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Most directly imaged giant exoplanets are fainter than brown dwarfs withsimilar spectra. To explain their relative underluminosity unusually cloudyatmospheres have been proposed. However, with multiple parameters varyingbetween any two objects, it remained difficult to observationally test thisidea. We present a new method, sensitive time-resolved Hubble Space Telescopenear-infrared spectroscopy, to study two rotating L/T transition brown dwarfs(2M2139 and SIMP0136). The observations provide spatially and spectrallyresolved mapping of the cloud decks of the brown dwarfs. The data allow thestudy of cloud structure variations while other parameters are unchanged. Wefind that both brown dwarfs display variations of identical nature: J- andH-band brightness variations with minimal color and spectral changes. Our lightcurve models show that even the simplest surface brightness distributionsrequire at least three elliptical spots. We show that for each source thespectral changes can be reproduced with a linear combination of only twodifferent spectra, i.e. the entire surface is covered by two distinct types ofregions. Modeling the color changes and spectral variations together revealpatchy cloud covers consisting of a spatially heterogenous mix oflow-brightness, low-temperature thick clouds and brighter, thin and warmclouds. We show that the same thick cloud patches seen in our varying browndwarf targets, if extended to the entire photosphere, predict near-infraredcolors/magnitudes matching the range occupied by the directly imaged exoplanetsthat are cooler and less luminous than brown dwarfs with similar spectraltypes. This supports the models in which thick clouds are responsible for thenear infrared properties of these underluminous exoplanets.

HST Spectral Mapping of L/T Transition Brown Dwarfs Reveals Cloud Thickness Variations