by a flat spectrum model. We derive a 3-$\sigma$ upper limit of 1570 ppm on the presence of water vapor absorption in the atmosphere of XO-2b.]{} In an appendix, we perform a similar analysis for the gas giant planet XO-1b.'
author:
- 'N. Crouzet, P. R. McCullough, C. Burke, D. Long'
bibliography:
- 'biblio.bib'
title: 'Transmission spectroscopy of exoplanet XO-2 observed with HST NICMOS'
---

Introduction
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Spectroscopy during planetary transits is a powerful tool to probe the atmospheres of exoplanets. The chemical composition and temperature profile can be derived, providing clues on the dynamics and chemical processes in the atmosphere. Molecular absorption in the atmosphere causes the planetary radius to vary with wavelength: the planet appears larger at wavelengths at which its atmosphere is more opaque. Thus, measuring the transit depth as a function of wavelength yields the spectrum of the planet’s atmosphere. Typically, atmospheric components lead to variations of the transit depth of a few 100 ppm. The transit, when the planet passes in front of the star, probes the terminator region of the atmosphere. The eclipse, when the planet passes behind the star, yields the dayside spectrum of the atmosphere.

XO-2b is a 2.6 day period transiting 