Water in the terrestrial planet-forming zone of the PDS 70 disk

TL;DR

This study uses JWST observations to detect water vapor in the inner disk of PDS 70, indicating in-situ formation and potential for habitable planet formation, with implications for disk chemistry and dynamics.

Contribution

First direct detection of water vapor in the inner disk of PDS 70, demonstrating in-situ formation and survival mechanisms in a planet-forming environment.

Findings

Water detected in the inner disk of PDS 70.

Water likely formed in-situ via reactions involving O, H2, and OH.

Inner disk shows variability in spectral energy distribution.

Abstract

Terrestrial and sub-Neptune planets are expected to form in the inner ($<10~$AU) regions of protoplanetary disks. Water plays a key role in their formation, although it is yet unclear whether water molecules are formed in-situ or transported from the outer disk. So far Spitzer Space Telescope observations have only provided water luminosity upper limits for dust-depleted inner disks, similar to PDS 70, the first system with direct confirmation of protoplanet presence. Here we report JWST observations of PDS 70, a benchmark target to search for water in a disk hosting a large ($\sim54~$AU) planet-carved gap separating an inner and outer disk. Our findings show water in the inner disk of PDS 70. This implies that potential terrestrial planets forming therein have access to a water reservoir. The column densities of water vapour suggest in-situ formation via a reaction sequence involving O, H$_2$, and/or OH, and survival through water self-shielding. This is also supported by the presence of CO$_2$ emission, another molecule sensitive to UV photodissociation. Dust shielding, and replenishment of both gas and small dust from the outer disk, may also play a role in sustaining the water reservoir. Our observations also reveal a strong variability of the mid-infrared spectral energy distribution, pointing to a change of inner disk geometry.