  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.
