Nearly six thousand exoplanets have been identified in the Milky Way, with Sub-Neptunes being the most frequent. These planets are smaller than Neptune yet more massive than Earth and feature rocky interiors with dense hydrogen-dominated atmospheres. Their composition makes them valuable for understanding how rocky planets such as Earth may have obtained their water, a key factor for life and planetary habitability.
Miozzi explained, "Our rapidly increasing knowledge about the vast diversity of exoplanets has enabled us to envision new details about the earliest stages of rocky planet formation and evolution. This opened the door to considering a new source for planetary water supplies - a long-debated mystery among Earth and planetary scientists - but experiments designed with this purpose in mind were absent."
The research is part of the AEThER (Atmospheric Empirical, Theoretical, and Experimental Research) project. This interdisciplinary initiative, led by Shahar and supported by the Alfred P. Sloan Foundation, brings together specialists in astronomy, cosmochemistry, planetary dynamics, petrology, and mineral physics to address the factors that allow rocky planets to develop life-friendly conditions. Their work seeks to understand connections between planetary atmospheres and the internal evolution of rocky bodies.
Earlier modeling suggested that water can form when atmospheric hydrogen interacts with iron-bearing magma during planetary formation, yet direct experimental results were lacking. Miozzi and Shahar's team - including researchers from IPGP and UCLA - pressed and heated samples to nearly 600,000 times atmospheric pressure (60 gigapascals) and over 4,000 degrees Celsius to simulate these early conditions.
The setup replicates a key phase in planet formation when dust and gas from a nascent star coalesce into massive, hot bodies that eventually melt into extensive magma oceans. These planets are typically covered by thick molecular hydrogen envelopes, which keep the magma ocean hot for billions of years.
"Our work provided the first experimental evidence of two critical processes from early planetary evolution," Miozzi stated. "We showed that a copious amount of hydrogen is dissolved into the melt and significant quantities of water are created by iron-oxide reduction by molecular hydrogen."
These results confirm that magma oceans can store large amounts of hydrogen while forming water, which has major implications for a planet's interior chemistry, core development, and atmospheric makeup.
Anat Shahar added, "The presence of liquid water is considered critical for planetary habitability. This work demonstrates that large quantities of water are created as a natural consequence of planet formation. It represents a major step forward in how we think about the search for distant worlds capable of hosting life."
Research Report:Experiments reveal extreme water generation during planet formation
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