Have Cambridge astronomers found the first signs of life on an exoplanet using the James Webb Space Telescope?

Have signs of life been discovered on a planet 110 light years away by an international team of scientists, led by the University of Cambridge?

The tantalising prospect emerged as data from the James Webb Space Telescope confirmed methane and carbon dioxide had been detected in the atmosphere of K2-18 b, an exoplanet in the ‘Goldilocks zone’ – not too hot and not too cold to support life.

It is the first time carbon-based molecules have been discovered in the atmosphere of an exoplanet in the habitable zone – but there was also another, weaker signal in the K2-18 b spectrum.

The researchers carried out several analyses and believe that signal could only be caused by a molecule called dimethyl sulphide (DMS).

On Earth, this is only produced by life – primarily microbial life like marine phytoplankton – which hints at the possibility of biological activity on K2-18 b.

The researchers are cautious, however, saying these signs are tentative and require further validation.

But what the findings do suggest is that K2-18 b and other such ‘Hycean planets’ could be our best chance to find life outside of our own solar system.

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

K2-18 b lies 110 light years from Earth in the constellation of Leo and is 8.6 times the size of Earth. It orbits a cool dwarf star, K2-18, in the habitable zone.

The Hubble Space Telescope provided us with the first insight into its atmosphere, but there has been debate over the findings.

The same research team examined it in 2021 and 2021, identifying it as belonging to this new class of habitable exoplanets called ‘Hycean’ worlds, which are hot, water-covered planets with a hydrogen-rich atmosphere.

Those findings prompted them to look again with the power of Hubble’s successor, the James Webb Space Telescope, to understand the constituent gases and physical conditions of its atmosphere.

In the race to find life elsewhere, this is an area of huge focus in astronomy but it is also challenging as the glare of much larger parent stars typically outshines the planets.

Astonishingly, the researchers solved this conundrum by analysing the light from K2-18 b’s parent star as it passed through the exoplanet’s atmosphere.

K2-18 b is what is known as a transiting exoplanet, meaning as it passes across the face of its host star, we can detect a drop in the stellar brightness – the method used to discover the planet in 2015.

During these transits, a tiny fraction of starlight passes through the exoplanet’s atmosphere before reaching Earth, leaving a trace in the stellar spectrum that astronomers can examine.

“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 Prof Madhusudhan, lead author of a paper being published in The Astrophysical Journal LettersThe Astrophysical Journal Letters. “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.”

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

Exoplanets such as K2-18 b, which are of sizes between that of Earth and Neptune, are unlike anything in our solar system and remain poorly understood.

“Our findings underscore the importance of considering diverse habitable environments in the search for life elsewhere,” said Prof Madhusudhan. “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 lack of ammonia – is indicative of an ocean beneath a hydrogen-rich atmosphere, but the findings on DMS are less certain.

“More observations are needed to determine whether it is in fact DMS that we’re seeing,” said Prof Madhusudhan.

“The possibility of DMS in the atmosphere is highly promising, but we are planning to take another look to robustly establish its presence.”

Despite that promise. K2-18 b might not be able to support life. With a radius 2.6 times that of Earth, researchers say the interior probably contains a large mantle of high-pressure ice, like Neptune, but with a thinner hydrogen-rich atmosphere and an ocean surface that may be too hot to be habitable, or be liquid.

More observations and theoretical work are needed, they say, and they plan follow-up work to validate their findings and provide further insight.

When they next use Webb, it will be to harness the telescope’s Mid-InfraRed Instrument (MIRI) spectrograph to scour K2-18 b’s atmosphere for biomarkers, including DMS, which could yet indicate the presence of biological activity.

“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.”

The results were presented on Monday at the First Year of JWST Science Conference in Baltimore, Maryland, USA.