Emergent Spectral Fluxes of Hot Jupiters: an Abrupt Rise in Day Side Brightness Temperature Under Strong Irradiation

TL;DR

This study analyzes a large sample of Spitzer secondary eclipses of hot Jupiters to identify an abrupt increase in day side brightness temperature at a specific temperature range, revealing insights into atmospheric processes.

Contribution

It provides a comprehensive re-analysis of 457 eclipses across 122 hot Jupiters, discovering a sharp rise in brightness temperature linked to atmospheric phase changes and exploring spectral properties related to metallicity and temperature inversions.

Findings

Abrupt brightness temperature increase occurs at 1714-1818K.

Magnetic drag and cloud dissipation are potential causes.

Fluxes are sensitive to metallicity below 1200K.

Abstract

We study the emergent spectral fluxes of transiting hot Jupiters, using secondary eclipses from Spitzer. To achieve a large and uniform sample, we have re-analyzed all secondary eclipses for all hot Jupiters observed by Spitzer at 3.6- and/or 4.5 microns. Our sample comprises 457 eclipses of 122 planets, including eclipses of 13 planets not previously published. We use these eclipse depths to calculate the spectral fluxes emergent from the exoplanetary atmospheres, and thereby infer temperature and spectral properties of hot Jupiters. We find that an abrupt rise in brightness temperature, similar to a phase change, occurs on the day side atmospheres of the population at an equilibrium temperature between 1714K and 1818K (99-percent confidence limits). The amplitude of the rise is 291 +/-49 Kelvins, and two viable causes are the onset of magnetic drag that inhibits longitudinal heat redistribution, and/or the rapid dissipation of day side clouds. We also study hot Jupiter spectral properties with respect to metallicity and temperature inversions. Models exhibiting 4.5 micron emission from temperature inversions reproduce our fluxes statistically for the hottest planets, but the transition to emission is gradual, not abrupt. The Spitzer fluxes are sensitive to metallicity for planets cooler than approximately 1200 Kelvins, and most of the hot Jupiter population falls between model tracks having solar to 30X-solar metallicity.