Transit infrared spectroscopy of the hot neptune around GJ 436 with the Hubble Space Telescope

TL;DR

This study uses Hubble Space Telescope infrared spectroscopy to precisely measure the size and atmospheric properties of the hot Neptune GJ 436b, finding a flat transmission spectrum and confirming orbital regularity.

Contribution

First high-cadence infrared spectroscopic observations of GJ 436b's transits, providing improved planetary and stellar radii and constraining atmospheric composition.

Findings

Confirmed planetary radius of 4.04 Earth radii

Detected no significant transit timing variations

Found a flat transmission spectrum with no water absorption signal

Abstract

The nearby transiting system GJ 436b offers a unique opportunity to probe the structure and atmosphere of an extra-solar "hot Neptune". In this Letter, we present the main results of observations covering two transit events with the NICMOS camera on the Hubble Space Telescope. The data consist in high-cadence time series of grism spectra covering the 1.1-1.9 micron spectral range. We find Rpl=4.04 +- 0.10 R_earth and Rstar= 0.446 +- 0.011 Rsun for the planet and star radius, confirming and improving earlier measurements with ground-based photometry and a Spitzer lightcurve at 8 microns, as opposed to a much higher value obtained with the Fine Guidance Sensor on the Hubble Space Telescope. We measure no departure from strict periodicity in the transits to the level of ~7 seconds. This strongly disfavours the proposed explanation of the orbital eccentricity of GJ 436b in terms of the perturbation by another close-by planet. We measure a flat transmission spectrum at the level of a few parts per 10 000 in flux, with no significant signal in the 1.4-micron water band to a level comparable to the maximum amplitude of the effect predicted by planetary atmosphere models.