Sublimating exocomets in the PDS 70 system

Young star systems are used to study ongoing or recent planet formation processes, by observations of protoplanets, gas, and planetesimals in their disks. The young protostar PDS 70 hosts two gas giant planets and a disk containing gas and dust that is being cleared by the emerging star. Recent observations by the James Webb Space Telescope (JWST) have indicated the presence of water in the inner 0.05 AU from the star (Perotti et al. 2023), but the origin of this water remains unclear. Here we report the discovery of variable absorption lines of neutral sodium in archival High Accuracy Radial velocity Planet Searcher (HARPS) spectra of PDS 70. These absorption lines are strongly Doppler-shifted and partially cover the projected stellar disk, indicating that the absorbing clouds are fast-moving and spatially confined. We interpret these sodium lines as sublimation of planetesimals as they pass close to the star on highly elliptical orbits, reminiscent of the exocomet phenomenon that has presently been observed only in a handful of other young debris-disk hosting systems (Ferlet et al. 1987, Montgomery & Welsh 2012, Kiefer et al. 2014, Kiefer et al. 2014b, Rappaport et al. 2018, Strøm et al. 2020). These HARPS data therefore suggest that infalling, sublimating icy exocomets could be responsible for sourcing the previously observed water in the inner, terrestrial planet-forming region of the PDS 70 system. This is the first evidence of exocomet activity in PDS 70, placing the system as the youngest system in which exocomets have been detected, at a time when planet formation is still ongoing. In the Solar System, planetesimals from the giant planet region are proposed as having been a source of delivery of volatiles to the inner parts of the system (O’brien et al. 2006, Raymond et al. 2009, Walsh et al. 2011, O’brien et al. 2014, Raymond & Izidoro 2017, O’Brien et al. 2018, Mandt et al. 2024), and a similar phenomenon may be presently occurring in PDS 70.