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The radius valley (or gap) in the observed distribution of exoplanet radii,which separates smaller super-Earths from larger sub-Neptunes, is a key featurethat theoretical models must explain. Conventionally, it is interpreted as theresult of the loss of primordial H/He envelopes atop rocky cores. However,planet formation models predict that water-rich planets migrate from regionsoutside the snowline toward the star. Assuming water to be in the form of solidice in their interior, many of these planets would be located in the radiusgap, in disagreement with observations. Here we use an advanced coupledformation and evolution model that describes the planets' origin and evolutionstarting from moon-sized, planetary seed embryos in the protoplanetary disk tomature Gyr-old planetary systems. Employing new equations of state and interiorstructure models to treat water as vapor mixed with H/He, we naturallyreproduce the valley at the observed location. The model results indicate thatthe valley separates less massive, in-situ, rocky super-Earths from moremassive, ex-situ, water-rich sub-Neptunes. Furthermore, the occurrence drop atlarger radii, the so-called radius cliff, is also matched by planets withwater-dominated envelopes. Owing to our statistical approach, we can assessthat the synthetic distribution of radii quantitatively agrees withobservations for the close-in population of planets; but only if atmosphericphotoevaporation is also acting, populating the super-Earth peak withevaporated rocky cores. Therefore, we provide a hybrid theoretical explanationof the radius gap and cliff caused by both formation (orbital migration) aswell as evolution (atmospheric escape).

A radius valley between migrated steam worlds and evaporated rocky cores