Roaring Storms in the Planetary-Mass Companion VHS 1256-1257 b: Hubble Space Telescope Multi-epoch Monitoring Reveals Vigorous Evolution in an Ultra-cool Atmosphere

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Photometric and spectral variability of brown dwarfs probes heterogeneous temperature and cloud distribution and traces the atmospheric circulation patterns.

• We present a new 42-hr Hubble Space Telescope (HST) Wide Field Camera 3 G141 spectral time series of VHS 1256$-$1257 b, a late L-type planetary-mass companion that has been shown to have one of the highest variability amplitudes among substellar objects.
• The light curve is rapidly evolving and best-fit by a combination of three sine waves with different periods and a linear trend.
• The amplitudes of the sine waves and the linear slope vary with wavelength, and the corresponding spectral variability patterns match the predictions by models invoking either heterogeneous clouds or thermal profile anomalies.