Characterization of three subneptunian worlds straddling the evaporation valley in the ν² Lupi planetary system with space telescopes

A new population of transiting (hence, characterizable) subneptunian worlds is emerging at long orbital periods of ~50-100 days. Unveiled thanks to the synergies between the space missions TESS and CHEOPS (Tuson et al. 2023, Ulmer-Moll et al. 2023, Garai et al. 2023, Osborn et al. 2023), these objects represent important steps in understanding the true nature of subneptunes, an ubiquitous yet mysterious family of planets, which are absent from our Solar System. The mild stellar irradiation bathing long-period subneptunes has presumably allowed them to remain much closer to their initial forming conditions than their scorched, hotter counterparts, maintaining their structures, compositions and atmospheres possibly unscathed. These worlds also possess large Hill spheres, allowing the search and potential detection of exomoons. The ν² Lupi system is arguably the herald of this new class of planets, featuring three subneptunian worlds first detected around a naked-eye star by velocimetry (Udry et al. 2019), and readily monitored in photometry by TESS. Rumors of transits from the two inner planets on periods of 12 and 28 days were already whispered across Iceland during Extreme Solar System IV (Kane et al. 2020), triggering the CHEOPS follow-up and the serendipitous discovery of the (partial) transit of the 108-day subneptune ν² Lupi d (Delrez et al. 2021). In this talk, I will report on the confirmation of this outstanding transit and a first exploration of the planet Hill sphere (Ehrenreich et al. 2023), as well as on its first spectrosopic follow-up to characterize the planet atmospheres. Performed with the Hubble Space Telescope, these UV observations aims at determining the photoevaporation status of warm subneptunes straddling the evaporation valley within one planetary system.