LHS 1140b, a planet 40 light-years away in the constellation Cetus, first caught astronomers' attention when it was discovered by the MEarth Project in 2017. At the time, it was one of the closest known rocky planets in the habitable zone — the region around a star where liquid water could exist on a planet's surface. But no one knew whether it had an atmosphere.
Now, using the James Webb Space Telescope and follow-up observations from ground-based telescopes, an international team has confirmed that LHS 1140b does have an atmosphere. The detection was made by analyzing starlight that passed through the planet's atmosphere as it transited its host star, a dim red dwarf. The signature of helium escaping from the upper atmosphere provided the first direct evidence, published in Science. Additional data suggests the likely presence of nitrogen, water vapor, and carbon dioxide — the same gases that make up Earth's atmosphere.
The planet is about 1.7 times the size of Earth and roughly 5.6 times its mass, placing it in the class of "super-Earths." Its location in the habitable zone means it receives about half the starlight Earth gets from the Sun — enough to keep water liquid if the atmosphere is thick enough. Models suggest LHS 1140b may be a "snowball" planet with a vast bull's-eye ocean on the side that always faces its star, kept warm by the greenhouse effect of its atmosphere.
The finding is a major milestone. While astronomers have detected atmospheres on gas giants and hot Jupiters before, this is the first confirmed atmosphere on a rocky, potentially habitable world. Future Webb observations will aim to identify biosignature gases such as methane and oxygen, bringing us closer than ever to answering whether life exists beyond Earth.
Knowledge takeaway: LHS 1140b, a super-Earth 40 light-years away, is the first rocky planet in the habitable zone confirmed to have an atmosphere — containing nitrogen, water vapor, and CO₂ — making it the most promising target yet for the search for life beyond our solar system.