Beyond Earth: Webb Space Telescope Identifies Key Molecules in Exoplanet K2-18 b

This artist’s concept shows what exoplanet K2-18 b might look like based on scientific data. K2-18 b, an exoplanet 8.6 times the size of Earth, orbits the small, cool star K2-18 in the habitable zone and lies 120 light-years from Earth. New research with NASA’s James Webb Space Telescope on K2-18 b has shown the presence of carbon-bearing molecules including methane and carbon dioxide. The abundance of methane and carbon dioxide, and the lack of ammonia, support the hypothesis that there may be a water ocean beneath the hydrogen-rich atmosphere on K2-18 b. Credit: NASA, ESA, CSA, Joseph Olmsted (STScI), Nikku Madhusudhan (IoA)

The data hints at an exoplanet with a possible liquid ocean.

Carbon-containing molecules have been found in the habitable atmosphere an exoplanet K2-18 b is an international group of astronomers using data from NASA‘s James Webb Space Telescope. These results are consistent with an exoplanet that may have a surface covered by an ocean beneath a hydrogen-rich atmosphere. The discovery provides a fascinating glimpse into a planet unlike anything in our Solar System, and raises exciting prospects for habitable worlds elsewhere in the Universe.

The spectra of K2-18 b, obtained with Webb’s NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph), show the abundance of methane and carbon dioxide in the atmosphere of the exoplanet, and the possible detection of a molecule called dimethyl sulfide (DMS). The detection of methane and carbon dioxide, along with the lack of ammonia, supports the hypothesis that there may be a water ocean beneath the hydrogen-rich atmosphere of K2-18 b. K2-18 b, 8.6 times the size of Earth, orbits the cool dwarf star K2-18 in the habitable zone and lies 120 light years from Earth. Credit: NASA, ESA, CSA, Ralf Crawford (STScI), Joseph Olmsted (STScI), Nikku Madhusudhan (IoA)

Webb Discovers Methane, Carbon Dioxide in the atmosphere of K2-18 b

New research with NASA’s James Webb Space Telescope on K2-18 b, an exoplanet 8.6 times as massive as Earth, has shown the presence of carbon-bearing molecules including methane and carbon dioxide. Webb’s findings add to recent studies suggesting that K2-18 b may be a Hycean exoplanet, one that has the potential to have a hydrogen-rich atmosphere and a surface covered by ocean water.

The first insight into the atmospheric properties of this habitable zone came from NASA’s Hubble Space Telescope observations, which inspired studies that changed our understanding of the system.

K2-18 b orbits the small, cool star K2-18 in the habitable zone and lies 120 light-years from Earth in the constellation Leo. Exoplanets like K2-18 b, with sizes between those of Earth and Neptune, they are unlike anything else on our planet. This lack of nearby planets means that these ‘sub-Neptunes’ are poorly understood, and the nature of Earth’s atmosphere is a matter of active debate among astronomers.

Implications for Exoplanet Life

The suggestion that the sub-Neptune K2-18 b may be a Hycean exoplanet is interesting, as some astronomers believe that these worlds are promising environments for searching for evidence of life on exoplanets.

“Our findings emphasize the importance of considering different habitats in the search for life elsewhere,” explained Nikku Madhusudhan, an astrophysicist at the University of Cambridge and lead author of the paper announcing the findings. “Traditionally, the search for life on exoplanets has focused mainly on small rocky planets, but the large Hycean world is more suitable for climate monitoring.”

The abundance of methane and carbon dioxide, and the lack of ammonia, support the hypothesis that there may be a water ocean beneath the hydrogen-rich atmosphere on K2-18 b. This first observation by Webb also provided the possible discovery of a molecule called dimethyl sulfide (DMS). On Earth, this is only produced by life. Most of the DMS in the Earth’s atmosphere is released from phytoplankton in the ocean.

The DMS inference is weak and needs further confirmation.

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

Characteristics of Exoplanet Atmospheres

While K2-18 b lies in the habitable zone, and is now known to store carbon-bearing molecules, this does not mean the planet can support life. The size of the giant planet – with a radius of 2.6 times the radius of Earth – means that inside this planet there is likely to be a large ice of high pressure, like Neptune, but with a bad hydrogen-rich atmosphere and a sea surface. The Hycean world is thought to have oceans of water. However, it is possible for the ocean to be too hot to live or become liquid.

“Although this type of planet does not exist in our solar system, sub-Neptunes are the most common type of planet known so far in the galaxy,” said team member Subhajit Sarkar of Cardiff University. “We found the most detailed sub-Neptune habitable zone to date, and this allowed us to use the molecules present in the atmosphere.”

Describing the atmospheres of exoplanets like K2-18 b – which means identifying their gases and physical conditions – is a very active area in astronomy. However, these planets outshine – literally – the luminosity of their massive parent stars, which makes studying exoplanet atmospheres more difficult.

The team countered this challenge by analyzing the light from the parent K2-18 b star as it passed the exoplanet’s atmosphere. K2-18 b is a transiting exoplanet, which means we can see a drop in light as it passes the face of its star. This is how the exoplanet was discovered for the first time in 2015 with NASA’s K2 mission. This means that during the journey a small fraction of the star’s light will pass through the exoplanet’s atmosphere before reaching telescopes like Webb. The passage of light through an exoplanet’s atmosphere leaves a trail that astronomers can piece together to see the gases in the exoplanet’s atmosphere.

James Webb’s Expertise and Future Research

“This result is only possible because of the extended wavelength range and the unprecedented sensitivity of the Webb, which allows for a robust detection of spectral features and only two motions,” said Madhusudhan. “By comparison, one study by Webb provided comparable accuracy to eight observations with Hubble conducted over a few years and a narrower wavelength range.”

“These results are the product of just two observations of K2-18 b, with many more on the way,” explained team member Savvas Constantinou of the University of Cambridge. “This means that our work here is an early indication of what Webb might see in habitable environments.”

The results of the group have been accepted for publication in the The Astrophysical Journal Letters.

The team is now looking to conduct a follow-up survey with the MIRI (Mid-Infrared Instrument) spectrograph telescope which they hope will further confirm their findings and provide new insights into the environmental conditions in K2-18 b.

“Our ultimate goal is the identification of life on a habitable exoplanet, which could revolutionize our understanding of our place in the universe,” concluded Madhusudhan. “Our findings are a promising step toward a deeper understanding of Hycean worlds in this quest.”

NASA’s James Webb Space Telescope is the world’s premier space telescope. It solves mysteries in our solar system, looks beyond the distant worlds around other stars, and explores the mysterious nature and origins of our universe and our place in it. Webb is an international program led by NASA and its partner, ESA (European Space Agency) and the Canadian Space Agency.