Jupiter is larger than Saturn by every major physical measure. Its equatorial diameter stretches about 143,000 kilometers compared to Saturn’s roughly 120,500 kilometers, making Jupiter about 19 percent wider. The mass gap is even more dramatic: Jupiter weighs in at about 318 Earth masses while Saturn tips the scales at roughly 95, so Jupiter is more than three times heavier. Yet the relationship between the two gas giants is more interesting than raw numbers suggest, because Saturn is surprisingly close to Jupiter in size given how much less massive it is.
Diameter, Volume, and the Bulging Middle
Jupiter’s equatorial radius of about 71,500 kilometers and Saturn’s of about 60,300 kilometers mean Jupiter is roughly 1.19 times wider at the equator. In volume, that width advantage compounds quickly: Jupiter could contain about 1,321 Earths, Saturn about 764. So Jupiter holds roughly 1.7 times Saturn’s volume. Neither planet is a perfect sphere, though. Both spin fast enough to bulge visibly at their equators and flatten at the poles, and Saturn is actually the more distorted of the two. Rotation causes Jupiter’s shape to deviate from a sphere by about 6.7 percent, while Saturn’s deviation reaches about 10.3 percent.1Monthly Notices of the Royal Astronomical Society. Structure and composition of Jupiter, Saturn, Uranus, and Neptune under different constraints and distortion due to rotation Saturn’s stronger oblateness comes from a combination of its rapid spin (a day on Saturn lasts only about 10.7 hours, just slightly longer than Jupiter’s roughly 9.9-hour day) and its lower density, which lets centrifugal force stretch the planet more easily.
That oblateness matters when you quote a planet’s “size.” Saturn’s equatorial diameter of roughly 120,500 km shrinks to about 108,700 km when measured pole to pole. Jupiter’s equatorial diameter of about 143,000 km drops to roughly 133,700 km pole to pole. If you happened to compare Saturn at its widest with Jupiter at its narrowest, the size gap would look much smaller than it really is. Astronomers handle this by specifying whether they mean equatorial or polar radius, and modern interior models account for rotational distortion when computing a planet’s structure.
The Mass Gap
Mass is where Jupiter truly dwarfs Saturn. Jupiter’s mass is 317.83 Earth masses; Saturn’s is 95.16 Earth masses.2arXiv. The Interiors of Jupiter and Saturn That makes Jupiter 3.34 times more massive. Both planets are composed overwhelmingly of hydrogen and helium, the same lightweight elements that make up the Sun, so almost all of Jupiter’s extra mass is just more of those gases piled on under tremendous gravitational pressure.
Because hydrogen and helium are compressible, piling on more mass does not increase the size of a gas giant in proportion. Jupiter has more than three times Saturn’s mass but only about 1.7 times its volume. This compressibility is why Jupiter is so much denser than Saturn, and it has a striking practical consequence for how big a gas giant can get: adding more hydrogen and helium to a Jupiter-sized planet would eventually make it barely any wider at all, because the extra weight crushes the interior more tightly. Jupiter is, in fact, close to the maximum diameter a hydrogen-helium planet can reach. A planet ten times Jupiter’s mass would scarcely be any larger in radius.
Density and Why Saturn Could Float
Jupiter’s mean density is about 1.33 grams per cubic centimeter. Saturn’s is about 0.69 g/cm³, famously less than water. The old thought experiment that Saturn would float in a sufficiently large bathtub, while Jupiter would sink, captures a real physical difference. Saturn is the least dense planet in the solar system, and it is not even close: no other planet has a density below 1 g/cm³.
This difference comes down to how much gravitational compression each planet experiences. Jupiter’s greater mass squeezes its interior to higher pressures and temperatures, packing the hydrogen and helium more tightly. Saturn, with less mass bearing down, stays puffier. Both planets are made of nearly the same stuff, but Jupiter’s deeper gravitational well does far more work compressing it. You can think of it like packing a suitcase: the same clothes take up much less room if you sit on the lid.
When astronomers plot the masses and radii of planets in the solar system against theoretical curves for different compositions, Jupiter and Saturn both land near the curve for pure hydrogen-helium mixtures.3The Astrophysical Journal. Mass–Radius Relationships for Exoplanets That placement confirms they are fundamentally the same type of object: gas giants dominated by the lightest elements. The rocky planets, by contrast, cluster along a completely different set of curves, and the ice giants Uranus and Neptune fall somewhere in between.
What Is Inside Each Planet
Both Jupiter and Saturn have deep interiors that are impossible to observe directly, so what we know comes from spacecraft gravity measurements, atmospheric probes, and theoretical modeling. The broad picture is similar for both planets: a thick envelope of hydrogen and helium surrounds a deeper region enriched with heavier elements like rock and ice. But the details differ in ways that reflect their size difference.
