I understand that this situation is hypothetical -- I just need a piece of advice to make it more realistic.

In my story, there is a superearth of 4,5 Earth masses and 1,35 Earth's radius, rotating around star similar to the Sun, except little lighter and dimmer. The planet has a moon... Composed mostly of iron.

Now, I've already made some calculations. For storytelling reasons (planet-moon physical transfer supposed to be much faster than Earth-Moon' is) the moon is supposed to rotate very close to the Superearth - I've estimated around 50,000 kilometers in average, considering Roche limit of roughly 28,000 kilometers due to the moon's enormous density. As itself, it is pretty small and light, but maintains a high surface gravity, again, due to the high density.

Now, here is one thing I am trying to understand right now the geological activity. It is clear that planet-moon system with only 50,000 km orbit will result in dramatic tidal heating, and, subsequently, plate tectonics and volcanism. In my case, first is countered by the planet's proximity from it's host star - it resides on cooler edge of habitable zone, and technically thick atmosphere and tidal heating are the only things that keep it warm enough for cold-acceptable vegetation and prevention of oceanic freezing. On the other hand, the major geological activity is diminished by superearth's incredibly thick crust without large "cracks", at least on surface level. I'm not sure what will be happening on the moon in that scenario, because tidal effect on it, caused by superearth's gravity, will be even stronger. I'm aware that cannonball planets probably get geologically dead pretty fast, but wouldn't this process stumble into said tidal effects? I just couldn't find any theories about iron MOONS, not planets.

How much of this has an actual scientific basis and what should I do to make it better?

Thank you in advance.


With my concerns about Roche limit, I totally forgot about the importance of mass - so there is some more calculations to make, but like I said, the moon is still relatively light. Also, in the initial idea most of heating of planet is atmospheric and not tidal - the latter might only contribute for a little oceanic heating.

  • $\begingroup$ "It is clear that planet-moon system with only 50,000 km orbit will result in dramatic tidal heating": It is not clear at all. It depends very strongly of the mass of the satellite. For example, humans have placed very many satellites around Earth on very much closer orbits, but they are so lighweight that no measurable tidal heating occurs. $\endgroup$
    – AlexP
    Mar 28, 2023 at 13:35
  • 1
    $\begingroup$ (a) To set an expectation: we're here to help you build imaginary worlds and we do permit Real World questions, but you're asking for Real World science about a world+moon combination humanity has never seen before and hasn't tried to model. No answer we give you will be "real." They'll be, at best, educated guesses. (b) For future reference, note that we permit only one question per post. Multiple posts are preferred (required) over one post with multiple questions. (c) A moon with enough mass to cause sufficient tidal heating to overcome the cold would cause breathtaking wind - literally. $\endgroup$
    – JBH
    Mar 28, 2023 at 13:55
  • $\begingroup$ Another consideration which might or might not be of interest, with such a massive planet it is unlikely that any chemical powered rocket would be capable of reaching orbit from the surface and even nuclear rockets might struggle (I haven't done the calcs but...) $\endgroup$
    – Slarty
    Mar 28, 2023 at 17:36
  • $\begingroup$ So... most of the planet's heat is coming from the sun? What do you mean by "atmospheric heating?" I'm pretty sure an exothermic atmosphere would be toxic. If the atmosphere is, e.g., like Earth's, then the atmosphere is heated by the sun or (to a much, much smaller degree) by the core via the mantle and/or volcanoes. $\endgroup$
    – JBH
    Mar 28, 2023 at 20:51
  • $\begingroup$ Will you re-phrase that, please? ‘Geological activity and of superearth - cannonball moon system’ doesn’t work in English, meaning anything that follows is largely guesswork. Only because 1,35 Earth's radius can hardly mean 1/35, I guess that 4,5 Earth masses means 4.5, rather than 4 or 5. Similarly, what does 'planet-moon physical transfer supposed to be much faster than Earth-Moon' is' mean? $\endgroup$ Apr 1, 2023 at 0:20

1 Answer 1


When I first saw the question, I thought I leave it to someone with who knows more about astronomy than me. Since this does not seem to happen, I give it a try. The followings points are more of educated guesses or gut feelings.

Tidal heating

Just of the top of my head, I would say this could work for the moon (at least it works for Io). So the moon could have geological activity with some impressive volcanoes that emit molten iron. I doubt it works for the planet, which is just so big that the heat dissipates too much especially for a very light moon. You will probably need radioactivity, if you want geological activity on your planet. I am not sure what you mean with atmospheric heating, but heat from the atmosphere will not be able to increase the core temperature of your planet.

Temperature of the surface

The temperature on the surface will be determined by the incoming radiation from the sun and eventually a greenhouse effect (depends on the atmosphere). Heat from the core of the planet will not have a significant impact on it, even if the core is very hot


Gravity on the surface is around 2.5g. This is a lot to just walk around. Launching rockets might be very difficult, as @Slarty already pointed out. The density of the planet seems very high to me, since it has 4.5 Earth masses but only 2.5 Earth volumes. A higher density than Earth would be expected, since it has a higher gravity and pressure, but this seems much.


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