A new rotation period and longitude system for Uranus

Abstract

Abstract In the absence of any visible solid surface, the rotation period of the giant planets has been inferred from periodic phenomena tied to the magnetic field produced in their deep interior. The main method relied on remote radio auroral observations, sometimes complemented by in situ magnetic measurements. For Uranus, such measurements acquired during the Voyager 2 flyby in 1986 yielded a rotation period of 17.24±0.01h1. This fundamental planetary parameter, referenced since then by the International Astronomical Union, is the basis of the Uranian longitude model2. Still, the period uncertainty limits its validity to a few years, after which the orientation of the magnetic axis was lost. Here, we use a novel approach, based on the long term (2011-2022) tracking of Uranus’ magnetic poles from Hubble Space Telescope images of its ultraviolet aurorae, to achieve a new rotation period of 17.247864±0.000010h. It is consistent with, although 28s longer than, the Voyager 2 period. This much more precise determination leads to a new longitude model now valid over decades, from before the Voyager 2 epoch up to the arrival of any future Uranus mission. It also has strong direct implications on formation scenarios, interior models, dynamo theories and studies of the magnetosphere. This novel approach stands as an alternate method to determine the rotation rate of any object hosting a magnetosphere and rotationally modulated aurorae, in our solar system and beyond.