The GAPS programme with HARPS-N at TNG: VI. The curious case of TrES-4b

We update the TrES-4 system parameters using high-precision HARPS-N radial-velocity measurements and new photometric light curves. A combined spectroscopic and photometric analysis allows us to determine a spectroscopic orbit with a semi-amplitude K = 51 ± 3 m s-1. The derived mass of TrES-4b is found to be Mp = 0.49 ± 0.04 MJup, significantly lower than previously reported. Combined with the large radius (Rp = 1.84 (+0.08) (−0.09) RJup) inferred from our analysis, TrES-4b becomes the transiting hot Jupiter with the second-lowest density known. We discuss several scenarios to explain the puzzling discrepancy in the mass of TrES-4b in the context of the exotic class of highly inflated transiting giant planets.