Metals and a potential extended outflow in the atmosphere of HAT-P-67b, the puffiest gas giant

Extremely low density exoplanets are tantalizing targets for atmospheric characterization, not only because of their promisingly large signals in transmission spectroscopy, but also because their atmospheres may help us understand their large radii. We present the first analysis of the atmosphere of the lowest-density known gas giant, HAT-P-67 b. This inflated Saturn-mass exoplanet sits at the boundary between hot and ultrahot gas giants, where thermal dissociation of molecules starts dominating atmospheric composition. We observed a transit of HAT-P-67 b at high spectral resolution with the CARMENES instrument. We searched for atomic and molecular species using cross correlation and likelihood mapping, and we explored potential atmospheric escape by targeting Hα and the metastable helium line. We detect different atomic species, including Na I and Ca II. The calcium signal seems to originate at high altitudes. Unlike in several ultrahot Jupiters, we do not measure a day-to-night wind. We detect strong variability in Hα and the helium triplet during the observations. These signals may originate in an extended planetary outflow that causes an early ingress and late egress. From an isothermal Parker wind model, we derive a mass loss rate of ~10¹³ g/s and an outflow temperature of T ~ 9900K. However, without a longer out-of-transit baseline, we cannot rule out stellar variability as the source of the Hα and He signals. Additional observations may confirm HAT-P-67b as an outstanding target for studies of hydrodynamic wind morphology.