Cartoon depicting the formation scenarios of the inner sub-Neptunes and their outer giant planets. Pebbles drift inward and release their volatiles at the water ice line, resulting in water vapor in the inner disk, where rocky pebbles are initially available to form rocky planetesimals. In scenario A, the giant planet forms outside the water ice line, blocking water-rich pebbles and thus preventing them from crossing the water ice line. The originally present water vapor diffuses inward and is lost over time. A dry sub-Neptune can then form in the inner disk. In scenario B the giant planet forms inside the water ice line, and thus the pebbles release their water vapor into the disk as they cross the water ice line. This water vapor can then diffuse inward because gas accretion onto giant planets is not 100% efficient. As a result, planets forming in the inner region can accrete water-rich vapor from the disk and form wet sub-Neptunes. In scenario C the sub-Neptune forms outside the water ice line, where it accretes a large amount of water ice and then migrates toward the inner edge of the disk, resulting in a very wet sub-Neptune. Here, the formation time and location of the giant planet do not matter. A sub-Neptune formed via scenario C could thus contain up to 50% of its total mass in water ice, while a sub-Neptune formed via scenario B would contain a water-rich atmosphere on top of a rocky core; as such, its overall water mass might be just a few percent. For the sake of simplicity, we do not show the movement of the water ice line over time or water-rich planetesimals. (For a discussion about diffusion, see Appendix B).