  The radius valley (or gap) in the observed distribution of exoplanet radii,
which separates smaller super-Earths from larger sub-Neptunes, is a key feature
that theoretical models must explain. Conventionally, it is interpreted as the
result of the loss of primordial H/He envelopes atop rocky cores. However,
planet formation models predict that water-rich planets migrate from regions
outside the snowline toward the star. Assuming water to be in the form of solid
ice in their interior, many of these planets would be located in the radius
gap, in disagreement with observations. Here we use an advanced coupled
formation and evolution model that describes the planets' origin and evolution
starting from moon-sized, planetary seed embryos in the protoplanetary disk to
mature Gyr-old planetary systems. Employing new equations of state and interior
structure models to treat water as vapor mixed with H/He, we naturally
reproduce the valley at the observed location. The model results indicate that
the valley separates less massive, in-situ, rocky super-Earths from more
massive, ex-situ, water-rich sub-Neptunes. Furthermore, the occurrence drop at
larger radii, the so-called radius cliff, is also matched by planets with
water-dominated envelopes. Owing to our statistical approach, we can assess
that the synthetic distribution of radii quantitatively agrees with
observations for the close-in population of planets; but only if atmospheric
photoevaporation is also acting, populating the super-Earth peak with
evaporated rocky cores. Therefore, we provide a hybrid theoretical explanation
of the radius gap and cliff caused by both formation (orbital migration) as
well as evolution (atmospheric escape).
