The Polar Vortex Hypothesis: Evolving, Spectrally Distinct Polar Regions Explain Short- and Long-term Light Curve Evolution and Color-Inclination Trends in Brown Dwarfs and Giant Exoplanets

Abstract

Recent studies revealed viewing-angle-dependent color and spectral trends in brown dwarfs, as well as long-term photometric variability (~100 hr). The origins of these trends are yet unexplained. Here, we propose that these seemingly unrelated sets of observations stem from the same phenomenon: The polar regions of brown dwarfs and directly imaged exoplanets are spectrally different from lower-latitude regions, and that they evolve over longer timescales, possibly driven by polar vortices. We explore this hypothesis via a spatio-temporal atmosphere model capable of simulating time-series, disk-integrated spectra of ultracool atmospheres. We study three scenarios with different spectral and temporal components: A null hypothesis without polar vortex, and two scenarios with polar vortices. We find that the scenarios with polar vortex can explain the observed infrared color-inclination trend and the variability amplitude-inclination trend. The presence of spectrally distinct, time-evolving polar regions in brown dwarfs and giant exoplanet atmospheres raises the possibility that one-dimensional, static atmospheric models may be insufficient for reproducing ultracool atmospheres in detail.