An international team of astronomers from the University of Montreal has confirmed a temperate exoplanet, LHS 1140 b, as the most promising candidate for a “super-Earth” covered in either ice or water. The finding, based on data from the James Webb Space Telescope, brings these researchers one step closer to finding probably habitable worlds beyond our solar system.
LHS 1140 b orbits a low-mass red-dwarf star about one-fifth the size of the Sun and is located about 48 light-years from Earth in the constellation Cetus. The exoplanet orbits well within that star’s habitable zone. More often, it is referred to as the “Goldilocks Zone”. The temperatures there could allow liquid water to exist, an element necessary for life as we know it.
“This is the first time we have ever seen a hint of an atmosphere on a habitable zone rocky or ice-rich exoplanet,” said Ryan MacDonald, a NASA Sagan Fellow in the University of Michigan’s Department of Astronomy. “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.”
Their finding, based on this transit observation, should be that LHS 1140 b is not a mini-Neptune—a small gas giant enriched with a thick hydrogen-rich atmosphere—but rather a super-Earth. New estimates say it is a rocky or water-rich planet 1.7 times larger in size than Earth and for quite a long period has attracted scientists’ attention for its potentially existing atmosphere and even liquid water.
Lead author Charles Cadieux, a doctoral student at the Université de Montréal, underlined this finding. “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 Cadieux. “This would be a major milestone in the search for potentially habitable exoplanets.”
JWST data taken in December 2023, along with all previous observations done by other space telescopes from Spitzer, Hubble, and TESS, provided new insight into LHS 1140 b. It quite securely rules out that it could have been a mini-Neptune at very good confidence levels, and there’s evidence to point toward an atmosphere rich in nitrogen, somewhat Earth-like, although further investigation will be required to confirm.
Models using independent estimates of this data place LHS 1140 b at densities lower than expected for a rocky, Earth-like composition—meaning that mass could be made up of 10 to 20 percent water. That makes LHS 1140 b quite a good candidate for a water world, perhaps an ice planet or even a snowball with a liquid ocean existing at the substellar point—the region on the surface of an exoplanet perpetually facing its star due to synchronous rotation.
MacDonald ran the atmospheric retrieval analysis and said LHS 1140 b could have an atmosphere rich in nitrogen, like Earth’s, which is 78 percent nitrogen. If this result holds — it’s still preliminary — such an atmosphere would point to a substantial atmosphere for the planet, and thus for conditions that could support liquid water.
“This is our first tantalizing glimpse of an atmosphere on a super-Earth in the habitable zone,” said MacDonald. “The star LHS 1140, compared to other known habitable exoplanets in their host stars’ habitable zones, for example, those in the TRAPPIST-1 system, appears to be quite a bit quieter.” That is to say, it is much easier to pull out LHS 1140’s atmosphere from stellar signals that are several orders of magnitude larger, such as those caused by starspots.
What makes LHS 1140 b a good candidate for future habitability studies is its potential atmosphere and proper conditions for liquid water. This exoplanet does offer the unique opportunity of studying a world that might otherwise be able to harbor life, being in the middle of the habitable zone and having an atmosphere to retain heat and maintain a stable climate.
René Doyon, one of JWST’s principal investigators for the NIRISS instrument, said, “A detection of an Earth-like atmosphere on a temperate planet pushes Webb’s capabilities to its limits: it’s doable; we just need lots of observing time.”. “The current hint of a nitrogen-rich atmosphere at the moment begs to be confirmed with more data. It will take at least one more year of observations to confirm that LHS 1140 b has an atmosphere, and likely two or three more to detect carbon dioxide.”
Already, the low visibility of LHS 1140 b with JWST implies years of observation to verify whether this exoplanet had habitable surface conditions. The discovery bodes well for the start of the search for life beyond Earth and delivers a mouthwatering foretaste of what could be waiting for us in the universe.