WASP-8b: Characterization of a Cool and Eccentric Exoplanet with Spitzer

TL;DR

WASP-8b, a cool and eccentric hot Jupiter, was characterized using Spitzer observations revealing its atmospheric properties, temperature profile, and the absence of thermal inversion, with some unresolved temperature discrepancies.

Contribution

This study provides the first detailed atmospheric characterization of WASP-8b, including thermal profile and composition, using multi-band infrared observations and advanced modeling techniques.

Findings

No thermal inversion detected in WASP-8b's atmosphere.

Brightness temperatures vary across wavelengths, with an anomalously high 3.6-micron temperature.

Modeling suggests rapid radiative timescales could explain temperature anomalies.

Abstract

WASP-8b has 2.18 times Jupiter's mass and is on an eccentric ($e=0.31$) 8.16-day orbit. With a time-averaged equilibrium temperature of 948 K, it is one of the least-irradiated hot Jupiters observed with the Spitzer Space Telescope. We have analyzed six photometric light curves of WASP-8b during secondary eclipse observed in the 3.6, 4.5, and 8.0 {\microns} Infrared Array Camera bands. The eclipse depths are $0.113\pm 0.018$%, $0.069\pm 0.007$%, and $0.093\pm 0.023$%, respectively, giving respective brightness temperatures of 1552, 1131, and 938 K. We characterized the atmospheric thermal profile and composition of the planet using a line-by-line radiative transfer code and a Markov-chain Monte Carlo sampler. The data indicated no thermal inversion, independently of any assumption about chemical composition. We noted an anomalously high 3.6-{\microns} brightness temperature (1552 K); by modeling the eccentricity-caused thermal variation, we found that this temperature is plausible for radiative time scales less than $\sim 10^{2}$ hours. However, as no model spectra fit all three data points well, the temperature discrepancy remains as an open question.