Probing the Atmospheric Circulation Patterns in the Highly-Variable Planetary-Mass Companion VHS 1256 B

Atmospheric dynamics determines the thermal, compositional, and cloud distributions in a planet, so observational constraints on these structures facilitate direct tests of general circulation models.Recent long-time-baseline photometric monitoring of variable brown dwarfs revealed vigorous evolution in substellar atmospheres, offering a powerful probe of atmospheric circulation. We present results from the first multi-epoch space-based spectroscopic monitoring program of a planetary-mass companion (PMC). Using Hubble Space Telescope Wide Field Camera 3 time-resolved spectra, we probe the atmospheric circulation patterns of the PMC VHS 1256 B over multiple timescales. Over one rotation period, VHS 1256 B’s light curve varies rapidly, suggesting fast-evolving waves and storms driven by atmospheric dynamics. On a long timescale spanning nearly one thousand rotation, the brightness and spectral variability is even greater. The J-band peak-to-valley amplitude is 38%, making VHS 1256 B the most variable substellar object known. The spectral time series qualitatively agrees with general circulation models containing equatorial waves and high-latitude storms, but the observed variability amplitude far exceeds the predicted value. The constraints on the rapid and vigorous evolution in VHS 1256 B’s atmosphere provide a crucial context for interpreting its upcoming JWST spectrum that will be collected in an Early Release Science program.