The Question

I am working on filling in the planets of the Ilus system. I am setting up a strange, huge, low density, sort of habitable world in the gap between two gas giants. Is there a way for a rocky planet thats roughly earth-like in mass and composition to form or be thrown into a stable orbit as a planet proper, and not become a moon of one of the gas giants?

Could it have once been one, but a passing interaction with some other celestial body in the early formation period have thrown it out and left it to become a real planet? I thought it would be interesting for a habitable Hycean-like world to orbit in a huge gap between Jupiters, and be a rather unique place to put a basecamp. It would have been were nitrogen was mined for terraforming the planet Ilus proper.

Also probably a wintery tundra for any intrepid explorer of the decaying infrastructure that once powered the system, a story idea actually.

  • 1
    $\begingroup$ Where the planet formed and where it is now can be two very different questions. $\endgroup$
    – AlexP
    Feb 9 at 19:07
  • $\begingroup$ The HD_191939 system is sort of similar to what you're looking for. Both d and f look like terrestrials ans while d has superearths or ice giants to its inner side, that might be good enough. Note that this is a very dense system, I did not run the numbers but it seems possible that there is a resonance as L.Dutch predicts. $\endgroup$ Feb 9 at 19:22
  • $\begingroup$ You need a different name. Ilus is the name of the planet that’s the setting for Cibola Burn, the fourth book in the Expanse series, and also in season 4 of the TV show based on the series. $\endgroup$
    – Mike Scott
    Feb 11 at 2:07
  • $\begingroup$ @MikeScott Wait really? It's also from somewhere-or-other from greek myth. The naming scheme for basically all the systems stems from greek and roman stuffs. Ilus, Aurea, Theia, Tanu, Rhea, Hyperion, and a few other ships, planets and other stuffs. $\endgroup$ Feb 11 at 3:24
  • $\begingroup$ Those being the star system, four different ships and a gas giant, all respectively. $\endgroup$ Feb 11 at 3:25

3 Answers 3


Is there a way for a rocky planet thats roughly earth-like in mass and composition to form or be thrown into a stable orbit as a planet proper, and not become a moon of one of the gas giants?


First, I just took a look in the list of known multiplanery systems, and ordered by the number of planets in a descending order.

I didn't need to go very far down on the list to find these:

HD 10180:

  • b: Rocky, but unconfirmed
  • c: Either gas giant or gas dwarf
  • i: Rocky, but unconfirmed
  • d: Either gas giant or gas dwarf
  • e: Gas giant
  • j: Very likely a rocky superearth, but unconfirmed
  • f: Gas giant
  • g: Gas giant
  • h: Gas giant

I.e., planets i and j are possibly what you are looking for.

HD 191939:

  • b: Probably a gas dwarf
  • c: Likely a gas dwarf
  • d: Superearth
  • e: Gas giant
  • g: Gas giant or gas dwarf
  • f: Gas giant

I.e., planet d definitely is what you are looking for.


This is very poorly constraint, but all the 6 planets are in the middle ground of superearths or gas dwarfs.

A possible scenario is that 4th and the 6th planets are gas dwarfs, and the 5th is a superearth, hence what you are looking for.

However, a more likely scenario is that the 5th is also a gas dwarf, thus, not what you are looking for.


  • b: Probably a rocky superearth, but possibly a gas dwarf
  • e: Rocky, Venus-like
  • c: Probably a gas dwarf
  • f: Rocky, Earth-like
  • g: Gas giant, unconfirmed
  • d: Gas giant

I.e., planet f and perhaps e would be what you are looking for.


  • b: Almost certainly a superearth. Unlikely, but possibly, a gas dwarf.
  • c: Probably a superearth, possibly a gas dwarf.
  • d: Likely a gas dwarf. Unlikely, but possibly, a superearth.
  • e: Surely a gas dwarf.
  • f: Probably a superearth. Unlikely, but possibly, a gas dwarf.
  • g: Almost certainly a gas giant.

There are 6 planets, By mass, there is a fair possibility that the 1st and the 5th are actually superearths and then the 5th would be what you are looking for.

However, further data including density says that all of them would be gas dwarfs. But since the data is very poorly constrained and we also have very poor knowledge about superearths and gas dwarfs, then we really don't know yet what is going on.

