Spitzer Secondary Eclipses of WASP-18b

TL;DR

This study reports detailed infrared observations of the exoplanet WASP-18b during secondary eclipses, revealing its extremely high temperature, likely thermal inversion, and minimal energy redistribution, contributing to understanding of hot Jupiter atmospheres.

Contribution

First infrared measurements of WASP-18b's secondary eclipses across multiple bands, indicating its extreme temperature and atmospheric properties.

Findings

WASP-18b has brightness temperatures around 3100-3300 K.

The planet likely has a thermal inversion in its atmosphere.

WASP-18b exhibits near-zero albedo and minimal heat redistribution.

Abstract

The transiting exoplanet WASP-18b was discovered in 2008 by the Wide Angle Search for Planets (WASP) project. The Spitzer Exoplanet Target of Opportunity Program observed secondary eclipses of WASP-18b using Spitzer's Infrared Array Camera (IRAC) in the 3.6 micron and 5.8 micron bands on 2008 December 20, and in the 4.5 micron and 8.0 micron bands on 2008 December 24. We report eclipse depths of 0.30 +/- 0.02%, 0.39 +/- 0.02%, 0.37 +/- 0.03%, 0.41 +/- 0.02%, and brightness temperatures of 3100 +/- 90, 3310 +/- 130, 3080 +/- 140 and 3120 +/- 110 K in order of increasing wavelength. WASP-18b is one of the hottest planets yet discovered - as hot as an M-class star. The planet's pressure-temperature profile most likely features a thermal inversion. The observations also require WASP-18b to have near-zero albedo and almost no redistribution of energy from the day-side to the night side of the planet.