New JWST images reveal the rosy glow of Uranus in unprecedented detail

See the rosy glow of Uranus in its full 3D glory

By Lee Billings edited by Claire Cameron

Among the solar system’s planets, Uranus is criminally overlooked. Like its outer solar system neighbor Neptune, this “ice giant” world is so far from the sun (and so visually bland) that we’ve only ever sent a single spacecraft, NASA’s Voyager 2, its way—and that was more than 40 years ago.

The lone flyby, conducted in late January 1986, barely probed the planet’s depths. And it happened just after a solar storm shattered Uranus’ magnetic field, limiting what scientists could learn about it from Voyager 2’s observations.

Despite its dreary appearance, Uranus may be crucial in solving several planetary mysteries. It is one of two major planets orbiting the Sun that rotates in a retrograde (clockwise) motion – and it is the only one with such an extreme axial tilt, where the axis of rotation is almost perpendicular to the orbital motion. In other words, Uranus moves around our star like a spinning top that is tipped over and spinning backwards. This celestial tilt likely came from Uranus being hit by a huge planetary collision early in the solar system’s history, giving the ice giant strange seasons spanning 42 Earth years. This may also have helped create Uranus’s skewed, chaotic magnetic field, which is misaligned with the planet’s center and spin.

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Yet as strange as Uranus is in some ways, in other ways it seems more typical: Most planetary systems astronomers have found around other stars are filled with so many worlds similar to Uranus in size and mass that this category of planet is probably the most common in the Milky Way. So if we want to understand how planets form and evolve, whether here or throughout the galaxy, we probably need to better understand Uranus.

That’s why new observations of the ice giant from NASA’s James Webb Space Telescope (JWST) offer much more than pretty pictures. Carried out by an international team and led by Paola Tiranti, a Ph.D. student at Northumbria University in England, the observations were published in Geophysical Research Letters February 19. Previous JWST observations of the planet have revealed a new moon, mapped the world’s subtle rings and more.

The new JWST data captures nearly a full day from Uranus, mapping the temperature and density of charged particles moving through the ice giant’s ionosphere, a high region of the atmosphere where auroras form and interact with the solar wind, as well as Uranus’s bizarre magnetic field. The data—which specifically tracks the abundance of H₃⁺an ion made of three hydrogen nuclei – constitutes the best three-dimensional map of the planet’s upper atmosphere to date.

“With Webb’s sensitivity, we can track how energy moves up through the planet’s atmosphere and even see the influence of the warped magnetic field,” Tiranti said in a statement.

JWST has a good view of how Uranus’ auroras sweep over and through lower atmospheric layers, she added. “Webb has now shown us how deep these effects reach into the atmosphere. By revealing Uranus’ vertical structure in such detail, Webb is helping us understand the energy balance of the ice giants. This is a crucial step towards characterizing giant planets outside our solar system.”

A mystery that the observations confirmed, but unfortunately did not solve, concerns Uranus’s peculiar falling temperature. For decades, scientists have found that the ice giant’s upper atmosphere is cooling unexpectedly – and these latest measurements show that the trend is still going. JWST saw an average temperature of around 150 degrees Celsius in Uranus’ upper atmosphere – lower than values ​​seen in previous observations.

The planet’s auroras appear as rosy, glowing patches that extend above the visible edges of Uranus’ atmosphere in the JWST images, which also capture the ice giant’s delicate ring system and the bright clouds around the polar cap. But in these images, Uranus’ rings and clouds are mostly just eye candy, says Heidi Hammel, an interdisciplinary researcher at JWST at the Association of Universities for Research in Astronomy, who was not involved in the work. Northern lights are the real scientific stars.

“These aurora detections are hugely important because they are a direct manifestation of the planet’s internal magnetic field,” says Hammel. “We really have no other way to probe the magnetic field remotely without a spacecraft on site.”

American astronomers still hope to send another spacecraft to Uranus in the coming years, but tight federal budgets—and the difficult timing required for energy-efficient interplanetary travel—could put such a mission uncomfortably far in the future. For now, scientists may have to make do with JWST’s remote but stunning views.

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