Measuring the mean plane of the distant Kuiper belt

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. 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 (98 per cent confidence) and (96 per cent confidence), but not at or. 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, and inclination 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 is expected to confirm or deny the existence of the warp reported here, and might detect the planet that may produce it.