So, I've got a fantasy world where there is a super-earth with Earth-like gravity (aim is approximately half-again the diameter of Earth) that is the homeworld of non-native humans(introduced to the world way back when) and orbiting it is a habitable moon with Earth-like gravity as well that is inhabited by non-native elves.

The premise that I had had in mind for this was that the super-earth would be composed of mostly lighter elements (barring the core and the like) while the moon is comprised of heavier elements. the idea being that the two would balance out. The moon is smaller but heavier while the super-earth being bigger but lighter.

Now, what immediately comes to mind as an issue is the moon; that thing is going to wreak havoc tidally and tectonically and I have highly tectonically and volcanically active regions, as well as some pretty damn high tides, being an issue for any form of coastal life beyond that which is adapted to it.

Things like the geology and topography are not set as of yet, beyond one loch-like-area in which the story is based, but I'm hoping it could be roughly parallel to that of earth.

A lot of what could be said about the planet is still in flux, but I am aiming for something fairly similar to that of the earth (no idea about the moon) and beyond the basics mentioned above I can work with what needs to be done to make it practical.

I did have an idea as to the moons closest point always being along the equator and that being where most of the tectonic activity happens with a byproduct being that a sand dune type desert had formed along the equator due to a variety of factors.

Anyway... thoughts? Mind this is a fantasy setting, but one that isn't too far out there and which is heading toward its enlightenment age. Due to the elves fleeing the moon because of a cataclysm much of the super-earth is known, it has been mapped by them through a telescope and far-seeing items. Furthermore, the diameter has been calculated properly with a metric style unit of measurement having been determined.

Much of the world is known, at least in the broad strokes. I'm just trying to hammer out the details.

  • $\begingroup$ Sorry, what is the exact question? $\endgroup$
    – DWKraus
    Sep 12, 2020 at 0:18
  • 1
    $\begingroup$ Here is somewhere to start phys.org/news/2014-12-binary-terrestrial-planets.html and en.wikipedia.org/wiki/Double_planet I think you are looking for two bodies with similar mass, but different sizes due to composition. $\endgroup$
    – DWKraus
    Sep 12, 2020 at 0:26
  • $\begingroup$ Basically I was double-checking my planet. And thanks! $\endgroup$
    – PseudoSim
    Sep 12, 2020 at 1:51
  • 1
    $\begingroup$ Mathematically a planet's gravity can be modeled as an infinitely small point. You have two such points with equal gravity. That means the two bodies are orbiting one another (not one orbiting the other) and the barycenter is half-way between the two. In short, you need to consider this a binary-planet, not a planet and a moon, and if you put them too close to one another, the two will pull into one another. $\endgroup$
    – JBH
    Sep 12, 2020 at 2:52
  • $\begingroup$ If the planet and the "moon" have the same surface gravity, then they have the same mass. Thus there is no moon. Instead, you have a "binary planet" pair, orbiting around a barycenter in empty space. Sure, one of the planets is a different size, but it's still a binary planet pair. $\endgroup$
    – cowlinator
    Sep 16, 2020 at 3:06

2 Answers 2


I think you get something wrong about such a system. Both planets would be locked to each other. The closer there two masses are to each other, the faster that would have happened.

Even the Earth and Moon will at some point be locked to each other. And since both of your Planets are of equal Mass, it is very hard to imagine that they wouldnt be locked to each other.

And the moment the two bodys are locked to each other, Tidal forces stop being a problem. The only reason those forces exsist is because on Body is moving faster or slower then the rotation of the other is. So in simple terms, the two Planets wouldnt rotate in respect to each other. The same side would always face the other Planet. Never to change.

Meaing, all the Problems you listed wouldnt exsist in the first place.

And if you think about it, this makes total sense. Since there is no relative change in position for both Planets, there is no force.

  • $\begingroup$ Strictly speaking the Earth will never become tidally locked to the Moon, because in the time it would take for that to occur, the sun will expand into a red giant and incinerate both the Earth and the Moon. $\endgroup$
    – jdunlop
    Feb 13, 2021 at 1:55

It is possible. If the Moon was the core of a Gasgiant who was destroid during the colisson with something else. Planets can catch other Objects and make them to moons. Mars has two small moons where it could be, that these have been astroids earlier.

And doubleplanets are possible too. Pluto and his Moon charon have a dimeter ratio from 1:2 and the Mass is 1:6,6 The center of the Mass is 1200km over the Pluto surface.

Here the Wikipedia aritcle to this constelation.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .