Astronomers have found the most compelling evidence yet that a rocky world beyond our solar system has a real atmosphere — and it sits squarely in its star's habitable zone.

The planet is LHS 1140b, a rocky world roughly 48 light-years from Earth, discovered in 2017 and orbiting a red dwarf star. In a paper published July 17 in the journal Science, researchers report they detected helium escaping from the planet's outer atmosphere — the clearest sign yet that small, rocky exoplanets can hold onto substantial atmospheres.

For astronomers hunting for potentially habitable worlds, this is a major milestone.

"It's been a major goal in the field of exoplanets to find atmospheres on rocky exoplanets," said Collin Cherubim, a planetary scientist at Harvard University and lead author of the study. "For Earth-like life to exist, an atmosphere, liquid water, and a rocky surface are required — and LHS 1140b could have all three."

The research team used the Magellan Clay telescope at Las Campanas Observatory in Chile, observing LHS 1140b during two separate windows — once in 2024 and once in 2025 — each time watching for six and a half hours as the planet passed in front of its star. Using near-infrared spectroscopy, they detected a strong signature of helium escaping from the planet's outer atmosphere.

Helium escaping from an atmosphere is a telltale sign: the planet has a substantial envelope of gas holding it together. Without an atmosphere, there is nothing to escape. The detection was clear and consistent across both observation sessions.

This matters most because LHS 1140b sits in the habitable zone — the orbital sweet spot where a planet could sustain liquid water on its surface given the right atmospheric conditions. Many astronomers already considered LHS 1140b one of the best candidates for biosignature searches among all known exoplanets. Now it has cleared a major bar.

The composition of the inner atmosphere remains unknown. Cherubim's models suggest it may contain water, carbon dioxide, and other small oxidized molecules — ingredients associated with temperate, potentially life-friendly conditions. But that's still speculative; the Magellan data speaks to helium escape in the upper atmosphere, not what sits below.

That mystery is exactly what comes next. Future observations — particularly with the James Webb Space Telescope — may be able to probe deeper into LHS 1140b's atmosphere and look for signatures of water vapor, carbon dioxide, or even biologically relevant molecules.

"This is an amazing missing piece of the puzzle as to whether rocky exoplanets can have atmospheres," said Sara Seager, an astrophysicist at MIT who was not involved in the study.

The result also strengthens a broader argument in exoplanet science. Small, rocky worlds orbiting red dwarf stars — once considered poor candidates for life because of intense stellar flares — can hold onto atmospheres long enough to be scientifically interesting, and perhaps biologically relevant.

Until recently, the evidence for rocky exoplanet atmospheres was nearly nonexistent. Most planets studied in this size range turned out to be airless rocks or covered in thick, inhospitable hydrogen envelopes. LHS 1140b has been the subject of persistent attention because it checks multiple boxes: rocky, temperate, large enough to hold an atmosphere, and close enough to Earth to observe in detail.

This detection does not prove LHS 1140b has life. But it confirms the planet has the structural conditions to support it — a rocky body with an atmosphere in a region where water could flow. That's a threshold no other rocky exoplanet in a habitable zone has definitively cleared before.

The universe is filled with planets. Finding the right kind is a different matter altogether. LHS 1140b just moved to the top of the list.