1140b in the category of helium worlds, a type of exoplanet that has been proposed but not definitively detected. “It’s different from what we would expect given the rocky planets in our own solar system,” says at Johns Hopkins University in Maryland. In some ways, a helium world could be more similar to a gas giant than any of the terrestrial worlds in our solar system, with a thick, heavy layer of atmosphere overlaying a rocky or icy surface.
“It’s exciting on its own merits, not as a new Earth,” says at the University of Arizona. “It might be habitable, but not in a conventional sense.” If it does have a thick atmosphere, pressures at the surface could be similar to those at the bottom of the ocean. If not, it could potentially be an ocean world more similar to icy moons in our solar system than any of the planets.
Right now, we only know that this world has an atmosphere that is leaking helium, but we know very little about the size and composition of the atmosphere itself. “Earth and Venus are very similar in size and both have atmospheres, but Venus’s surface isn’t habitable,” says Allen. “I think to have a real picture of habitability on this planet, we need to understand the bulk atmosphere.” As-yet-unpublished data from the James Webb Space Telescope could help clear that up, she says.
However, the very presence of an atmosphere, along with the fact that LHS 1140b orbits its star in what is known as the habitable zone, where temperatures are mild enough to preserve liquid water on its surface, makes it extremely promising. “This planet has now become the best place to search for life outside of our solar system,” says Cherubim.
There is a caveat, though. When Cherubim and his colleagues observed LHS 1140b again with the same setup in 2025, they didn’t see the signature helium line. They pored over the 2024 and 2025 data, consulted with other researchers and even had a separate team reanalyse all the data, and the results remained the same: the excited helium was there in 2024, and not in 2025.
That was surprising, but not altogether abnormal, says Cherubim. “When you make a measurement of escaping helium and then you go back and look at it months and years later, it is usually different and sometimes it isn’t there – and we don’t understand it,” he says.
This particular helium signature is poorly understood, says Ranjan, so we can’t be sure why it might have appeared and then disappeared. It is possible that the shape of the outflow changed, or that shifted the nature of the helium in the outflow. If the helium is still there but not in an excited state, we would expect the signature to disappear. Cherubim has plans to continue observing LHS 1140b in an attempt to figure it out: “Maybe we’ll never see it again, maybe we’ll see it every time, maybe we’ll see it half the time, but I’m feeling really good about it,” he says.
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