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Terrestrial and sub-Neptune planets are expected to form in the inner($<10~$AU) regions of protoplanetary disks. Water plays a key role in theirformation, although it is yet unclear whether water molecules are formedin-situ or transported from the outer disk. So far Spitzer Space Telescopeobservations have only provided water luminosity upper limits for dust-depletedinner disks, similar to PDS 70, the first system with direct confirmation ofprotoplanet presence. Here we report JWST observations of PDS 70, a benchmarktarget to search for water in a disk hosting a large ($\sim54~$AU)planet-carved gap separating an inner and outer disk. Our findings show waterin the inner disk of PDS 70. This implies that potential terrestrial planetsforming therein have access to a water reservoir. The column densities of watervapour 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 bythe presence of CO$_2$ emission, another molecule sensitive to UVphotodissociation. Dust shielding, and replenishment of both gas and small dustfrom the outer disk, may also play a role in sustaining the water reservoir.Our observations also reveal a strong variability of the mid-infrared spectralenergy distribution, pointing to a change of inner disk geometry.

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