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SESK: Stuff Everyone Should Know · sesk.info/aero-saturn-low-density

Which planet has the lowest average density?

Saturn, whose average density is lower than water's.

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  1. Earth
  2. Saturn ✔
  3. Mercury

Why: Saturn has enormous volume but relatively little mass for its size. That makes it the least dense planet in the solar system.

Huh, didn't know that: Saturn's atmosphere is mostly hydrogen and helium.

The dad joke
Saturn is a heavyweight planet with a lightweight average.
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References

Answer: Saturn. Saturn has enormous volume but relatively little mass for its size. That makes it the least dense planet in the solar system.

  1. Cassini Mission FAQ science.nasa.gov/mission/cassini/faq
    Such a large volume with so little mass makes Saturn the least dense of all the planets
    Checked 2026-10-10.

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