The world I’m building is approximately 1/3 of the size of earth and it has 3 moons orbiting it. The orbits are all different but occasionally they have either a full moon or a new moon on the same day. I have a couple of questions. First, if all these moons were small (totaling to about 1/3 the size of earth’s moon) and slightly different sizes with different orbits, would the planet still be able to support life? If the answer to the first question is yes, then what would happen to tides and earthquakes and volcanoes? Would it be possible for a three mooned planet to function pretty similarly to Earth?

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    $\begingroup$ Arrrr, matey, The sea tides with three weenie moons be a complex beast, but of little ferocity. You can safely leave port without a spare copy of the tide tables. $\endgroup$
    – user535733
    Commented Jul 30, 2020 at 22:11
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    $\begingroup$ To support @user535733's observation, 1/3 the moon, 1/3 the gravity, and the further the moon the less impact it has. Consequently, even when all three moons are lined up, the total gravimetric influence is less than Luna has on Earth. $\endgroup$
    – JBH
    Commented Jul 31, 2020 at 4:50
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    $\begingroup$ So the size of the moons relative to the size of the planet doesn’t matter for it’s gravitational effect? I just want to make sure I’m understanding correctly. $\endgroup$
    – Amber Blue
    Commented Jul 31, 2020 at 15:11
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    $\begingroup$ What will happen to that world? Eventually its star will become a red giant and overheat the planet, destroying all life, and maybe swallow the planet entirely. The same thing that happens to all planets eventually. $\endgroup$ Commented Jul 31, 2020 at 15:33
  • $\begingroup$ “Size” is an ambiguous term. When you say “totalling to about 1/3 the size of the Earth’s moon”, do you mean that their total volume is 1/3 of our moon’s volume, that their total visible area in the sky is 1/3 of our moon’s visible area, that their total surface area is 1/3 of our moon’s surface area or that the sum off their radii is 1/3 of our moon’s radius? These will give four very different results. $\endgroup$
    – Mike Scott
    Commented Aug 1, 2020 at 9:28

1 Answer 1


It is highly unlikely that a world as small as a third the size of the Earth would be able to retain an atmosphere so life would be unlikely unless perhaps in some form of ocean under an ice crust.

The tides would be very small and the moons themselves would be in danger of destruction if they approached too close to Earth due to Earth's tidal forces.

Such small worlds orbiting each other might better be described as a binary / tertiary planet depending on exact sizes.

  • $\begingroup$ the world is a third the size of earth, not the moon. $\endgroup$
    – ths
    Commented Jul 31, 2020 at 13:10
  • $\begingroup$ Thanks - I have now corrected it. But I don't think it would make much difference to the final conclusion. $\endgroup$
    – Slarty
    Commented Aug 1, 2020 at 9:26
  • $\begingroup$ Depends on how you measure size. By diameter, Mars is more than half the size of Earth, but only about 10% of its mass, with about 1/3 the surface gravity: mars.nasa.gov/all-about-mars/facts $\endgroup$
    – jamesqf
    Commented Aug 1, 2020 at 17:02

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