Uranus’ rotation back on schedule
Determining the internal rotation speed of a giant planet is a major scientific challenge, as its atmosphere masks the planetary core.
To achieve this, we can follow the periodic recurrence of phenomena linked to the magnetic field produced in the planetary core. For Uranus, a rotation period of 17 hours, 14 minutes, 24 sec +/- 36 sec was obtained by continuous 30-day observation of its auroral radio emissions during the Voyager 2 probe’s flyby of the planet in 1986.
It was this value that defined Uranus’ longitude system linked to the magnetic field (known as System III). However, the inaccuracy of the rotation period rendered the longitude system invalid just a few months later. To overcome this limitation, the researchers had to completely rethink the measurement method.
The team led by Laurent Lamy, assistant astronomer at LIRA (Laboratoire d’Instrumentation et de Recherche en Astrophysique) at Paris Observatory - PSL and LAM (Laboratoire d’Astrophysique de Marseille), and including Renée Prangé, CNRS Research Director at LIRA, and Jérôme Berthier, astronomer at LTE (Laboratoire Temps-Espace), two laboratories attached to Paris Observatory - PSL, developed an innovative approach: use ultraviolet aurorae, observed over the long term by the Hubble Space Telescope, to find the position of the magnetic poles in order to deduce the planet’s internal rotation.
A breakthrough for planetology and space exploration
"This measurement provides a key reference for the entire planetology community. This work has also made it possible to recover the position of the magnetic poles, lost four decades ago. With our new longitude system, we can now compare observations spread over a century, which is crucial for the long-term study of the Uranian magnetosphere and preparing future missions to Uranus" explains Laurent Lamy.
Combining the expertise of Paris Observatory in the service of scientific progress
The combined expertise of the LIRA, specialized, among other things, in the study of planetary magnetospheres, and the LTE, a national reference for ephemeris calculations and the modeling of time and space, has made it possible to construct a robust and precise rotation reference frame.
"The LTE has participated in this breakthrough in our knowledge of Uranus by making available an ephemeris calculation tool that has enabled us to implement this new method exploiting observations made over the long term. It now enables us to provide more accurate physical ephemerides of Uranus. For example, a 28-second longer rotation period means that between Voyager 2’s flyby on January 24, 1986 and the publication of this article on April 7, 2025, Uranus has made 9 fewer revolutions than previously thought. This gain in precision in calculating the physical ephemeris of Uranus is essential for preparing future space missions to explore the planet, which will in turn provide new data.” explains Jérôme Berthier (LTE).
For Renée Prangé (LIRA), this new reference opens up an era of in-depth study, including the development of better constrained theoretical models, of the Uranian magnetosphere across the wide variety of its geometric configurations relative to the Sun:
“”We will thus be able to access the structure of its distinctive magnetic field and understand its interaction processes with the solar wind for all configurations encountered along its 84-year orbit around the Sun. These models, involving very diverse conditions - and fundamentally different from those observed for traditional Solar System planets - could also pave the way for future studies of interactions in star-exoplanet systems."
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