When it comes to considering the possibility of life existing away from Earth, one of the key qualities scientists look for is the existence of liquid water, which is crucial for living things as we understand them. 

In other star systems, the region at which water can exist in its liquid state is defined as the “habitable zone” or the “Goldilocks zone” due to the fact these planets, like the bears’ porridge preferred by the fairy tale character of the same moniker, are neither too hot nor too cold — in this case, to support water.

The habitable zone is fairly well-defined for rocky or terrestrial planets like Earth, which have understandably dominated scientists’ minds when they think of finding life on extra-solar planets or “exoplanets”. After all, there is only one planet in the Universe we are aware of that hosts life, so it is little wonder the investigation into life outside the solar system has hunted conditions similar to those found on our own planet.  

In 2021, however, scientists, including University of Cambridge professor of astrophysics and exoplanetary science, Nikku Madhusudhan, really started thinking outside the box and outside the habitable zone when it comes to planets that could support life.

“The word hycean stands for the combination of a hydrogen-rich atmosphere over an ocean world, basically,” Madhusudhan said. “These are planets that are expected to have planet-wide oceans that are habitable underneath atmospheres, thin atmospheres that are hydrogen-rich.”

Madhusudhan and colleagues suggested that ocean worlds, called “hycean” exoplanets, could be prime targets in the hunt for extraterrestrial life, and should perhaps reshape  this fascinating investigation. 

Redefining the Goldilocks zone

These worlds stand to redefine the concept of a habitable planet because as a result of their hydrogen-rich atmospheres, they would be capable of hanging on to their liquid oceans slightly closer to their stars than rocky planets. Even more excitingly, this hycean habitable zone could extend indefinitely away from that parent star, with water protected from turning to ice.

“These planets can be as far as you want from the star and still be habitable. In fact, these planets can also be free-floating in space and still be habitable,” Madhusudhan said. “That is because the atmosphere of hycean worlds is hydrogen-rich and the hydrogen can act as a very efficient greenhouse gas and keep the surface of the planet warm enough for liquid water to be sustained away from the heat of a star.”

And as well as enjoying a much wider habitable zone around their target stars, hycean worlds also benefit from the fact that their watery composition makes them less dense than Earth-like planets. This means that given two planets with the same mass, the hycean planet can reach a much larger size than the terrestrial planet.

While that doesn’t make them any likelier to host life, it does make hycean exoplanets targets that our telescopes are more likely to spot, This greater size also makes it easier to investigate hycean planets than smaller terrestrial planets.

“You can detect them more, and you can observe their atmospheres better, and there are more of them compared to rocky planets in the habitable zone,” Madhusudhan said. “We know of many more planets that fall in this hycean category around nearby stars.”

New results from the James Webb Space Telescope (JWST) obtained by a team of researchers led by Madhusudhan have indeed verified that the focus on hycean exoplanets and the excitement around them is warranted.

K2–18 b a very significant exoplanet

Studying the exoplanet and suspected hycean world K2–18 b with the JWST, Madhusudhan and his team found that the planet’s atmosphere contains traces of carbon molecules in the form of carbon dioxide and methane. Also particularly significant about this detection is what was missing in the atmosphere of the planet.

“We found some molecules, especially carbon-rich molecules, methane, and we did not find ammonia,” Madhusudhan said. “That is a smoking gun, if you will; very strong evidence that there ought to be some sort of an ocean surface underneath a hydrogen rich atmosphere on K2–18 b.”

The researcher added that scientists are currently aware of dozens of other candidate hycean planets and that is type of investigation is exactly what they need to be doing in order to discover if these worlds possess liquid water oceans.

After this is confirmed, scientists can start to hunt for signs of living processes called “biomarkers” to confirm if there is indeed life in these oceans. Excitingly, the JWST investigation of K2–18 b has indeed turned-up very provisional evidence of what could turn out to be such a biomarker.

Madhusudhan and his colleagues detected traces of dimethyl sulfide around K2–18 b, which is located 120 light years from our planet.  This finding is significant because on Earth, the presence of this molecule in our atmosphere is a clear indicator of life. To be more precise, dimethyl sulfide is primarily generated by phytoplankton through the process of photosynthesis here on our own planet.

Madhusudhan does urge caution when considering this detection, however. Not only does it need to be confirmed with further investigation with the JWST but even then its presence won’t confirm that this ocean world does indeed harbour life as the team will need to rule out it isn’t being created as the result of non-biological processes.

Even if that is the case, K2–18 b will remain a special world for Madhusudhan and a significant one for exoplanet researchers. That is because not only could it be the first confirmed hycean world, but it is the planet that got Madhusudhan and his colleagues pondering about the possibility of ocean plants to begin with two years ago.

“The first atmospheric observations were coming in for K2-18 b and they were showing some spectral features initially thought to be due to water vapor,” he said. “Historically if you think of habitable planets, the notion was that they have to be rocky and roughly Earth-like and Earth-size to be habitable but this planet is two and a half times Earth radius, and then about nine times Earth’s mass.

“So it was too big for people to think that it could be habitable.”

Nevertheless, the team tried to understand what is the possible interior composition of K2–18 b was and while exploring all possibilities, they found although this planet is significantly larger than Earth and it likely has a hydrogen rich atmosphere with surface liquid water that could mean its habitable, perhaps only by simple organisms.

Madhusudhan believes that K2–18 b is the tip of the iceberg for hycean worlds and thinks the next decade of exoplanet research will heavily focus on these ocean bearing planets. 

“Hycean worlds present our best opportunity to search for habitable environments and life elsewherein the universe over the next 10 years with with the JWST,” Madhusudhan concluded. “If we can observe these these hycean world candidates, we could make some major advancements in our understanding of planetary habitability and life away from Earth.”

References: N. Madhusudhan., et al., Habitability and Biosignatures of Hycean Worlds, The Astrophysical Journal, (2021), DOI: 10.3847/1538–4357/abfd9c

N. Madhusudhan., et al., Carbon-bearing Molecules in a Possible Hycean Atmosphere, (accepted for publication in The Astrophysical Journal), (2023), pre-print: https://arxiv.org/abs/2309.05566

Feature image credit: Pixabay