Exoplanetary transmission spectroscopy in the near-infrared using Hubble/NICMOS is currently ambiguous because different observational groups claim different results from the same data , depending on their analysis methodologies .
Spatial scanning with Hubble/WFC3 provides an opportunity to resolve this ambiguity .
We here report WFC3 spectroscopy of the giant planets HD 209458b and XO-1b in transit , using spatial scanning mode for maximum photon-collecting efficiency .
We introduce an analysis technique that derives the exoplanetary transmission spectrum without the necessity of explicitly decorrelating instrumental effects , and achieves nearly photon-limited precision even at the high flux levels collected in spatial scan mode .
Our errors are within 6 % ( XO-1 ) and 26 % ( HD 209458b ) of the photon-limit at a resolving power of \lambda / { \delta \lambda } \sim 70 , and are better than 0.01 % per spectral channel .
Both planets exhibit water absorption of approximately 200 ppm at the water peak near 1.38 \mu m. Our result for XO-1b contradicts the much larger absorption derived from NICMOS spectroscopy .
The weak water absorption we measure for HD 209458b is reminiscent of the weakness of sodium absorption in the first transmission spectroscopy of an exoplanet atmosphere by Charbonneau et al .
( 2002 ) .
Model atmospheres having uniformly-distributed extra opacity of 0.012 cm ^ { 2 } g ^ { -1 } account approximately for both our water measurement and the sodium absorption .
Our results for HD 209458b support the picture advocated by Pont et al .
( 2013 ) in which weak molecular absorptions are superposed on a transmission spectrum that is dominated by continuous opacity due to haze and/or dust .
However , the extra opacity needed for HD 209458b is grayer than for HD 189733b , with a weaker Rayleigh component .