An international team of astronomers using the NASA/ESA Hubble Space Telescope have made new measurements of Uranus’ interior rotation rate with a novel technique, achieving a level of accuracy 1000 times greater than previous estimates. By analysing more than a decade of Hubble observations of Uranus’ aurorae, researchers have refined the planet’s rotation period and established a crucial new reference point for future planetary research.
Determining a planet’s interior rotation rate is challenging, particularly for a world like Uranus, where direct measurements are not possible. A team led by Laurent Lamy (of LIRA, Observatoire de Paris-PSL and LAM, Aix-Marseille Univ., France), developed an innovative method to track the rotational motion of Uranus’ aurorae: spectacular light displays generated in the upper atmosphere by the influx of energetic particles near the planet’s magnetic poles. This technique revealed that Uranus completes a full rotation in 17 hours, 14 minutes, and 52 seconds — 28 seconds longer than the estimate obtained by NASA’s Voyager 2 during its 1986 flyby.
“Our measurement not only provides an essential reference for the planetary science community but also resolves a long-standing issue: previous coordinate systems based on outdated rotation periods quickly became inaccurate, making it impossible to track Uranus’ magnetic poles over time,” explains Lamy. “With this new longitude system, we can now compare auroral observations spanning nearly 40 years and even plan for the upcoming Uranus mission.”
This breakthrough was made possible thanks to Hubble’s long-term monitoring of Uranus. Over more than a decade, Hubble has regularly observed its ultraviolet auroral emissions, enabling researchers to track the position of the magnetic poles with magnetic field models.
“The continuous observations from Hubble were crucial,” says Lamy. “Without this wealth of data, it would have been impossible to detect the periodic signal with the level of accuracy we achieved.”
Unlike the aurorae of Earth, Jupiter, or Saturn, Uranus’ aurorae behave in a unique and unpredictable manner. This is due to the planet’s highly tilted magnetic field, which is significantly offset from its rotational axis. The findings not only help astronomers understand Uranus’ magnetosphere but also provide vital information for future missions.
The Planetary Science Decadal Survey in the US prioritized the Uranus Orbiter and Probe concept for future exploration.
These findings set the stage for further studies that will deepen our understanding of one of the most mysterious planets in the Solar System. With its ability to monitor celestial bodies over decades, the Hubble Space Telescope continues to be an indispensable tool for planetary science, paving the way for the next era of exploration at Uranus.
These results are based on observations acquired with Hubble programmes GO #12601, 13012, 14036, 16313 and DDT #15380 (PI: L. Lamy). The team’s paper has been published today in Nature.