Have you ever wondered what happens thousands of kilometers above the clouds of a planet that literally rolls around the Sun on its side? What if that planet's magnetic field is so crooked, so off-kilter, that its auroras paint wild, sweeping patterns across the sky unlike anything we've ever seen?
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On January 19, 2025, the James Webb Space Telescope stared at Uranus for nearly 15 hours straight — almost one full rotation of the ice giant — and what it found has rewritten what we know about this distant world's upper atmosphere. An international team led by PhD student Paola Tiranti of Northumbria University has delivered the first-ever three-dimensional map of Uranus's ionosphere, revealing temperatures, ion densities, auroras, and a decades-long cooling mystery that no one has fully explained .
Grab a coffee. Settle in. This one's worth the read.
📑 Table of Contents
Webb's First 3D Map of Uranus: What Its Glowing Ionosphere Tells Us
Why Is Uranus the Strangest Planet in Our Solar System?
A Planet Tipped on Its Side
Let's paint the picture. Uranus isn't like the other planets. It doesn't spin upright like Earth or Jupiter. It tumbles along its orbit with an axial tilt of 97.8 degrees — basically lying on its side . Most scientists think a massive collision early in the solar system's history knocked it over, back when it was still forming, likely much closer to the Sun than it is today .
Discovered by William Herschel in 1781, Uranus sits about 2.9 billion kilometers (roughly 19 AU) from the Sun . It spins once every 17 hours and takes approximately 84 Earth years to complete a single orbit . During that slow journey, its poles take turns pointing almost directly at the Sun — and at us. That geometry opens a rare window for telescopes like Webb.
But the tilt isn't the only odd thing. Uranus's magnetic field is tilted and offset from the planet's rotation axis, making its magnetosphere one of the most bizarre in the solar system . While Earth's magnetic poles sit fairly close to its geographic poles, Uranus's magnetic axis is skewed at a steep angle. The charged particles streaming along those crooked field lines produce auroras that sweep across the atmosphere in patterns nothing like the neat rings we see at Earth's poles.
"Uranus's magnetosphere is one of the strangest in the Solar System," said Tiranti. "It's tilted and offset from the planet's rotation axis, which means its auroras sweep across the surface in complex ways" .
And until Webb looked, we had almost no idea what was happening in the thin, charged layers of atmosphere where those auroras live.
How Did JWST Observe Uranus's Ionosphere?
15 Hours Watching a Planet Spin
On January 19, 2025, JWST program #5073 (led by Principal Investigator Henrik Melin of Northumbria University) aimed Webb's Near-Infrared Spectrograph (NIRSpec) at Uranus for 15.4 hours — just shy of a full Uranian day . That continuous stare captured the planet through nearly a complete rotation, so the team could sample every longitude.
The instrument used its Integral Field Unit (IFU) mode with the F290LP/G395H filter and grating, covering wavelengths from 2.87 to 5.14 micrometers . Within that range, strong infrared emission lines from a molecule called H₃⁺ (trihydrogen cation) glow between 3.29 and 4.10 micrometers — faint but detectable with Webb's extraordinary sensitivity.
Twenty observations were collected using a 5-point cycling pattern to capture the planet's limb. Each exposure lasted about 6.1 minutes . The team then binned spectra in 350-kilometer altitude steps from 475 km up to 5,025 km above the 1-bar pressure level, producing vertical profiles at every 10° of longitude .
What Is H₃⁺ and Why Does It Matter?
Here's where the chemistry gets fun.
H₃⁺ is a three-atom hydrogen ion — one of the most abundant molecular ions in the universe. In giant planet atmospheres, it forms when solar ultraviolet light (or energetic particles) strips electrons from hydrogen molecules. The resulting H₂⁺ quickly reacts with surrounding H₂ to produce H₃⁺ .
What makes H₃⁺ so useful? It's a near-perfect thermometer. Because it reaches thermal equilibrium with its surroundings, measuring its infrared glow tells us the local temperature and density of the upper atmosphere . First detected at Uranus by Trafton and colleagues back in 1993, H₃⁺ has been our best remote-sensing tool for probing the ionospheres of giant planets ever since .
Before this study, though, no one had ever measured H₃⁺ vertical profiles at Uranus. We knew the molecule was there. We didn't know how it was stacked — how temperature and density changed with altitude, longitude by longitude.
Webb changed that.
What Did the Temperature Profiles Reveal?
A Planet That Keeps Getting Colder
The global temperature profile tells a clear story. Starting at 419 ± 7 K at 475 km altitude, temperatures climb roughly linearly to a peak of 470 ± 4 K at 3,625 km, then drop again at higher altitudes . The overall column-weighted temperature comes out to 426 ± 2 K — about 150 °C .
