A Near-infrared Transmission Spectrum for the Warm Saturn Hat-p-12b

We present a HST WFC3 transmission spectrum for the transiting exoplanet HAT-P-12b. This warm (1000 K) sub-Saturn-mass planet has a smaller mass and a lower temperature than the hotJupiters that have been studied so far. We find that the planet’s measured transmission spectrum lacks the expected water absorption feature for a hydrogen-dominated atmosphere, and is instead bestdescribed by a model with high-altitude clouds. Using a frequentist hypothesis testing procedure, we can rule out a hydrogen-dominated cloud free atmosphere to 4.9σ. When combined with other recent WFC3 studies, our observations suggest that clouds may be common in exo-planetary atmospheres. Subject headings: planetary systems — planets and satellites: atmospheres — radiative transfer– methods: data analysis–planets and satellites: individual(HAT-P-12b)