We present the transmission spectrum of HAT-P-12b through a joint analysis of data obtained from the Hubble Space Telescope Space Telescope Imaging Spectrograph ( STIS ) and Wide Field Camera 3 ( WFC3 ) and Spitzer , covering the wavelength range 0.3–5.0 \mu m. We detect a muted water vapor absorption feature at 1.4 \mu m attenuated by clouds , as well as a Rayleigh scattering slope in the optical indicative of small particles .
We interpret the transmission spectrum using both the state-of-the-art atmospheric retrieval code SCARLET and the aerosol microphysics model CARMA .
These models indicate that the atmosphere of HAT-P-12b is consistent with a broad range of metallicities between several tens to a few hundred times solar , a roughly solar C/O ratio , and moderately efficient vertical mixing .
Cloud models that include condensate clouds do not readily generate the sub-micron particles necessary to reproduce the observed Rayleigh scattering slope , while models that incorporate photochemical hazes composed of soot or tholins are able to match the full transmission spectrum .
From a complementary analysis of secondary eclipses by Spitzer , we obtain measured depths of 0.042 \% \pm 0.013 \% and 0.045 \% \pm 0.018 \% at 3.6 and 4.5 \mu m , respectively , which are consistent with a blackbody temperature of 890 ^ { +60 } _ { -70 } K and indicate efficient day–night heat recirculation .
HAT-P-12b joins the growing number of well-characterized warm planets that underscore the importance of clouds and hazes in our understanding of exoplanet atmospheres .