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.
One aspect has been common to both worlds, though. As in science fiction, real-world scientists have long viewed rocky planets that are just the right distance from their respective stars (or “within the habitable zone”) as being ideal candidates for supporting life. Given that life exists on Earth, is it not reasonable to suppose that any other inhabited planet will have conditions similar to our own?
Not anymore. Without ruling out the possibility that a rocky planet somewhere out there could be home to some form of life, a new school of scientific thought has been exploring another kind of world that could be potentially inhabitable — or even inhabited. Such as K2-18b, the subject of a promising new study published in The Astrophysics Journal Letters.
K2-18b is 2.6 times larger than Earth and an exoplanet, or a planet far from our Solar System: 124 light years away in this case, and orbiting its star in the habitable zone. It has now shown the strongest signs yet of a possible signature of life, scientists have announced — while also calling for caution in interpreting their findings, which they arrived at using data from NASA’s James Webb Space Telescope. K2-18b is not a rocky planet but a potential “Hycean world”, a new kind of inhabitable world first proposed by the same team of scientists.
Apart from the potential biosignatures themselves, the takeaways from the announcement include Webb’s role in performing what it was primarily designed for, the cautionary reminder that the findings may not mean what scientists hope they mean, and the promise of a new area to explore. All these years, were scientists looking in the wrong place?
Hycean worlds
The team behind the announcement has been pitching Hycean worlds, assuming these do exist, as ideal candidates to explore when looking for signs of alien life. This does not presume in any way that Hycean worlds are likelier to host life than rocky planets — no one can be sure where life exists until they actually detect it. The reason Hycean worlds are ideal candidates, the team behind the concept argues, is that they are easier to study than rocky planets. To put it simply, their larger atmospheric rings make them more suitable for the methods traditionally used.
So, what are Hycean worlds? Proposed by Cambridge University astronomer Nikku Madhusudhan and his team in 2021, these are a subset of a class of planets called sub-Neptunes, which are larger than Earth and smaller than Neptune. In September 2023, a scientist on the team had discussed the concept with Hindustan Times. Subhajit Sarkar, an astrophysicist with the University of Cardiff, is also among the authors of the latest paper on K2-18b.
“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.
