I know that objects with lower mass tends to orbit objects with higher mass, is it possible to have a Moon sized planet who's mass is so heavy that an Earth sized planet orbit it as it's Moon in a stable orbit?

  • The Moon sized planet has to be an actual planet, not a black hole or some other singularity.
  • The Earth sized Moon can be a gas planet, a rock planet, a water world, or any other natural combination.
  • Both need to be naturally made so that limits their building materials to naturally occurring ones and not theoretical only materials.
  • For this question I don't care if the mass and\or orbit is not Earth like or Moon like, only that the size (diameter\circumference) is Earth & Moon like.
  • $\begingroup$ You say no singularity--but what about a black dwarf for the "planet"? $\endgroup$ Sep 9, 2018 at 5:11
  • $\begingroup$ "I know that objects with lower mass tends to orbit objects with higher mass" Inaccurate. Both objects orbit the center of mass of the system. $\endgroup$
    – Ben Voigt
    Sep 9, 2018 at 16:25

1 Answer 1



So, the densest a planet can reasonably get is effectively making it out of solid iron. (Making it out of denser materials is unlikely given the relative rarity of those materials, and a big chunk of iron could be something like a piece of a stellar remnant after some star-shattering cataclysm.) The moon has a volume of approximately 21.9 billion cubic km. Iron has a density of 7.3 billion tonnes per cubic km. A moon-sized hunk of iron, then, would mass approximately 1.6 * 10^23 kg. That's another order of magnitude denser than it currently is, but still only about a tenth of Earth's mass (rather than its current figure, 1%).

So, can we make Earth lighter? The least dense planetary masses in the solar system are the gas giants, but they're also the most massive. Happily, the good people at Harvard have a lifeline for us. Turns out, if you're a good distance away from the star, the minimum core mass for a gas giant is circa 0.2 earth masses. The authors in fact postulate that Uranus and Neptune were originally formed as much smaller planets.

If we assume (not wholly accurate, but we're working with Fermi numbers here) that the core of such a planet is composed entirely of ice, and that the overall density of the planet averages 0.9 g/cm^3 (heavier than Saturn, but a much lower gas-to-core ratio), we still can only manage to get an earth-volume planet down to around 9 * 10^23 kg. This definitely would result in a more equal gravitational relationship (rather than the 100-to-1 relationship with our moon), but the disparity in diameters is so tremendous that there's no explicable natural way (without dipping into neutron stars or black holes, which you said you'd prefer not to do) for the larger to orbit the smaller.

Geometry does tell us that you'd only need to increase the radius of your super-dense moon fivefold to give it a hundredfold increase in volume and mass (at which point the planet/moon relationship could be established), but it would still be five times bigger than the moon.

  • $\begingroup$ That's a great answer, but your last sentence that increasing the radius of the super-dense moon fivefold would make it five times bigger than the Moon is rather vague. Increase the radius fivefold would make it five times the diameter of the moon (and thus about 5/4 the diameter of Earth), 25 times the surface area, and 125 times the volume, which would make it 125 times the mass of the Moon except that it is supposed to be about 10 times as dense as the Moon and so would have about 1,250 times the mass of the Moon. "Bigger" is very ambiguous. $\endgroup$ Sep 9, 2018 at 17:47
  • $\begingroup$ @M.A.Golding - an excellent point; I was mostly aiming for equivalent Fermi number masses to the Earth/Moon relationship (100 to 1), without considering how thoroughly that would violate the OP's desired configuration. $\endgroup$
    – jdunlop
    Sep 9, 2018 at 19:11

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