"Water on planets is much more limited than previously believed," said ETH Zurich professor Caroline Dorn. The study, conducted with the Max Planck Institute for Astronomy and UCLA, shows that these planets cannot retain massive oceans beneath hydrogen-rich atmospheres, overturning the Hycean planet concept.
Researchers modeled how young sub-Neptunes interacted chemically when enveloped by magma oceans and hydrogen gas. Simulations of 248 planets demonstrated that water molecules are destroyed as hydrogen and oxygen bind with metallic compounds, sending them deep into planetary interiors. The remaining surface water is limited to just a few percent of total mass.
The results reveal that sub-Neptunes are unlikely to harbor oceans comprising 10-90 percent of their mass. Instead, their water content resembles that of Earth, suggesting our planet is more typical than exceptional. "According to the calculations, there are no distant worlds with massive layers of water where water makes up around 50 percent of the planet's mass," Dorn noted.
Paradoxically, planets formed within the snow line-closer to their star-tend to have more water-rich atmospheres, not because of ice accretion, but through chemical reactions between silicate magma and hydrogen. This challenges long-held assumptions about water delivery during planetary formation.
The findings narrow the scope for habitable exoplanets, as liquid water on the surface may be confined to smaller, Earth-like worlds, detectable only with future observatories beyond the James Webb Space Telescope.
Research Report:Sub-Neptunes Are Drier Than They Seem: Rethinking the Origins of Water-Rich Worlds
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