Uranus, the seventh planet from the Sun, is classed as an ice giant distinguished by its extreme axial tilt of 98 degrees to the plain of the Solar System. This unusual orientation creates 42 year long seasons of continuous sunlight or darkness at its poles. It appears pale blue-green in colour due to atmospheric methane absorbing red light and it possesses a system of thin rings and 27 known moons. Despite being discovered in 1781 by William Herschel, the planet has been visited only once by spacecraft, Voyager 2 in 1986, leaving many aspects of this distant world still really quite mysterious.
Images of Uranus spanning a decade (Credit : ESA)
One such mystery is the planet’s rotation. Measuring the rotation of Uranus and the other giant planets without solid surfaces means that either the magnetic field or atmospheric features must be tracked. Many atmospheric features however travel at different speeds at different latitudes creating complex differential rotation patterns. For Uranus especially, distinctive atmospheric markers are scarce so instead, core rotation must be inferred indirectly through gravitational measurements, radio emissions, or magnetic field variations.
Using the Hubble Space Telescope, a team of astronomers have achieved unprecedented accuracy in measuring the rotation of Uranus by tracking its auroral displays over a decade. Led by Laurent Lamy (from LIRA, a space research laboratory in France) the international team developed the novel technique that determined Uranus rotates in 17 hours, 14 minutes, and 52 seconds, 28 seconds longer than NASA’s Voyager 2 estimated during its 1986 flyby. This auroral tracking method achieved accuracy 1,000 times greater than previous measurements and overcomes the significant challenge of measuring interior rotation in a gas giant where direct observation is impossible.
Image of the Hubble Space Telescope (Credit : ESA)
“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for the planetary science community but also resolves a long-standing issue: previous coordinate systems based on outdated rotation periods quickly became inaccurate, making it impossible to track Uranus’ magnetic poles over time,” – Laurent Lamy from (LIRA, Observatoire de Paris-PSL and LAM, Aix-Marseille Univ., France)
The study wouldn’t be possible without Hubble’s long-term monitoring of Uranus with its regular observations of ultraviolet auroral emissions spanning more than a decade. The data enabled researchers to track the position of the planet’s magnetic poles with unprecedented precision. Lamy emphasised that these continuous observations were essential, as the periodic signal could not have been detected with such accuracy without Hubble’s data.
Observations of Uranus’ aurora present challenges though due to their unpredictable behaviour, primarily caused by the planet’s highly tilted magnetic field that significantly offsets from its rotational axis. The results of this study will guide upcoming missions to Uranus and sets the foundation for deeper studies of this wonderfully elusive and mysterious ice giant.
Source : Hubble helps determine Uranus’ rotation rate with unprecedented precision