That number might sound hot. But for Uranus, it's cold — colder than any ground-based telescope or earlier spacecraft has recorded. The planet's upper atmosphere has been cooling steadily since the early 1990s, and this measurement confirms the trend is still going .
What's driving the cooldown? One hypothesis points to reduced solar wind power over the decades . But that explanation remains debated — recent work by Jasinski and colleagues (2025) argues the solar wind alone can't account for it . Whatever the cause, the cooling has real consequences: a colder thermosphere means a smaller scale height for molecular hydrogen, which reshapes the entire ionospheric structure.
Across all longitudes, temperatures stay between 380 K and 480 K — a surprisingly tight range . That limited variability suggests either that auroral heating at Uranus is weaker than the several-hundred-kelvin polar enhancements at Jupiter and Saturn, or that energy redistributes efficiently across the planet .
Data: Tiranti et al. (2026), Geophysical Research Letters
Where Do Uranus's Auroras Form?
Two Bright Bands and a Mysterious Dark Gap
When we think of auroras, we picture Earth's northern lights — shimmering curtains of green and violet dancing around the poles. Uranus has auroras too, but they're strange. Because the magnetic field is so lopsided, the auroral zones don't sit neatly over the geographic poles. They wander.
Webb's observations revealed two bright bands of H₃⁺ emission near the magnetic poles :
- Band 1: Between 50°–110° W longitude
- Band 2: Between 220°–290° W longitude
These match the expected positions of the northern and southern auroral zones as predicted from earlier UV observations .
Between those two bright bands, something unexpected showed up: a distinct drop in both emission intensity and ion density, centered between 190°–240° W . The team calls this the "Emission Dip" region. In this zone, H₃⁺ densities at the density peak altitude are about 25% lower than the average of all other regions .
What causes the dip? Tiranti and colleagues suggest it could be linked to the geometry of Uranus's magnetic field — specifically, transitions in the field line topology that control how charged particles travel through the upper atmosphere . Similar dark regions have been observed at Jupiter, where the magnetic field geometry creates what scientists call "magnetic silhouettes" in the ionosphere .
It's as if the magnetic field itself casts shadows on the glowing atmosphere.
How Do the Observations Compare to Predictions?
Here's where things get really interesting. For decades, scientists have relied on computer models to guess what Uranus's ionosphere should look like. Now we have real data — and the models don't quite match.
The team compared their results to the 1-D ionospheric models by Moore et al. (2019). The measured H₃⁺ densities turned out to be significantly lower than predicted . The peak density from JWST sits at (2.58 ± 0.13) × 10⁸ m⁻³ at 1,175 km for a non-auroral southern profile, while the model predicts values ranging from 2.24 × 10⁸ m⁻³ (dawn) to a whopping 3.35 × 10⁹ m⁻³ (noon) — more than an order of magnitude higher at midday .
Data: Tiranti et al. (2026) and Moore et al. (2019)
Why the mismatch? The paper identifies several reasons :
- Geometry matters. Webb's limb observations sample near the terminator (the day-night boundary), where ion production rates differ from the noon conditions most models assume.
- The thermosphere is cooler than expected. Models built on Voyager 2-era temperature profiles (which were warmer) overestimate the scale height of molecular hydrogen, inflating predicted H₃⁺ densities.
- Vibrational chemistry is uncertain. The conversion rate from H⁺ to H₃⁺ depends on vibrationally excited H₂. At Uranus's cooler temperatures, this population may be smaller than models assume based on Jupiter analogs.
- Magnetic field tilt complicates transport. Uranus's highly tilted field reduces vertical plasma transport compared to the near-vertical field lines the models assume, lowering H₃⁺ densities further.
These discrepancies aren't failures — they're opportunities. Every gap between prediction and measurement is a chance to refine our understanding.
What Is the Giant Planet Energy Crisis?
Here's a puzzle that has nagged planetary scientists for decades. The upper atmospheres of all four giant planets — Jupiter, Saturn, Uranus, and Neptune — are hundreds of degrees hotter than sunlight alone can explain . Solar ultraviolet radiation simply can't deliver enough energy to heat these thermospheres to the temperatures we observe. Something else is pumping energy in, and we don't fully know what.
This is called the giant planet energy crisis, and Uranus is right in the middle of it.
Webb's observations add a new piece to the puzzle. The temperature profiles show that energy isn't deposited uniformly. Temperatures peak between 3,000 and 4,000 km — near the exobase, where the atmosphere thins into space . The weak longitudinal temperature gradients and elevated high-altitude temperatures suggest that something beyond simple collisional heat transfer is at work. Gravity wave dissipation, thermal conduction, and magnetic-field-driven energy redistribution are all on the table .
