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LHS 1140b is the second-closest temperate transiting exoplanet to Earth with a temperature low enough to support surface liquid water.
LHS 1140b may be a world completely covered in ice (left) similar to Jupiter’s moon Europa or be an ice world with a liquid ocean and a cloudy atmosphere (center). Image credit: B. Gougeon / Université de Montréal.
LHS 1140 is a cool low-mass star approximately 39 light-years away in the constellation of Cetus.
Also known as Gliese 3053, GJ 3053 and 2MASS J00445930-1516166, the star is about one-fifth the size of the Sun and approximately 5 billion years old.
LHS 1140 is known to host three exoplanets: LHS 1140b, LHS 1140c and LHS 1140d.
Earlier this year, astronomers reported new mass and radius estimates for LHS 1140b 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 1140b was whether it is a mini-Neptune type exoplanet or a super-Earth.
The 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 Webb.
An analysis of the new Webb observations strongly excluded the mini-Neptune scenario, with tantalizing evidence suggesting exoplanet LHS 1140b is a super-Earth that may even have a nitrogen-rich atmosphere.
If this result is confirmed, LHS 1140b 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 1140b 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 1140b being a compelling water world, likely resembling a snowball or ice planet with a potential liquid ocean at the sub-stellar point, 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.
An artist’s impression of the planetary system LHS 1140. Image credit: Sci.News.
“Of all currently known temperate exoplanets, LHS 1140b 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, a Ph.D. student at the Université de Montréal.
“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 1140b 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 1140b has an Earth-like atmosphere, it would be a snowball planet with a vast bull’s-eye ocean measuring about 4,000 km in diameter, equivalent to half the surface area of the Atlantic Ocean.
The surface temperature at the center of this alien ocean could even be a comfortable 20 degrees Celsius.
LHS 1140b’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 its star’s habitable zone and the likelihood of its having an atmosphere that can retain heat and support a stable climate,” the astronomers said.
The team’s paper will be published in the Astrophysical Journal Letters.
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Charles Cadieux et al. 2024. Transmission Spectroscopy of the Habitable Zone Exoplanet LHS 1140b with JWST/NIRISS. ApJL, in press; arXiv: 2406.15136
