Abstract

Heterogeneous clouds in the atmospheres of brown dwarfs and exoplanets introduce wavelength-dependent rotational modulations in their light curves. High-precision time-resolved observations of the rotational modulations can reveal condensate cloud properties such as altitude and covering fractions, which are essential constraints on cloud formation and atmospheric circulation models. Using HST/WFC3 grism spectroscopy and Spitzer time-resolved observations, we found that VHS1256b, a low surface gravity substellar companion that spectroscopically resembled HR8799bcd planets, is variable at the 25% level in its near infrared light curves. This is among the strongest variability amplitudes that have ever been discovered in a substellar object. In this talk, I will summarize the observational results from multi-wavelength time-series observations of VHS1256b. I will then discuss the interpretations of the results by comparing the chromatic variations in the modulation amplitude to the ultra-cool atmospheric models. Our observations agree with the predictions from patchy cloud models, supporting the theory of dissipation of clouds in photospheres as the source of the rotational modulation.