Lead: Scientists using fresh radio-occultation data from NASA’s Juno spacecraft report that Jupiter is very slightly smaller than long-standing estimates indicated. Published in Nature Astronomy, the revised dimensions subtract roughly 15 miles from Jupiter’s polar extent and about 5 miles from its equatorial width. The team, including Yohai Kaspi of the Weizmann Institute and Scott Bolton of the Southwest Research Institute, derived the new figures from 24 Juno occultation passes; the planet measures about 83,000 miles pole-to-pole and about 89,000 miles at the equator. Researchers say the differences are minor for Jupiter’s rank among planets but significant enough to merit updates to reference materials.
Key Takeaways
- New measurements are based on 24 radio-occultation events recorded by Juno while orbiting Jupiter since 2016.
- Revised polar diameter is approximately 83,000 miles—about 15 miles less than decades-old estimates.
- Revised equatorial diameter is about 89,000 miles—around 5 miles smaller than earlier values.
- Earlier size estimates came from Voyager and Pioneer missions in the 1970s using radio occultation but did not fully account for strong atmospheric winds.
- The study reporting these values appears in the peer-reviewed journal Nature Astronomy.
- Lead and co-authors note that while the numerical changes are small, they affect precision in planetary models and educational references.
Background
Measurements of Jupiter’s size have long relied on radio-occultation methods: spacecraft transmit radio signals through the planet’s upper atmosphere while passing behind it from Earth’s perspective, and scientists infer atmospheric density and planetary radius from how those signals are delayed and refracted. In the 1970s, Voyager and Pioneer used that technique to produce the standard figures recorded in textbooks for decades. However, those early analyses were limited by the available data and by incomplete modeling of the planet’s powerful zonal winds and their effect on atmospheric density profiles.
Juno, launched in 2011 and in orbit around Jupiter since 2016, carries instrumentation and an orbital geometry that permit repeated, precisely timed occultations. By collecting a larger number of high-quality radio-occultation events and applying updated models for atmospheric dynamics, the Juno team could revisit the planet’s apparent shape with finer resolution. The result is a modest downward revision of both polar and equatorial measures—changes small in absolute terms but outside the formal uncertainties of prior values.
Main Event
During certain orbits, Juno slips behind Jupiter relative to Earth; as that occurs the spacecraft’s radio signal passes through varying depths of Jupiter’s atmosphere and is temporarily blocked or distorted. The team analyzed 24 such occultations to map where the atmosphere becomes dense enough to refract radio waves and thereby to define an effective planetary boundary. That boundary is where the atmosphere transitions from tenuous to optically significant at radio wavelengths, and it yields practical measures of the planet’s polar and equatorial extents.
The analysis trimmed roughly 15 miles from the previously accepted polar height and about 5 miles from the equatorial width. Those adjustments produce the rounded values the team reports—about 83,000 miles from pole to pole and about 89,000 miles across at the equator. Co-author Yohai Kaspi characterizes the result as requiring corrections to standard references, while Scott Bolton emphasizes the value of Juno’s repeated, high-precision occultation dataset for refining planetary metrics.
Researchers stress that these changes do not imply any physical shrinking of Jupiter; rather, the new figures reflect improved measurement of the planet’s apparent radius at radio wavelengths under currently modeled atmospheric conditions. The work also illustrates how persistent winds and subtle atmospheric structure can bias size inferences if not fully incorporated into retrieval algorithms.
Analysis & Implications
The downward revisions are minute relative to Jupiter’s overall dimensions: a 15-mile change on an ~83,000-mile scale is a fractional difference of order 2×10^-4. For broad public understanding and Jupiter’s status as the largest planet, the numerical change is immaterial. However, for precision planetary science the update matters because radius is an input for models of gravity, internal structure, and atmospheric dynamics. Slight shifts in the reference radius alter inferred density profiles and can change the normalization used in gravity-field inversions.
Accounting for wind-induced departures from a simple hydrostatic atmosphere is particularly important for gas giants. Jupiter’s fast zonal winds redistribute mass horizontally and vertically; if not properly modeled, those flows can bias the location where radio signals are refracted and thus the deduced radius. Juno’s data permit a consistent treatment that couples occultation geometry with independently derived wind and gravity measurements, improving internal-consistency among planetary parameters.
Longer term, the result demonstrates the importance of revisiting ‘settled’ numbers as missions deliver higher-quality data. For comparative planetology and exoplanet studies, having rigorously determined radii for Solar System benchmarks reduces systematic error when scaling or interpreting measurements of distant gas giants. Educational materials and reference tables will likely be updated to reflect the refined numbers, even as scientists continue to refine the models and uncertainties.
Comparison & Data
| Parameter | Prior (Voyager/Pioneer era) | New (Juno, 24 occultations) |
|---|---|---|
| Pole-to-pole diameter | ~83,015 miles (approx.) | ~83,000 miles |
| Equatorial diameter | ~89,005 miles (approx.) | ~89,000 miles |
The table shows rounded values: the paper reports reductions of roughly 15 miles at the poles and about 5 miles at the equator compared with estimates that have been widely used since the 1970s. Although the absolute differences are small, they exceed the formal uncertainties of some earlier retrievals and therefore represent a measurable improvement in planetary geodesy.
Reactions & Quotes
Study co-authors and mission scientists framed the update as a precision correction enabled by Juno’s dataset and improved atmospheric modeling. Laboratory and mission teams say the change underscores the interplay between atmospheric dynamics and remote-sensing retrievals.
“Textbooks will need to be updated.”
Yohai Kaspi, Weizmann Institute of Science
Kaspi’s remark, offered in the study announcement, highlights the educational and reference-book implications of the updated numbers despite their small magnitude. The comment was presented as a concise summary of why even minor numerical revisions matter for standard references.
“This enables an accurate measurement of Jupiter’s size.”
Scott Bolton, Southwest Research Institute
Bolton’s statement refers to Juno’s repeated occultation geometry, which provides a larger, more consistent dataset than earlier single-pass encounters. He and co-authors emphasize that combining multiple occultations reduces random error and reveals systematic effects linked to winds and atmospheric structure.
Unconfirmed
- No independent reanalysis of the Voyager/Pioneer occultation datasets has yet reconciled all methodological differences between older and newer retrievals.
- Potential downstream impacts on some internal-structure inferences remain to be evaluated quantitatively by follow-up studies.
Bottom Line
Juno’s radio-occultation measurements have produced a slight but robust downward revision of Jupiter’s measured polar and equatorial extents—about 15 miles smaller at the poles and roughly 5 miles smaller across the equator—leading scientists to call for updates to reference materials. The changes do not alter Jupiter’s status as the largest planet but do improve the precision of a key planetary parameter used in many models.
Beyond textbooks, the main value of the update is scientific: refined radius values reduce systematic uncertainty in gravity and interior models and set a higher standard for future measurements of Solar System and exoplanetary giants. Researchers will continue to integrate Juno’s occultations with gravity and atmospheric datasets to test implications for Jupiter’s deep winds and internal structure.