Using data from NASA’s James Webb Space Telescope, an international team of astronomers detected carbon-containing molecules in the atmosphere of exoplanet K2-18 b, located in the habitable zone. These results are consistent with the existence of an exoplanet that may contain an ocean-covered surface beneath a hydrogen-rich atmosphere. The discovery provides a fascinating look at a planet unlike any other in our solar system and opens interesting perspectives on potentially habitable worlds elsewhere in the universe.

A new study of K2-18 b, an exoplanet 8.6 times larger than Earth, using NASA’s James Webb Space Telescope, has revealed the presence of carbon-containing molecules, including methane and carbon dioxide. Webb’s discovery adds to recent research suggesting that K2-18 b could potentially be a Hycean exoplanet with a hydrogen-rich atmosphere and a surface covered by a watery ocean.

The first information about the atmospheric properties of this habitable exoplanet came from observations with NASA’s Hubble Space Telescope, which led to further research that transformed our understanding of the system.

K2-18 b orbits the cool dwarf star K2-18 in the habitable zone and is located 120 light-years from Earth in the constellation Leo. Exoplanets like K2-18 b, which lies between Earth and Neptune in size, are unlike anything else in our solar system. The lack of nearby equivalent planets means that these “sub-Neptunes” are poorly understood, and the nature of their atmospheres is a subject of active debate among astronomers.

The suggestion that sub-Neptunian K2-18 b may be a Hycean exoplanet is intriguing because some astronomers believe these worlds are promising environments to search for evidence of life on exoplanets.

“Our findings highlight the importance of looking at a variety of habitable environments in the search for life elsewhere,” said Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author of the paper describing the results. “Traditionally, the search for life on exoplanets has focused mainly on smaller rocky planets, but the larger Hyckeian worlds are much more amenable to atmospheric observations.”

The large amounts of methane and carbon dioxide, as well as the lack of ammonia, support the hypothesis that there may be a water ocean beneath the hydrogen-rich atmosphere in K2-18b. These initial observations by Webb also led to the possible discovery of a molecule called dimethyl sulfide (DMS). It is produced only by life on Earth. Most of the DMS in the Earth’s atmosphere is emitted from phytoplankton in the marine environment. The DMS result is less robust and requires further validation.

“Future Webb observations will be able to confirm whether DMS is indeed present at significant levels in the atmosphere of K2-18 b,” Madhusudhan explained.

Although K2-18b is in the habitable zone and is now known to contain carbon-containing molecules, this does not necessarily mean that the planet can host life. The planet’s large size (its radius is 2.6 times that of Earth) means that the planet’s interior likely contains a large high-pressure ice mantle similar to that of Neptune, but with a thinner, hydrogen-rich atmosphere and surface ocean. Hycaean worlds are assumed to have water oceans. But it is also possible that the ocean is too hot to be habitable or liquid.

“Although no such planets exist in our solar system, sub-Neptunians are currently the most common type of planet known in the galaxy,” said team member Subhajit Sarkar from Cardiff University. “We obtained the most detailed spectrum to date of the sub-Neptunian habitable zone, allowing us to identify molecules present in its atmosphere.”

Characterizing the atmospheres of exoplanets such as K2-18 b, that is, determining their gases and physical conditions, is a very active field in astronomy. However, these planets are literally eclipsed by the brightness of their much larger parent stars, making the study of exoplanet atmospheres particularly difficult.

The team solved this problem by analyzing the light of the parent star K2-18 b as it passed through the exoplanet’s atmosphere. K2-18 b is a transiting exoplanet; This means we may see a drop in brightness as it passes across the face of the host star. Thus, the exoplanet was first discovered in 2015 during the NASA K2 mission. This means that during transit, a very small fraction of the starlight will pass through the exoplanet’s atmosphere before reaching telescopes like Webb. The passage of starlight through an exoplanet’s atmosphere leaves traces that astronomers can piece together to identify gases in the exoplanet’s atmosphere.

“This result was only possible thanks to Webb’s extended wavelength range and unprecedented sensitivity, which allows reliable detection of spectral features in just two passes,” said Madhusudhan. “In comparison, one Webb transit observation provided sensitivity comparable to eight Hubble observations over several years and in a relatively narrow wavelength range.”

“These results come from just two observations of K2-18 b, with many more to come,” said team member Savvas Constantinou from the University of Cambridge. “This means that our work here is only an early demonstration of what Webb can observe on exoplanets in the habitable zone.”

The team’s results have been accepted for publication. Astrophysics Journal Letters.

The team now plans to conduct further studies using the MIRI (Mid Infrared Instrument) spectrograph; They hope this will confirm their findings and provide new information about environmental conditions at K2-18 b.

“Our ultimate goal is to detect life on a habitable exoplanet, which will change our understanding of our place in the universe,” Madhusudhan concluded. “Our discoveries are a promising step in this quest towards a deeper understanding of the Hykean worlds.” Source