An international team of astronomers led by the University of Montreal has made an exciting discovery about the temperate exoplanet LHS 1140 b: it could be a promising “super-Earth” covered in ice or water.
When the exoplanet LHS 1140 b was first discovered, astronomers speculated that it might be a mini-Neptune: an essentially gaseous planet, but very small in size compared to Neptune. But after analyzing data from the James Webb Space Telescope (JWST) collected in December 2023 — combined with previous data from other space telescopes including Spitzer, Hubble and TESS — scientists have come to a very different conclusion.
Located about 48 light-years from Earth in the constellation Cetus, LHS 1140 b appears to be one of the most promising exoplanets in its star's habitable zone, potentially harboring an atmosphere and even an ocean of liquid water. The results of this discovery by astronomers from the Université de Montréal are available on ArXiv and will soon be published in The letters of the astrophysical journal.
An exoplanet in the 'Goldilocks Zone'
LHS 1140 b, an exoplanet orbiting a low-mass red dwarf star about one-fifth the size of the sun, has fascinated scientists because it is one of the closest exoplanets to our solar system that lies within its star's habitable zone. Exoplanets found in this “Goldilocks zone” have temperatures that allow water to exist on them in liquid form — liquid water is a crucial element for life as we know it on Earth.
Earlier this year, researchers led by Charles Cadieux, a PhD candidate at UdeM's Trottier Institute for Research on Exoplanets (iREx), under the supervision of Professor René Doyon, reported new mass and radius estimates for LHS 1140 b with exceptional precision, comparable to those of the known TRAPPIST-1 planets: 1.7 times the size of Earth and 5.6 times more massive.
One of the critical questions about LHS 1140 b has been whether it is a mini-Neptune-type exoplanet (a small gas giant with a thick hydrogen-rich atmosphere) or a super-Earth (a rocky planet larger than Earth). The latter scenario included the possibility of a so-called “Hycean world” with a global liquid ocean surrounded by a hydrogen-rich atmosphere that would show a clear atmospheric signal that could be detected with the powerful Webb telescope.
New insights from Webb data
Through a highly competitive process, the team of astronomers was awarded valuable “director's discretionary time” (DDT) on Webb last December, during which two transits of LHS 1140 b were observed with the Canadian-built NIRISS (Near-Infrared Imager and Slitless Spectrograph) instrument. This DDT program is only the second devoted to the study of exoplanets in Webb's nearly two years of operation, underscoring the importance and potential impact of these findings.
Analysis of these observations strongly ruled out the mini-Neptune scenario, with tantalizing evidence suggesting that exoplanet LHS 1140 b is a super-Earth that may even have a nitrogen-rich atmosphere. If this result is confirmed, LHS 1140 b would be the first temperate planet to show evidence of a secondary atmosphere, formed after the planet's original formation.
Estimates based on all the collected data reveal that LHS 1140 b is less dense than expected for a rocky planet with a composition similar to Earth, suggesting that 10 to 20 percent of its mass is water. This discovery suggests that LHS 1140 b is a fascinating waterworld, likely resembling a snowball or ice planet with a potential liquid ocean at the substellar point, the region of the planet's surface that would always face the system's host star due to the planet's expected synchronous rotation (much like Earth's Moon).
“Of all the currently known temperate exoplanets, LHS 1140 b may be our best chance to one day indirectly confirm liquid water on the surface of an alien world outside our solar system,” said Cadieux, lead author of the new study. “This would be a major milestone in the search for potentially habitable exoplanets.”
Possible presence of an atmosphere and an ocean
Although only preliminary, the presence of a nitrogen-rich atmosphere on LHS 1140 b would suggest that the planet has retained a substantial atmosphere, creating conditions that could support liquid water. This discovery favors the waterworld/snowball scenario as the most plausible.
Current models indicate that if LHS 1140 b has an atmosphere similar to Earth's, it would be a snowball planet with a huge “bull's-eye” ocean about 4,000 kilometers (2,500 miles) in diameter, which is half the surface of the Atlantic Ocean. The surface temperature at the center of this strange ocean could be a comfortable 20 degrees Celsius (68 degrees Fahrenheit).
LHS 1140 b’s potential atmosphere and favorable conditions for liquid water make it an exceptional candidate for future habitability studies. This planet offers a unique opportunity to study a world that could support life, given its position in the habitable zone of its star and the likelihood that it has an atmosphere that can trap heat and support a stable climate.
Several years of observation ahead
To confirm the presence and composition of LHS 1140 b's atmosphere and to distinguish between the snowball planet and bull's-eye ocean planet scenarios, further observations are needed. The research team emphasized the need for additional transit and eclipse measurements with the Webb telescope, focusing on a specific signal that could reveal the presence of carbon dioxide. This feature is crucial for understanding the atmospheric composition and detecting potential greenhouse gases that could indicate habitable conditions on the exoplanet.
“Detecting an Earth-like atmosphere on a temperate planet pushes Webb's capabilities to the limit — it's doable; we just need a lot of observing time,” said Doyon, who is also the principal investigator for the NIRISS instrument. “The current indication of a nitrogen-rich atmosphere needs to be confirmed with more data. We need at least another year of observations to confirm that LHS 1140 b has an atmosphere, and probably two or three more years to detect carbon dioxide.” According to Doyon, the Webb telescope will likely have to observe this system for several years at every possible opportunity to determine whether LHS 1140 b has habitable surface conditions.
Given the limited visibility of LHS 1140 b from Webb (a maximum of eight visits per year are possible), astronomers will need several years of observations to detect carbon dioxide and confirm the presence of liquid water on the planet's surface.
