Scientists Explore 'Steam Worlds' for Life Beyond Earth

Scientists are intensifying their investigation into so-called "steam worlds," a class of exoplanets known as sub-Neptunes that are rich in water but orbit so close to their stars that their surfaces are enveloped in thick steam atmospheres. On September 15, 2025, researchers from the University of California, Santa Cruz, announced a breakthrough in modeling these exotic worlds, offering new insights into their composition and evolution. This work, published in The Astrophysical Journal, leverages both laboratory data and observations from the James Webb Space Telescope (JWST) to refine our understanding of how water behaves under extreme conditions found on these planets.

New Models and Observational Breakthroughs

Traditional models for characterizing water-rich exoplanets were based on the icy moons of our solar system, such as Europa and Enceladus. However, sub-Neptunes are much larger and hotter, lacking icy crusts and liquid oceans. Instead, they possess thick steam atmospheres and layers of supercritical water—a phase neither gas nor liquid. The new models developed by the UC Santa Cruz team account for these unique conditions, incorporating experimental data on water's behavior at high pressures and temperatures. This approach is crucial as JWST has recently confirmed the presence of steam on several sub-Neptunes, and is expected to observe many more in the near future.

Implications for the Search for Life

The discovery and characterization of steam worlds could significantly broaden the scope of astrobiology. While these planets are too hot for liquid water on their surfaces, their interiors may host exotic phases of water, such as superionic ice, which could create new niches for life. Professor Natalie Batalha, head of UC Santa Cruz's astrobiology initiative, emphasized that understanding the most common types of planets in the universe could help scientists focus on rarer, potentially habitable worlds. The interiors of these planets serve as natural laboratories for studying conditions that are difficult to replicate on Earth, potentially leading to unforeseen scientific applications.

Next Steps and Future Observations

The L 98-59 system, located just 35 light-years from Earth, exemplifies the diversity of exoplanetary systems. Recent studies using data from TESS, HARPS, ESPRESSO, and JWST have revealed a range of rocky and water-rich planets, including one in the habitable zone where liquid water could exist. These findings highlight the importance of combining space- and ground-based observations with innovative data analysis techniques. As JWST and other instruments continue to probe these worlds, scientists hope to unravel the mysteries of their atmospheres and interiors, bringing us closer to answering the age-old question of whether life exists beyond Earth.