An international team of scientists studying potential water worlds has determined that although sub-Neptune exoplanets made up of mostly water are likely rare, Earthlike planets may be much more common than previously thought.
Led by researchers from ETH Zurich, the Max Planck Institute for Astronomy in Heidelberg, and the University of California, Los Angeles (UCLA), the findings may decrease the overall chances of life in the cosmos since water worlds were a common target for astrobiologists, but increase the chances of finding life on Earthlike planets that are yet to be discovered.
In the 30 years since astronomers first discovered a planet outside of our solar system, over 5,000 exoplanets have been spotted orbiting distant stars. Most are larger worlds since they are easier to spot with current telescopes. Although many of these larger worlds are gas giants like Jupiter that would be incapable of supporting life as we know it, scientists searching the cosmos for signs of life have focused a lot of attention on a smaller class of common exoplanets called sub-Neptunes.
Larger than Earth but smaller than Neptune, many of these planets formed far away from their host stars beyond what is known as the ice barrier, where surface water would freeze. Some planet formation models have suggested that these sub-Neptunes gather their H2O beyond the ice barrier and then the ice melts to form largely ocean worlds with hydrogen-rich atmospheres as they drift closer to the host star. Sometimes called water worlds, scientists refer to these planets as Hycean, a combination of “hydrogen” and “ocean.”
Because life on Earth requires water, astrobiologists have targeted water worlds as one potential place to look. The excitement was raised further in April 2024 when University of Cambridge astronomers spotted a sub-Neptune hycean world called K2-18b that looked like a perfect place for life to thrive.
In a statement announcing the new study, the researchers highlighted what they termed a “fundamental vulnerability” in the water world theory. Specifically, sub-Neptunes likely undergo a developmental phase where they are covered in a hot magma ocean, creating a hydrogen shell that persists for millions of years. However, previous models appear to have ignored any “chemical coupling” that occurred between the atmosphere and the planet’s interior.
To conduct new sub-Neptune formation simulations that account for chemical coupling, the team calculated what they called the “equilibrium state” of 26 different components for a total of 248 model planets.
“We have now factored in the interactions between the planet’s interior and its atmosphere,” explains Aaron Werlen, a researcher on Dorn’s team and lead author of the study.
According to the statement, the computer simulations revealed that the previously unaccounted-for chemical processes “destroy most H2O water molecules.” This means that Hydrogen (H) and oxygen (O) attach themselves to metallic compounds that largely “disappear” into the core of the planet. As a result, these worlds have much less water than previously believed.
“We focus on the major trends and can clearly see in the simulations that the planets have much less water than they originally accumulated,” Werlen explained. “The water that actually remains on the surface as H2O is limited to a few per cent at most.”
Caroline Dorn, a professor of exoplanets at ETH Zurich and study author, said their calculations showed that there are “no distant worlds with massive layers of water where water makes up around 50 percent of the planet’s mass,” as most previous planetary formation models had predicted.
“Hycean worlds with 10-90 percent water are therefore very unlikely,” the researcher added.
One unexpected result from the study was that, although water worlds were less likely to exist, the number of Earth-like planets with the potential to support life was more common than previously thought.
“The Earth may not be as extraordinary as we think. In our study, at least, it appears to be a typical planet,” Dorn explained.
In their conclusion, the researchers note that their findings will have “far-reaching implications for theories of planetary formation and the interpretation of exoplanetary atmospheres.” Although their findings may decrease the overall chances of finding life beyond Earth, the increase in Earthlike planets likely increases the chances of discovering life forms similar to those found on Earth.
The study authors also note that future observatories that are more powerful than those currently available will likely be needed to confirm the prevalence of Earthlike planets discovered by their models.
“Conditions conducive to life, with sufficient liquid water on the surface, are likely to exist only on smaller planets, which will probably be observable only with observatories even better than the James Webb Space Telescope,” they explained.
The study “Sub-Neptunes Are Drier Than They Seem: Rethinking the Origins of Water-Rich Worlds” was published in the Astrophysical Journal Letters.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.
