Methane and carbon dioxide found in atmosphere of habitable-zone exoplanet

Exoplanets such as K2-18 b, which have sizes between those of Earth and Neptune, are unlike anything in our Solar System. This lack of analogous nearby planets means that these ‘sub-Neptunes’ are poorly understood and the nature of their atmospheres is a matter of active debate between astronomers. 

“Our findings underscore the importance of considering diverse habitable environments in the search for life elsewhere,” said lead author Professor Nikku Madhusudhan, from Cambridge’s Institute of Astronomy. “Traditionally, the search for life on exoplanets has focused primarily on rocky planets, but Hycean worlds are significantly more conducive to atmospheric observations.”

The abundance of methane and carbon dioxide, and dearth of ammonia, are consistent with the presence of an ocean underneath a hydrogen-rich atmosphere in K2-18 b. The inference of DMS, however, is less robust and requires further validation. “More observations are needed to determine whether it is in fact DMS that we’re seeing,” said Madhusudhan. “The possibility of DMS in the atmosphere is highly promising, but we are planning to take another look to robustly establish its presence.”

While K2-18 b looks like a highly promising candidate in the search for life elsewhere, it is possible that it may not be able to support life. The planet’s large size — with a radius 2.6 times the radius of Earth — means that the planet’s interior likely contains a large mantle of high-pressure ice, like Neptune, but with a thinner hydrogen-rich atmosphere and an ocean surface. It is possible that the ocean is too hot to be habitable or be liquid. More observations and theoretical work are needed to establish this conclusively.

“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,” said co-author 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.”

Characterising the atmospheres of exoplanets like K2-18 b — meaning identifying their constituent gases and physical conditions — is an area of frenzied activity in astronomy. Determining the chemicals present in the atmospheres of exoplanets is vital to understanding these alien worlds and provides tantalising hints about habitability elsewhere in the Universe. However, these planets are outshone — literally — by the glare of their much larger parent stars, which makes exploring exoplanet atmospheres challenging. 

The team sidestepped this challenge by analysing light from K2-18 b’s parent star as it passed through the exoplanet’s atmosphere. K2-18 b is a transiting exoplanet, meaning that we can detect a drop in the stellar brightness as it passes across the face of its host star. This is how the exoplanet was first discovered in 2015. This means that during transits a tiny fraction of starlight will pass through the exoplanet’s atmosphere before reaching Earth. The starlight’s passage through the atmosphere leaves ghostly traces in the stellar spectrum that astronomers can piece together to determine the constituent gases of 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,” said Madhusudhan. “For comparison, one transit observation with Webb provided comparable precision to eight observations with Hubble conducted over a few years in a shorter wavelength range.”

“These results are the product of just two observations of K2-18 b, with many more on the way,” said co-author Savvas Constantinou, also from Cambridge’s Institute of Astronomy. “This means our work here is but an early demonstration of what Webb can observe in habitable zone exoplanets.”

The team now intends to conduct follow-up research that they hope will further validate their findings and provide new insights into the environmental conditions on K2-18 b. The team's next round of Webb observations will use the telescope's Mid-InfraRed Instrument (MIRI) spectrograph to scour K2-18 b's atmosphere for tell-tale chemical signatures called biomarkers, including DMS, which could potentially indicate the presence of biological activity.

“Our ultimate goal is the identification of life on a habitable exoplanet, which would transform our understanding of our place in the Universe,” said Madhusudhan. “Our findings are a promising first step in this direction.”

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Reference:
Nikku Madhusudhan et al. ‘Carbon-bearing Molecules in a Possible Hycean Atmosphere.’ Paper presented at The First Year of JWST Science Conference. Baltimore, Maryland, USA.

Adapted from a story by the European Space Agency (ESA).