Dry or water world? How the water contents of inner sub-Neptunes constrain giant planet formation and the location of the water ice line

TL;DR

This paper investigates how the water content of inner sub-Neptunes can reveal the formation location of water ice lines and giant planets in protoplanetary disks, using pebble accretion models and implications for planetary system architecture.

Contribution

It introduces a model linking water content in sub-Neptunes to the formation and blocking of icy pebbles by giant planets, providing a new method to constrain planetary formation conditions.

Findings

Water content in sub-Neptunes depends on giant planet position relative to the water ice line.

Blocking of icy pebbles outside the ice line results in dry inner disks.

Sub-Neptune water content can be used to infer giant planet formation locations.

Abstract

In the pebble accretion scenario, the pebbles that form planets drift inward from the outer disk regions, carrying water ice with them. At the water ice line, the water ice on the inward drifting pebbles evaporates and is released into the gas phase, resulting in water-rich gas and dry pebbles that move into the inner disk regions. Large planetary cores can block the inward drifting pebbles by forming a pressure bump outside their orbit in the protoplanetary disk. Depending on the relative position of a growing planetary core relative to the water ice line, water-rich pebbles might be blocked outside or inside the water ice line. Pebbles blocked outside the water ice line do not evaporate and thus do not release their water vapor into the gas phase, resulting in a dry inner disk, while pebbles blocked inside the water ice line release their water vapor into the gas phase, resulting in water vapor diffusing into the inner disk. As a consequence, close-in sub-Neptunes that accrete some gas from the disk should be dry or wet, respectively, if outer gas giants are outside or inside the water ice line, assuming that giant planets form fast, as has been suggested for Jupiter in our Solar System. Alternatively, a sub-Neptune could form outside the water ice line, accreting a large amount of icy pebbles and then migrating inward as a very wet sub-Neptune. We suggest that the water content of inner sub-Neptunes in systems with giant planets that can efficiently block the inward drifting pebbles could constrain the formation conditions of these systems, thus making these sub-Neptunes exciting targets for detailed characterization (e.g., with JWST, ELT, or ARIEL). In addition, the search for giant planets in systems with already characterized sub-Neptunes can be used to constrain the formation conditions of giant planets as well.