A team of astronomers has identified a temperate exoplanet as a promising extraterrestrial world of ice or water. The findings, led by the University of Montreal, suggest that the habitable zone exoplanet LHS 1140 b is unlikely to be a small Neptune, a so-called gas giant (large planets composed mostly of gas), and a mini-Neptune with a thick, hydrogen-rich atmosphere. Located about 48 light-years away in the direction of the constellation China, the planet is emerging as one of the most promising known candidates for a habitable exoplanet zone, potentially containing an atmosphere and even a liquid water ocean.

Data from the James Webb Space Telescope (JWST) were collected in December 2023 and added to previous data from other Space Telescopes Spitzer, Hubble and TESS to confirm this result. Astrophysics Journal Letters this week and the preprint server is currently available on the site arXiv.

“This is the first time we’ve seen a hint of an atmosphere on a rocky or ice-rich exoplanet in the habitable zone. Detecting atmospheres on small rocky worlds is a key goal for JWST, but seeing these signals is much harder than seeing the atmospheres of giant planets,” said Ryan Macdonald, a NASA Sagan Scientist in the University’s Department of Astronomy who was instrumental in analyzing LHS 1140’s atmosphere.

“LHS 1140 b is one of the best small exoplanets in the habitable zone that could support a thick atmosphere, and we could find evidence of weather on this world.”

LHS 1140 b, a potentially habitable world

LHS 1140 b, an exoplanet orbiting a low-mass red dwarf star about one-fifth the size of the Sun, has attracted the attention of scientists because it is one of the closest exoplanets to the Solar System, located in the star’s habitable zone. Exoplanets in this “Goldilocks Zone” have temperatures that allow liquid water, a key element for life as we know it on Earth, to exist on them.

One of the critical questions about LHS 1140 b was whether it was a mini-Neptune-type exoplanet or a rocky or water-rich super-Earth larger than Earth.

“Of all the temperate exoplanets currently known, LHS 1140 b may one day be our best chance to indirectly confirm the existence of liquid water on the surface of an alien world outside our solar system,” said Charles Cadier, lead author of the paper and a doctoral student at the University of Montreal. “This will be a significant milestone in the search for potentially habitable exoplanets.”

JWST data leads to new insights

The team’s analysis of the observations strongly ruled out the mini-Neptune scenario; there is evidence that exoplanet LHS 1140 b is a super-Earth that could have a nitrogen-rich atmosphere similar to Earth’s. However, the team cautions that additional observations, in addition to those from JWST, will be needed to confirm the signs of nitrogen gas.

Estimates based on all the accumulated data suggest that LHS 1140 b is less dense than expected for a rocky planet with an Earth-like composition, suggesting that 10–20% of its mass could be water. This discovery makes LHS 1140 b an intriguing candidate for a water world; it would likely resemble a snow globe or ice planet with a potential liquid ocean in a substellar spot or region of the planet’s surface, and would always face the system’s parent star via the synchronous rotation of the planet (similar to Earth’s Moon).

An atmosphere and an ocean are possible

Macdonald conducted an atmospheric analysis of LHS 1140 b, which showed that it has a nitrogen-rich atmosphere, potentially similar to Earth’s, which is 78% nitrogen. Although this is only a preliminary result, the presence of a nitrogen-rich atmosphere suggests that the planet has a significant amount of atmosphere, creating conditions that could support liquid water.

This discovery supports the water world/snow globe scenario as the most plausible. Current models suggest that if LHS 1140 b had an Earth-like atmosphere, it would be a snow planet with an ocean about 4,000 kilometers across, equivalent to half the surface area of ​​the Atlantic Ocean. The surface temperature at the center of this alien ocean could be as high as 20 degrees Celsius.

LHS 1140 b’s potential atmosphere and conditions suitable for liquid water make it an outstanding candidate for future habitability studies. Given its location in the habitable zone and the possibility of an atmosphere capable of retaining heat and maintaining a stable climate, this planet offers a unique opportunity to study a world that could support life.

“This is our first striking look at the atmosphere on a super-Earth in the habitable zone. Compared to other known habitable zone exoplanets, such as those in the TRAPPIST-1 system, LHS 1140’s star appears quieter and less active, making it much easier to separate LHS 1140 b’s atmosphere from stellar signals caused by starspots,” MacDonald said.

“Our initial JWST study of LHS 1140 b revealed that it is perhaps the best-known habitable-zone exoplanet to date in terms of atmospheric characterization. While we need more JWST observations to confirm the nitrogen-rich atmosphere and search for other gases, this is a very promising start.”