Hubble data reveals Uranus’ day is 28 seconds longer than scientists thought

Tapping more than a decade of Hubble Space Telescope observations, astronomers have determined Uranus’s rotation period with ultra-high accuracy — roughly 1,000 times greater than previous estimates — by tracking the planet’s unique ultraviolet aurorae. This new measurement refines the Uranian day to 17 hours, 14 minutes, and 52 seconds, establishing a vital baseline for planetary science and future exploration missions.

The research, published in Nature Astronomy, tackled the challenge of measuring the interior rotation of an ice giant like Uranus, where direct observation is impossible. An international team, under the guidance of Laurent Lamy of the Observatoire de Paris-PSL, developed a novel technique using long-term Hubble data to monitor the movement of Uranus’s ultraviolet auroral emissions. These aurorae are tied to the planet’s magnetic field, which rotates with its deep interior.

The analysis revealed Uranus completes a rotation in 17 hours, 14 minutes, and 52.31 seconds (17.247864 ± 0.000010 hours). This is significantly more precise than, and 28 seconds longer than, the estimate derived from NASA’s Voyager 2 flyby back in 1986.

This refined measurement resolves a major challenge for scientists studying the ice giant. “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for the planetary science community but also resolves a long-standing issue: previous coordinate systems based on outdated rotation periods quickly became inaccurate, making it impossible to track Uranus’ magnetic poles over time,” explained lead author Laurent Lamy in a corresponding NASA release. He added, “With this new longitude system, we can now compare auroral observations spanning nearly 40 years and even plan for the upcoming Uranus mission.” Achieving this level of accuracy hinged on the telescope’s consistent observations over many years. “The continuous observations from Hubble were crucial,” Lamy stated. “Without this wealth of data, it would have been impossible to detect the periodic signal with the level of accuracy we achieved.”