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The hot Jupiter WASP-80b has been identified as a possible excellent targetfor detecting and measuring HeI absorption in the upper atmosphere. We observed4 primary transits of WASP-80b in the optical and near-IR using the HARPS-N andGIANO-B high-resolution spectrographs, focusing on the HeI triplet. We furtheremployed a three-dimensional hydrodynamic aeronomy model to understand theobservational results. We did not find any signature of planetary absorption atthe position of the HeI triplet with an upper limit of 0.7% (i.e. 1.11planetary radii; 95% confidence level). We re-estimated the stellar high-energyemission that we combined with a stellar photospheric model to generate theinput for the hydrodynamic modelling. We obtained that, assuming a solar He toH abundance ratio, HeI absorption should have been detected. Considering astellar wind 25 times weaker than solar, we could reproduce the non-detectiononly assuming a He to H abundance ratio about 16 times smaller than solar.Instead, considering a stellar wind 10 times stronger than solar, we couldreproduce the non-detection only with a He to H abundance ratio about 10 timessmaller than solar. We attempted to understand this result by collecting allpast HeI measurements looking for correlations with stellar high-energyemission and planetary gravity, but without finding any. WASP-80b is not theonly planet with a sub-solar estimated He to H abundance ratio, suggesting thepresence of efficient physical mechanisms (e.g. phase separation, magneticfields) capable of significantly modifying the He to H content in the upperatmosphere of hot Jupiters. The planetary macroscopic properties and the shapeof the stellar spectral energy distribution are not sufficient for predictingthe presence or absence of detectable metastable He in a planetary atmosphere,as also the He abundance appears to play a major role.

The GAPS Programme at TNG. XXXII. The revealing non-detection of metastable HeI in the atmosphere of the hot Jupiter WASP-80b