Gliese 163:

5 planets, but their masses are poorly constrained. The last is definitely a gas giant. The 4th is either a gas dwarf or a gas giant.

It is possible that the 2nd and the 3rd are superearths while the 1st is a gas dwarf.

HD 108236:

5 planets, but their masses and radius are very poorly constrained, so we really don't know for sure. But it seems that the 1st and the 5th are rocky, the 2nd and 4th are gas dwarfs and the 3rd either a superearth or a gas dwarf.


5 planets. Accordingly to whoever edited this article on wikipedia, one of them is rocky and the other 4 are gas giants, but the article doesn't says which one is rocky. Looking to the planetary system table there, I think that the 3rd would be the rocky planet.

Gliese 3293:

4 planets. The 1st is a superearth. The 2nd and the 4th are gas giants. The 3rd might be either a superearth or a gas dwarf.

Superearths between gas dwarfs!? Meh, what about Earth-sized between Jupiter-sized?

None known as far as I can tell. We don't know those yet because:

  • a. Those might be very rare.
  • b. Detecting Earth-sized or Mars-sized exoplanets is incredible hard and very few of those are known.
  • c. Most of the known exoplanets are very near to their host stars because our detection methods perceive them much easier than those far away, and this introduces a severe observational bias.
  • d. Very few exoplanets were directly photographed, and all of those are very large gas giants in wide orbits. So, if there are rocky planets orbiting between them, we just don't know yet.

As we are studying and discovering more and more about stellar systems other than the Sun's, more and more we discover that a large part of them deviates a lot from what we expected from our planet-formation theories. And just 30 years ago, we knew nothing about them, so it might be just a matter of time and luck until we look to the right star and find a system that is what you are looking for, and possible one of those listed above is exactly that, just waiting for better-constrained data.

A possible scenario is that in a thick part of the formation disk, three planets starts to form, one in the inner side but still somewhat far from the inner edge, one right in the middle, and one in the outer side, but also far from the outer edge. Then, the inner and the outer planets starve the middle one from forming material, but not enough to prevent the formation of a rocky planet there. Also, it is important that the inner planet don't migrate outward after being formed nor that the outer migrate inward (the opposite is ok).

  • 1
    $\begingroup$ Well heck! You certainly did more research than I could begin to- Anyhow! Thanks! I have some more questions about this Hycean planet-thing, so I will be posting more on this later. +1 and I'll probably accept this answer. $\endgroup$ Feb 10 at 4:49

Orbiting between two gas giants might be possible, if some orbital resonances can happen to stabilize the orbits, something like the 4:2:1 orbital resonance between Ganymede, Europa and Io.

However, for a rocky planet to form between two gas giant is very tricky, considering the disturbance that the two bodies will exert on anything that it is in their neighborhood. Keep in mind that Jupiter alone has been able to stop the aggregation of the bodies which now form the asteroid belt.

Therefore the only remaining mechanism is that the planet forms somewhere else and then, through gravitational shots, migrates and is placed between the two big boys. How easy is for that to happen I cannot tell, but I have the feeling it has to be some lucky shot.

  • $\begingroup$ Well if it is in the range of 'lucky... but plausible' is good enough for me! I was worried about a flat 'no, thats impossible, and will break the suspension of disbelief'. I guess it could actually be part of the point as to why build a research base on it, trying to figure out how it formed and why it didn't get destroyed or became a normal moon with a thick atmosphere, like titan or something. $\endgroup$ Feb 9 at 22:19

Not according to Accretion Theory

Accretion Theory states that the different kinds of planets form based on distance to the star. Rocky planets form close to the sun because most of the substances with lower boiling points either react with minerals forming silicates and such or they are blasted away with the solar wind. This allows planets to only form thin atmospheres and hydrosphere's at thier surfaces.

As you get farther out, the solar wind is weaker and temperatures are colder. This means gasses can accrete into gas giants and many things that are gases close to the star become liquid or ice allowing them to bury any heavy elements under so much "not rock" that we would not classify it as a rocky planet. While outer moons and planets my have rocky or metallic cores, they inevitably form massively think ice layers, atmospheres that turn them into either things like Jupiter's Icey moons or actual gas giants.

This means rocky planets should in theory only form inside of the Goldilocks zone; so, if your system has a planet out that far, something happened to move it there, or it is on an elliptical orbit that only sometimes passes into the outer solar system


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