Recent modeling at Jupiter suggests that an offset magnetic field can drive significant ion drag, carrying auroral energy toward the equator . Uranus's magnetic field is even more offset and tilted than Jupiter's. Similar transport processes could play a role here too — but nobody has modeled it yet in detail.
"By revealing Uranus's vertical structure in such detail, Webb is helping us understand the energy balance of the ice giants," Tiranti noted .
Will We Ever Send a Spacecraft Back to Uranus?
Only one spacecraft has ever visited Uranus: Voyager 2, which flew past on January 24, 1986 . That brief encounter — just a few hours of close observations — gave us nearly everything we knew about the planet's magnetic field, ring system, and major moons until telescopes like Hubble and now Webb arrived.
That's almost 40 years ago. Think about that. The phone you're reading this on is more powerful than the computers that guided Voyager.
The planetary science community has been vocal about returning. A mission concept called the Uranus Orbiter and Probe sits on NASA's conceptual drawing boards, with a possible launch window in the 2030s or beyond . The idea is straightforward: put a spacecraft into orbit around Uranus, study the planet and its moons and rings for years, and send a probe plunging into the atmosphere to measure its composition and structure from the inside out.
If this mission flies, it could answer the questions Webb is raising: What drives the ionospheric cooling? How does the tilted magnetic field shape particle precipitation and auroral dynamics? What is Uranus actually made of at its core?
Until that day comes, we lean on JWST and ground-based observatories. And honestly, the data they're delivering is nothing short of remarkable.
Final Thoughts
Let's step back and take in what just happened. A PhD student and her team pointed the most powerful space telescope in human history at a pale blue-green dot nearly 3 billion kilometers away, watched it spin for 15 hours, and extracted — for the first time ever — a three-dimensional portrait of its invisible, electrified upper atmosphere .
They measured temperatures that confirmed a 30-year cooling trend nobody fully explains yet. They found auroras shaped by a magnetic field so weird it has no real analog in the solar system. They spotted a mysterious dark gap in the ionosphere that might be the magnetic field's own shadow. And they showed that our best models still have a lot of catching up to do .
That's what science looks like in 2026. Not just answers — better questions.
For those of you who made it this far: you're our kind of people. The kind who don't switch off your mind at the end of a long day. The kind who see a strange number like 4.45 × 10⁸ m⁻³ and want to know the story behind it. We wrote this article for you — here at FreeAstroScience.com, where we take the hardest ideas the universe throws at us and break them down until they make sense.
Come back soon. There's always more to learn, and the universe isn't going to stop being surprising anytime soon.
📚 Sources & Further Reading
Every claim in this article traces back to peer-reviewed research or official agency releases. We encourage you to explore the original material — science belongs to everyone.
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Tiranti, P. I., Melin, H., Moore, L., Thomas, E. M., Knowles, K. L., Stallard, T. S., Roberts, K., & O'Donoghue, J. (2026). "JWST Discovers the Vertical Structure of Uranus' Ionosphere." Geophysical Research Letters, 53, e2025GL119304.
https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025GL119304
— The original peer-reviewed paper presenting the first 3D ionospheric profiles of Uranus from JWST/NIRSpec. Open access. -
ESA/Webb Science Release weic2602 (19 February 2026). "Webb Maps the Mysterious Upper Atmosphere of Uranus."
https://esawebb.org/news/weic2602/
— Official ESA/Webb press release with imagery credits (ESA/Webb, NASA, CSA, STScI, P. Tiranti, H. Melin, M. Zamani). -
Collins Petersen, C. (27 February 2026). "JWST Digs Into the Uranian Ionosphere." Universe Today.
https://www.universetoday.com/articles/jwst-digs-into-the-uranian-ionosphere
— Science journalism summary covering Voyager 2 context, JWST findings, and the Uranus Orbiter and Probe mission concept. -
[4]
Moore, L., Melin, H., O'Donoghue, J., Stallard, T., Moses, J., Galand, M., et al. (2019). "Modelling H₃⁺ in Planetary Atmospheres: Effects of Vertical Gradients on Observed Quantities." Philosophical Transactions of the Royal Society A, 377(2154), 20190067.
https://doi.org/10.1098/rsta.2019.0067
— The 1-D ionospheric model used as the primary comparison benchmark in the Tiranti et al. study. -
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Melin, H., Fletcher, L., Stallard, T., Miller, S., Trafton, L., Moore, L., et al. (2019). "The H₃⁺ Ionosphere of Uranus: Decades-Long Cooling and Local-Time Morphology." Philosophical Transactions of the Royal Society A, 377(2154), 20180408.
https://doi.org/10.1098/rsta.2018.0408
— Key paper documenting the long-term cooling trend in Uranus's upper atmosphere since the early 1990s. -
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Melin, H., Fletcher, L. N., Hammel, H. B., Milam, S. N., Moore, L., Stallard, T. S., et al. (2025). "The Ionosphere of Uranus as Revealed by JWST." Geophysical Research Letters, 52(22), e2025GL118301.
