Uranus’ Rotation Rate with Unprecedented Precision

  Uranus is full of surprises after 20 years of observations of Astronomers through Hubble 

An international team of astronomers using the NASA/ESA Hubble Space Telescope has made new measurements of Uranus' interior rotation rate with a novel technique, achieving a level of accuracy 1,000 times greater than previous estimates. After watching Uranus for decades and, until recently, it seemed like a fairly calm, blue-green planet. But new findings, based on two decades of observations from NASA’s Hubble Space Telescope, paint a different picture. 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. Uranus is full of surprises, with shifting patterns of haze, uneven distributions of methane, and giant seasonal changes which take place on a timescale of decades. These changes have big implications not only for our understanding of this strange ice giant, but also for how we might interpret data from similar exoplanets in distant star systems.

Images from the NASA/ESA Hubble Space Telescope showcase the dynamic aurora on Uranus in October 2022. These observations were made by the Space Telescope Imaging Spectrograph (STIS) and includes both visible and ultraviolet data. An international team of astronomers used Hubble to make new measurements of Uranus' interior rotation rate by analysing more than a decade of the telescope’s observations of Uranus’ aurorae. This refinement of the planet’s rotation period achieved a level of accuracy 1000 times greater than previous estimates and serves as a crucial new reference point for future planetary research. When Voyager 2 passed by Uranus in 1986, it revealed what looked like a smooth billiard ball with a teal hue. The same impression stuck for a long time. Over the following twenty years, it was discovered that Uranus isn’t static at all. Instead, sunlight shifts the planet’s appearance as it tilts throughout its 84-year orbit around the Sun. But starting in 2002, a team of researchers led by Erich Karkoschka from the University of Arizona, and Larry Sromovsky and Pat Fry from the University of Wisconsin used Hubble’s Space Telescope Imaging Spectrograph (STIS) to take a closer look. 

Uranus is mainly composed of hydrogen, helium and methane, with the methane responsible for its cyan colour because it reflects blue-green light and absorbs red. Yet the Hubble data show that methane is not spread evenly around the planet. It’s severely depleted at both poles but remains more abundant at mid and lower latitudes. On top of that, researchers see big swings in aerosol hazes over time, especially near the poles. The dramatic tilt of Uranus, which essentially rolls around the Sun like a barrel, means its poles each get about 42 years of continuous sunlight followed by 42 years of darkness. In the last two decades, the north pole has gradually come into more direct sunlight. Hubble’s observations show that this brightening sunlight appears to correlate with thicker haze, making the north pole shine more vividly. Meanwhile, the south pole is losing light and has been dimming. This is why the planet doesn’t look quite the same from year to year. An area that was relatively dark a decade ago could become much brighter as it moves into the spotlight. Conversely, the region on the far side of the planet recedes into darkness, causing its cloud features to fade from view.

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. Scientists expected that methane, the main ingredient behind Uranus’s distinctive colour, would be key to understanding the planet’s atmosphere. But what they found is that methane distribution varies more dramatically than anticipated, hinting at complex circulation patterns. Their data point toward upwelling in some areas, where methane-rich gas rises to the top, and strong down welling at the poles, which flushes methane out of the visible layers of the atmosphere. This interplay also seems tied to changes in cloud cover and haze formation. In false-colour images from Hubble, researchers can pick out details which are invisible to the naked eye. They can see exactly where methane is absent, where it’s abundant, and how these conditions tie to different latitudes or altitudes. The change of Uranus for the four years that STIS observed Uranus across a 20-year period. Over that span of time, the researchers watched the seasons of Uranus as the south polar region darkened going into winter shadow while the north polar region brightened as northern summer approaches. 

“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 by the team. “With this new longitude system, we can now compare auroral observations spanning nearly 40 years and even plan for the upcoming Uranus mission.” Uranus is not just an oddity at the edge of our solar system. Ice giants like Uranus and Neptune are common throughout the galaxy, and many exoplanets we discover may be similar. Learning how an ice giant’s atmosphere responds to sunlight, chemical processes and extreme axial tilt can guide us in interpreting data from faraway star systems. Some NASA scientists and engineers are already discussing sending a dedicated orbiteer to Uranus in the coming decades. If built, that spacecraft could peel back even more layers of mystery: sampling the planet’s magnetic field, studying its faint ring system and measuring wind speeds from within. But in the meantime, Hubble’s slow, steady watch continues to reveal new details each time Uranus comes into view.

Image of Uranus’ aurorae was taken by the NASA/ESA Hubble Space Telescope on 10 Oct, 2022. The observations were made by the Space Telescope Imaging Spectrograph (STIS) and includes both visible and ultraviolet data. An international team of astronomers used Hubble to make new measurements of Uranus' interior rotation rate by analysing more than a decade of the telescope’s observations of Uranus’ aurorae. This refinement of the planet’s rotation period achieved a level of accuracy 1000 times greater than previous estimates and serves as a crucial new reference point for future planetary research. For Earth, seasons last a few months. For Uranus, one season can stretch over two decades. That’s what makes Hubble’s long-term observations so valuable: short missions like Voyager’s quick flyby provide just a snapshot. Now, by capturing repeated views over many years, astronomers can piece together how the planet’s weather and seasons evolve. The data from 2002 through 2022 capture what amounts to “late spring” in Uranus’s northern hemisphere, heading into northern summer solstice in 2030. Over this span, the planet’s north pole has gone from modestly bright to intensely reflective, likely due to changes in the thickness or composition of haze. On the opposite side, the south polar region has moved deeper into shadow.

This breakthrough was 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 produce magnetic field models which successfully match the changing position of the magnetic poles with time. “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.” For now, researchers are sifting through this wealth of Hubble data to refine their theories about Uranus’s climate. The findings confirm that even at great distances from the Sun, solar radiation can dramatically impact weather and haze formation. They also highlight how crucial long-duration missions like Hubble are for understanding planetary atmospheres. A single pass from a space probe could never have captured the unfolding story of Uranian seasons quite like this.

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. These findings set the stage for further studies which 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. In short, the once “boring” Uranus now appears to be a place of shifting hazes, peculiar circulation, and slow but strong seasonal makeovers. Observing this unlikely show at the solar system’s edge, astronomers are constantly reminded that no planet can be written off as uninteresting, especially not one that literally travels on its side and is full of hidden surprises.

These results are based on observations acquired with Hubble programs GO #12601, 13012, 14036, 16313 and DDT #15380. The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries which make our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Centre in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.