Building wet planets through high-pressure magma-hydrogen reactions

Area of Science:

• Planetary Science
• Exoplanet Research
• Geochemistry

Background:

• Sub-Neptune exoplanets are common, with models suggesting they are either dry (hydrogen-rich) or wet (water-rich).
• Water-rich sub-Neptunes were thought to form in the outer solar system and migrate inward.
• Current models predict limited water content in hydrogen-rich planets.

Purpose of the Study:

• To investigate reactions between hydrogen and silicate melts at high pressures.
• To determine the potential for water generation within sub-Neptune interiors.
• To re-evaluate the link between exoplanet composition and formation location.

Main Methods:

• Experimental simulations of warm, dense hydrogen fluid interacting with silicate melt under high pressure.
• Analysis of silicon release from magma, formation of alloys and hydrides.
• Quantification of water produced from oxygen liberated from the silicate melt.

Main Results:

• Experimental evidence shows silicon is released from magma, forming alloys and hydrides.
• Significant amounts of water (up to tens of weight per cent) are produced.
• Water production is much higher than predicted by low-pressure extrapolations.
• These reactions can create a spectrum of water contents, potentially leading to water-rich compositions from initially hydrogen-rich planets.

Conclusions:

• Planetary interior reactions can generate substantial water in sub-Neptunes.
• This challenges the assumption that water-rich exoplanets form only in cold, outer regions.
• The presence of abundant water in exoplanet atmospheres may not solely indicate inward migration.
• An evolutionary link between hydrogen-rich and water-rich planets is plausible.