The study, led by Universite de Montreal, indicates that LHS 1140 b is unlikely to be a mini-Neptune, a type of small gas giant. Located about 48 light-years away in the constellation Cetus, this exoplanet is one of the most promising candidates in the habitable zone, potentially featuring an atmosphere and possibly a liquid water ocean.
James Webb Space Telescope (JWST) data collected in December 2023, along with earlier data from Spitzer, Hubble, and TESS, confirmed this finding, which will be published in The Astrophysical Journal Letters this week.
"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 world is a major goal from 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 the analysis of 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 small red dwarf star about one-fifth the size of the sun. It is one of the nearest exoplanets in its star's habitable zone, where conditions might allow liquid water to exist-a crucial ingredient for life as we know it.
"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," said Charles Cadieux, lead author of the science paper and doctoral student at Universite de Montreal. "This would be a major milestone in the search for potentially habitable exoplanets."
The analysis excluded the possibility of LHS 1140 b being a mini-Neptune, suggesting it is a super-Earth with a possible nitrogen-rich atmosphere. However, more JWST observations are needed to confirm this.
Estimates suggest that LHS 1140 b is less dense than a rocky planet with an Earth-like composition, implying that 10 to 20% of its mass could be water. This points to LHS 1140 b potentially being an ice world with a liquid ocean at its sub-stellar point, the area always facing its star due to synchronous rotation.
MacDonald's atmospheric analysis hints at a nitrogen-rich atmosphere, similar to Earth's. Although tentative, such an atmosphere suggests conditions that could support liquid water.
Current models suggest that if LHS 1140 b has an Earth-like atmosphere, it could be an ice world with a central ocean about 4,000 kilometers in diameter, akin to half the surface area of the Atlantic Ocean, with temperatures at the center potentially around 20 degrees Celsius.
"This is our first tantalizing glimpse of an atmosphere on a super Earth in the habitable zone. Compared to other known habitable zone exoplanets, such as those in the TRAPPIST-1 system, the star LHS 1140 appears to be calmer and less active, making it significantly less challenging to disentangle LHS 1140 b's atmosphere from stellar signals caused by starspots," MacDonald said.
"Our initial reconnaissance of LHS 1140 b with JWST has revealed this to be perhaps the best habitable zone exoplanet currently known for atmospheric characterisation. While we need more JWST observations to confirm the nitrogen-rich atmosphere, and to search for other gases, this is a very promising start."
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