2022AJ....164..143W

The atmospheres of gas giant planets are thought to be inhomogeneous due to weather and patchy clouds. We present two full nights of coronagraphic observations of the HR 8799 planets using the CHARIS integral field spectrograph behind the SCExAO adaptive optics system on the Subaru Telescope to search for spectrophomometric variability. We did not detect significant variability signals, but placed the lowest variability upper limits for HR 8799c and d. Based on injection-recovery tests, we expected to have a 50% chance to detect signals down to 10% H-band photometric variability for HR 8799c and down to 30% H-band variability for HR 8799d. We also investigated spectral variability and expected a 50% chance to recover 20% variability in the H/K flux ratio for HR 8799c. We combined all the data from the two nights to obtain some of the most precise spectra obtained for HR 8799c, d, and e. Using a grid of cloudy radiative-convective-thermochemical equilibrium models, we found all three planets prefer supersolar metallicity with effective temperatures of ∼1100 K. However, our high signal-to-noise spectra show that HR 8799d has a distinct spectrum from HR 8799c, possibly preferring more vertically extended and uniform clouds and indicating that the planets are not identical.