Spin-orbit alignment of the $\beta$ Pictoris planetary system

Abstract

A crucial diagnostic that can tell us about processes involved in the formation and dynamical evolutionof planetary systems is the angle between the rotation axis of a star and a planet's orbital angular momentum vector ("spin-orbit" alignment or "obliquity"). Here we present the first spin-orbit alignment measurement for a wide-separation exoplanetary system, namely on the directly-imaged planet $\beta$ Pictoris b. We use VLTI/GRAVITY spectro-interferometry with an astrometric accuracy of 1 $\mu$as (microarcsecond) in the Br$\gamma$ photospheric absorption line to measure the photocenter displacement associated with the stellar rotation. Taking inclination constraints from astroseismology into account, we constrain the 3-dimensional orientation of the stellar spin axis and find that $\beta$ Pic b orbits its host star on a prograde orbit. The angular momentum vectors of the stellar photosphere, the planet, and the outer debris disk are well-aligned with mutual inclinations $<3\pm 5^{\circ }$, which indicates that $\beta$ Pic b formed in a system without significant primordial misalignments. Our results demonstrate the potential of infrared interferometry to measure the spin-orbit alignment for wide-separation planetary systems, probing a highly complementary regime to the parameter space accessible with the Rossiter-McLaughlin effect. If the low obliquity is confirmed by measurements on a larger sample of wide-separation planets, it would lend support to theories that explain the obliquity in Hot Jupiter systems with dynamical scattering and the Kozai-Lidov mechanism.

6 pages, 4 figures, accepted for ApJL

References in corpus (10)

• Dynamical Outcomes of Planet-Planet Scattering
• Molecular Gas Clumps from the Destruction of Icy Bodies in the $\beta$ Pictoris Debris Disk
• HST/ACS Multiband Coronagraphic Imaging of the Debris Disk around Beta Pictoris
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• The Orbit and Transit Prospects for $\beta$ Pictoris b constrained with One Milliarcsecond Astrometry
• Tidal Dissipation and Obliquity Evolution in Hot Jupiter Systems
• New HST data and modeling reveal a massive planetesimal collision around Fomalhaut
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Cited by in corpus (11)

• Precise Dynamical Masses and Orbital Fits for $\beta$ Pic b and $\beta$ Pic c
• Rotation Periods, Inclinations, and Obliquities of Cool Stars Hosting Directly Imaged Substellar Companions: Spin-Orbit Misalignments are Common
• 2023 Astrophotonics Roadmap: pathways to realizing multi-functional integrated astrophotonic instruments
• On the Detection of Exomoons Transiting Isolated Planetary-Mass Objects
• TOI-942b: A Prograde Neptune in a ~60 Myr old Multi-transiting System
• The $\beta$ Pictoris b Hill sphere transit campaign. Paper I: Photometric limits to dust and rings
• Dynamical Mass of the Young Brown Dwarf Companion PZ Tel B
• Formation of the warped debris disc around $\beta$ Pictoris
• Integral field spectroscopy with the solar gravitational lens
• Effects of Rotation on the Spectra of Brown Dwarfs
• Radial velocity analysis of stars with debris discs