In an exciting discovery, a team led by Université de Montréal (UdeM) astronomers have excluded the possibility of the temperate exoplanet LHS 1140 b being a mini-Neptune, instead identifying it as a promising super-Earth ice or water world. This planet, located about 48 light-years away in the constellation Cetus, emerges as one of the most promising habitable zone exoplanet candidates known, potentially harboring an atmosphere and even a liquid water ocean. Precious data from the James Webb Space Telescope (JWST) were collected in December 2023 and added to previous data from other space telescopes, including Spitzer, Hubble, and TESS, to solidify this result published in The Astrophysical Journal Letters on July 10, 2024.
LHS 1140 b, an exoplanet orbiting a low-mass red dwarf star roughly one-fifth the size of the Sun, has captivated scientists due to it being 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 would allow water to exist on them in liquid form — liquid water being a crucial element for life as we know it on Earth.
Earlier this year, research led by Charles Cadieux, Ph.D. student at the Trottier Institute for Research on Exoplanets (iREx) and UdeM, under the supervision of Prof. René Doyon, reported new mass and radius estimates for LHS 1140 b with exceptional accuracy, comparable to those of the well-known TRAPPIST-1 planets: 1.7 times the size of Earth and 5.6 times its mass.
One of the critical questions about LHS 1140 b was 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). This latter scenario included the possibility of a so-called “Hycean world” with a global liquid ocean enveloped by a hydrogen-rich atmosphere which would exhibit a distinct atmospheric signal that could be observed using the powerful Webb Telescope.
Charles Cadieux, Ph.D. student at the Trottier Institute for Research on Exoplanets and the Université de Montréal, is the paper’s lead author. (Credit: C. Cadieux)
Through an extremely competitive process, the team of astronomers obtained 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 programme is only the second dedicated to the study of exoplanets in the nearly two years of Webb’s operations, underscoring the importance and potential impact of these findings.
Analysis of these observations strongly excluded the mini-Neptune scenario, with tantalizing evidence suggesting 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 initial formation.
Estimates based on all accumulated data reveal that LHS 1140 b is less dense than expected for a rocky planet with an Earth-like composition, suggesting that 10 to 20% of its mass may be composed of water. This discovery points to LHS 1140 b being a compelling candidate water world, likely resembling a snowball or ice planet with a potential liquid ocean at the sub-stellar point, or the area of the planet’s surface that would always be facing the system’s host star due to the planet’s expected synchronous rotation (much like the Earth’s Moon).
“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,” affirms Charles Cadieux, lead author of the science paper. “This would be a major milestone in the search for potentially habitable exoplanets.”
While it is still only a tentative result, the presence of a nitrogen-rich atmosphere on LHS 1140 b would suggest the planet has retained a substantial atmosphere, creating conditions that might support liquid water. This discovery favors the water world/snowball scenario as the most plausible. Current models indicate that if LHS 1140 b has an Earth-like atmosphere, it would be a snowball planet with a bull’s-eye ocean about 4,000 kilometers in diameter, equivalent to half the surface area of the Atlantic Ocean. The surface temperature at the centre of this alien ocean could even be a comfortable 20 degrees Celsius.
LHS 1140 b’s potential atmosphere and favorable conditions for liquid water make it an exceptional candidate for future habitability studies. This planet provides a unique opportunity to study a world that could support life, given its position in the habitable zone and the likelihood of having an atmosphere that can retain heat and support a stable climate.
Confirming the presence and composition of LHS 1140 b’s atmosphere and discerning between the snowball planet and bull’s-eye ocean planet scenarios require further observations. The research team has emphasised the need for additional transit and eclipse measurements with the Webb Telescope, focusing on a specific signal that could unveil 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 this exoplanet.
“Detecting an Earth-like atmosphere on a temperate planet is pushing Webb’s capabilities to its limits. It’s feasible; we just need lots of observing time,” says René Doyon, who is also the Principal Investigator of the NIRISS instrument. “The current hint of a nitrogen-rich atmosphere begs for confirmation with more data. We need 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.” According to Doyon, the Webb Telescope will likely have to observe this system at every possible opportunity for several years to determine whether LHS 1140 b has habitable surface conditions.
Given LHS 1140 b’s limited visibility with Webb — a maximum of only eight visits per year are possible — astronomers will require several years of observations to detect carbon dioxide and confirm the presence of liquid water on the planet’s surface. LHS 1140 b stands out as a prime candidate for continued study among nearby exoplanets and astronomers’ quest to find habitable worlds beyond our Solar System. Given the possibility of confirming the presence of an atmosphere and, indirectly, liquid water on its surface, committing future observing time to LHS 1140 b should be well worth it!
About this study
The paper “Transmission Spectroscopy of the Habitable Zone Exoplanet LHS 1140 b with JWST/NIRISS” was published on July 10, 2024 in Astrophysical Journal Letters. The lead author is Charles Cadieux, Ph.D. student at the Trottier Institute for Research on Exoplanets at the Université de Montréal. Other iREx researchers that contributed to this paper are René Doyon (UdeM), Étienne Artigau (UdeM), Olivia Lim (UdeM), Michael Radica (UdeM), Salma Salhi (UdeM), Lisa Dang (UdeM), Loïc Albert (UdeM), Louis-Philippe Coulombe (UdeM), Nicolas Cowan (McGill), David Lafrenière (UdeM), Alexandrine L’Heureux (UdeM), Caroline Piaulet-Ghorayeb (UdeM), Björn Benneke (UdeM), Neil Cook (UdeM), and Marylou Fournier-Tondreau (UdeM and University of Oxford). Additional contributors are based out of the University of Michigan, the Centre national de recherche scientifique (France), NASA Goddard Space Flight Center, the American University, McGill University, McMaster University, and the University of Toronto. C. Cadieux and the UdeM team acknowledge financial support from the Canadian Space Agency for this study.
Scientific Contacts
Charles Cadieux
Ph.D. Student
Trottier Institute for Research on Exoplanets
Université de Montréal
charles.cadieux.1@umontreal.ca
Tel: 514 503-0176
René Doyon
Director
Trottier Institute for Research on Exoplanets
Université de Montréal
rene.doyon@umontreal.ca
Tel: 514 349-5779
Media Contact
Nathalie Ouellette
JWST Outreach Scientist
Trottier Institute for Research on Exoplanets
Université de Montréal
nathalie@astro.umontreal.ca
Tel: 613 531-1762
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