Exoplanets transiting bright nearby stars are key objects for advancing our knowledge of planetary formation and evolution. The wealth of photons from the host star gives detailed access to the atmospheric, interior and orbital properties of the planetary companions. \(\nu^2\) Lupi (HD 136352) is a naked-eye (\(V=5.78\)) Sun-like star that was discovered to host three low-mass planets with orbital periods of 11.6, 27.6 and 107.6 d via radial-velocity monitoring. The two inner planets (b and c) were recently found to transit, prompting a photometric follow-up by the brand new Characterising Exoplanets Satellite (CHEOPS). Here, we report that the outer planet d is also transiting, and measure its radius and mass to be \(2.56\pm0.09\) \(R_\oplus\) and \(8.82\pm0.94\) \(M_\oplus\), respectively. With its bright Sun-like star, long period and mild irradiation (\(\sim 5.7\) times the irradiation of Earth), \(\nu^2\) Lupi d unlocks a completely new region in the parameter space of exoplanets amenable to detailed characterization. We refine the properties of all three planets: planet b probably has a rocky mostly dry composition, while planets c and d seem to have retained small hydrogen-helium envelopes and a possibly large water fraction. This diversity of planetary compositions makes the \(\nu^2\) Lupi system an excellent laboratory for testing formation and evolution models of low-mass planets.
Authors: Laetitia Delrez, David Ehrenreich, Yann Alibert, Andrea Bonfanti, Luca Borsato, Luca Fossati, Matthew J. Hooton, Sergio Hoyer, Francisco J. Pozuelos, Sébastien Salmon, Sophia Sulis, Thomas G. Wilson, Vardan Adibekyan, Vincent Bourrier, Alexis Brandeker, Sébastien Charnoz, Adrien Deline, Pascal Guterman, Jonas Haldemann, Nathan Hara, Mahmoudreza Oshagh, Sergio G. Sousa, Valérie Van Grootel, Roi Alonso, Guillem Anglada-Escudé, Tamás Bárczy, David Barrado, Susana C. C. Barros, Wolfgang Baumjohann, Mathias Beck, Anja Bekkelien, Willy Benz, Nicolas Billot, Xavier Bonfils, Christopher Broeg, Juan Cabrera, Andrew Collier Cameron, Melvyn B. Davies, Magali Deleuil, Jean-Baptiste Delisle, Olivier D. S. Demangeon, Brice-Olivier Demory, Anders Erikson, Andrea Fortier, Malcolm Fridlund, David Futyan, Davide Gandolfi, Antonio Garcia Muñoz, Michaël Gillon, Manuel Guedel, Kevin Heng, László Kiss, Jacques Laskar, Alain Lecavelier des Etangs, Monika Lendl, Christophe Lovis, Pierre F. L. Maxted, Valerio Nascimbeni, Göran Olofsson, Hugh P. Osborn, Isabella Pagano, Enric Pallé, Giampaolo Piotto, Don Pollacco, Didier Queloz, Heike Rauer, Roberto Ragazzoni, Ignasi Ribas, Nuno C. Santos, Gaetano Scandariato, Damien Ségransan, Attila E. Simon, Alexis M. S. Smith, Manfred Steller, Gyula M. Szabó, Nicolas Thomas, Stéphane Udry, Nicholas A. Walton