LHS 1140b becomes first rocky exoplanet in habitable zone with atmosphere

LHS 1140b becomes first rocky exoplanet in habitable zone with atmosphere

Astronomers have detected an atmosphere surrounding LHS 1140b, a rocky exoplanet orbiting within the habitable zone of a distant star. The discovery, detailed in a study published July 16 in the journal Science, represents the first time an atmosphere has been found on a rocky planet in the "Goldilocks zone" of another star.

Located approximately 48 to 49 light-years from Earth in the constellation Cetus, LHS 1140b orbits a red dwarf star. Red dwarfs are the most common type of star in the galaxy, though they are typically smaller and cooler than the sun. The star LHS 1140 is five times smaller than the sun and about half a percent as bright, with surface temperatures ranging from 3,100°F to 5,800°F.

For a planet to be considered potentially habitable, it must meet three primary criteria: a rocky composition, temperatures capable of sustaining liquid water, and an atmosphere to stabilize surface temperatures and shield the surface from ionizing radiation. While hundreds of planets have been found in habitable zones, and a few dozen of those are small and rocky, LHS 1140b is the first to check all three boxes.

"This is the first time anyone has found an atmosphere on a rocky planet in the habitable zone of another star."

Collin Cherubim, lead author, via statement

A New Class of "Helium World"

The discovery was driven by computer models developed by Dr. Collin Cherubim, a PhD graduate from Harvard University. These models simulate the evolution of exoplanet atmospheres over billions of years and predicted that LHS 1140b would possess a helium-dominated atmosphere that is slowly escaping into space.

To verify this, researchers used the transit method, observing the planet with the Magellan Clay telescope at Las Campanas Observatory in Chile. As the planet passed in front of its star, the team identified absorption markers showing that starlight was being filtered through a helium-rich upper atmosphere. This "helium bleed" served as a detectable marker for the more complex atmosphere beneath.

Cherubim suggests this finding identifies a new class of planet known as a helium world. While the identified helium in the upper atmosphere is depleted of hydrogen and cannot support life on its own, the study indicates that other gases may exist at lower altitudes. Theoretical predictions from Cherubim's models suggest the lower atmosphere may contain water, carbon dioxide, carbon monoxide, and small amounts of molecular oxygen.

Challenges to Habitability

Despite the breakthrough, LHS 1140b differs significantly from Earth. The planet is roughly 70% larger in diameter than Earth and 5.6 times its mass, resulting in gravity that is nearly twice as strong. It is also tidally locked, meaning one side faces the star in permanent daytime while the other remains in permanent night.

The environment is also shaped by the volatility of M-class dwarf stars. These stars often emit high-energy X-ray and extreme UV radiation that typically strips the atmospheres from rocky planets early in their life cycles. However, LHS 1140 is described as a quiet one, bathing the planet in only 10 times the X-ray energy that Earth receives. For comparison, Proxima Centauri b receives up to 400 times the X-ray radiation Earth does.

Researchers are not yet certain if the planet possesses a rocky surface or is entirely covered by an ocean, though climate models predict the presence of significant water.

The Broader Search for Life

The discovery follows a long history of exoplanet research that began in 1992. Astronomers have now identified more than 6,000 worlds orbiting distant stars, including over 6,200 according to other records.

Other candidates for habitability remain under scrutiny, though with mixed results:

• K2-18b: A sub-Neptune where scientists previously reported tentative evidence of dimethyl sulfide. However, a Nasa-led reanalysis in 2025 found the signal too weak to confirm and suggested the gas can form without biology.
• TRAPPIST-1 system: This system contains seven Earth-sized planets. NASA's James Webb Space Telescope has ruled out an Earth-like atmosphere on TRAPPIST-1d, while data for TRAPPIST-1e remain inconclusive.
• Mars: Researchers still consider Mars the best chance for discovering life due to possible biosignatures already identified in its soil.

Robin Wordsworth, a professor of Earth and planetary sciences at Harvard, noted that while the existence of terrestrial-type planets is now known to be common, the primary question was whether they could retain an atmosphere. According to Wordsworth, the discovery of LHS 1140b confirms that at least one has.