Modern interior models suggest Jupiter’s core contains roughly 40 Earth masses of heavy elements, while Saturn’s holds about 25 Earth masses.4Monthly Notices of the Royal Astronomical Society. Structure and composition of Jupiter, Saturn, Uranus, and Neptune under different constraints and distortion due to rotation – Section: 5 INTERIOR MODEL RESULTS Those heavy-element cores are not the sharp, well-defined rocky balls that the word “core” usually brings to mind, though. Data from NASA’s Juno mission at Jupiter and Cassini’s final orbits at Saturn have pushed scientists toward the idea that both planets have “fuzzy cores,” meaning central regions enriched in heavy elements that gradually blend into the surrounding hydrogen-helium envelope rather than sitting as a distinct lump.5AGU Advances. The Fuzzy Cores of Jupiter and Saturn The boundary between “core” and “envelope” is more like a gradient than a wall.
The formation story behind these cores helps explain the size gap between the two planets. The leading theory holds that both Jupiter and Saturn started as rocky and icy cores of roughly ten to thirty Earth masses, massive enough to gravitationally capture enormous envelopes of hydrogen and helium from the surrounding solar nebula.6ScienceDirect. Formation of the giant planets Jupiter’s core apparently reached the critical threshold to trigger runaway gas capture earlier and accumulated far more envelope gas before the nebula dispersed. Saturn started the same process but got a late start or less favorable conditions, ending up with a smaller share of the available gas.
Helium Rain and Internal Heat
One of the more fascinating internal differences between Jupiter and Saturn involves helium. Deep inside both planets, pressures are high enough that hydrogen becomes metallic, behaving like a liquid metal rather than a gas. At certain pressure and temperature combinations, helium stops mixing smoothly with metallic hydrogen and condenses into droplets that sink, a phenomenon called helium rain. This process releases gravitational energy as heat, and it plays a much bigger role in Saturn than in Jupiter.
In Jupiter, the helium rain region appears to extend only a few tens of kilometers and produces a relatively modest effect on the planet’s overall energy budget.7The Planetary Science Journal. Stable Stratification of the Helium Rain Layer Yields Vastly Different Interiors and Magnetic Fields for Jupiter and Saturn In Saturn, helium rain occurs much deeper and over a much more extended region. The latent heat released by helium condensation may account for a large fraction, possibly close to all, of Saturn’s intrinsic heat output. Evolutionary models confirm that helium de-mixing in Jupiter is modest, while in Saturn it is significant enough to create a large helium gradient and what researchers describe as a “helium ocean” deep in the interior.8Astronomy & Astrophysics. Evolution of Jupiter and Saturn with helium rain Saturn’s atmospheric helium content has been depleted as a result, with estimates placing its helium mass fraction between 0.13 and 0.16, noticeably lower than Jupiter’s atmosphere.
This difference helps solve a longstanding puzzle. Saturn radiates more than twice as much energy as it absorbs from the Sun, and simple cooling from formation heat cannot fully account for the surplus. Helium rain provides the missing energy source. Jupiter, being larger and hotter internally, does not need helium rain to explain its heat output, because its primordial heat alone does the job. So the same basic process, helium separating from hydrogen under extreme pressure, plays out very differently because of the size and mass difference between the two planets.
Magnetic Field Strength
Jupiter has the strongest magnetic field of any planet in the solar system, with an equatorial surface field of roughly 4.3 gauss, about ten times stronger than Earth’s. Scientists initially expected Saturn’s field to follow a similar scaling, since Saturn is also a giant planet with a deep metallic hydrogen interior that should generate a strong dynamo. Instead, Saturn’s equatorial surface field turned out to be only about 0.20 gauss, three to five times weaker than scaling predictions based on Jupiter and Earth would suggest.9PubMed. Saturn’s Magnetic Field and Magnetosphere
The likely explanation connects back to helium rain. Saturn’s extended helium rain layer may act as a partially insulating barrier between the region where the magnetic field is generated and the outer envelope. This barrier weakens and filters the field, suppressing the non-axisymmetric components and making Saturn’s field look unusually simple and symmetric compared to Jupiter’s more complex field geometry. The same helium rain that explains Saturn’s excess heat also helps explain why its magnetic field is surprisingly tame for a planet of its size. Jupiter, with its thinner helium rain layer, does not experience the same damping effect.7The Planetary Science Journal. Stable Stratification of the Helium Rain Layer Yields Vastly Different Interiors and Magnetic Fields for Jupiter and Saturn
The practical upshot for magnetosphere size is dramatic. Jupiter’s magnetosphere extends millions of kilometers into space, easily the largest structure in the solar system aside from the Sun’s own heliosphere. Saturn’s magnetosphere is considerably smaller, though still enormous by terrestrial standards. If you could see magnetospheres with the naked eye, Jupiter’s would appear about five times the diameter of the full Moon as seen from Earth, while Saturn’s would be a much more modest affair.
