Astronomers have detected evidence of an atmosphere on LHS 1140 b, a rocky exoplanet 40 light-years away.
We’ve seen atmospheres on Neptune-like worlds, but never on any that were similar to Earth. LHS 1140 b is a rocky entity with 5.6 times the mass of Earth and 1.7 times its radius, orbiting a red dwarf. The data suggests we aren’t looking at a barren rock or a gas ball, but something in the transition: a hybrid between a water-world and a giant similar to Uranus and Neptune.
The planet receives about 42% of the stellar radiation Earth does. This has allowed scientists to determine that its temperature is suitable for it to possess liquid water, although it is still unknown if planets of this size have surfaces like Earth’s.
The Helium Signal and the 2025 Disappearance
The breakthrough came from a disappearing act. In 2024, the spectrograph at the Magellan Clay telescope in Chile captured a clear signature of escaping helium. In astrophysics, a spectrum is a way of studying the characteristics of a celestial object, including its composition, speed, and movement; by decomposing the light of the host star as it passes through the planet’s atmosphere, scientists can determine which elements are present.
Then, in 2025, the helium signal vanished.
By combining these observations from previous years with sophisticated models of exoplanet evolution, the team concluded that LHS 1140 b possesses a stratified, multi-layered atmosphere. The helium exists in the upper atmosphere, while other chemical species, such as water, are trapped at lower altitudes, closer to the surface.
This suggests a planetary architecture that can protect a surface from the radiation of a red dwarf.
Water-World vs. Ice Giant: The Compositional Divide
LHS 1140 b has a composition similar to that of Earth. The core question now is whether it’s a rocky planet with a thin skin of water or a global ocean hundreds of kilometers deep.
- The Ocean World Theory: A planet completely covered in water, with oceans hundreds of kilometers deep.
- The Terrestrial Theory: A rocky surface with localized oceans, similar to Earth’s geography.
- The Ice Giant Baseline: Worlds like Uranus or Neptune, dominated by hydrogen.
José Antonio Caballero, an expert in star and planet evolution at the Centro español de Astrobiología (CAB CSIC), describes the planet as a midpoint between an oceanic world and a hydrogen-based ice giant. This distinction is critical because it determines the habitability of the planet.
The Contrast of the LHS 1140 System
The system provides its own control group. Researchers also analyzed LHS 1140 c, a smaller planet in the same neighborhood. Unlike its larger sibling, LHS 1140 c is subjected to greater irradiation and shows no evidence of an atmosphere.
The technical precision required for this find is staggering. This is the result of a large group of researchers from universities in the United States, reported in the journal Science, to validate a discovery that provides the most solid evidence to date of the existence of worlds with conditions of composition and temperature similar to those of Earth to harbor life beyond the solar system.
The Long Game for Exoplanetary Science
As Caballero notes, this is the first detection of an atmosphere on a planet similar to Earth, but the infrastructure is now in place to find many more. The transition from detecting atmospheres on worlds similar to Uranus and Neptune to worlds similar to Earth is the astronomical equivalent of a match where, after the first goal, many more will come.
Now that we know an atmosphere exists, the hunt shifts to other chemical species.
For now, the data stands: LHS 1140 b is a rocky world, it has a complex, layered atmosphere, and it likely harbors liquid water. It is the most solid evidence to date of the existence of worlds with conditions of composition and temperature similar to those of Earth to harbor life beyond the solar system.
