A. G. Sreejith, L. Fossati, M. Lendl, P. E. Cubillos, C. Broeg, D. Ehrenreich, B. -O. Demory, D. Piazza, I. Argyriou, V. Bourrier, M. Braam, L. Carone, Y. Carteret, H. Chakraborty, A. Deline, O. Demangeon, C. Farret Jentink, G. Giono, Ch. Helling, T. Jeřábková, P. Kabáth, T. A. van Kempen, A. Suárez Mascareño, J. C. Morales, M. Munari, C. Opitom, R. Ottensamer, G. Parma, L. Roth, J. Skottfelt, R. -S. Taubner, A. Tanzer, T. G. Wilson, B. Edwards, G. Scandariato, V. Van Grootel, J. Venturini, A. Vorburger, S. Ambily, A. Brandeker, T. Belenguer, E. Bolmont, S. Boro Saikia, H. M. Cegla, Q. Changeat, C. Danielski, K. Fahrion, B. Flemming, K. France, E. Gillen, K. Hakim, C. Haswell, R. Helled, M. Ivchenko, J. Jacobs, K. Kislyakova, V. Korol, T. Kupfer, A. Kuzucan, J. McCormac, C. Mordasini, J. Munday, G. Nelemans, A. F. Pala, I. Ribas, J. Sanz-Forcada, C. Snodgrass, S. Sousa, J. Žák, Y. Alibert, S. P. D. Borthakur, S. R. Carberry Mogan, J. M. Carrasco, C. Dorn, J. P. D. Gow, M. Güdel, F. Kerschbaum, D. Kitzmann, G. Laky, E. McDonald, H. Ottacher, P. A. Sossi, V. Trigler, R. Wallner

Understanding the origin and evolution of planetary atmospheres, and the connection between their lower and upper atmospheric layers, remains a fundamental challenge in exoplanet science. Addressing these questions requires observations spanning a broad wavelength range, from the ultraviolet to the near-infrared, together with the ability to simultaneously characterise planetary and stellar variability. The Wide-band Atmospheric Laboratory for Transiting Exoplanet Research (WALTzER) is an ESA F3 mission candidate designed to address this need through simultaneous low-resolution spectroscopy and high-precision photometry of transiting exoplanets. WALTzER covers simultaneously the near-ultraviolet (NUV: 240--325 nm) and visible (VIS: 450--820 nm) bands with two medium resolution spectrographs and the near-infrared (NIR: 900--1700 nm) range with a broadband photometer. We outline the main science drivers and present the mission concept and instrument design. WALTzER's broad, simultaneous wavelength coverage enables the detection and characterisation of the atmospheres of more than 30 gas-rich exoplanets and provides data relevant for a broad range of Solar System and Galactic astrophysics science cases. The payload consists of a 35 cm Cassegrain telescope feeding three co-aligned channels (i.e. NUV, VIS, NIR), sharing a common optical train to minimise instrumental systematics, while maximising observational efficiency.