  We demonstrate that the deep volatile storage capacity of magma oceans has
significant implications for the bulk composition, interior and climate state
inferred from exoplanet mass and radius data. Experimental petrology provides
the fundamental properties on the ability of water and melt to mix. So far,
these data have been largely neglected for exoplanet mass-radius modeling.
Here, we present an advanced interior model for water-rich rocky exoplanets.
The new model allows us to test the effects of rock melting and the
redistribution of water between magma ocean and atmosphere on calculated planet
radii. Models with and without rock melting and water partitioning lead to
deviations in planet radius of up to 16% for a fixed bulk composition and
planet mass. This is within current accuracy limits for individual systems and
statistically testable on a population level. Unrecognized mantle melting and
volatile redistribution in retrievals may thus underestimate the inferred
planetary bulk water content by up to one order of magnitude.
