Abstract EANA2026-58

The Wide-band Atmospheric Laboratory for Transiting Exoplanet Research (WALTzER) is an ESA F-class mission concept designed to characterize the atmospheres of planets outside our Solar System. The mission's primary objective is to investigate the composition, vertical structure, and evolution of exoplanet atmospheres by analysing them from their lower layers up to the exosphere where they escape into space. During its 3-year nominal mission, WALTzER will conduct a core survey using transmission spectroscopy to track the temporal evolution of more than 30 gas-rich exoplanets, including gas giants, inflated Neptunian worlds, and young low-mass planets. These observations will address key scientific questions about the origin and evolution of planetary atmospheres and the interaction between their lower and upper layers.

The WALTzER payload consists of a 35 cm telescope that simultaneously directs light into three instrument channels: a near-ultraviolet (NUV) spectrograph (240-325 nm), a visible (VIS) spectrograph (450-820 nm), and a near-infrared (NIR) photometer (~900-1700 nm). This design will fill the critical ultraviolet capability gap expected after the Hubble Space Telescope (HST) is decommissioned, offering competitive performance while being the first instrument to provide simultaneous coverage from the NUV to the NIR. This capability will allow WALTzER to probe atmospheric layers from the lower atmosphere up to the exosphere, characterizing aerosols and quantifying atmospheric mass loss – processes that shape planetary environments. WALTzER will provide data that directly complements the infrared observations of the JWST and Ariel missions.

In addition to its core survey, WALTzER will function as a versatile observatory for the broader astrophysics’ community. There are two consortium surveys planned, one on the radiation environment of exoplanets and the other on active bodies in the Solar System. The first will allow insights into planetary habitability, whilst providing detailed knowledge of stellar physical processes (e.g. magnetic evolution) – eventually aiming to characterise targets for future observatories like the Habitable Worlds Observatory (HWO). In the frame of the second consortium survey, WALTzER will study active bodies, such as long-period comets and the moons of giant planets. WALTzER observations will probe sublimation and thermal evolution in cometary nuclei, map activity and composition along their orbits, and disentangle intrinsic compositional differences from temperature-driven outgassing. Regarding icy moons, WALTzER will map and monitor emission from S⁺, O⁺, O, Na, K, and other species in the Jovian system complementing ESA’s Juice and NASA’s Europa Clipper narrow field-of-view observations. Further, WALTzER will monitor Enceladus’ plumes through NUV OH and visible Na and K observations, constraining long-term plume variability. These measurements will help clarify the processes driving Enceladus’ activity and support future exploration, including ESA’s planning of the Enceladus orbiter and South polar lander.

WALTzER is a partnership between ESA and a consortium of ten member states, and 25% of its observing time will be made available to the global scientific community through open calls, ensuring a wide scientific benefit in the post-HST era.