A NEAR-INFRARED TRANSMISSION SPECTRUM FOR THE WARM SATURN HAT-P-12b

Abstract

We present a HST WFC3 transmission spectrum for the transiting exoplanet\nHAT-P-12b. This warm (1000 K) sub-Saturn-mass planet has a smaller mass and a\nlower temperature than the hot-Jupiters that have been studied so far. We find\nthat the planet's measured transmission spectrum lacks the expected water\nabsorption feature for a hydrogen-dominated atmosphere, and is instead\nbest-described by a model with high-altitude clouds. Using a frequentist\nhypothesis testing procedure, we can rule out a hydrogen-dominated cloud free\natmosphere to 4.9$\\sigma$. When combined with other recent WFC3 studies, our\nobservations suggest that clouds may be common in exo-planetary atmospheres.\n