Ann Arbor Times

A team of astronomers, including a University of Michigan researcher, has identified the exoplanet LHS 1140 b as a promising super-Earth ice or water world. The findings, led by Université de Montréal, suggest that this habitable zone exoplanet is unlikely to be a mini-Neptune—a small gas giant with a thick hydrogen-rich atmosphere. Located approximately 48 light-years away in the constellation Cetus, LHS 1140 b emerges as one of the most promising candidates for harboring an atmosphere and potentially even a liquid water ocean.

Data from the James Webb Space Telescope (JWST), collected in December 2023 and supplemented by previous data from other space telescopes such as Spitzer, Hubble, and TESS, have solidified these results. The study has been accepted for publication in The Astrophysical Journal Letters.

“This is the first time we have ever seen a hint of an atmosphere on a habitable zone rocky or ice-rich exoplanet. Detecting atmospheres on small, rocky worlds is a major goal for JWST, but these signals are much harder to see than for giant planet atmospheres,” said Ryan MacDonald, NASA Sagan Fellow in the U-M Department of Astronomy who was key in analyzing LHS 1140 b’s atmosphere. “LHS 1140 b is one of the best small exoplanets in the habitable zone capable of supporting a thick atmosphere, and we might just have found evidence of air on this world.”

LHS 1140 b orbits a low-mass red dwarf star roughly one-fifth the size of the sun and lies within its star’s habitable zone—an area where temperatures could allow water to exist in liquid form. This proximity makes it one of the closest known exoplanets to our solar system that could potentially support life.

“One critical question about LHS 1140 b was whether it is a mini-Neptune type exoplanet or a super-Earth—a rocky or water-rich planet larger than Earth,” said Charles Cadieux, lead author of the study and doctoral student at Université de Montréal. “Of all currently known temperate exoplanets, LHS 1140 b could well be our best bet to one day indirectly confirm liquid water on the surface of an alien world beyond our solar system.”

The team's analysis strongly excludes the mini-Neptune scenario and suggests that LHS 1140 b may have a nitrogen-rich atmosphere similar to Earth's. However, additional observations with JWST will be necessary to confirm this hypothesis.

Estimates based on accumulated data indicate that LHS 1140 b is less dense than expected for a rocky planet with an Earth-like composition. This suggests that up to 20% of its mass may consist of water. Consequently, LHS 1140 b could resemble an ice planet with potential liquid oceans at its sub-stellar point—the area always facing its host star due to synchronous rotation.

MacDonald conducted atmospheric retrieval analysis indicating that LHS 1140 b might possess an atmosphere rich in nitrogen—similar to Earth's which contains about 78% nitrogen. Although still tentative, this finding suggests that LHS 1140 b has retained substantial atmospheric conditions favorable for liquid water.

Current models predict that if LHS 1140 b has an Earth-like atmosphere, it would be akin to a snowball planet with an ocean around four thousand kilometers in diameter—half the surface area of the Atlantic Ocean—with central temperatures possibly reaching comfortable levels around twenty degrees Celsius.

LHS 1140 b's potential atmosphere and conditions conducive to liquid water make it an exceptional candidate for future habitability studies. This discovery offers unique opportunities for studying worlds capable of supporting life given their position within habitable zones and likely atmospheric retention.

“This is our first tantalizing glimpse of an atmosphere on a super-Earth in the habitable zone," MacDonald added. "Compared to other known habitable zone exoplanets like those in TRAPPIST-1 system, star LHS 1140 appears calmer and less active making it easier to distinguish its planetary signals from stellar noise caused by starspots."

He concluded: “Our initial reconnaissance with JWST has revealed this perhaps as the best-known habitable zone exoplanet currently available for atmospheric characterization. While more JWST observations are needed to confirm its nitrogen-rich atmosphere and search for other gases; this marks a very promising start.”