## Sub-Neptune Exoplanets Likely Lack Extensive water Layers, New Research Suggests
Recent research from a team led by Christoph Dorn at ETH Zurich indicates that sub-Neptune exoplanets are unlikely to possess the vast, water-rich compositions previously hypothesized, possibly impacting the search for extraterrestrial life. The findings, published in *The Astrophysical journal Letters* (https://doi.org/10.3847/2041-8213/adff73), incorporate interactions between a planet’s interior adn its atmosphere for a more thorough understanding of planetary evolution.
The study centers on the premise that young sub-Neptunes initially developed deep, hot magma oceans sustained by a surrounding hydrogen gas shell for millions of years. Researchers,led by Aaron Werlen,investigated how chemical interactions between these magma oceans and the atmospheres effect the water content of these exoplanets.
To achieve this,the team combined an existing planetary evolution model with a new model calculating chemical processes occurring between atmospheric gases and the metals and silicates within the magma. Simulations, encompassing 248 model planets and 26 different components, revealed that chemical reactions largely destroy water molecules (H2O). Hydrogen and oxygen bind with metallic compounds, effectively removing them from the surface and depositing them within the planet’s core.
While acknowledging limitations in calculation accuracy,the researchers are confident in the overall trend: planets retain significantly less water than initially accumulated,with surface water limited to a maximum of a few percent. This builds upon previous work from Dorn’s group demonstrating that most of a planet’s water is typically hidden within its interior.
The research challenges the possibility of “Hycean” worlds – planets with water comprising 10-90% of their mass – finding them highly improbable. This conclusion suggests that conditions suitable for life, requiring substantial liquid water on the surface, might potentially be limited to smaller planets, potentially requiring more advanced observatories than the James Webb Space Telescope for detection.Interestingly, the study also found that planets forming *within* the snow line - the region where ice can condense – tend to have more water-rich atmospheres than those forming beyond it. This water isn’t derived from accumulated ice, but rather created through chemical reactions between hydrogen in the atmosphere and oxygen from silicates in the magma ocean.
“These findings challenge the classic link between ice-rich formation and water-rich atmospheres,” Werlen stated, emphasizing the crucial role of the equilibrium between magma ocean and atmosphere in determining planetary composition. Dorn added that Earth might potentially be more typical than previously thought, possessing a water content similar to many othre planets. The findings have importent implications for planetary formation theories and the interpretation of exoplanetary atmospheres as observed by instruments like the James Webb Space Telescope.
*Source: ETH Zurich*
