For decades, we only had four spacecraft—Voyager 1 and 2, and Pioneer 10 and 11—to measure Jupiter’s size. In the 1970s, those probes flew by and used radio beams to guess how big the planet was. four spacecraft—Voyager 1
The values they found became the ones that were printed in textbooks and reference books. No one thought those numbers would be very wrong, and they aren’t. But they were just right enough to hide some important information about the real shape of Jupiter. real shape of
Since 2016, Juno has been orbiting Jupiter and has now given us a much clearer picture. Researchers used Juno’s radio signals to figure out the gas giant’s size to within about 400 meters in each direction in a new study that came out in the journal Nature Astronomy. This is very precise for a planet that is more than 140,000 kilometres wide. much clearer picture
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How much smaller is Jupiter than it used to be?
The new work doesn’t show a smaller Jupiter in a dramatic way. The changes are measured in kilometres, not thousands of kilometres. But for scientists who study planets, those small changes are very important. very important
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MeasurementValue before: Value after: Difference
The polar radius from the center to the pole is about 41,546 miles (66,854 km) and 41,534 miles (66,842 km). It is 7.5 miles (12 km) shorter. polar radius from
The distance from the center to the equator is about 44,424 miles (71,492 km), or 44,421 miles (71,488 km) less than 2.5 miles (−4 km). distance from the
So, Jupiter is only a few kilometres smaller than we thought, and its shape is a little more flat because it spins so quickly. few kilometres smaller
When modellers try to match gravity measurements, magnetic data, and atmospheric winds all at once, a few kilometres can make or break a theory about what is inside Jupiter. gravity measurements, magnetic
How bending radio waves can show what a planet looks like
Juno is always sending radio waves back to Earth. The signals don’t go in straight lines as they pass through Jupiter’s atmosphere. The gas layers bend the radio waves in the same way that a lens bends light. sending radio waves
In the new study, scientists carefully watched how the radio beam bent and faded as Juno moved behind Jupiter from Earth’s point of view and then came back out. Researchers were able to find the “edge” of Jupiter in different directions because the planet completely blocked the signal. radio beam bent
Those edges aren’t perfectly smooth. Deep winds and fast jet streams change the shape of the planet’s outer layers, making some areas bulge and pulling others in. fast jet streams
The team was able to separate the effects of winds and find the planet’s underlying, more regular shape by carefully modelling how the atmosphere bends radio signals. underlying, more regular
The result is a more accurate map of Jupiter’s radius at the poles and the equator, as well as a better understanding of how oblate, or flattened, the gas giant really is. more accurate map
Why Jupiter looks like a spinning water balloon
Even though Jupiter is very big, it only takes about 10 hours for it to complete one rotation. That fast spin makes a strong outward force at the equator that fights against gravity. strong outward force
Like a water balloon spun on a string, the effect makes the middle bulge and the top and bottom flatten. The poles are about 3,000 miles (about 4,800 km) closer to the center than the equator of Jupiter. middle bulge and
Scientists can figure out how the mass is spread out inside the bulge by knowing exactly how big it is. The planet’s final shape depends on how it responds to rotation and gravity, which is affected by a denser core or a different arrangement of hydrogen and helium. planet’s final shape
Rebuilding models of the hidden inside of Jupiter
The clouds on Jupiter are just the surface of a much deeper and stranger world. The planet slowly changes from gas to a thick, hot liquid, and then to strange “metallic hydrogen” that can carry electricity. This happens under the clouds. Scientists think that there is a dense core of rock and heavier elements somewhere below that. dense core of
Researchers need to match several types of data at the same time to test different interior models: different interior models
- The way Jupiter’s gravity pulls on spacecraft is how we measure it.
- The shape and flattening of the planet, which Juno’s radio work has made more precise.
- Jet streams and winds in the atmosphere move mass around the planet.
- Juno and other missions have measured temperature and composition.
Scientists had to make some tough choices because of the earlier size values. Some models could match the gravity data but not the official radii, or the other way around. The new numbers make those limits more in line with each other, which means that simulations can make a Jupiter that is more consistent. more consistent
By moving the official radius in by a few kilometres, the interior models now match both the gravity readings and what Juno sees in the atmosphere better. official radius in
That, in turn, makes it easier to guess how big Jupiter’s core might be, how deep its famous storms and bands go, and how the planet has moved heavier elements around since it formed. famous storms and
Why people who look for exoplanets care about how big Jupiter really is
For Solar System experts, refining Jupiter might seem like doing chores, but the effects go far beyond our own neighbourhood. far beyond our
Astronomers have found thousands of planets that go around other stars. A lot of them are “hot Jupiters,” “warm Jupiters,” or other gas giants that are roughly the same size and mass as our own. Astronomers often use Jupiter as a standard when they try to figure out the bulk density of these worlds. bulk density of
We change our expectations for worlds that are like the reference planet if its size or structure changes. Scientists can better tell if an exoplanet’s density means it has a puffy, low-mass envelope, a dense heavy-element core, or something in between with more accurate models of Jupiter. dense heavy-element core
People also think that Jupiter was the first planet to form in our Solar System. It took in most of the gas before the young Sun blew the rest of it away. Learning about its insides helps us understand the early history of the whole planetary family, including how Earth got to where it is now. early history of
Why this means that textbooks will really change
Planetary reference tables in textbooks for school and college usually show standard values from official sources and famous missions. Once those numbers are changed, everything from posters in the classroom to encyclopedias will change too. standard values from
Future editions will probably change the listed equatorial and polar radii of Jupiter, and some may even add a note saying that these values come from measurements taken during the Juno mission instead of the 1970s flybys. Juno mission instead
The planet itself hasn’t gotten smaller; our measuring tape just got sharper because we used better tools and spent ten years in orbit. measuring tape just
Some helpful words that are behind the news
For people who don’t know much about planetary jargon, here are a few important ideas that can help make sense of this change: important ideas that
Radius: the distance from the center of a planet to its surface. Because gas giants don’t have solid surfaces, scientists use a certain pressure level in the atmosphere as the “surface.” distance from the
Oblateness is a way to measure how flat a spinning planet is. There is no oblateness in a perfect sphere, but a gas giant that spins quickly has a clear bulge at its equator. clear bulge at
Metallic hydrogen is a high-pressure form of hydrogen that lets electrons move freely, which makes the fluid able to conduct electricity. It probably has a big effect on Jupiter’s magnetic field. high-pressure form of
Gravity field: The way the mass of a planet is spread out. Spacecraft that are in orbit can pick up on tiny changes, like when something speeds up or slows down. mass of a
Planetary scientists often use computers to run simulations in which they change these properties and see which combinations create the same shape and gravity that we see. Even a small change in Jupiter’s official radius puts those simulations in a new area and can rule out older ideas. small change in
People are now thinking about using similar methods on other worlds. Radio bending and precise tracking may also be used on future missions to Saturn, Uranus, or Neptune to change the sizes and shapes of those planets. That could change how we group them. For example, there is still a debate about whether Uranus and Neptune are better called “ice giants” or “rock giants” with thick gaseous blankets. similar methods on
Jupiter’s small shrinkage is a reminder that even planets we know well can still surprise us, and that our most reliable numbers can change when a new spacecraft looks at them more closely. most reliable numbers
