Astronomers have discovered more than 6000 planets orbiting other stars—many of them small and rocky like Earth. But despite years of searching, firm evidence that such worlds have stable atmospheres has remained elusive. Now, researchers report in Science that the relatively nearby super-Earth LHS 1140b appears to be leaking helium gas into space, implying it still has a substantial atmosphere billions of years after its birth.
“This is an extraordinary result if confirmed,” says astronomer René Doyon of the University of Montréal, who was not involved in the work. “The signal is definitely there.”
The search for alien atmospheres, especially around temperate planets where liquid water can exist, is key to finding planets that could support life. Astronomers have found plenty of gas giant exoplanets, similar to Jupiter or Saturn, but in addition to lacking a rocky surface, many orbit so close to their stars that they are scorchingly hot, and thus uninhabitable.
Detecting atmospheres around small, rocky planets is especially difficult. The bright light from the planet’s parent star makes spotting the world hard enough. Detecting the far fainter signal of its atmosphere is harder still. To make things easier, astronomers have focused on small dim stars known as M dwarfs which, luckily, seem to have plenty of rocky planets around them. In many studies, scientists look for distinctive frequencies of light absorbed by atmospheric gases when planets pass in front of their home star.
But astronomers have not turned up anything convincing so far. For example, the seven Earth-size planets orbiting an M dwarf star known as TRAPPIST-1, just 41 light-years from Earth, have so far displayed no signs of atmospheres. That’s led many to worry that M dwarfs, which are often turbulent when young, blast away the atmospheres of their planets.
To settle the question, JWST’s managers have devoted 500 hours of telescope time to determining whether any small rocky planets around M dwarfs have atmospheres. They’re targeting nine exoplanets around various stars using an indirect method to detect the presence of atmospheres. JWST measures the temperature of the star-facing side of the planet. If this dayside is as hot as expected, the planet is probably a bare rock. But a cooler than expected dayside suggests winds are whisking heat to the nightside of the planet, implying an atmosphere.
The campaign recently released data from its first target, the Earth-size planet GJ 3929b, 52 light-years away. Initial analysis of those data, posted on the preprint server arXiv last month, reveals a world that shows every sign of being a bare rock. Still, the authors can’t exclude the possibility of a thin atmosphere.
The team publishing today took a different tack. Leader Collin Cherubim, a planetary scientist at Harvard University, had built a computer model of how exoplanetary atmospheres evolve over time. In particular, the model looked at how lighter elements, such as hydrogen and helium, would tend to drift up to the planet’s outer atmosphere and be the first to be boiled off by the heat and outbursts of the host star. As a result, heavier gases such as nitrogen and oxygen—held more tightly by the planet’s gravity—become enriched lower down.
Cherubim applied the model to LHS 1140b, a well-studied planet 49 light-years away whose density is slightly lower than Earth’s. That means it likely has a substantial atmosphere or is wrapped in water or ice, or a mixture of all three. Earlier studies of the world firmly ruled out a dense hydrogen atmosphere, although inconclusive hints of heavier gases such as nitrogen were found. Cherubim’s model predicted that LHS 1140b would have an upper atmosphere dominated by helium. “I was just following the numbers,” he says. “My model predicted escaping helium and a lot of it.”
To find that helium, the team used the 6.5-meter Magellan Telescopes in Chile, which recently had a new high-resolution spectrometer installed. “It was sensitive to this helium line in the near infrared, so it was kind of perfect,” Cherubim says. In 2024, the team observed the LHS 1140 system on a night when LHS 1140b and another smaller and closer in planet, LHS 1140c, both passed in front of their star. When the larger planet transited, the researchers saw starlight dropping in brightness at a wavelength known to be absorbed by helium. When the smaller planet crossed, they saw no such signal.
The team repeated the observation the following year and did not see the same signal, suggesting the helium loss varies with time. Doyon points out that M dwarf stars sometimes show the same helium absorption signal because the gas exists in their own atmospheres. But he says it would be “quite a coincidence” for variable helium in the stellar atmosphere to mimic a planetary signal precisely during a transit.
The variability is still a puzzle, Doyon says. “This begs for more observations by other instruments and independent analysis to be absolutely sure.”
Many teams are zooming in on LHS 1140b, because of its many charms. Besides being nearby, it could be hiding other gases in its lower atmosphere, such as nitrogen and carbon dioxide—much like Earth. And if those gases provide an element of greenhouse warming, it could possess a liquid water ocean warm enough to swim in. “I expect many, many telescopes to take a look at it,” Doyon says.