2025NatAs...9.1511B

Lava planets are rocky exoplanets that orbit so close to their host star that their dayside is hot enough to melt silicate rock. Their short orbital periods ensure that lava planets are tidally locked into synchronous rotation, with permanent day and night hemispheres. Such asymmetric magma oceans have no analogues in the Solar System and their internal dynamics and evolution are still poorly understood. Here we report the results of numerical simulations showing that solid-liquid fractionation has a major impact on the composition and evolution of lava planets. We explored two different interior thermal states. If the interior is fully molten, the atmosphere will reflect the planet's bulk silicate composition, and the nightside solid surface is gravitationally unstable and constantly replenished. If the interior is mostly solid with only a shallow magma ocean on the dayside, the outgassed atmosphere will lack in Na, K and FeO, and the nightside will have an entirely solid mantle with a cold surface. We show that these two end-member cases can be distinguished with observations from JWST, offering an avenue to probe the thermal and chemical evolution of exoplanet interiors.