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

Abstract

We constrain the angular momentum architecture of VHS J125601.92-125723.9, a 140 ± 20 Myr old hierarchical triple system composed of a low-mass binary and a widely separated planetary-mass companion, VHS 1256 b. VHS 1256 b has been a prime target for multiple characterization efforts, revealing the highest measured substellar photometric variability to date and the presence of silicate clouds and disequilibrium chemistry. Here we add a key piece to the characterization of this super-Jupiter on a Tatooine-like orbit: we measure its spin-axis tilt relative to its orbit, i.e., the obliquity of VHS 1256 b. We accomplish this by combining three measurements. We find a projected rotation rate $v\sin {i}_{p}=8.7\pm 0.1\,\mathrm{km}\,{{\rm{s}}}^{-1}$ for VHS 1256 b using near-infrared high-resolution spectra from Gemini/IGRINS. Combining this with a published photometric rotation period indicates that the companion is viewed edge on, with a line-of-sight spin axis inclination of i _p = 90° ± 18°. We refit available astrometry measurements to confirm an orbital inclination of ${i}_{{\rm{o}}}={23}_{-13}^{+10}$ degrees. Taken together, VHS 1256 b has a large planetary obliquity of ψ = 90° ± 25°. In total, we have three measured angular momentum vectors for the system: the binary orbit normal, companion orbit normal, and companion spin axis. All three are misaligned with respect to each other. Although VHS 1256 b is tilted like Uranus, their origins are distinct. We rule out planet-like scenarios including collisions and spin–orbit resonances, and suggest that top-down formation via core/filament fragmentation is promising.

Keywords

• Astrostatistics
• High resolution spectroscopy
• Exoplanet evolution
• Exoplanet formation
• Exoplanet systems