Ring Systems and Visible Appearance
Saturn’s rings are its most famous feature and make it appear far larger through a telescope than its physical globe alone would suggest. The main ring system extends from about 7,000 km above Saturn’s cloud tops out to roughly 80,000 km, giving the ring system an outer diameter of about 270,000 km. Jupiter also has a ring system, but it is thin, faint, and essentially invisible without specialized instruments. Jupiter’s rings were not even discovered until Voyager 1 flew past in 1979.
The contrast is ironic given Jupiter’s greater size and stronger gravity. Saturn’s rings are thought to be relatively young in cosmic terms, possibly only a hundred million years old, and may be the debris of a destroyed moon or captured comet. Jupiter’s weaker ring system is maintained by dust knocked off its small inner moons. So while Jupiter wins on nearly every physical metric, Saturn wins the visual contest hands down, at least for anyone with a backyard telescope.
The apparent size of the two planets as seen from Earth also depends on orbital distance. At its closest approach (opposition), Jupiter spans about 50 arcseconds across, while Saturn’s disk reaches about 20 arcseconds. Factor in Saturn’s rings and the total visual extent grows to about 45 arcseconds, nearly rivaling Jupiter’s apparent diameter. This is why Saturn often looks comparable to Jupiter in a telescope eyepiece even though its physical globe is substantially smaller.
How the Two Giants Compare Among Exoplanets
The discovery of thousands of exoplanets has given astronomers a much broader perspective on where Jupiter and Saturn fit in the spectrum of possible planetary sizes. Many known exoplanets are “hot Jupiters,” gas giants that orbit extremely close to their host stars. These planets can be significantly puffier than Jupiter despite having similar masses, because intense stellar heating inflates their atmospheres. A hot Jupiter with one Jupiter mass might have 1.5 or even 2 times Jupiter’s radius.
Interior models of our solar system’s giant planets are highly compatible with the mass-radius relationships seen in the broader exoplanet population.10Monthly Notices of the Royal Astronomical Society. Structure and composition of Jupiter, Saturn, Uranus, and Neptune under different constraints and distortion due to rotation – Section: ABSTRACT Jupiter and Saturn land right where you would expect for cool, hydrogen-helium-dominated planets at their respective masses. Saturn’s position on the mass-radius diagram is particularly useful as a calibration point: it sits in a mass range where the radius is still climbing steeply with added mass, while Jupiter sits closer to the plateau where additional mass barely increases size. Together, the two planets bracket the transition between “bigger means wider” and “bigger means denser.”
This context also reveals something about why Saturn exists at the size it does. In a different solar system, a planet with Saturn’s core mass might have accumulated more or less envelope gas depending on the disk’s lifetime and the planet’s orbital location. Saturn-mass exoplanets are indeed observed, though they are harder to detect than their larger cousins because they produce smaller signals in both transit and radial-velocity surveys. The ones we have found span a range of radii depending on their temperature and composition, but cool Saturn analogs in wide orbits remain at the edge of current detection capability. Our own Saturn may be a fairly typical outcome for a core that started accumulating gas but ran out of supply before reaching Jupiter’s mass.
Saturn’s Rings and the Gravity They Depend On
One less obvious way that size and mass shape these two planets is through their tidal influence on surrounding material. The region around any massive body where its tidal forces would rip apart a loosely held object is called the Roche limit, and it scales with the planet’s density and the orbiting material’s properties. Saturn’s rings sit almost entirely within its Roche limit, which is why the ring particles never coalesce into a moon. Jupiter’s Roche limit is at a larger absolute distance because of its greater mass, but the region inside it is relatively empty aside from the faint dusty rings and the small inner moons Metis and Adrastea.
The difference in ring prominence between the two planets remains one of planetary science’s open questions. Jupiter’s stronger gravity should, in principle, be better at capturing and retaining ring material. Yet Saturn has the spectacular ring system while Jupiter has almost nothing. Recent work suggests this may partly be a matter of timing and luck: Saturn may have experienced a relatively recent catastrophic event that supplied fresh ring material, while Jupiter has not had a similar event in the recent past. The rings are not a permanent feature and are thought to be slowly decaying, which means we happen to live in an era when Saturn still has them. A few hundred million years from now, Saturn’s rings may be gone, and any future comparison of the two planets will lack their most visually striking difference.