Hycean worlds and the promise of extraterrestrial life

The possibility of the existence of hydrogen worlds represents one of the most exciting ideas in astrobiology. Although their existence has not yet been confirmed, the idea of planets with vast oceans and hydrogen-rich atmospheres opens new doors in the search for life beyond Earth. In today's Astrobito, we tell you how these hypothetical planets could transform our understanding of the cosmos and bring us closer to an answer to one of humanity's biggest questions: are we alone in the universe?

Paper details:

Title: Prospects for biological evolution on Hycean worlds

Authors: Emily G. Mitchell and Nikku Madhusudhan

First-author institution: Department of Zoology, University of Cambridge, Cambridge, UK

Status: Open access in arXiv and published on MNRAS

The search for extraterrestrial life has long captured the imagination of humanity. For centuries, we've gazed at the stars, pondering whether anyone—or anything—is out there. With the rapid advancements in our space exploration capabilities, we are closer than ever to answering this age-old question. One of the most exciting developments in this quest could be the discovery of a new class of exoplanets: Hycean worlds.

Hycean worlds offer a fresh and promising angle on the hunt for life beyond our solar system. These planets are unique in that they feature vast, ocean-covered surfaces and thick, hydrogen-rich atmospheres. While they may not look exactly like Earth, these planets possess the potential to support life—specifically, microbial life that could resemble the earliest forms of life that evolved in Earth’s primordial oceans.

In today’s astrobito, we explore how Hycean worlds could be the key to unlocking the search for biosignatures—indicators that life might exist or have once existed. With new observational tools like the James Webb Space Telescope (JWST) coming online, these planets are quickly becoming prime targets for exploration.

The Ingredients for Life: Oceans, Atmospheres, and Chemistry

Hycean worlds differ from Earth in several key ways, but they still have one crucial similarity: the presence of liquid water. These planets feature expansive oceans, which could provide a stable environment for microbial life to thrive. Beneath these oceans, their volatile-rich interiors create conditions conducive to life, making Hycean planets a compelling focus for astrobiologists.

One of the most important features of these worlds is their hydrogen-rich atmospheres. The combination of abundant hydrogen and carbon dioxide in the atmosphere could provide the chemical energy required for microbial life to evolve. On Earth, the first living organisms emerged in a similarly hostile, primordial environment, using available chemicals to produce energy and develop into more complex life forms.

Additionally, these planets’ large sizes and extended atmospheres make them ideal for observation. According to this study, the JWST has already made strides in detecting carbon-bearing molecules—such as methane and carbon dioxide—in the atmosphere of one candidate Hycean world, K2-18b. These discoveries suggest that Hycean worlds might harbour the ingredients for life, providing tantalising possibilities for future research.

Temperature: A Critical Factor for Life

One of the central findings in this study is the role of temperature in supporting life. On Hycean worlds, even relatively small changes in ocean surface temperature can significantly impact the rate at which life evolves. For example, the authors found that a mere 10°C increase in ocean surface temperature could double the rate of biological evolution. This suggests that warmer Hycean worlds might experience faster rates of speciation—the process by which new species emerge—than we observe here on Earth. In fact, the authors propose that life on these planets could evolve so quickly that key microbial groups, such as phytoplankton, might emerge in just 1.19 billion years, far sooner than they did on Earth.

The rate of biological evolution on Earth can accelerate considerably with small increases in temperature. The red lines show the rate of evolution on Earth when the temperature increases by 10°C, and the blue lines show it when the temperature decreases by 10°C compared to normal Earth temperatures (black line).

This faster evolutionary pace could be crucial for detecting biosignatures. If life evolves rapidly, we could potentially detect biomarkers—such as dimethyl sulphide (DMS), a compound produced by plankton on Earth—in the atmospheres of these planets much earlier than we might on colder, slower-evolving worlds.

Conversely, the study suggests that a decrease in surface temperature—by just 10°C below Earth's—could significantly delay the origin of complex life forms. In such conditions, the authors found that only simpler microbial groups would evolve, with more complex organisms like eukaryotes and oxygen-producing phytoplankton failing to emerge. This could limit the types of biosignatures we could detect on cooler Hycean worlds.

Hycean Worlds: A New Frontier in the Search for Life

The promise of Hycean worlds lies not only in their potential to support life, but also in their ability to increase the number of habitable planets we can study. Most exoplanets discovered to date have been rocky and orbiting sun-like stars. However, Hycean worlds orbit cooler, smaller stars known as M-dwarfs, which are far more abundant throughout the galaxy. This presents an exciting opportunity to discover more potentially habitable planets—and possibly life—across a larger portion of the cosmos.

The study’s authors also point out that Hycean worlds are more likely to have atmospheric conditions that are conducive to the detection of life. Their thick, hydrogen-rich atmospheres offer a greater chance of revealing chemical signatures through telescopic observation. As a result, these planets are emerging as key targets for future research, especially with tools like JWST capable of detecting biomarkers in their atmospheres.

K2-18b, one of the best-known candidate Hycean worlds, is a prime example. The planet is around 2.4 billion years old, much younger than Earth, which means it could provide a glimpse into the early stages of life’s evolution. If K2-18b harbours microbial life, the faster rate of evolution predicted by the authors could make it a prime candidate for detecting biosignatures much earlier in the planet’s history than we would on Earth-like planets.

An artist's impression of exoplanet K2-18b. Image taken from https://science.nasa.gov/exoplanet-catalog/k2-18-b/

Looking Ahead: The Future of the Search for Life

As we look ahead, the possibility of discovering life on Hycean worlds represents one of the most exciting frontiers in astrobiology. This study makes it clear that, depending on a planet’s temperature and atmospheric conditions, microbial life on warmer Hycean worlds could evolve much faster than on Earth, increasing the chances of detecting biosignatures. In contrast, cooler worlds may harbour simpler life forms, if any at all.

The findings of this research provide a roadmap for future studies, showing that a modest increase in temperature on Hycean worlds can lead to faster evolutionary rates and more complex biosignatures. This means that warmer Hycean planets—especially those that are younger than Earth—could be prime targets for future missions focused on detecting life.

Ultimately, the discovery of Hycean worlds could shift the focus of the search for life beyond Earth, giving us new and promising targets to explore. With continued advances in technology and observation, we may soon be able to answer one of humanity’s most profound questions: Are we alone in the universe?

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