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Leaning Sideways: VHS 1256-1257b is a Super-Jupiter with a Uranus-like Obliquity

Presentation #401.06 in the session Dynamics, Obliquities, and Tides.

Published onApr 03, 2024
Leaning Sideways: VHS 1256-1257b is a Super-Jupiter with a Uranus-like Obliquity

Planet obliquities, the orientation between a planet’s spin axis and its orbit normal, are a new window into the formation and evolutionary histories of exoplanets. Up until 2020, only our solar system planets had measured obliquities – Uranus rotates on its side, Venus spins upside-down, and Saturn is tilted by 27 degrees, pointing to histories of processes like giant impacts and secular spin-orbit resonances. Today there are only three measured exoplanetary obliquities. Here we present the fourth, targeting an extrasolar object of exceptional interest. VHS 1256-1257b is a planetary-mass companion orbiting a brown dwarf binary in a hierarchical triple system. It is the only planetary-mass companion targeted with spectroscopy by the JWST High Contrast ERS Program, which obtained a remarkable 1-20 um spectrum indicating the presence of silicate clouds and disequilibrium chemistry. Extensive photometric monitoring has additionally shown VHS 1256b to be the most variable substellar object known. Here we add a key piece to the characterization of this super-Jupiter on a Tatooine-like orbit – we measure a large planetary obliquity of 90 ± 25 deg. To do this, we measure a projected equatorial velocity (vsini) with Gemini/IGRINS, and the combination of vsini, rotation period, and orbit yields the planet obliquity. Although VHS 1256b is tilted like Uranus, their origins are definitely distinct. We rule out formation scenarios such as collisions and spin-orbit resonances by estimating, and assert that top-down formation via turbulent fragmentation is promising.

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