Unusual “steam worlds” could hold the key to understanding where life may exist beyond Earth, according to new findings by astronomers.
Recent models from UC Santa Cruz examine how water behaves under the extreme pressures and temperatures found inside sub-Neptune planets. In these environments, thick vapor atmospheres may conceal unusual phases of water, such as supercritical fluids and superionic ice.
These findings could help guide observations from the James Webb Space Telescope and other future missions to explore the most common type of planet found in our galaxy.
Worlds Wrapped in Steam
Sub-Neptunes, which are larger than Earth but smaller than Neptune, account for the majority of exoplanets identified to date. Unlike the icy moons found in our solar system, these planets orbit close to their stars and are covered by dense vapor atmospheres. Due to temperatures exceeding conditions for liquid water to exist, researchers have questioned whether these environments could still support the chemistry needed for life.
Although ‘steam worlds’ were first proposed about twenty years ago, they are now supported by observations. The James Webb Space Telescope has detected water vapor around several sub-Neptunes, with many more expected to be studied in the near future. For now, details on the planet interiors beneath the atmosphere remain relatively unknown.
Modeling Exotic Water
To address this, the UC Santa Cruz team, led by postdoctoral researcher Artem Aguichine, along with Natalie Batalha and Jonathan Fortney, developed a new modeling approach for water-rich planets. Their study, published in The Astrophysical Journal, incorporates laboratory data on water under extreme conditions, extending beyond earlier models that were designed for icy moons such as Europa or Enceladus.
Within sub-Neptunes, water can exist in forms beyond vapor or liquid. Under high heat and pressure, it may become a supercritical fluid, which has properties of both liquids and gases. Water may also assume the form of superionic ice, an unusual state in which hydrogen ions migrate through a fixed pattern of oxygen atoms. Scientists have managed to replicate both conditions for short periods in the lab, but predicting their full behavior remains a challenge.
“The interiors of planets are natural ‘laboratories’ for studying conditions that are difficult to reproduce in a university laboratory on Earth,” Batalha explained. “The water worlds are especially exotic in this sense.”
From Atmospheres to Interiors
By integrating these exotic phases into their models, the researchers aim to better connect what telescopes observe in a planet’s atmosphere with what is happening deep within the planet. “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,” Aguichine said.
Sub-Neptunes differ from icy moons in both mass and structure; they are ten to a hundred times more massive and lack a frozen surface. However, their dense steam atmospheres and supercritical water layers could support unfamiliar types of chemistry.
Exploring Signs of Life
The team’s models also account for how sub-Neptunes change over time by simulating the movement of water within a planet’s interior over millions or billions of years. This evolutionary perspective may be important for identifying signs of habitability in different planetary systems.
Upcoming observations will provide opportunities to test these predictions. The James Webb Space Telescope will study additional sub-Neptunes, while the European Space Agency’s PLAnetary Transits and Oscillations of stars (PLATO) mission will contribute additional data in the future.
Although sub-Neptunes may be too hot to support life as we know it, studying their water-rich interiors could influence the broader search for habitable environments. Since these are the most commonly found type of planet in our galaxy, exploring their structure helps astronomers further understand how water, one of the universe’s most abundant molecules, impacts planetary evolution and the potential for life.
“In the future, we may find that a subset of these water worlds represents new niches for life in the galaxy,” Batalha concludes.
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds a Master of Business Administration and a Bachelor of Science in Business Administration, along with a certification in Data Analytics. His work combines analytical training with a focus on emerging science, aerospace, and astronomical research.