I designed my Kepler Bb planet to have 4 major moons. They each have a differing trajectory and are in resonant orbits for stabilization. I also designed the planet to have earth gravity but be bigger than earth.

Planet characteristics:

Mass = 4 earth masses

Radius = 4 earth radii

Gravity = 1g

This should be possible since if you increase it by 1 earth radius per earth mass, you should get the same density in all layers as earth. The core has minute dark energy in case it isn't possible via mass and radius alone. The dark energy isn't enough to rip the planet apart but it is enough to keep the gravity at a certain amount(in this case 1g).

Now why earth gravity? Well the hill sphere is only dependent on radius(it is the orbit that is gravity dependent). Plus earth gravity is the best for humanoid, carbon based lifeforms that live mostly underground(which my Kepler Bb humanoids are).

So anyway, it has 4 major moons. I know it would lead to very complex tide cycles that would lead to high tide at the same time in multiple places, more than 2 spring tides or neap tides etc. But what I am asking is whether this is habitable or not because I am concerned that the tidal forces of 4 major moons would be enough to break the planet into planetoids or at least make the planet more like Venus than Earth in terms of climate. These planetoids could crash into other planets with great force which is another concern since planetoids are much bigger than asteroids or meteroids, both of which can crash into a planet with a lot of force. And while yes the high tech Kepler B# people have been able to withstand every mass extinction so far, a planetoid might just be enough to kill off the species and I don't want that to happen since that is the same species as the people on Kepler Bb.

So would a planet like Kepler Bb be able to have 4 major moons in resonant orbits and not break into planetoids or be exposed to lava every second and kill off all lifeforms and become a Venus analog?

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    $\begingroup$ It might be important to add the sizes/masses of the four moons, for those who are in the know of the equations to calculate that. But as far as I've been able to discern, yes it is quite possible. $\endgroup$
    – Fayth85
    Commented Aug 9, 2016 at 1:10
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    $\begingroup$ In order to get 1g at the surface with a mass four times that of Earth, you need to double the radius, not increase it four-fold. $\endgroup$
    – Mark
    Commented Aug 9, 2016 at 1:17
  • $\begingroup$ I suspect all the moons with their resonant orbits will have a massive amount of geological activity. Your planet may be bombarded with volcanic debris, or even have massive earthquakes of its own due to tidal flexing. I also wonder about the stability of the system, since the tidal drag of all these moons will slow down the planetary rotation quickly in astronomical terms, and the moons themselves will be gradually moving away from the planet, much like our own moon. $\endgroup$
    – Thucydides
    Commented Aug 9, 2016 at 1:33
  • $\begingroup$ «This should be possible since if you increase it by 1 earth radius per earth mass, you should get the same density in all layers as earth.» no, the volume does not scale linearly with the radius! $\endgroup$
    – JDługosz
    Commented Aug 9, 2016 at 3:01
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    $\begingroup$ @Caters, Newton's formula for gravity, adjusted to give surface gravity: a = Mg/d^2. If you quadruple both M and d, then a (surface gravity) is reduced to a quarter of its original value. $\endgroup$
    – Mark
    Commented Aug 9, 2016 at 4:25

1 Answer 1


There is no issue with the planet getting torn apart by the moons. Tides on Earth from the Moon are not all that strong, especially compared with the tides experienced by planets very close to their stars that we know are still held together, e.g. very hot rocky planets like Kepler-78b or Corot-7b.

In practice, tidal forces are extremely sensitive to the orbital distance, much more so than gravity (gravity scales as the orbital separation r^-2 whereas tides are r^-6 or so). So, if all 4 moons are similar in mass, your planet would feel much stronger tides from the innermost moon than the outer ones. I can imagine that tides would be a little stronger when the moons' positions are aligned but not by all that much.

It's a good call to put the moons in resonance to stabilize them. This would happen in any case as the moons' orbits were pushed outward by tides (e.g., Jupiter's Galilean moons).

I don't really understand the setup of your planet's gravity. With 4 Earth masses and 4 Earth radii the gravity should be 1/4 of Earth's (g ~ M r^-2). Your planet sounds more like a "mini-Neptune" than a "super-Earth". In fact, many mini-Neptunes (and super-Earths too) have been discovered by the Kepler space telescope (see here: https://planetplanet.net/2014/03/03/hot-super-earths-and-mini-neptunes/).

  • $\begingroup$ I was just trying to get identical density and thus gravity and I thought that if I increased the radius and the mass both by 1, since volume and density both scale cubically in relation to radius, I would get the same density in all layers as earth. It turns out that I only need to double the radius for a 4 earth mass planet to get earth gravity. I'm glad I don't have to worry about planetoids or even an earth analog becoming a venus analog because that would be bad for civilization in either case. $\endgroup$
    – Caters
    Commented Aug 9, 2016 at 11:54
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    $\begingroup$ @Caters: Gravity does not come from density, it comes from mass and radius. When you design a bigger planet with the same gravity as Earth, you need to lower the density, because the volume goes with r³. $\endgroup$ Commented Aug 9, 2016 at 12:12

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