LHS 1140b is the second-closest temperate transiting exoplanet to Earth, with temperatures cold enough for liquid water to exist on its surface.
LHS 1140b could be a completely icy world, like Jupiter's moon Europa (left), or it could be an icy world with a liquid ocean and cloud-like atmosphere (centre). Image courtesy of B. Gougeon / University of Montreal.
LHS 1140 is a cool, low-mass star located about 39 light-years from Earth in the constellation Cetus.
Also known as Gliese 3053, GJ 3053 and 2MASS J00445930-1516166, the star is about one-fifth the size of our Sun and is about 5 billion years old.
LHS 1140 is known to be home to three exoplanets: LHS 1140b, LHS 1140c, and LHS 1140d.
Earlier this year, astronomers reported new mass and radius estimates for LHS 1140b with extraordinary precision, matching estimates for the better-known TRAPPIST-1 planet: 1.7 times the size and 5.6 times the mass of Earth.
One of the key questions about LHS 1140b was whether it was a mini-Neptune-type exoplanet or a super-Earth.
The latter scenario included the possibility of the existence of so-called Hythean worlds with global liquid oceans enveloped in a hydrogen-rich atmosphere, which would show a clear atmospheric signal observable using Webb.
Analysis of Webb's new observations completely rules out the mini-Neptune scenario and provides compelling evidence suggesting that exoplanet LHS 1140b is a super-Earth, possibly with a nitrogen-rich atmosphere.
If confirmed, this result would make LHS 1140b the first temperate planet to show evidence of a secondary atmosphere that formed after the planet's initial formation.
Estimates based on all the accumulated data suggest that LHS 1140b is less dense than expected for a rocky planet with an Earth-like composition, and that 10-20% of its mass may be made up of water.
The discovery indicates that LHS 1140b is a fascinating watery world, possibly similar to a snowball or ice planet, and the planet's expected synchronous rotation means that a region of the planet's surface could always harbor a liquid ocean at its substellar point, facing the system's host star.
Artist's impression of planetary system LHS 1140. Image courtesy of Sci.News.
“Among the currently known temperate exoplanets, LHS 1140b may be the best candidate for future indirect confirmation of the existence of liquid water on the surface of an alien world outside our solar system,” said Charles Cadieux, a doctoral student at the University of Montreal.
“This will be a major milestone in the search for potentially habitable exoplanets.”
Although still preliminary, the presence of a nitrogen-rich atmosphere on LHS 1140b suggests that the planet could retain a significant amount of atmosphere, creating the conditions for liquid water to exist. This finding makes the water-world/snowball scenario the most plausible.
Current models suggest that if LHS 1140b had an Earth-like atmosphere, it would be a snowball planet with a huge bull's-eye shaped ocean about 4,000 km in diameter, equivalent to half the surface area of the Atlantic Ocean.
Surface temperatures in the core of this alien ocean could reach a comfortable 20 degrees Celsius.
LHS 1140b has favorable conditions for a potential atmosphere and liquid water, making it an excellent candidate for future habitability studies.
“The planet provides a unique opportunity to study worlds that could potentially support life, as it is located in the habitable zone of its star and likely has an atmosphere capable of retaining heat and supporting a stable climate,” the astronomers said.
Team paper will be published in Astrophysical Journal Letters.
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Charles Cadieux others2024. Transmission spectroscopy of the habitable zone exoplanet LHS 1140b with JWST/NIRISS. Apu JL,in press; arXiv:2406.15136
Source: www.sci.news
