Hycean worlds: A new frontier in search for extraterrestrial life

Scientists have found strong signs of possible life on exoplanet K2-18b, a potential "Hycean world," suggesting new avenues in the search for alien life.

Public perceptions of alien life, shaped largely by science fiction, often revolve around images of humanoid beings or caricatures of animals roaming a planet similar to our own. In the real world, scientists usually look for no more than microbial life; in fact, they look only for signs that indicate such life could exist away from Earth.

A handout artist's impression released on April 17, 2025 by N. Madhusudhan/University of Cambridge, shows the K2-18b super-Earth, a hycean world, such as exoplanet K2-18b which astronomers say they have found the strongest yet �hints� of the life outside this solar system. (Photo by Handout / University of Cambridge / AFP) (AFP)

One aspect has been common to both worlds, though. As in science fiction, real-world scientists have long viewed rocky planets that are just

“We do not have any sub-Neptunes in our own Solar System. The interior structures of such planets are not well understood and different possible solutions exist to explain their densities. One of these solutions allows for a shallow hydrogen-rich atmosphere above a water ocean layer. This is the ‘Hycean world’ model,” Sarkar had said then.

In essence, there could be various scenarios that give sub-Neptunes such densities, and one possible scenario includes the water ocean layer. And if such water does exist, it raises the possibility of life. The hunt for life, therefore, hinges on detecting any signs this hypothesised water is willing to show an observer on Earth.

This is the essence of any search for signs of alien life. It is unrealistic to expect an alien lifeform, whatever their size, to show up from light years away on any telescope, even one as powerful as Webb. What scientists look for, therefore, is biosignatures based on chemistry.

If water exists on a planet, it will result in the presence of certain chemicals in the planet’s atmosphere. If organisms themselves are producing chemicals that are not known to be formed by inorganic means, these chemicals will leave their own signatures on the atmosphere. Different chemicals in the atmosphere interact differently with light, impacting the way this light reaches faraway Earth. If the light observed has a certain wavelength, it could indicate the presence of a certain chemical that is known to emit light at that wavelength. The science we know as transmission spectroscopy reads these wavelengths, and scientists interpret these readings to predict what chemicals exist on the planet, and then hypothesise what — or who — could be producing these chemicals.

In rocky planets, transmission spectroscopy is difficult because the process relies on how the atmospheric ring absorbs the background starlight. In a sub-Neptune, on the other hand, the atmosphere is rather expanded because of its composition, and the planet itself is large, which means more background starlight can be blocked leading to stronger spectral features, and a better chance to see any molecular signatures.

“So, while at this time, we have no idea if Hycean planets are more or less likely to host life than rocky planets, if biosignatures did exist, they would be easier to detect on the Hycean planet,” Sarkar had said.

What the new findings mean

Using Webb data, the team has now detected signs that indicate the presence of two chemicals that are strongly associated with life as earthlings know it. These are dimethyl sulphide (DMS) and dimethyl disulphide (DMDS); the signs indicate either or both could be present.

On Earth, these chemicals are only produced by life, primarily a certain section of marine algae. This finding comes with promise as well as a red flag. While it provides very strong evidence for a potential biosignature, what if there is some unknown chemical process that is producing these molecules in K2-18b’s atmosphere?

This is not the first time such signatures have been detected. In September 2023, HT had reported findings (against based on Webb readings) about the possible presence of DNS in the exoplanet’s atmosphere. Back then, Sarkar had been cautious while explaining this to HT: the signal was quite weak. Now it is stronger, thanks to the use of better technology on Webb.

The question researchers ask themselves in such cases is: could the observations have happened by chance? There is a 0.3% probability that they did, Cambridge University said in a statement. This is called the “three-sigma” level of statistical significance. To be formally classified as a scientific discovery, the observations have to cross the “five-sigma” level (or a 0.00006% probability of being a fluke).

That said, the findings are an improvement the previous results, which used Webb instruments that covered wavelengths in the 0.8-5-micron range. The new findings using a different instrument are also independent, covering an entirely different range of wavelengths in the 6-12-micron range.

In a statement on the Cambridge website, Madhusudhan remains as cautious as last time: “It’s important that we’re deeply sceptical of our own results, because it’s only by testing and testing again that we will be able to reach the point where we’re confident in them. That’s how science has to work.”

It may be some time before science can confirm if the chemicals associated with life on Earth do exist in the exoplanet’s atmosphere, or if there is indeed an unknown chemical process that is producing these chemicals without involving life. As of now, the very announcement reinforces a newly growing promise. If science wants to detect life, a rocky planet may not be the only place to look.