New radio-tracking data from NASA’s Juno spacecraft show that Jupiter is marginally smaller and more oblate than long-standing figures indicate. Using signal bending as Juno’s radio transmissions passed through and were occulted by Jupiter’s atmosphere, researchers recalculated the planet’s equatorial and polar radii with tighter precision. The study, published Feb. 2, 2026 in Nature Astronomy, finds the polar radius is about 41,534 miles (66,842 km) and the equatorial radius about 44,421 miles (71,488 km), shifts of roughly 7.5 miles (12 km) and 2.5 miles (4 km) respectively from previous standard values. Those adjustments are small in absolute terms but meaningful for interior models and comparative studies of other gas giants.
Key Takeaways
- Juno radio occultation data refined Jupiter’s shape to an estimated precision of roughly 1,300 feet (400 meters) in each direction.
- The newly derived polar radius is 41,534 miles (66,842 km), about 7.5 miles (12 km) smaller than prior values from Voyager/Pioneer-era measurements.
- The updated equatorial radius is 44,421 miles (71,488 km), about 2.5 miles (4 km) smaller than the historical standard.
- Earlier standard radii were based on six radio occultation measurements by Voyager 1 and 2 and Pioneer 10 and 11 made roughly 50 years ago.
- Researchers accounted for atmospheric winds and refraction effects in the radio signals to isolate geometric shape from transient atmospheric dynamics.
- These revised dimensions help reconcile gravity and atmospheric observations with models of Jupiter’s internal structure.
- Improved measurements of Jupiter set a more reliable baseline for interpreting sizes and shapes of gas giants beyond the solar system.
Background
For decades the canonical values for Jupiter’s radii have come from a small set of radio occultation experiments performed by the Voyager and Pioneer spacecraft in the early 1970s and 1980s. Those historic one-off passes provided the community with working numbers that were adopted as standards because no comprehensive, higher-resolution alternative existed. The Juno mission, which arrived at Jupiter in 2016, has been collecting large volumes of radio tracking and occultation data over many orbits, enabling repeated, high-precision probes of how Jupiter’s atmosphere bends and blocks signals.
Planetary radii for gas giants are not fixed solid-surface measures but are defined relative to atmospheric pressure levels and the planet’s rotation and wind fields; consequently, winds and differential rotation change the planet’s apparent oblateness. Accurate radii are crucial inputs for interior models that combine gravity, rotation and atmospheric profiles to infer core size, composition gradients and deep-zone dynamics. Because Jupiter likely formed earliest in our solar system, refining its physical parameters affects theories of solar-system formation and the initial conditions used to model exoplanetary gas giants.
Main Event
The research team tracked changes in the Juno-to-Earth radio link as the signal grazed Jupiter and then was occulted by the planet, measuring how the path bent and the point at which the carrier was cut off. By aggregating many such occultation events across different latitudes and longitudes, the authors separated effects of atmospheric refraction and zonal winds from the underlying spheroidal geometry. They also compared Juno-derived occultation radii with the legacy Voyager/Pioneer values to quantify systematic offsets rather than random scatter.
Accounting for wind-driven shape changes was a key step: strong zonal jets alter the atmosphere’s mass distribution and thus the planet’s figure. The team modeled those wind contributions and removed them from the occultation-derived profiles to estimate the geopotential surface that corresponds to the commonly used reference pressure level. That produced the polar and equatorial radii reported in the paper and yielded an internal consistency between the measured gravity harmonics and atmospheric observations.
Though the numerical adjustments—on the order of a few kilometers—are modest, the authors emphasize that these shifts improve the fit of interior models to both gravity and atmospheric datasets. Co-author Eli Galanti (Weizmann Institute) and colleagues argued that the updated radii reduce residuals in model fits and thus inform parameters such as density gradients and core mass estimates. The changes also refine the baseline used when comparing Jupiter to large exoplanets whose observed radii are often interpreted through models anchored to our solar-system giants.
Analysis & Implications
Small changes in radius propagate through interior models because inferred mass distribution and moment of inertia depend on the assumed figure. A 12 km change at the pole and a 4 km change at the equator shift how gravity harmonics are mapped to depth-dependent density variations, which in turn affects estimates of whether Jupiter has a compact core or a diffuse heavy-element gradient. With improved radii, some interior models that previously required ad hoc adjustments now fit the combined gravity and atmospheric constraints more naturally.
Beyond Jupiter, the result matters for exoplanet science: researchers calibrate planetary-structure codes using solar-system exemplars. If Jupiter’s basic dimensional inputs change, predicted radii and internal properties for similar-mass exoplanets may be nudged as well, altering inferred core masses or heavy-element fractions from transit and mass measurements. The magnitude of that effect will depend on how model sensitivities map the few-kilometer shifts into quantities accessible for distant planets.
Operationally, the study also demonstrates the value of long-lived, repeated occultation campaigns. The Voyager/Pioneer era provided a handful of high-quality points; Juno’s sustained presence allowed many more occultations under diverse viewing geometries, reducing statistical and systematic uncertainties. The approach used here—combining radio occultations, wind corrections and gravity constraints—sets a methodological standard for future missions studying gas giants in situ or by remote radio techniques.
Comparison & Data
| Measurement | Polar radius | Equatorial radius | Net change |
|---|---|---|---|
| Legacy (Voyager/Pioneer) | 41,541 mi (66,854 km) | 44,423.5 mi (71,492 km) | — |
| Juno (this study) | 41,534 mi (66,842 km) | 44,421 mi (71,488 km) | −7.5 mi (−12 km) polar; −2.5 mi (−4 km) equatorial |
The table above summarizes the shift from legacy values to the Juno-derived radii reported in Nature Astronomy. Though absolute differences are small, the relative adjustments are significant enough to change model fits when combined with high-precision gravity harmonics and atmospheric profiles.
Reactions & Quotes
“A shift of only a few kilometers can materially change how interior models reconcile gravity and atmospheric constraints,”
Eli Galanti, Weizmann Institute of Science
Galanti, listed as a co-author, explained that the refined radii help close gaps between competing interior interpretations by reducing systematic mismatch in model inputs.
“By tracking distance and rotation with more sophisticated methods, we can measure Jupiter’s shape with unprecedented accuracy,”
Yossi Kaspi, study co-author
Kaspi highlighted the methodological advance: dense radio-occulation sampling and explicit wind corrections enable the improved geometric estimates reported in the paper.
Unconfirmed
- Whether textbook figures must be universally revised immediately: the degree and timing of curriculum updates vary by field and publisher and are not determined by the paper alone.
- Any single interior-model solution being declared definitive: the revised radii narrow possibilities but do not uniquely identify Jupiter’s internal layering or exact core mass.
- Long-term temporal variability: while the study accounts for winds, whether Jupiter’s figure changes measurably over decades due to deep atmospheric shifts remains to be monitored.
Bottom Line
The Juno spacecraft’s expanded radio-occultation dataset yields slightly smaller polar and equatorial radii for Jupiter than the multi-decade-old standards, with reductions of about 12 km at the pole and 4 km at the equator. Those adjustments are minor on planetary scales but important for models that combine gravity, rotation and atmospheric data to infer internal structure and composition.
For planetary scientists and modelers, the result tightens constraints and reduces systematic mismatches; for educators and communicators, it signals that widely cited reference numbers can shift as measurement techniques improve. Continued Juno observations and complementary analyses will further refine Jupiter’s figure and deepen its role as the calibration anchor for gas-giant studies both in and beyond our solar system.