The Search for Life on Hycean Worlds: Could These Ocean Planets Host Microbial Life?

Hycean worlds are a recently proposed class of exoplanets with ocean-covered surfaces and hydrogen-rich atmospheres. They differ from rocky planets like Earth in size and atmosphere but may offer promising environments for life. The detection of carbon-bearing molecules, such as methane and carbon dioxide, in the atmosphere of the candidate Hycean world K2-18b with the James Webb Space Telescope (JWST) has sparked significant interest in these planets. This paper investigates how conditions on Hycean planets might influence the evolution of microbial life, focusing on how temperature impacts evolutionary rates and the potential for biosignature detection.

Modelling Evolution on Hycean Planets

To understand the possible evolution of life on Hycean planets, the authors use a neutral model of evolution that focuses on unicellular organisms like bacteria, archaea, and simple eukaryotic life. These models do not rely on specific environmental or biological pressures but instead simulate evolution based on changes in temperature and metabolic rates. This approach helps predict how long it would take for different groups of microorganisms to evolve under varying temperature conditions.

Methods

The study relies on the Metabolic Theory of Ecology (MTE), which links temperature and body size to metabolic rates and evolutionary speeds. The researchers calculate how much time it would take for specific groups of organisms to evolve under different median ocean temperatures compared to Earth. By adjusting the temperature by ±10 K (Kelvin), they estimate how evolutionary rates and origination times change.

The researchers focus on key groups of unicellular organisms, such as Cyanobacteria, Methanococcaceae, and Dinoflagellates, which play essential roles in Earth’s biosphere. These organisms are used as analogs to predict the potential biosphere composition on Hycean worlds.

Results

The study finds that even small changes in median ocean temperature can have a significant impact on evolutionary rates. A 10 K increase in temperature can more than double the evolutionary rate, allowing major groups of unicellular organisms to appear up to 2 billion years earlier than they did on Earth. Conversely, a 10 K decrease can severely slow evolution, delaying the emergence of key groups by several billion years.

For instance, in a warmer Hycean world, simple life forms could evolve into more complex organisms much faster than they did on Earth. In contrast, cooler Hycean worlds may remain dominated by simpler life forms for much longer periods.

Key Findings on Phytoplankton and Biosignatures

Phytoplankton, especially groups like Dinoflagellates, Coccolithophores, and Diatoms, are crucial for producing detectable biosignatures such as dimethyl sulfide (DMS). The study shows that on warmer Hycean planets, these phytoplankton could evolve within 1.3 billion years, producing biosignatures relatively early. On cooler planets, the appearance of these key biosignature producers might be significantly delayed or absent altogether, affecting our ability to detect life on such worlds.

Implications

This research suggests that Hycean planets with slightly warmer temperatures than Earth may be the best places to search for life. These worlds could host complex microbial biospheres at relatively young ages, making it easier to detect biosignatures in their atmospheres. On the other hand, colder Hycean planets may still harbor life, but it is likely to be simpler and less detectable.

The findings are particularly relevant for selecting future exoplanet targets for biosignature studies. For example, the candidate Hycean world K2-18b, with its potentially warm oceans, might be an excellent place to start searching for life beyond Earth.

Conclusion and Future Work

The study highlights how temperature plays a pivotal role in the evolution of life on oceanic worlds. Future research could explore other factors that may affect evolution on Hycean planets, such as gravity, pressure, and the availability of essential chemicals. Additionally, understanding how multicellular life might evolve in these environments could provide deeper insights into the potential complexity of extraterrestrial biospheres.

Source: Mitchell