A Spitzer Search for Water in the Transiting Exoplanet HD189733b

TL;DR

This study uses Spitzer observations of HD189733b's transit at 3.6 and 5.8 microns to measure planetary radii and assess the feasibility of detecting atmospheric water vapor, highlighting the need for multiple transits for precise spectroscopy.

Contribution

First simultaneous infrared transit observations of HD189733b at two wavelengths, providing precise radius measurements and insights into observational challenges for atmospheric characterization.

Findings

Planet radii determined with 1% accuracy.

Uncertainties hinder detection of atmospheric water vapor.

Multiple transits needed for high-precision transmission spectroscopy.

Abstract

We present Spitzer Space Telescope observations of the extrasolar planet HD189733b primary transit, obtained simultaneously at 3.6 and 5.8 microns with the Infrared Array Camera. The system parameters, including planetary radius, stellar radius, and impact parameter are derived from fits to the transit light curves at both wavelengths. We measure two consistent planet-to-star radius ratios, (Rp/Rs)[3.6$\mu$m] = 0.1560 +/- 0.0008(stat) +/- 0.0002(syst) and (Rp/Rs)[5.8$\mu$m] = 0.1541 +/- 0.0009(stat) +/- 0.0009(syst), which include both the random and systematic errors in the transit baseline. Although planet radii are determined at 1%-accuracy, if all uncertainties are taken into account the resulting error bars are still too large to allow for the detection of atmospheric constituants like water vapour. This illustrates the need to observe multiple transits with the longest possible out-of-transit baseline, in order to achieve the precision required by transmission spectroscopy of giant extrasolar planets.