Saturn, the enormous gas giant, could float on water (if you could find a big enough bathtub)

Saturn, the enormous gas giant, could float on water (if you could find a big enough bathtub)

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Among the many strange and fascinating facts about our Solar System, one of the most mind-boggling is this: Saturn, the second-largest planet in our celestial neighbourhood, is less dense than water.

Facts about space and astronomy. Credit: Westend61 / Getty Images
Credit: Westend61 / Getty Images

In fact, if you could find a bathtub big enough, Saturn would float in it.

This idea might sound like science fiction, but it’s rooted in science fact.

Saturn ring plane crossing 23 March 2025
Credit: NASA/JPL

Saturn is full of gas

They don't call Saturn a 'gas giant' for nothing.

Saturn's average density is just 0.687 grams per cubic centimetre, while water has a density of 1 gram per cubic centimetre.

That means Saturn is about 30% less dense than water, which seems like a strange property for a planet.

Why is Saturn so light for its size? The key lies in its composition.

Saturn is a gas giant, made up mostly of hydrogen and helium, the two lightest elements in the Universe.

These gases don’t weigh much, but Saturn is enormous, with a diameter about nine times that of Earth.

Illustration showing how Saturn compares to Earth in size. Credit: Mark Garlick / Science Photo Library / Getty Images
Illustration showing how Saturn compares to Earth in size. Credit: Mark Garlick / Science Photo Library / Getty Images

All that gas gives the planet a huge volume, but relatively little mass compared to a rocky planet like Earth.

And that's the distinction: Saturn's volume is the amount of space it occupies, but its mass is the amount of matter – 'stuff' – in it.

The result of Saturn's high volume and low mass is a low overall density.

If you wanted to put Saturn into a bathtub of water, the bath would probably need to be about 120,000km wide, since Saturn itself is 116,460km wide!

Saturn and its rings James Webb Space Telescope, 30 June 2023 Credit: NASA, ESA, CSA, STScI, Matt Tiscareno (SETI Institute), Matt Hedman (University of Idaho), Maryame El Moutamid (Cornell University), Mark Showalter (SETI Institute), Leigh Fletcher (University of Leicester), Heidi Hammel (AURA)
Saturn and its rings, as seen by the James Webb Space Telescope, 30 June 2023. Credit: NASA, ESA, CSA, STScI, Matt Tiscareno (SETI Institute), Matt Hedman (University of Idaho), Maryame El Moutamid (Cornell University), Mark Showalter (SETI Institute), Leigh Fletcher (University of Leicester), Heidi Hammel (AURA)

Saturn's gravity

So how does this affect Saturn's gravity?

Saturn's mass is 95 times the mass of Earth, yet its Saturn's gravity is only slightly stronger than Earth –about 1.08 times Earth's gravity, according to NASA – because it's such a large planet.

Beneath Saturn's thick cloud layers lies a core of heavier elements surrounded by metallic hydrogen, where pressures and temperatures are extreme, far beyond anything found on Earth.

Saturn remains one of the most visually stunning and scientifically intriguing objects in our Solar System, and the more we learn about it, the more it seems to amaze us.

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