Coreless exoplanets induced by metal-silicate miscibility.

The rocky interiors of exoplanets are generally expected to differentiate into a silicate mantle and a metallic core due to the immiscibility of metal and silicate under typical planetary formation conditions. Here, we use large-scale molecular dynamics simulations driven by machine-learning potentials of ab initio quality to explore the phase behavior of metallic liquids and silicate melts under the extreme pressure-temperature conditions of the exoplanetary interiors. The results map out the solvus closure curve, above which a single homogeneous liquid phase of the iron and silicate mixture is stable. Under the assumptions of our bulk Earth composition and interior models, the inferred solvus closure curve permits deep partially mixed or, in extreme cases, fully mixed metal-silicate interiors in some hot super-Earths and sub-Neptunes with thick gas envelopes. Compared with differentiated planets, coreless planets exhibit nearly identical mass-radius relationships but possess markedly less compressed interiors, with central pressures reduced by up to a factor of two. This reduction in internal compression may strongly influence their thermophysical properties and interior dynamics. Coreless planets also possess a larger fluid Love number, a quantity potentially observable and testable in future missions.