Measuring the Mean Plane of the Distant Kuiper Belt

TL;DR

This paper measures the mean plane of the distant Kuiper belt, finds a warp at certain distances, and suggests it could be caused by an unseen planet, with future surveys like LSST poised to confirm these findings.

Contribution

Introduces a new bias-independent method to measure the Kuiper belt's mean plane and identifies a potential warp indicating an unseen planet.

Findings

Detected a warp in the Kuiper belt at 80-200 AU with high confidence.

Proposed a hypothetical planet between Mercury and Earth in mass and 100-200 AU in semimajor axis as a possible cause.

Future LSST observations could confirm the warp and detect the hypothesized planet.

Abstract

In the absence of any unseen planetary-mass bodies in the outer solar system, the mean plane of the distant Kuiper belt should be the same as the plane orthogonal to the angular momentum vector of the solar system -- the invariable plane. Here, we measure the mean plane of the non-resonant Kuiper belt across semimajor axes 50 - 400 AU. We introduce a new method to measure the mean plane that we demonstrate to be independent of observational bias. In particular, our results are not biased by surveys that look only at limited areas on the celestial sphere. We find a warp relative to the invariable plane at semimajor axes of 80 - 400 AU (98% confidence) and 80 - 200 AU (96% confidence), but not at 50 - 80 AU or 200 - 400 AU. If it is not spurious, a possible explanation for this warp is an unseen planet in the outer solar system. With $n$-body simulations, we find that a planet with mass between that of Mercury and the Earth, semimajor axis in the range 100 - 200 AU, and inclination $\gtrsim 10^{\circ}$ to be the most likely cause of the warp; however, parameters outside of these ranges are still possible. Such a body is distinct in both mass and semimajor axis from the various versions of an unseen planet invoked to explain apsidal clustering in the outer solar system. The Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) is expected to confirm or deny the existence of the warp reported here, and might detect the planet that may produce it.