Thermal emission at 4.5 and 8 micron of WASP-17b, an extremely large planet in a slightly eccentric orbit

TL;DR

This study detects thermal emission from WASP-17b at 4.5 and 8 microns, revealing its unusually large size and low eccentricity, challenging existing models of gas giant planet inflation.

Contribution

It provides the first thermal emission measurements of WASP-17b and reveals its anomalously large radius and slight orbital eccentricity, questioning current theories of planetary inflation.

Findings

WASP-17b's radius is 2.0 Rjup, larger than any known planet.

The planet's brightness temperatures are 1881 K at 4.5 microns and 1580 K at 8 microns.

The orbit is slightly eccentric, with e between 0.0012 and 0.070.

Abstract

We report the detection of thermal emission at 4.5 and 8 micron from the planet WASP-17b. We used Spitzer to measure the system brightness at each wavelength during two occultations of the planet by its host star. By combining the resulting light curves with existing transit light curves and radial velocity measurements in a simultaneous analysis, we find the radius of WASP-17b to be 2.0 Rjup, which is 0.2 Rjup larger than any other known planet and 0.7 Rjup larger than predicted by the standard cooling theory of irradiated gas giant planets. We find the retrograde orbit of WASP-17b to be slightly eccentric, with 0.0012 < e < 0.070 (3 sigma). Such a low eccentricity suggests that, under current models, tidal heating alone could not have bloated the planet to its current size, so the radius of WASP-17b is currently unexplained. From the measured planet-star flux-density ratios we infer 4.5 and 8 micron brightness temperatures of 1881 +/- 50 K and 1580 +/- 150 K, respectively, consistent with a low-albedo planet that efficiently redistributes heat from its day side to its night side.