Scientists have made exciting progress in understanding “steam worlds.” These are planets larger than Earth but smaller than Neptune, and they are too hot for liquid water. Instead, their atmospheres are filled with water vapor. While steam worlds are unlikely to support life, studying them can help us learn more about ocean planets. This knowledge is crucial as we search for life beyond our solar system.
The most commonly found extrasolar planets are called sub-Neptune planets. They are generally rich in water and often orbit very close to their stars, making their surfaces too hot for liquid water. Instead, water exists in unique forms, creating steamy atmospheres.
Recent research has introduced a new, more accurate model for these steam worlds. This model helps scientists understand the composition and origin of these planets by accounting for the complex behavior of water in extreme conditions that we can’t easily replicate on Earth.
Artem Aguichine, a researcher at the University of California, Santa Cruz, noted, “When we understand how the most commonly observed planets in the universe form, we can shift our focus to less common exoplanets that could actually be habitable.” Water plays a key role in the complexity needed for life, making this research particularly important.
Interest in steam worlds has surged since October 2024, when the James Webb Space Telescope discovered the exoplanet GJ 9827 d. This planet, twice the size of Earth and 100 light-years away, has an atmosphere almost entirely made of water vapor, marking it as the first confirmed steam world.
Since then, the James Webb Space Telescope has identified steam in several sub-Neptune planets. This highlights the urgency of developing models to connect their atmospheres with internal structures. Previous models focused on icy moons in our solar system, like Saturn’s Enceladus and Jupiter’s Europa, but those don’t provide a good comparison for steam worlds.
The difference is stark: sub-Neptune planets can be ten to 100 times more massive than typical icy moons. Additionally, they orbit much closer to their stars, resulting in steamy atmospheres rather than icy surfaces. This means their water exists in unusual states, including something known as “supercritical water,” which has properties of both gas and liquid but behaves differently under extreme pressure.
Natalie Batalha, another researcher, explained, “The interiors of planets are natural ‘laboratories’ for studying conditions difficult to reproduce here.” By understanding water under different states, scientists can gain insights into potential future discoveries. She added, “In the future, we may find that some of these water worlds represent new niches for life in the galaxy.”
Understanding how these planets change over time is also vital. The new model considers their evolution over billions of years, not just their current state. This modeling could be tested further with the European Space Agency’s upcoming PLATO mission, which aims to find Earth-sized planets in their stars’ habitable zones. Aguichine stated, “PLATO will help us verify our models and guide our future research on life beyond Earth.”
The research findings were published in The Astrophysical Journal on July 24. For more details, you can find the full study here.
