Saturn is so light it would float in water if you had a big enough tub

Drop most planets into a large enough body of water and they would sink without a second thought. Saturn is the one exception in the solar system, a gas giant so puffed up with hydrogen and helium that its average density comes in lower than water itself. It sounds like a trivia-night oddity, but the fact says something real about what the planet is actually made of.

A Giant Made Almost Entirely of Gas

Saturn is the second-largest planet in the solar system, with an equatorial diameter of roughly 74,897 miles, about nine times wider than Earth. Despite that enormous size, the planet has no solid surface at all. Like fellow gas giant Jupiter, Saturn is built almost entirely from hydrogen and helium, the same two elements that dominate the composition of the sun, arranged in layers of gas that gradually thicken into liquid the deeper they go.

That composition is laid out in detail on NASA’s Saturn fact sheet, which notes that the planet’s bulk volume is so large relative to its mass that its average density works out to less than that of water, making Saturn the only planet in the solar system with that particular distinction.

How Astronomers Measure the Density of a Planet No One Can Touch

Calculating the density of a planet nobody has ever landed on requires combining two separate measurements rather than a single direct reading. Mass comes from watching how strongly a planet’s gravity tugs on something nearby, typically one of its moons, and applying the same orbital-mechanics equations that describe how the Moon orbits Earth. Volume comes from measuring the planet’s apparent size through a telescope and combining that with its known distance from Earth to work out its true diameter. Dividing the resulting mass by the resulting volume produces the average density, without ever needing a spacecraft to touch the planet’s clouds.

Why Average Density, Not Size, Is What Matters

Density measures how much mass is packed into a given volume, and it is entirely possible for an enormous object to have a low average density if most of that volume is filled with something light. Saturn’s rings, moons, and overall bulk make it a genuine giant by any measure of size, but the vast majority of that volume is taken up by low-density hydrogen and helium gas rather than dense rock or metal. The result is a planet roughly 95 times the mass of Earth that would still bob at the surface if it were somehow dropped into a body of water large enough to hold it.

What’s Actually Inside a Planet That Would Float

Beneath its gaseous outer layers, Saturn does contain denser material. At the planet’s center sits a core of metals such as iron and nickel, described in more detail on the planet’s Wikipedia entry, surrounded by rocky material compressed by intense heat and pressure and wrapped in a deep layer of liquid metallic hydrogen. That dense core, however, makes up only a small fraction of the planet’s overall volume. Nearly everything above it is gas transitioning into liquid, which is why the planet’s average density stays so low even though its interior contains genuinely heavy elements.

The One Planet That Would Pass This Test

None of the solar system’s other planets come close to matching that property. Jupiter, despite being built from similar material, is massive enough that its own gravity compresses its interior into a higher average density than Saturn’s. Rocky planets like Earth and Mars are dense enough that the comparison is not even close. Saturn’s unusual combination of enormous size, relatively low mass for that size, and a composition dominated by the lightest elements in the universe leaves it as the sole planet in the solar system that would, in principle, float.

A Planet of Extremes Beyond Just Density

Saturn’s low density is not the only oddity that comes with being built almost entirely from gas. The planet completes a full rotation, its version of a day, in only about 10.7 hours, the second-shortest day of any planet in the solar system, even though a full trip around the sun takes roughly 29.4 Earth years. Its ring system, made of billions of icy and rocky fragments ranging from dust grains to house-sized chunks, spans up to about 175,000 miles from the planet, yet stands only around 30 feet thick through most of its main rings.

Saturn’s atmosphere adds another layer of strangeness on top of the density record. Around the planet’s north pole sits a persistent, six-sided jet stream pattern roughly 20,000 miles across, first spotted in images from the twin Voyager spacecraft in the early 1980s and studied in far more detail by the Cassini orbiter afterward. As of 2025, Saturn was known to host 274 confirmed moons, more than any other planet in the solar system, ranging from the haze-covered surface of Titan to small, crater-pocked bodies still awaiting official names.

This article was produced with the assistance of AI and reviewed by Morning Overview editors.

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