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

TL;DR

This study uses Hubble's WFC3 in spatial scanning mode to obtain high-precision transmission spectra of exoplanets HD209458b and XO-1b, revealing water absorption features and addressing previous ambiguities in near-infrared exoplanet spectroscopy.

Contribution

Introduces a novel analysis technique that derives exoplanet transmission spectra without explicit decorrelation of instrumental effects, achieving near photon-limited precision.

Findings

XO-1b shows water absorption of ~200 ppm near 1.38 microns.

HD209458b exhibits weak water absorption, consistent with haze or dust effects.

Results challenge previous NICMOS measurements and support models with additional atmospheric opacity.

Abstract

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 HD209458b 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-percent (XO-1) and 26-percent (HD209458b) of the photon-limit at a spectral resolving power of 70, and are better than 0.01-percent per spectral channel. Both planets exhibit water absorption of approximately 200 ppm at the water peak near 1.38 microns. Our result for XO-1b contradicts the much larger absorption derived from NICMOS spectroscopy. The weak water absorption we measure for HD209458b 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 per gram account approximately for both our water measurement and the sodium absorption in this planet. Our results for HD209458b 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 HD209458b is grayer than for HD189733b, with a weaker Rayleigh component.