Scientists have now detected the first evidence of an atmosphere on a rocky terrestrial exoplanet that is also located in the habitable zone. The exoplanet, dubbed LHS 1140 b, is located circling a red dwarf star about 48 light years away and is estimated to have about 5.6 times the mass of Earth and about 2.4 times the diameter.

To test their prediction, the team used the Warm Infrared Echelle (WINERED) Spectrograph on the Magellan Telescope at Las Campanas Observatory in Chile. They observed a rare alignment, where LHS 1140 b and another planet [in the system] transited their star on the same night.

Although one planet showed no evidence of an atmosphere, the other, LHS 1140 b, showed helium escaping from around it, confirming that it retains an atmosphere.

You can read their paper here [pdf]. The discovery is of significance, as it proves finally — more than two decades after the first detection of an exoplanet — that rocky exoplanets outside our solar system can have an atmosphere. There is also evidence that this loss of helium might be seasonal in nature. From their abstract:

Helium absorption is detected in 2024 but not in 2025, indicating time-variable atmospheric escape. We interpret these results as indicating an upper atmosphere dominated by helium and depleted in hydrogen, with other volatile species trapped at lower altitudes, consistent with atmospheric fractionation models. No helium absorption is detected for LHS 1140c, a smaller and more strongly irradiated
exoplanet in the same system.

The planet has an orbit 24.7 days, and is located in the habitable zone, though overall it gets only 42% of the radiation from its star compared to Earth. Thus, it is very cold on this planet, with a gravity significant heavier than Earth’s. In other words, even if it has an atmosphere, it remains a very alien place.