The stable climate of KELT-9b

TL;DR

This study presents multi-band photometric observations of the ultra-hot Jupiter KELT-9b, revealing efficient heat redistribution, negligible albedo, and stable atmospheric temperatures, enhancing understanding of extreme planetary atmospheres.

Contribution

The paper provides the first combined analysis of CHEOPS, TESS, and Spitzer data for KELT-9b, deriving detailed atmospheric temperature maps and constraints on heat redistribution and albedo.

Findings

Heat redistribution efficiency of ~0.3 confirmed.

Bond albedo consistent with zero.

No brightness temperature variability detected.

Abstract

Even among the most irradiated gas giants, so-called ultra-hot Jupiters, KELT-9b stands out as the hottest planet thus far discovered with a dayside temperature of over 4500K. At these extreme irradiation levels, we expect an increase in heat redistribution efficiency and a low Bond albedo owed to an extended atmosphere with molecular hydrogen dissociation occurring on the planetary dayside. We present new photometric observations of the KELT-9 system throughout 4 full orbits and 9 separate occultations obtained by the 30cm space telescope CHEOPS. The CHEOPS bandpass, located at optical wavelengths, captures the peak of the thermal emission spectrum of KELT-9b. In this work we simultaneously analyse CHEOPS phase curves along with public phase curves from TESS and Spitzer to infer joint constraints on the phase curve variation, gravity-darkened transits, and occultation depth in three bandpasses, as well as derive 2D temperature maps of the atmosphere at three different depths. We find a day-night heat redistribution efficiency of $\sim$0.3 which confirms expectations of enhanced energy transfer to the planetary nightside due to dissociation and recombination of molecular hydrogen. We also calculate a Bond albedo consistent with zero. We find no evidence of variability of the brightness temperature of the planet, excluding variability greater than 1% (1$\sigma$).