Spitzer Transit and Secondary Eclipse Photometry of GJ 436b

TL;DR

This study uses Spitzer infrared data to precisely measure GJ 436b's radius, detect its secondary eclipse, and infer its orbital eccentricity and temperature, revealing a gaseous envelope and possible tidal heating effects.

Contribution

First precise infrared transit and eclipse measurements of GJ 436b allowing improved radius determination and insights into its atmospheric composition and orbital dynamics.

Findings

GJ 436b's radius is 4.33 Earth radii, larger than a naked ocean planet.

Detected secondary eclipse at phase 0.587, indicating orbital eccentricity.

Brightness temperature of 712K suggests tidal heating and possible gravitational perturbations.

Abstract

We report the results of infrared (8 micron) transit and secondary eclipse photometry of the hot Neptune exoplanet, GJ436b using Spitzer. The nearly photon-limited precision of these data allow us to measure an improved radius for the planet, and to detect the secondary eclipse. The transit (centered at HJD = 2454280.78149 +/-0.00016) shows the flat-bottomed shape typical of infrared transits, and it precisely defines the planet-to-star radius ratio (0.0839 +/-0.0005), independent of the stellar properties. However, we obtain the planetary radius, as well as the stellar mass and radius, by fitting to the transit curve simultaneously with an empirical mass-radius relation for M-dwarfs (M=R). We find Rs=Ms=0.47 +/-0.02 in solar units, and Rp=27,600 +/-1170 km (4.33 +/-0.18 Earth radii). This radius significantly exceeds the radius of a naked ocean planet, and requires a gasesous hydrogen-helium envelope. The secondary eclipse occurs at phase 0.587 +/-0.005, proving a significant orbital eccentricity (e=0.15 +/-0.012). The amplitude of the eclipse (5.7 +/-0.8e-4) indicates a brightness temperature for the planet of T=712 +/-36K. If this is indicative of the planet's physical temperature, it suggests the occurrence of tidal heating in the planet. An uncharacterized second planet likely provides ongoing gravitational perturbations, to maintain GJ436b's orbit eccentricity over long time scales.