A new rotation period and longitude system for Uranus

Abstract

The rotation period of Uranus was estimated to be 17.24 ± 0.01 h in 1986 from radio auroral measurements during the brief Voyager 2 flyby. This value is the basis for the Uranian SIII longitude system still in use. However, the poor period uncertainty limited its validity to a few years, after which the orientation of the magnetic axis was lost. Alternate, conflicting, rotation periods have also been proposed since then. Here we use the long-term tracking of Uranus’ magnetic poles between 2011 and 2022 from Hubble Space Telescope images of its ultraviolet aurorae to achieve an updated, independent, extremely precise rotation period of 17.247864 ± 0.000010 h, only consistent with the Voyager 2 estimate. Its 28-s-longer value and improved accuracy yields a new longitude model now valid over decades, up to the arrival of any future Uranus mission, which will allow the reanalysis of the whole set of Uranus observations. In addition, it has strong direct implications for formation scenarios, interior models, dynamo theories and studies of the magnetosphere. This approach stands as a new method to determine the rotation rate of any object hosting a magnetosphere and a rotationally modulated aurorae, in our Solar System and beyond.