https://doi.org/10.1029/2025GL118301
— Earlier JWST disk-averaged observations of Uranus's ionosphere (415 K), providing context for the new vertical profiles. -
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Trafton, L., Geballe, T., Miller, S., Tennyson, J., & Ballester, G. (1993). "Detection of H₃⁺ from Uranus." Astrophysical Journal, 405, 761–766.
https://doi.org/10.1086/172404
— The landmark first detection of H₃⁺ at Uranus, which launched three decades of infrared ionospheric monitoring. -
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Jasinski, J. M., Melin, H., Sinclair, J. A., Zomerdijk-Russell, S., Achilleos, N., & Paty, C. (2025). "Uranus' Long-Term Thermospheric Cooling Is Unlikely to Be Primarily Driven by the Solar Wind." Geophysical Research Letters, 52(24), e2025GL119362.
https://doi.org/10.1029/2025GL119362
— A recent study challenging the hypothesis that reduced solar wind power alone explains Uranus's decades-long cooling. -
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Masters, A., Szalay, J., Zomerdijk-Russell, S., & Kao, M. (2024). "Solar Wind Power Likely Governs Uranus' Thermosphere Temperature." Geophysical Research Letters, 51(22), e2024GL111623.
https://doi.org/10.1029/2024GL111623
— Presents the competing argument that solar wind power does govern Uranus's upper atmospheric temperature. -
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Knowles, K., Stallard, T., O'Donoghue, J., Moore, L., Agiwal, O., Melin, H., et al. (2025). "Magnetic Silhouettes in Jupiter's Non-Auroral Ionosphere." Journal of Geophysical Research: Space Physics, 130(5), e2025JA033868.
https://doi.org/10.1029/2025JA033868
— Describes magnetic field topology-driven dark regions in Jupiter's ionosphere, analogous to the emission dip found at Uranus. -
[11]
Müller-Wodarg, I. C. F., Iñurrigarro, P., Moore, L., Koskinen, T. T., & Medvedev, A. S. (2025). "Temperatures of Jupiter's Upper Atmosphere: The Role of the Planetary Magnetic Field." The Astrophysical Journal Letters, 990(1), L22.
https://doi.org/10.3847/2041-8213/adfa21
— Models how Jupiter's offset magnetic field drives ion drag and equator-ward energy transport — a process that may also operate at Uranus. -
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Miller, S., Tennyson, J., Geballe, T. R., & Stallard, T. (2020). "Thirty Years of H₃⁺ Astronomy." Reviews of Modern Physics, 92(3), 035003.
https://doi.org/10.1103/RevModPhys.92.035003
— Comprehensive review of H₃⁺ as a diagnostic tool for giant planet ionospheres, explaining its role as a near-perfect thermometer. -
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Thomas, E. M., Melin, H., Stallard, T. S., Chowdhury, M. N., Wang, R., Knowles, K., & Miller, S. (2023). "Detection of the Infrared Aurora at Uranus with Keck-NIRSPEC." Nature Astronomy, 7(12), 1473–1480.
https://doi.org/10.1038/s41550-023-02096-5
— The first confirmed detection of Uranus's infrared aurora from a ground-based telescope, a precursor to the JWST campaign. -
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Simon, A., Nimmo, F., & Anderson, R. (2021). Uranus Orbiter and Probe: Journey to an Ice Giants System. National Aeronautics and Space Administration.
https://science.nasa.gov/wp-content/uploads/2023/10/uranus-orbiter-and-probe.pdf
— The NASA Planetary Science and Astrobiology Decadal Survey concept study for a flagship mission to Uranus. -
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Ness, N. F., Acuña, M. H., Behannon, K. W., Burlaga, L. F., Connerney, J. E., Lepping, R. P., & Neubauer, F. M. (1986). "Magnetic Fields at Uranus." Science, 233(4759), 85–89.
https://doi.org/10.1126/science.233.4759.85
— Voyager 2's discovery of Uranus's highly tilted and offset magnetic field — the foundation for understanding the planet's bizarre magnetosphere. -
[16]
Tiranti, P. I. (2025). "Dataset for JWST Discovers the Vertical Structure of Uranus' Ionosphere" [Dataset]. Zenodo.
https://doi.org/10.5281/zenodo.17036022
— Open data repository with all fitted H₃⁺ values (temperature, density, radiance) as a function of longitude, local time, latitude, and altitude.
Image Credit: ESA/Webb, NASA, CSA, STScI, P. Tiranti, H. Melin, M. Zamani (ESA/Webb). All sources verified as of February 28, 2026.
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