A ground-based Ks-band detection of the thermal emission from the transiting exoplanet WASP-4b

TL;DR

This study demonstrates ground-based near-infrared photometry can reliably detect the thermal emission of transiting exoplanets, exemplified by a clear detection of WASP-4b's secondary eclipse at 2.2 μm, revealing insights into its atmospheric properties.

Contribution

First ground-based high-cadence Ks-band detection of thermal emission from a transiting exoplanet, providing a new method for characterizing exoplanet atmospheres.

Findings

Detected thermal emission of WASP-4b at 2.2 μm with high significance

Estimated brightness temperature of 1995 ± 40 K for the planet

Indicated inefficient heat redistribution on the planet's day-side

Abstract

Secondary eclipses are a powerful tool to measure directly the thermal emission from extrasolar planets, and to constrain their type and physical parameters. We started a project to obtain reliable broad-band measurements of the thermal emission of transiting exoplanets. Ground-based high-cadence near-infrared relative photometry was used to obtain sub-millimagnitude precision light curve of a secondary eclipse of WASP-4b -- a 1.12 M_J hot Jupiter on a 1.34 day orbit around G7V star. The data show a clear ~10-\sigma detection of the planet's thermal emission at 2.2 \mu m. The calculated thermal emission corresponds to a fractional eclipse depth of 0.185^{+0.014}_{-0.013}%, with a related brightness temperature in Ks of T_B = 1995 \pm 40 K, centered at T_C = 2455102.61162^{+0.00071}_{-0.00077} HJD. We could set a limit on the eccentricity of e cos \omega=0.0027 \pm 0.0018, compatible with a near-circular orbit. The calculated brightness temperature, as well as the specific models suggest a highly inefficient redistribution of heat from the day-side to the night-side of the planet, and a consequent emission mainly from the day-side. The high-cadence ground-based technique is capable of detecting the faint signal of the secondary eclipse of extrasolar planets, making it a valuable complement to space-based mid-IR observations.