# Statistical Characterization of Hot Jupiter Atmospheres using Spitzer's Secondary Eclipses

## TL;DR

This study analyzes 78 secondary eclipse measurements of 36 hot Jupiters from Spitzer data, revealing new insights into their orbital eccentricities, atmospheric temperatures, and spectral differences from blackbody models, with implications for atmospheric structure.

## Contribution

First comprehensive statistical analysis of hot Jupiter atmospheres using Spitzer secondary eclipse data, identifying trends in eccentricity and spectral deviations from blackbody predictions.

## Key findings

- Eccentricity increases with orbital period in hot Jupiters.
- Detected systematic spectral differences from blackbody models related to temperature.
- Found planets with temperatures >2200K have zero albedo and uniform heat redistribution.

## Abstract

We report 78 secondary eclipse depths for a sample of 36 transiting hot Jupiters observed at 3.6- and 4.5 microns using the Spitzer Space Telescope. Our eclipse results for 27 of these planets are new, and include highly irradiated worlds such as KELT-7b, WASP-87b, WASP-76b, and WASP-64b, and important targets for JWST such as WASP-62b. We find that WASP-62b has a slightly eccentric orbit e cos(omega) = 0.00614+/- 0.00064, and we confirm the eccentricity of HAT-P-13b and WASP-14b. The remainder are individually consistent with circular orbits, but we find statistical evidence for eccentricity increasing with orbital period in our range from 1 to 5 days. Our day-side brightness temperatures for the planets yield information on albedo and heat redistribution, following Cowan and Agol (2011). Planets having maximum day side temperatures exceeding ~ 2200K are consistent with zero albedo and distribution of stellar irradiance uniformly over the day-side hemisphere. Our most intriguing result is that we detect a systematic difference between the emergent spectra of these hot Jupiters as compared to blackbodies. The ratio of observed brightness temperatures, Tb(4.5)/Tb(3.6), increases with equilibrium temperature by 100 +/- 24 parts-per-million per Kelvin, over the entire temperature range in our sample (800K to 2500K). No existing model predicts this trend over such a large range of temperature. We suggest that this may be due to a structural difference in the atmospheric temperature profile between the real planetary atmospheres as compared to models.