Condensate clouds fundamentally impact the atmospheric structure and spectra of exoplanets and brown dwarfs but the connections between surface gravity, cloud structure, dust in the upper atmosphere, and the red colors of some brown dwarfs remain poorly understood. Rotational modulations enable the study of different clouds in the same atmosphere, thereby providing a method to isolate the effects of clouds. Here we present the discovery of high peak-to-peak amplitude (8%) rotational modulations in a low-gravity, extremely red (J-K s =2.55) L6 dwarf WISEP J004701.06+680352.1 (W0047). Using the Hubble Space Telescope (HST) time-resolved grism spectroscopy we find a best-fit rotational period (13.20±0.14 hours) with a larger amplitude at 1.1 micron than at 1.7 micron. This is the third largest near-infrared variability amplitude measured in a brown dwarf, demonstrating that large-amplitude variations are not limited to the L/T transition but are present in some extremely red L-type dwarfs. We report a tentative trend between the wavelength dependence of relative amplitude, possibly proxy for small dust grains lofted in the upper atmosphere, and the likelihood of large-amplitude variability. By assuming forsterite as haze particle, we successfully explain the wavelength dependent amplitude with submicron-sized haze particles sizes of around 0.4 µm. W0047 links the earlier spectral and later spectral type brown dwarfs in which rotational modulations have been observed; the large amplitude variations in this object make this a benchmark brown dwarf for the study of cloud properties close to the L/T transition.

The Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) near-IR channel is extensively used in time-resolved observations, especially for transiting exoplanet spectroscopy and brown dwarf and directly imaged exoplanet rotational phase mapping. The ramp effect is the dominant source of systematics in the WFC3 for time-resolved observations, which limits its photometric precision. Current mitigation strategies are based on empirical fits and require additional orbits "to help the telescope reach a thermal equilibrium." We show that the ramp effect profiles can be explained and corrected with high fidelity using charge trapping theories. We also present a model for this process that can be used to predict and to correct charge trap systematics. Our model is based on a very small number of parameters that are intrinsic to the detector. We find that these parameters are very stable between the different datasets, and we provide best-fit values. Our model is tested with more than 120 orbits (∼ 40 visits) of WFC3 observations, and is proved to be able to provide near photon noise limited corrections for observations made with both staring and scanning modes of transiting exoplanets as well as for starting-mode observations of brown dwarfs. After our model correction, the light curve of the first orbit in each visit has the same photometric precision as subsequent orbits, so data from the first orbit need no longer be discarded. Near IR arrays with the same physical characteristics (e.g., JWST/NIRCam) may also benefit from the extension of this model if similar systematic profiles are observed.

Context. Of the presently known ≈ 3900 exoplanets, sparse spectral observations are available for ≈ 100. Ultra-hot Jupiters have recently attracted interest from observers and theoreticians alike, as they provide observationally accessible test cases. Aims. We aim to study cloud formation on the ultra-hot Jupiter HAT-P-7b, the resulting composition of the local gas phase, and how their global changes affect wavelength-dependent observations utilised to derive fundamental properties of the planet. Methods. We apply a hierarchical modelling approach as a virtual laboratory to study cloud formation and gas-phase chemistry. We utilise 97 vertical 1D profiles of a 3D GCM for HAT-P-7b to evaluate our kinetic cloud formation model consistently with the local equilibrium gas-phase composition. We use maps and slice views to provide a global understanding of the cloud and gas chemistry. Results. The day/night temperature difference on HAT-P-7b (∆T ≈ 2500 K) causes clouds to form on the nightside (dominated by H 2 /He) while the dayside (dominated by H/He) retains cloud-free equatorial regions. The cloud particles vary in composition and size throughout the vertical extension of the cloud, but also globally. TiO 2 [s]/Al 2 O 3 [s]/CaTiO 3 [s]-particles of cm-sized radii occur in the higher dayside-latitudes, resulting in a dayside dominated by gas-phase opacity. The opacity on the nightside, however, is dominated by 0.01 . . . 0.1 µm particles made of a material mix dominated by silicates. The gas pressure at which the atmosphere becomes optically thick is ∼ 10 −4 bar in cloudy regions, and ∼ 0.1 bar in cloud-free regions. Conclusions. HAT-P-7b features strong morning/evening terminator asymmetries, providing an example of patchy clouds and azimuthally-inhomogeneous chemistry. Variable terminator properties may be accessible by ingress/egress transmission photometry (e.g., CHEOPS and PLATO) or spectroscopy. The large temperature differences of ≈2500 K result in an increasing geometrical extension from the night-to the dayside. The chemcial equilibrium H 2 O abundance at the terminator changes by < 1 dex with altitude and 0.3 dex (a factor of 2) across the terminator for a given pressure, indicating that H 2 O abundances derived from transmission spectra can be representative of the well-mixed metallicity at P 10 bar. We suggest the atmospheric C/O as an important tool to trace the presence and location of clouds in exoplanet atmospheres. The atmospheric C/O can be sub-and supersolar due to cloud formation. Phase curve variability of HAT-P-7b is unlikely to be caused by dayside clouds.