The Electric Lives of Giant Planets
Uranus: The Sideways Planet Is Still Cooling Down
On February 19th, Paola Tiranti and colleagues published the first three-dimensional map of Uranus' upper atmosphere. Using JWST's Near InfraRed Spectrograph (NIRSpec), they watched the planet for nearly a full rotation — 15 hours of continuous staring — and traced H₃⁺ emissions from the cloud tops up to 5,000 kilometres into the ionosphere.
What they found was surprising, even by Uranus standards.
Temperatures in the ionosphere peak between 3,000 and 4,000 km altitude. The ions themselves are densest much lower, around 960 km. But the headline result is a simple and puzzling one: Uranus' upper atmosphere is about 426 Kelvin cooler than when Voyager 2 flew past in 1986. It's been cooling steadily since at least the early 1990s, and nobody is quite sure why. For a planet that already struggles to explain why its upper atmosphere is hundreds of degrees hotter than sunlight alone could account for (the so-called "energy crisis" of the ice giants), the fact that it's also losing that heat is deeply strange.
JWST also mapped the auroras. Uranus has a magnetic field tilted about 60 degrees from its rotation axis — which is itself tilted 98 degrees from vertical. The result is a magnetosphere that tumbles through space like a spinning top knocked sideways. The auroras don't sit neatly over the poles. Instead, they sweep across the surface in complex, shifting patterns. JWST resolved two bright auroral bands near the magnetic poles, separated by dark regions where emission and ion density drop sharply — gaps in the electric curtain that trace where the magnetic field lines transition between open and closed.
"Uranus' magnetosphere is one of the strangest in the solar system," Tiranti said. "Webb has now shown us how deeply those effects reach into the atmosphere."
Jupiter: When Moons Leave Fingerprints
Two weeks later, on March 3rd, Katie Knowles and colleagues published the first spectral analysis of something astronomers have known about but never truly examined: the auroral footprints of Jupiter's moons.
Here's the mechanism. Jupiter's magnetic field rotates with the planet, completing a full turn every 10 hours. But its moons orbit much more slowly — Io takes 42.5 hours. This mismatch means the magnetic field is constantly sweeping past the moons, and where it interacts with each moon, powerful electrical currents are generated. These currents funnel charged particles down the field lines and into Jupiter's atmosphere, creating bright glowing spots — auroral footprints — that trace the position of each moon onto the planet's upper atmosphere like a moving tattoo.
We've known these footprints existed since the 1990s. We've imaged them in ultraviolet and infrared. But we'd only ever measured their brightness. Knowles' team used JWST's NIRSpec during a 22-hour observation window in September 2023 to do something new: measure their physical properties — temperature, H₃⁺ density, and how both change over time.
The results were dramatic. Inside Io's auroral footprint, the team found a cold spot — a region where temperatures dropped to just 538 Kelvin (265°C), compared to 766 Kelvin (493°C) in Jupiter's main aurora. At the same time, the H₃⁺ density in this cold spot was three times higher than in the surrounding aurora, with density variations of up to 45 times within the same small region.
Those numbers are extraordinary. They mean the stream of electrons crashing into Jupiter's atmosphere from Io is not a steady beam — it's flickering, surging, and dying on timescales of minutes. The upper atmosphere is responding to Io's volcanic output in near-real-time. Io ejects about a tonne of material into space every second from its volcanoes, feeding a doughnut-shaped cloud of plasma called the Io plasma torus. As Io plows through this environment, it generates electrical currents so fierce they leave an atmospheric bruise that changes faster than anyone expected.
"We found extreme variability in both temperature and density within Io's auroral footprint that happened on the timescale of minutes," Knowles said. "This tells us that the flow of high-energy electrons crashing into Jupiter's atmosphere is changing incredibly rapidly."
The team only caught the cold spot in one of their five snapshots. Is it common? Does it switch on and off? To find out, Knowles was awarded 32 hours of observation time on NASA's Infrared Telescope Facility in Hawaii across six nights in January 2026. That data is now being analysed.
One Molecule, Two Worlds, One Telescope
What connects these two papers isn't just institutional affiliation or publication timing. It's the method: using H₃⁺ as an atmospheric probe. The same molecule, measured the same way, revealing the hidden electrical lives of worlds separated by two billion kilometres.
At Jupiter, the story is about violence — a volcanic moon hammering the planet's atmosphere with such ferocity that temperatures and densities swing wildly within minutes. At Uranus, it's about mystery — a planet slowly losing heat that it shouldn't have had in the first place, its auroras painting patterns that no simple model can explain.
Both studies lay groundwork for something bigger. Every giant exoplanet we discover has an upper atmosphere shaped by similar physics — magnetic fields, charged particles, auroral heating. When we observe the spectra of hot Jupiters transiting their stars, understanding the behaviour of H₃⁺ in our own system helps us interpret what we see in distant ones. These aren't just local stories. They're calibration points for a much larger science.
And there's a practical dimension too. A Uranus orbiter mission was ranked as the top priority flagship in the most recent Planetary Science Decadal Survey. Studies like Tiranti's demonstrate what JWST can achieve from Earth orbit — and how much more we'd learn with a spacecraft actually there, tasting the atmosphere directly.
For now, we have H₃⁺ and a telescope at the L2 point. It's turning out to be enough to rewrite what we know about worlds we thought we'd already explored.
Sources:
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Tiranti, P. I. et al. (2026). "JWST Discovers the Vertical Structure of Uranus' Ionosphere." Geophysical Research Letters. DOI: 10.1029/2025GL119304
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Knowles, K. L. et al. (2026). "Short-Term Variability of Jupiter's Satellite Footprints as Spotted by JWST." Geophysical Research Letters. DOI: 10.1029/2025GL118553