Potentially Habitable Ice-Covered Super-Earth Discovered by Astronomers

Potentially Habitable Ice-Covered Super-Earth Discovered by Astronomers
by Clarence Oxford
Los Angeles CA (SPX) Jul 10, 2024

An international team of astronomers led by the Universite de Montreal has discovered that the exoplanet LHS 1140 b might be a "super-Earth" covered in ice or water, presenting a promising target for further study.

Initially, LHS 1140 b was thought to be a mini-Neptune, a small gaseous planet. However, data from the James Webb Space Telescope (JWST) collected in December 2023, alongside previous data from Spitzer, Hubble, and TESS, have led scientists to reconsider this classification.

Located 48 light-years from Earth in the constellation Cetus, LHS 1140 b resides in its star's habitable zone. The findings from the Universite de Montreal are available on ArXiv and will soon be published in The Astrophysical Journal Letters.

Exoplanet in the Habitable Zone
LHS 1140 b orbits a low-mass red dwarf star about one-fifth the size of the Sun. It's one of the closest exoplanets within a habitable zone, where temperatures allow liquid water-a crucial element for life as we know it-to exist.

Earlier this year, researchers led by Charles Cadieux, a Ph.D. student at UdeM's Trottier Institute for Research on Exoplanets (iREx), provided new mass and radius estimates for LHS 1140 b. Their measurements indicated that the planet is 1.7 times the size of Earth and 5.6 times its mass.

A key question was whether LHS 1140 b was a mini-Neptune or a super-Earth. The latter scenario suggests the possibility of a "Hycean world," a rocky planet with a global liquid ocean and a hydrogen-rich atmosphere. This distinct atmospheric signal can be detected using the Webb Telescope.

Insights from Webb Data
The team obtained "director's discretionary time" (DDT) on Webb last December, observing two transits of LHS 1140 b with the NIRISS (Near-Infrared Imager and Slitless Spectrograph) instrument. This DDT program is significant, as it is only the second focused on exoplanet study in Webb's operations.

The analysis excluded the mini-Neptune scenario, instead suggesting that LHS 1140 b is a super-Earth, possibly with a nitrogen-rich atmosphere. If confirmed, it would be the first temperate planet with a secondary atmosphere formed post-formation.

Data indicates that LHS 1140 b is less dense than a rocky planet with an Earth-like composition, suggesting 10 to 20 percent of its mass may be water. This points to LHS 1140 b being an ice-covered world, possibly with a liquid ocean at its sub-stellar point, where the planet's surface faces its host star due to synchronous rotation.

"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, lead author of the new study. "This would be a major milestone in the search for potentially habitable exoplanets."

Atmosphere and Ocean Possibility
A nitrogen-rich atmosphere on LHS 1140 b suggests it has retained a substantial atmosphere, supporting conditions for liquid water. This supports the water-world/snowball planet hypothesis.

Current models propose that if LHS 1140 b has an Earth-like atmosphere, it would be a snowball planet with a vast "bull's-eye" ocean around 4,000 kilometers in diameter. The central temperature of this ocean could be about 20 degrees Celsius.

LHS 1140 b's atmosphere and potential for liquid water make it an exceptional candidate for habitability studies, offering a unique opportunity to study a potentially life-supporting world.

Years of Observation Ahead
Confirming LHS 1140 b's atmosphere and determining whether it is a snowball or bull's-eye ocean planet requires further observations. The research team plans additional transit and eclipse measurements with the Webb Telescope to detect carbon dioxide, which is crucial for understanding the atmospheric composition and potential greenhouse gases.

"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," said 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."

Given the limited visibility of LHS 1140 b with Webb - only eight visits per year - astronomers will need several years of observations to confirm liquid water on its surface.

Research Report:Transmission Spectroscopy of the Habitable Zone Exoplanet LHS 1140 b with JWST/NIRISS