Webb finds methane and Carbon Dioxide in Atmosphere of exoplanet K2-18 b.

An multinational team of scientists used data from the NASA/ESA/CSA James Webb Space Telescope to investigate K2-18 b, an exoplanet 8.6 times the mass of Earth, and discovered the presence of carbon-bearing compounds such as methane and carbon dioxide. The discovery adds to prior research indicating that K2-18 b could be a Hycean exoplanet, one with a hydrogen-rich atmosphere and an ocean-covered surface.

Observations with the NASA/ESA Hubble Space Telescope provided the first insight into the atmospheric features of this habitable-zone exoplanet, prompting further research that have since transformed our knowledge of the system. New observations were made aboard the NASA/ESA/CSA James Webb Space Telescope using the Canadian-contributed NIRISS and European-contributed NIRSpec instruments.

K2-18 b

K2-18 b is 120 light-years from Earth in the constellation Leo and orbits the cool dwarf star K2-18 in the habitable zone. Exoplanets like K2-18 b, with sizes between Earth and Neptune, are unlike anything else in our Solar System. Because there are no equivalent neighboring planets, these'sub-Neptunes' are poorly known, and the nature of their atmospheres is a source of intense discussion among astronomers. The possibility that K2-18 b being a Hycean exoplanet is intriguing, as some astronomers believe these worlds are excellent conditions for searching for evidence of life on exoplanets.

The Habitable Zone is the region around a star where the conditions could potentially be suitable to sustain life on a planet within this region, for example allowing the presence of liquid water on its surface.

“Our findings underscore the importance of considering diverse habitable environments in the search for life elsewhere,” explained Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author of the paper announcing these results. “Traditionally, the search for life on exoplanets has focused primarily on smaller rocky planets, but the larger Hycean worlds are significantly more conducive to atmospheric observations.”

Spectra of K2-18 b, obtained with Webb’s NIRISS

Hydrogen-rich atmosphere on K2-18 b

The abundance of methane and carbon dioxide, as well as the scarcity of ammonia, support the theory that an ocean exists beneath a hydrogen-rich atmosphere on K2-18 b. These early Webb observations also suggested the presence of a chemical known as dimethyl sulphide (DMS). Only life on Earth can produce this. The majority of DMS in the atmosphere is emitted by phytoplankton in maritime areas.

The inference of DMS is less robust and requires further validation. “Upcoming Webb observations should be able to confirm if DMS is indeed present in the atmosphere of K2-18 b at significant levels,” explained Madhusudhan.

While K2-18 b is in the habitable zone and has been found to contain carbon-bearing compounds, this does not necessarily imply that the planet is capable of supporting life. With a radius 2.6 times that of Earth, the planet's interior is anticipated to possess a huge mantle of high-pressure ice, similar to Neptune, but with a thinner hydrogen-rich atmosphere and an ocean surface. Oceans of water are expected for hycean worlds. It is possible, however, that the ocean is too hot to be habitable or liquid.

“Although this kind of planet does not exist in our solar system, sub-Neptunes are the most common type of planet known so far in the galaxy,” explained team member Subhajit Sarkar of Cardiff University. “We have obtained the most detailed spectrum of a habitable-zone sub-Neptune to date, and this allowed us to work out the molecules that exist in its atmosphere.”

Characterizing the atmospheres of exoplanets like K2-18 b, that is, identifying their gases and physical conditions, is a hot topic in astronomy. However, these planets are literally outshone by the light of their much larger parent stars, making researching exoplanet atmospheres exceedingly difficult.

The team avoided this problem by analyzing light emitted by K2-18 b's parent star as it travelled through the exoplanet's atmosphere. K2-18 b is a transiting exoplanet, which means that its brightness decreases when it passes across the face of its host star. This is how the exoplanet was discovered in the first place. This means that a minuscule proportion of starlight will travel through the exoplanet's atmosphere before reaching observatories like Webb during transits. The passage of starlight through the atmosphere of an exoplanet leaves traces that astronomers can piece together to estimate the gases in the exoplanet's atmosphere.

“This result was only possible because of the extended wavelength range and unprecedented sensitivity of Webb, which enabled robust detection of spectral features with just two transits,” continued Madhusudhan. “For comparison, one transit observation with Webb provided comparable precision to eight observations with Hubble conducted over a few years and in a relatively narrow 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 our work here is but an early demonstration of what Webb can observe in habitable-zone exoplanets.”

The team plans to do additional study using the telescope's Mid-Infrared Instrument (MIRI) spectrograph, which they hope will corroborate their findings and provide new insights into the environmental conditions on K2-18 b.

“Our ultimate goal is the identification of life on a habitable exoplanet, which would transform our understanding of our place in the Universe," concluded Madhusudhan. "Our findings are a promising step towards a deeper understanding of Hycean worlds in this quest.”

The team’s results are accepted for publication in The Astrophysical Journal Letters.

James Webb Telescope

Webb is the largest, most powerful telescope ever launched into space. Under an international collaboration agreement, ESA provided the telescope’s launch service, using the Ariane 5 launch vehicle. Working with partners, ESA was responsible for the development and qualification of Ariane 5 adaptations for the Webb mission and for the procurement of the launch service by Arianespace. ESA also provided the workhorse spectrograph NIRSpec and 50% of the mid-infrared instrument MIRI, which was designed and built by a consortium of nationally funded European Institutes (The MIRI European Consortium) in partnership with JPL and the University of Arizona.

Webb is an international partnership between NASA, ESA and the Canadian Space Agency (CSA).

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