Astronomers discover surprising icy world in habitable zone with JWST data

Astronomers discover surprising icy world in habitable zone with JWST data

Temperate exoplanet LHS 1140 b may be a world completely covered in ice (left), similar to Jupiter’s moon Europa, or an icy world with a liquid substellar ocean and a cloudy atmosphere (center). LHS 1140 b is 1.7 times the size of Earth (right) and is the most promising exoplanet in the habitable zone found so far in our search for liquid water outside the solar system. Credit: B. Gougeon/Université de Montréal

A team of astronomers has identified a temperate exoplanet as a promising super-Earth with ice or water.

The findings, led by the Université de Montréal, show that the habitable-zone exoplanet, LHS 1140 b, is likely not a mini-Neptune, a small so-called gas giant — large planets composed mostly of gas — with a thick hydrogen-rich atmosphere. The planet, located about 48 light-years away in the constellation Cetus, emerges as one of the most promising candidates for habitable-zone exoplanets known, potentially harboring an atmosphere and even an ocean of liquid water.

Data from the James Webb Space Telescope (JWST) were collected in December 2023 and added to earlier data from other space telescopes Spitzer, Hubble and TESS to confirm this result, which has been accepted for publication in The letters of the astrophysical journal this week and currently available on the arXiv preprint server.

“This is the first time we’ve seen a hint of a habitable-zone atmosphere on a rocky or ice-rich exoplanet. Detecting atmospheres on small, rocky worlds is an important goal for JWST, but these signals are much harder to see than for the atmospheres of giant planets,” said Ryan MacDonald, a NASA Sagan Fellow in the U-M Department of Astronomy who played a key role in analyzing LHS 1140 b’s atmosphere.

“LHS 1140 b is one of the best small exoplanets in the habitable zone that can support a thick atmosphere. We may have found evidence of air on this world.”

LHS 1140 b, a potentially habitable world

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 the solar system that lies within its star’s habitable zone. Exoplanets found in this “Goldilocks Zone” have temperatures that allow liquid water to exist on them, a crucial element for life as we know it on Earth.

One of the crucial questions about LHS 1140 b was whether it is a Neptune-like mini-exoplanet or a super-Earth: a rocky or water-rich planet larger than Earth.

“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 Charles Cadieux, lead author of the paper and a doctoral student at the Université de Montréal. “This would be a major milestone in the search for potentially habitable exoplanets.”

JWST data leads to new insights

Analysis of the team’s observations strongly ruled out the mini-Neptune scenario, with evidence suggesting that the exoplanet LHS 1140 b is a super-Earth that could even have a nitrogen-rich atmosphere like Earth’s. However, the team cautions that additional observations using JWST observations will be needed to confirm the nitrogen gas signature.

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% of its mass is water. This discovery suggests that LHS 1140 b is a compelling candidate for a waterworld, likely resembling a snowball or ice planet with a potential liquid ocean at the substellar point, or the region of the planet’s surface that would always face the system’s host star due to the planet’s synchronous rotation (much like Earth’s Moon).

Possible presence of an atmosphere and ocean

MacDonald performed the atmospheric retrieval analysis that suggests that LHS 1140 b has a nitrogen-rich atmosphere, possibly similar to Earth’s atmosphere, which is 78% nitrogen. While still only a preliminary result, the presence of a nitrogen-rich atmosphere 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 an ocean with a rosette about 4,000 kilometers in diameter, which is half the surface of the Atlantic Ocean. The surface temperature at the center of this strange 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 offers a unique opportunity to study a world that could support life, given its position in the habitable zone and the likelihood of an atmosphere capable of trapping heat and supporting a stable climate.

“This is our first tantalizing glimpse of an atmosphere on a super-Earth in the habitable zone. Compared to other known exoplanets in the habitable zone, such as those in the TRAPPIST-1 system, the star LHS 1140 appears to be quieter and less active, making it significantly less challenging to disentangle the atmosphere of LHS 1140 b from stellar signals caused by starspots,” MacDonald said.

“Our initial exploration of LHS 1140 b with JWST has revealed that this may be the best habitable zone exoplanet currently known for atmospheric characterization. While we need more JWST observations to confirm the nitrogen-rich atmosphere and to search for other gases, this is a promising start.”

More information:
Charles Cadieux et al, Transmission spectroscopy of the habitable zone of exoplanet LHS 1140 b with JWST/NIRISS, arXiv (2024). DOI number: 10.48550/arxiv.2406.15136

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