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Exoplanetary transmission spectroscopy in the near-infrared usingHubble/NICMOS is currently ambiguous because different observational groupsclaim different results from the same data, depending on their analysismethodologies. Spatial scanning with Hubble/WFC3 provides an opportunity toresolve this ambiguity. We here report WFC3 spectroscopy of the giant planetsHD209458b and XO-1b in transit, using spatial scanning mode for maximumphoton-collecting efficiency. We introduce an analysis technique that derivesthe exoplanetary transmission spectrum without the necessity of explicitlydecorrelating instrumental effects, and achieves nearly photon-limitedprecision even at the high flux levels collected in spatial scan mode. Ourerrors are within 6-percent (XO-1) and 26-percent (HD209458b) of thephoton-limit at a spectral resolving power of 70, and are better than0.01-percent per spectral channel. Both planets exhibit water absorption ofapproximately 200 ppm at the water peak near 1.38 microns. Our result for XO-1bcontradicts the much larger absorption derived from NICMOS spectroscopy. Theweak water absorption we measure for HD209458b is reminiscent of the weaknessof sodium absorption in the first transmission spectroscopy of an exoplanetatmosphere by Charbonneau et al. (2002). Model atmospheres havinguniformly-distributed extra opacity of 0.012 cm^2 per gram accountapproximately for both our water measurement and the sodium absorption in thisplanet. Our results for HD209458b support the picture advocated by Pont et al.(2013) in which weak molecular absorptions are superposed on a transmissionspectrum that is dominated by continuous opacity due to haze and/or dust.However, the extra opacity needed for HD209458b is grayer than for HD189733b,with a weaker Rayleigh component.

Infrared Transmission Spectroscopy of the Exoplanets HD209458b and XO-1b Using the Wide Field Camera-3 on the Hubble Space Telescope