Which planet would float in water?
Quick answer
Saturn is the planet that would float in water, if you could find a bathtub big enough. Its average density is about 0.69 g/cm³, lower than water’s 1 g/cm³. It’s the only planet in our solar system less dense than water, because it’s made mostly of hydrogen and helium, the lightest elements.
Key facts
- Average density
- About 0.69 g/cm³ (water = 1.0)
- Mass
- About 95 times Earth’s
- Volume
- About 760 Earths could fit inside
- Main ingredients
- Hydrogen and helium
Big but light
Saturn is the second-largest planet, about 9 times wider than Earth, but its huge volume is filled mostly with gas. Mass spread over that much space gives it a very low average density.
How the planets compare
| Planet | Average density (g/cm³) | Would it float? |
|---|---|---|
| Earth | 5.5 (densest planet) | No |
| Mercury | 5.4 | No |
| Venus | 5.2 | No |
| Mars | 3.9 | No |
| Neptune | 1.6 | No |
| Jupiter | 1.3 | No |
| Uranus | 1.3 | No |
| Saturn | 0.69 | Yes |
For more about the biggest planet, see what the largest planet is.
Why it couldn’t really float
This is a thought experiment. In reality:
- An ocean big enough would need to be larger than Saturn, and its own gravity would make it collapse into a planet or star.
- Saturn has no solid surface; its gas would mix with and dissolve into the water.
- Its core is likely dense rock and ice, and its deep layers are compressed far beyond water’s density. Only the average is low.
Saturn is slightly squashed
Saturn spins so fast, a day lasts about 10.5 hours, and its low density lets it bulge at the equator. It’s the flattest planet, about 10% wider at the equator than pole to pole. See which planet has the shortest day.
Its famous rings
Saturn’s rings are made of billions of chunks of ice and rock, but they’re surprisingly thin, often just tens of meters thick despite being hundreds of thousands of kilometers wide.
Saturn’s largest moon has a thick atmosphere; see what the largest moon of Saturn is.
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