Analysis of 26 radio measurements from the Juno mission, when the spacecraft passed behind Jupiter, found that the planet is about 8 km narrow at the equator and about 24 km flat at the poles – a change that refines models of the internal structure and winds.
"Textbooks will need to be updated.,” says Prof. Yochai Caspi of the Department of Earth and Planetary Sciences at the Weizmann Institute of Science. For more than fifty years, it seemed that the dimensions of Jupiter – the largest planet in the solar system – were well known. Now, thanks to advanced measurement methods and new data, the numbers are being updated, and with them our understanding of the shape of the gas giant.
In a study published on 2-2-2026 in the journal Nature Astronomy An international research team, led by scientists from the Weizmann Institute, has presented the most precise measurement yet of the size and shape of Jupiter. The result sounds “small” in everyday terms, but it is very significant for planetary science: Jupiter is slightly narrower at the equator, and flatter at the poles, than previously estimated.
How to measure the dimensions of Jupiter, and why it's not "just" a photograph
At first glance, it sounds simple: if you know the distance to Jupiter and track its rotation, you can deduce its size and shape. But in practice, to achieve an accuracy of a few kilometers, much more sophisticated methods are needed.
The measurements on which textbooks were based for many years relied on Only six measurements, which were made almost five decades ago as part of NASA's Voyager and Pioneer missions. The measurements were received using radio signals sent from the spacecraft to Earth. These signals made it possible to calculate, among other things, how Jupiter's environment affects the propagation of radio waves, and from this to estimate the planet's dimensions.
The new breakthrough came thanks to the Juno mission. The spacecraft was launched in 2011 and entered orbit around Jupiter in 2016. In 2021, the mission was extended, and as part of the extension, the trajectory was updated so that from Earth's perspective, the spacecraft also passed Behind JusticeThis change seems technical, but it opened up a unique measurement possibility: When the spacecraft passes behind the planet, the radio signal it transmits is blocked, and during the passage it is also “bent” by Jupiter’s atmosphere.
This is where the physical principle behind the measurement comes in: the atmosphere is not “empty space.” It is layers of gases at different densities and temperatures. Changes in density and temperature change the way radio waves travel. If we measure with great precision how the signal bends and attenuates as it passes “behind” the planet, we can deduce from it the location of Jupiter’s contours with high precision, and the contribution of the atmosphere to the overall picture.
Juno's principal investigator, Dr. Scott Bolton of the Southwest Research Institute in San Antonio, Texas, emphasizes that the new orbit is also an opportunity for new scientific goals. He says that when the spacecraft passes behind the planet, the atmosphere blocks and distorts the radio signal - thus allowing for a more precise measurement of Jupiter's dimensions.
What they found: A narrower, flatter justice
The Weizmann Institute research team took advantage of the new opportunity to analyze a much larger data set than in the past. Instead of six historical measurements, this time they analyzed 26 New measurements collected as part of the Juno mission. According to Dr. Eli Galanti, the old measurements laid important foundations for understanding Jupiter, but now it was possible to examine the planet using a richer and more up-to-date database, and with a measurement method that produces exceptionally high sharpness.
To turn the radio signals into detailed physical information, advanced data processing was needed. Dr. Maria Smirnova, a doctoral student in Prof. Caspi’s group, developed a unique method for processing the new data from Juno. According to the study description, the team tracked how the radio signals “bent” as they passed through Jupiter’s atmosphere. They converted this information into detailed maps of temperature and density, and from that, they constructed an exceptionally sharp picture of size and shape.
And what did the data show?
- In the area of the equator Jupiter Narrower by about 8 kilometers Compared to previous estimates.
- In the polar regions it is Flatter by about 24 kilometers Compared to what was previously thought.
The geometric meaning is clear: Jupiter is more flattened, meaning the difference between the equator and the poles is slightly greater than the old “official number.” Prof. Caspi emphasizes that the planet itself has not changed. What has changed is the precision and method of measurement.
Here it is important to understand why such “a few kilometers” bother scientists for good. The models of Jupiter’s internal structure, of the density distribution within it and of the relationship between the atmosphere and the interior depth, rely on reference values. When the reference values change, even slightly, the conditions under which the models must fit both gravity and atmospheric measurements become more precise or change.
Dr. Galanti explains that these few kilometers are “very significant,” because a small change in the planet’s radius allows models to better fit both gravity data and atmospheric measurements. Maayan Ziv, a doctoral student in Prof. Caspi’s group, examined this implication using advanced models of Jupiter’s internal density structure. According to the study, the updated models show that the new shape does bridge gaps that existed between predictions and observations.
Not Just Size: Winds, Storms, and What It Means for Gaseous Planets
Prof. Caspi points out another aspect that strengthened the conclusions: previous measurements did not fully take into account the The powerful winds of JupiterIncorporating winds into the calculations helped the team resolve long-standing discrepancies in past measurements. He explains that it's very difficult to know what's going on below the clouds, but radio data opens a window into the depth of high-latitude winds and the power of hurricanes.
A better understanding of the winds is not a “side bonus.” It is related to a deeper question: To what extent is Jupiter’s atmosphere connected to the planet’s interior? Is what we see on the surface a relatively shallow layer, or a manifestation of deeper processes? According to the article, this understanding also connects to another study by Prof. Caspi, led by Dr. Nimrod Gabriel, recently published in PNAS. That study used Juno’s measurements of the movement of giant cyclones at the poles to predict how deep they penetrate into Jupiter. According to the description, the prediction was later verified using new radiometric measurements from Juno.
There’s also a broader message for planetary science: Jupiter is a key “reference point” for understanding gas giants, both in the solar system and beyond. Measuring Jupiter more precisely makes it easier to assess properties of other gas planets, and to understand which models work well and which need to be revised.
According to the latest measurements, Jupiter's equatorial radius is about 7% larger than its polar radius. On Earth, for comparison, the difference between the radii is only 0.33%. This means that Jupiter is about 20 times more flattened than our planet. The reasons for this are Jupiter's rapid rotation rate, its complex internal structure, and its atmospheric winds.
Ultimately, says Prof. Caspi, such studies help us understand how planets form and evolve. Jupiter was likely the first planet to form in the solar system, so understanding it brings us closer to understanding the formation of the entire solar system, including Earth.
Looking ahead, the methods developed in the study are also expected to be used by Caspi's group in analyzing data that will begin flowing from the JUICE mission, an unmanned spacecraft of the European Space Agency that will be launched in 2023. According to the article, the spacecraft carries A device developed at the Weizmann Institute, and is expected to allow an even deeper look into the gas giant's atmosphere.
It is also noted that the Juno mission is managed by JPL – a division of Caltech – for NASA's Science Mission Directorate (SMD) in Washington.
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