On the misalignment of the directly imaged planet β Pictoris b with the system's warped inner disk

Abstract

The vertical warp in the debris disk Beta Pictoris -- an inclined inner disk extending into a flat outer disk -- has long been interpreted as the signpost of a planet on an inclined orbit. Direct images spanning 2004-2010 have revealed Beta Pictoris b, a planet with a mass and orbital distance consistent with this picture. However, it was recently reported that the orbit of planet b is aligned with the flat outer disk, not the inclined inner disk, and thus lacks the inclination to warp the disk. We explore three scenarios for reconciling the apparent misalignment of the directly imaged planet Beta Pictoris b with the warped inner disk of Beta Pictoris: observational uncertainty, an additional planet, and damping of planet b's inclination. We find that, at the extremes of the uncertainties, the orbit of Beta Pictoris b has the inclination necessary to produce the observed warp. We also find that if planet b were aligned with the flat outer disk, it would prevent another planet from creating a warp with the observed properties; therefore planet b itself must be responsible for the warp. Finally, planet b's inclination could have been damped by dynamical friction and still produce the observed disk morphology, but the feasibility of damping depends on disk properties and the presence of other planets. More precise observations of the orbit of planet b and the position angle of the outer disk will allow us to distinguish between the first and third scenario.