The 2025 ESA Distinguished Lecture will feature Professor Leigh Fletcher, who is a Professor of Planetary Science at the University of Leicester, specialising in the exploration of our Solar System’s giant planets via robotic spacecraft missions, ground-based astronomical facilities, and space telescopes. He earned a Natural Science degree from Cambridge, a PhD in Planetary Physics from Oxford, and has since worked as a NASA fellow at the Jet Propulsion Laboratory and as a Research Fellow at Oxford. He held a prestigious Royal Society University Research Fellowship (URF) between 2013 and 2020, and a European Research Council Consolidator Grant between 2017 and 2024. He was the recipient of the 2016 Harold C. Urey prize for outstanding achievements in planetary science by an early-career scientist, awarded by the Division for Planetary Sciences (DPS) of the American Astronomical Society. He is a team member for the Cassini mission to Saturn, the Juno and JUICE missions to Jupiter, and is a passionate advocate for future exploration of the distant Ice Giants. He currently leads the planetary atmospheres team at the University of Leicester.

Title: JWST as a New Voyager for Giant Planet Systems
Abstract: The first three years of JWST scientific operations have provided a treasure trove of new discoveries in our Solar System. Not since Voyager’s Grand Tour have we had a facility capable of spatially-resolved infrared spectroscopy of the four giant planet systems with shared techniques and instrumentation, enabling a comprehensive comparative planetology of the gas and ice giants, their rings, and myriad icy “ocean worlds”. JWST is opening a new window for discovery, accessing spectral domains that were too dark, too cold, or simply overlooked in planetary mission design over previous decades. I will review recent discoveries enabled by JWST, from a combination of guaranteed-time, early-release and guest-observer programmes. 

For the atmospheres of the giant planets, JWST has detected previously unseen jet streams on Jupiter, revealed the chemical composition of the Great Red Spot and polar vortices, explored seasonally-reversing stratospheric circulations on Saturn, mapped the stratospheric chemistry and circulation of Uranus for the first time, and caught enormous storm activity on Neptune. Emissions from H₃⁺, methane, and carbon monoxide from their ionospheres have revealed delicate wave patterns on Jupiter, never-before-seen structures on Saturn, and the discovery of H₃⁺ on Neptune, providing windows onto magnetospheric processes shaping the wider systems. JWST spectroscopy has revealed the extreme crystallinity of Saturn’s rings, including the “heavy water” fraction, and filtered imaging is revealing radial trends in icy composition of rings and moons in both Ice Giant systems. JWST has also constrained the gases erupting from Io’s powerful volcanism, searched for (but not detected) evidence of Europa plume activity, discovered polar hydrogen peroxide and a tenuous CO₂ exosphere on Ganymede, and revealed how CO₂ ice is co-located with the chaotic terrains of Europa, implying a connection to the deep surface ocean. These new discoveries will help to shape the exploration from future giant planet missions, such as ESA’s JUICE mission, NASA’s Europa Clipper and Dragonfly, and future endeavours like Uranus Orbiter and Probe.

The ESA Office established the ESA Distinguished Lecture Series in 2021. The series aims to showcase the science from ESA missions, recognize talented early-career European astronomers, and foster collaborations in the broader international community. Each lecture features an in-person visit to STScI, where the Distinguished Lecturer has the opportunity to meet with staff, students, and postdocs. The lecture will be held at STScI in the Bahcall Auditorium.