Reassessing the Evidence for Time Variability in the Atmosphere of the Exoplanet HAT-P-7 b

TL;DR

This study reevaluates claims of atmospheric variability in exoplanet HAT-P-7 b, finding that stellar noise likely explains the observed signals rather than true planetary atmospheric changes.

Contribution

It demonstrates that stellar supergranulation noise can mimic planetary atmospheric variability signals in Kepler data.

Findings

Variations initially attributed to HAT-P-7 b's atmosphere are likely caused by stellar noise.

Background stellar supergranulation can produce signals similar to planetary atmospheric variability.

Analysis shows that stellar activity must be carefully accounted for in exoplanet atmospheric studies.

Abstract

We reassess the claimed detection of variability in the atmosphere of the hot Jupiter HAT-P-7 b, reported by Armstrong et al. (2016). Although astronomers expect hot Jupiters to have changing atmospheres, variability is challenging to detect. We looked for time variation in the phase curves of HAT-P-7 b in Kepler data using similar methods to Armstrong et al. (2016), and identified apparently significant variations similar to what they found. Numerous tests show the variations to be mostly robust to different analysis strategies. However, when we injected unchanging phase curve signals into the light curves of other stars and searched for variability, we often saw similar levels of variations as in the HAT-P-7 light curve. Fourier analysis of the HAT-P-7 light curve revealed background red noise from stellar supergranulation on timescales similar to the planet's orbital period. Tests of simulated light curves with the same level of noise as HAT-P-7's supergranulation show that this effect alone can cause the amplitude and phase offset variability we detect for HAT-P-7 b. Therefore, the apparent variations in HAT-P-7 b's atmosphere could instead be caused by non-planetary sources, most likely photometric variability due to supergranulation on the host star.