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Nine times wider than Earth

Saturn Would Float in a Big Enough Bath

It is the second largest thing orbiting the Sun. It is also lighter, for its size, than water.

Saturn: 95 timesthe mass of Earth And still lighter,litre for litre, thana bath
Both facts are true at once, because they are answers to different questions.

  1. Step 1 of 5

    Heavy and dense are not the same question.

    How much is there? 95 Earths' worth ofstuffHow tightly packed? 0.69 grams in everycubic centimetre

    A bathful of feathers weighs more than a marble. That does not make feathers heavy. Weight asks how much there is; density asks how tightly it is packed. Saturn is the biggest example anyone has of the two answers pointing opposite ways.

    Water is 1.00 gram per cubic centimetre by definition. Saturn comes in under it.

  2. Step 2 of 5

    Almost all of Saturn is gas being squashed.

    Cloud topsAmmonia ice: thestripesHydrogen and helium gasThickening thefurther you fallLiquid hydrogenSqueezed until itflowsMetallic hydrogenPressure makes itconductA small rocky coreA few Earthmasses, buried
    Nowhere on the way down is there a floor.

    There is no surface to stand on. You would sink through cloud, then thickening fog, then a fluid that behaves like a metal because the pressure leaves it no choice. Only right at the centre is there anything rocky.

    Jupiter is built the same way and is also mostly hydrogen — but it is heavy enough to crush itself denser than water.

  3. Step 3 of 5

    Hydrogen is the lightest thing there is.

    About 96 per centhydrogenMost of the rest isheliumEnormously wide, sothe weight is spreadthinNot massive enough tosqueeze itself dense

    One hydrogen atom is a single proton with a single electron. Nothing in the universe is made of less. A planet built almost entirely from the lightest possible ingredient, and not quite massive enough to crush it hard, ends up under the density of water.

  4. Step 4 of 5

    So in an imaginary bath, it rises.

    Saturn in the bathPush from the water it displacesIts own weight
    The upward push wins, so it rises — that is all floating ever means.

    Floating is a contest. The water pushes up with the weight of the water shoved aside; the object pulls down with its own weight. Anything less dense than water wins that contest and rises. Saturn, litre for litre, is less dense than water.

    The bath is not a real proposal. There is no bath, and a planet's own gravity would pull the water into a ball around it long before any floating happened.

  5. Step 5 of 5

    Put the numbers side by side and it stops sounding silly.

    Saturn0.69 g/cm³Water1.00 g/cm³Earth5.51 g/cm³ — thedensest planetIron7.87 g/cm³
    Only the first bar is below water.

    Density is a plain measurement, and Saturn's is simply low. It is the only planet in the solar system that comes in under water, and it does it comfortably rather than by a whisker.

The short version

Saturn is 95 Earths of mostly hydrogen spread over an enormous volume, which leaves it less dense than water — heavy and light are answers to different questions.

Try it yourself

Weigh an empty drink bottle, fill it with water and weigh it again. The difference in grams is very close to its volume in millilitres, because water is one gram per millilitre. Everything denser than that sinks.

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