Is there life in other worlds? We still don't know, but the hunt has picked up since the launch of the James Webb Space Telescope in 2021, and intriguing new possibilities have emerged.
Beyond the Solar System, 120 light years from Earth, an exoplanet called K2-18b orbits the star K2. Data from its atmosphere, captured by Webb, has now revealed the presence of carbon dioxide and methane, both strong indications of life. This does not necessarily establish that life exists there, but two additional aspects make K2-18b an exciting candidate for research.
One is a "tentative" finding, which will need to be validated: the atmosphere of K2-18b also appears to contain dimethyl sulphide, a chemical very strongly associated with life. On Earth, it is produced by algae and related lifeforms; no other way of production is known.
The other exciting aspect is the possibility that K2-18b belongs to a proposed class of celestial bodies called Hycean worlds. Those behind the idea pitch Hycean worlds as ideal candidates for research into the possibility of extraterrestrial life.
"The concept of Hycean worlds was only introduced recently, in 2021, and was a theoretical advancement," astrophysicist Nikku Madhusudan of Cambridge University said in an email response. Madhusudan had led the team that first proposed the concept (The Astrophysics Journal, 2021) and is also the lead author of the latest research on K2-18b. The new paper has been accepted for publication in The Astrophysical Journal Letters, he said.
"Hycean worlds are substantially more accessible for atmospheric observations and are therefore more likely to reveal possible signatures of life than rocky planets, if they indeed harbour life," Madhusudan said.
Before going into what Hycean worlds are like, it helps to understand first what it means to search for "signatures of life".
What to look for
If there is life on a planet or faraway moon, the signs would likely be in the mix of gases in its atmosphere. If the mix is similar to that in Earth's atmosphere, with oxygen, carbon dioxide and methane, these would be an indication (not confirmation) of life.
To study the atmospheric composition, scientists use transmission spectroscopy, the science of examining the spectra of light in the atmosphere, as observed from Earth. Since different substances absorb and emit light at different wavelengths, reading those wavelengths helps identify those substances.
Spectroscopy is also used to examine the chemistry of nearer celestial bodies. Like Jupiter's moon Europa, for instance. In two new studies published in Science, scientists have inferred that the solid carbon dioxide on Europa's surface (detected earlier) was produced from an internal carbon source. Carbon dioxide was likely formed directly in the ocean beneath Europa's icy crust, the papers propose, although they do not rule out its formation on the surface (from matter sourced from the ocean).
"In all of those possibilities, the carbon we see on the surface is primarily originating from the internal ocean, which is an important finding for eventually understanding the habitability of Europa's ocean," planetary scientist Samantha Trumbo of Cornell University, author of one of the two papers, told HT.
At times, results can look promising but end in disappointment. In 2018, Nature published a paper suggesting that the atmosphere of Venus contained phosphine, a gas that is synthesised by bacteria on Earth. A subsequent paper, however, found errors in that study and ruled out phosphine.
To come back to exoplanets, the search for signs of life involves some key considerations. One, the planet must be in the "habitable zone", or at just the right distance from its star for water to exist as a liquid. Two, what kind of exoplanet is ideal for atmospheric studies? This is where Hycean worlds come in.
The new possibilities
Among exoplanets are a group called sub-Neptunes, with sizes between those of Earth and Neptune. Studies have made different possible models for what they are made of, and one of these is the "Hycean world" model. A Hycean world is a sub-Neptune with a shallow hydrogen-rich atmosphere above a water ocean layer.
"Further theoretical work indicated that if such an ocean exists, we should see a specific pattern of gases in the atmosphere. In our observation of K2-18 b we in fact see such a pattern, and so we think a water ocean could be present… and so this raises the possibility of such worlds harbouring life," said astrophysicist Subhajit Sarkar of Cambridge, one of the authors of the new paper.
So far, rocky planets in the habitable zone have been the focus in the search for life because they are thought likely to host liquid water oceans, like Earth does. Now, the idea of Hycean worlds with oceans offers a possible alternative. For various technical reasons, it is harder to obtain an atmospheric spectrum from rocky planets than it is from sub-Neptunes. "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 said.
As for the findings, Nasa has announced the detection of carbon dioxide and methane on K2-18b but made it clear that the inference on dimethyl sulphide is "less robust and requires further validation". Madhusudan and Sarkar, too, stressed this point.
The detection of methane itself is a cause for excitement. While methane is expected to be abundant in cool hydrogen-rich atmospheres, it had not been found previously in exoplanets with such conditions. "Our detections of methane and carbon dioxide are the first detections of any carbon-bearing molecules in a habitable-zone exoplanet," Madhusudan said.
It is important to note here that methane, carbon dioxide or even dimethyl sulphide, if validated, are "signatures of life", not life itself. In the search for these signs, the best that scientists might manage is an estimate of probability of life, Nasa says on its website.
"Still, an exoplanet with, say, a 95% probability of life would be a game changer of historic proportions."
We are getting closer to first contact.