Fresh measurements from NASA’s Juno spacecraft now show those figures were slightly off, forcing scientists to rethink the gas giant’s shape and what lies beneath its swirling clouds.
Jupiter turns out to be a touch smaller, a bit more squashed at the poles and stretched at the equator, than official values long suggested.
Juno rewrites a 50‑year‑old picture of Jupiter
For decades, our measurements of Jupiter’s size came from just four spacecraft: Voyager 1 and 2, and Pioneer 10 and 11. Those probes flew past in the 1970s and used radio beams to estimate the planet’s dimensions.
Their results became the standard values printed in textbooks and reference tables. Nobody expected those numbers to be wildly wrong, and they aren’t. But they were just precise enough to hide some crucial details of Jupiter’s true shape.
Juno, orbiting Jupiter since 2016, has now provided a far sharper view. In a new study published in the journal Nature Astronomy, researchers used Juno’s radio signals to pin down the gas giant’s size to within about 400 metres in each direction — razor-thin accuracy on a planet over 140,000 kilometres across.
How much smaller is Jupiter, really?
The new work does not reveal a shrunken Jupiter in the dramatic sense. The changes are measured in kilometres, not thousands of kilometres. Yet for planetary scientists, those small shifts matter a great deal.
| Measurement | Old value | New Juno value | Difference |
|---|---|---|---|
| Polar radius (centre to pole) | ≈ 41,546 miles (66,854 km) | 41,534 miles (66,842 km) | −7.5 miles (−12 km) |
| Equatorial radius (centre to equator) | ≈ 44,424 miles (71,492 km) | 44,421 miles (71,488 km) | −2.5 miles (−4 km) |
So Jupiter is only a few kilometres smaller than we thought, and its shape is slightly more flattened by its rapid rotation.
For modelers trying to match gravity measurements, magnetic data and atmospheric winds all at once, a few kilometres can make or break a theory of Jupiter’s interior.
How bending radio waves reveal a planet’s shape
Juno constantly sends radio signals back to Earth. As those signals skim Jupiter’s atmosphere, they do not travel in straight lines. The gas layers bend the radio waves, much like a lens bends light.
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In the new study, scientists tracked exactly how the radio beam curved and faded as Juno slipped behind Jupiter from Earth’s point of view, and then re-emerged. As the planet blocked the signal entirely, researchers could pinpoint where the “edge” of Jupiter lay along different directions.
Those edges are not perfectly smooth. Fast jet streams and deep winds distort the planet’s outer layers, bulging some regions and tugging others inwards.
By carefully modelling how the atmosphere bends radio signals, the team could peel away the influence of winds and isolate the underlying, more regular shape of the planet itself.
The result is a refined map of Jupiter’s radius at the poles and equator, and a stronger grasp on just how oblate — or flattened — the gas giant really is.
Why Jupiter is squashed like a spinning water balloon
Jupiter completes one rotation in about 10 hours, despite its huge size. That rapid spin generates a powerful outward force at the equator, which competes with gravity.
The effect is similar to a water balloon spun on a string: it bulges around the middle and flattens at the top and bottom. Jupiter’s equator sticks out around 3,000 miles (about 4,800 km) farther from the centre than the poles.
Knowing the exact amount of this bulge helps scientists work backwards to the internal distribution of mass. A denser core or a different layering of hydrogen and helium changes how the planet responds to rotation and gravity, and therefore its final shape.
Rebuilding models of Jupiter’s hidden interior
Jupiter’s clouds are just the visible skin of a far deeper and stranger world. Beneath the cloud tops, the planet gradually transitions from gas to a thick, hot fluid, then to exotic “metallic hydrogen” that conducts electricity. Somewhere below that, scientists suspect a compact core of rock and heavier elements.
To test different interior models, researchers must match several types of data at once:
- Jupiter’s gravity field, measured by how it tugs on spacecraft.
- The planet’s shape and flattening, now refined by Juno’s radio work.
- Atmospheric winds and jet streams, which carry mass around the planet.
- Measurements of temperature and composition from Juno and past missions.
The earlier size values forced scientists into awkward compromises: some models could match the gravity data but not the official radii, or vice versa. The new figures bring those constraints closer into line, so simulations can produce a more consistent Jupiter.
By nudging the official radius inward by just a few kilometres, the interior models now better fit both gravity readings and what Juno sees in the atmosphere.
That, in turn, sharpens estimates of how large Jupiter’s core might be, how deep its famous bands and storms reach, and how the planet has redistributed heavier elements since it formed.
Why exoplanet hunters care about Jupiter’s true size
Refining Jupiter might sound like housekeeping for Solar System specialists, but the knock-on effects stretch far beyond our local neighbourhood.
Thousands of planets have been found orbiting other stars. Many of them are “hot Jupiters”, “warm Jupiters” or other gas giants loosely similar in mass and radius to our own. When astronomers interpret the bulk density of these worlds, they often lean on Jupiter as a benchmark.
If the reference planet’s size and structure shift, so do our expectations for worlds that resemble it. More accurate Jupiter models help scientists judge whether an exoplanet’s density points to a puffy, low-mass envelope, a dense heavy-element core, or something in between.
Jupiter is also thought to be the first planet that formed in our Solar System, grabbing the lion’s share of gas before the young Sun blew the remaining material away. Understanding its interior sheds light on the entire planetary family’s early history, including how Earth ended up where it is.
Why this means textbooks really will change
Planetary reference tables in school and university texts generally list standard values drawn from official agencies and landmark missions. Once those numbers are updated, everything from classroom posters to encyclopaedias will follow.
Future editions are likely to tweak Jupiter’s listed equatorial and polar radii, and some may add a note that these values are based on Juno-era measurements instead of the 1970s flybys.
The planet itself hasn’t shrunk; our measuring tape just got sharper thanks to better instruments and a decade-long orbital mission.
Some useful terms behind the headlines
For readers less familiar with planetary jargon, a few key concepts help make sense of this shift:
- Radius: The distance from a planet’s centre to its surface. Gas giants do not have a solid surface, so scientists define a specific pressure level in the atmosphere as the “surface”.
- Oblateness: A measure of how flattened a spinning planet is. A perfect sphere has no oblateness; a rapidly rotating gas giant shows a clear equatorial bulge.
- Metallic hydrogen: A high-pressure form of hydrogen where electrons can move freely, allowing the fluid to conduct electricity. It likely plays a major role in Jupiter’s magnetic field.
- Gravity field: The way a planet’s mass is distributed. Tiny variations can be sensed by orbiting spacecraft as slight speed-ups or slow-downs.
Planetary scientists often run computer simulations where they adjust these properties and see which combinations reproduce the observed shape and gravity. Changing Jupiter’s official radius even slightly forces those simulations into new territory and can rule out older ideas.
Similar techniques are now being considered for other worlds. Future missions to Saturn, Uranus or Neptune may use radio bending and precision tracking to revise those planets’ sizes and shapes as well. That could affect how we classify them — such as the ongoing debate over whether Uranus and Neptune are better described as “ice giants” or “rock giants” with thick gaseous blankets.
For now, Jupiter’s modest shrinkage is a reminder that even familiar planets still hold surprises, and that our most trusted numbers can shift when a new spacecraft looks just a little more closely.
Originally posted 2026-02-20 16:45:32.