My and my teams' works take place on a moon orbiting as giant about 3 times the mass of jupiter (possessing also a massive moon system) in a solar system with an F-star and be either the first, third or fifth of about 10 to 12 planets (I haven't properly figured that out yet). I wanted to know what you thought about the formation scenario for the moon.

The idea is essentially that as the solar system was stabilizing the gas giant moved closer to the star and into the habitable zone as its moons were still forming, during this transit it would have captured a few massive bodies and debries, which would explain the fact that it has numerous galilean moons (about 10 over several orbits) including a particularly massive and iron rich planetoid that would become the habitable moon.

Do you think that this formation scenario is plausible?

Addendum. The gas giant's axis would have a 30 degrees tilt, thus partially removing prolonged eclipse periods and giving seasons to the moon. The giant would also be located at about 2 AUs from the star. Plus the moon would be on an orbit that would grant it an orbital period similar to earth's about 36 hours (544000 kms distance) or 90 hours with a 5:2 spin-orbit resonance allowing for a rotation of 36 hours (1003000 kms), plus effects such as tidal heating would be very relevant to the planet. Also the mass of the moon would be about 0.7 times the mass of earth


3 Answers 3


Fundamentally there are three ways a moon may form. Impact, as in the case of Earth-Luna, conformation, as in the case of the major moones of the gas giants, and finally capture, as in the case of Neptune-Triton. Another important fact is that in every solar system there is a so called frost-line. Beyond this line, the largest gas giant will form as this is where all the volatiles (water, ammonia, methane) from the inner system accumulate here. Inside the frost line rocky planets with an of magnitude more or less water than Earth will be dominant. Outside of this line volatile rich planets will be dominant.

The issue with a capture scenario is that the captured object is most likely going to end up in an irregular and excentric orbit. While the orbit will circularise over time or settle into a spin orbit resonance, it will cause havoc with the original moon system. Especially something Earth-sized will be bad. A super-earth could while out every other moon. Neptune lacks any other major moon but captured Triton.

So my guess is that the capture event will eject all but the innermost regular moons or one or two outer ones, who got into a resonance with the intruder early.

  • $\begingroup$ TheDyingOfLight Hmmm so it would be plausible for the gas giant to retain it's major moons and a ring system post migration and capture of the earth-like object? That would be my main concern really. If you need to know, the habitable moon would have a mass of 0.7 earths. $\endgroup$ Aug 16, 2021 at 14:18
  • $\begingroup$ @JuimyTheHyena Especially ring systems are probably short lived (read tens of millions of years) phenomena in general. The chance that the moon system will survive a capture event intact is miniscule. Maybe if the new moon has a really far out orbit near the Hill-spheres edge. We might posit a collision with one of the original moons instead. This could have enriched the original moons with metals and silicates. Additionally, the subsequent bombardment could have boiled of enoght water to make some inner moons surfaces relatively dry... maybe. $\endgroup$ Aug 16, 2021 at 15:03
  • $\begingroup$ TheDyingOfLight Ooooh! That's actually a pretty good idea...What if the migration and capture/impact happened relatively early during the formation of the solar system, causing a chain reaction that essentially caused the moon formation process to start anew as the giant entered the habitable zone? Eventually leading to the existence of a particularly massive earth-like moon and 10 other galilean moons? $\endgroup$ Aug 16, 2021 at 15:11
  • $\begingroup$ TheDyingOfLight Also I was speaking specifically about this: "So my guess is that the capture event will eject all but the innermost regular moons or one or two outer ones, who got into a resonance with the intruder early." Are you implying that the inner moons would be relatively safe in the case of a capture of a 0.7 times earth masses and stabilizing itself on an orbit about 1003000 kms away from the planet? $\endgroup$ Aug 16, 2021 at 15:14

Yes, this is definitely plausible, and an analogue has been explored in the context of our Solar System. The Nice model and its variants postulate that the four giant planets underwent a series of migrations based on unstable resonances. It's believed that these interactions could also have enabled the planets to capture planetesimals from the disk and retain them as satellites (Nesvorný et al. 2007)! You mention that the planet you're discussing would be one of several, so it's quite believable that it experienced a similar process.

As a side note: I had originally been worried about whether resonances could destabilize the orbits of moons during migration - and that's quite possible (Spalding et al. 2015)! On the other hand, it seems like it should primarily effect satellites with orbiting their planet at distances less than about 10 planetary radii (which your moons certainly exceed) and would be even less of a problem if the satellites were captured towards the end of the migration process.



The planet is massive, so you'll easily get tidal locking of the moon, that is the rotation of the moon's axis will be synchronous to the orbit duration (month) around the planet. For earth's moon, this duration is about 28 days, for your moon it will be a multiple of that.

The backside of your moon, turned away from the planet, will not get any sunlight for about half the moon's orbit duration, or full sunlight, the other half of the month.

The other side (turned toward the planet) would have no sunlight when the back side has.. and for a large part of the month, a solar eclipse occurs: the sun is covered completely by the huge planet.

I'm not sure about the inhabitable zone as a function of distance, but tidal lock could always present a problem for habitability by humans.. perceived day and night become very long !

On average, this moon will be very cold. There is no region inhabitable, because the "moderate zone" (or meridian) will also suffer from the lengthy solar eclipses, when the moon is behind the planet.

In order to be inhabitable, this moon should have

  • its own rotation and less solar eclipse: a considerable distance to its planet, allowing every part of the moon getting sunlight and warm up, and
  • a large orbit inclination angle i.r.t. to the planet's orbit around the sun. In case of tidal lock, you may have some narrow, butterfly-shaped zone with relatively moderate temperatures.

Moon formation scenario

Q: "the gas giant moved closer to the star and into the habitable zone as its moons were still forming, during this transit it would have captured a few massive bodies "

Suppose the above conditions are met, letting the moon have about the same daily cycle as earth and its orbit have a large inclination angle, solar eclipses will be rare.

Goldilock approach vector

When the small planet that becomes a moon is attracted by the big planet, it will proceed its orbit, but the vector of gravity will add to its movement.. in order to yield (eventually) a circular orbit, it should pass the planet with some kind of "goldilock vector": too close will mean the small planet will get a very elongated, elliptical orbit, too far away will just result in orbit disturbance and instability, it will not "take over" the planet easily. Between these cases, there should be a vector that results - in the end - in proper orbit. I've no means to put formulas or investigate..

Goldilock distance to the sun

The usual "Goldilock" condition also counts: distance to the sun. I started this answer worrying about the weather (temperature). I can add: at these masses/scales, it would require this giant planet to occupy a much closer distance to the sun. I wonder if a Goldilock-orbit would be allowed for this giant mass planet ? You could make it a blue star, or a heavier star with higher temperature, yielding some more energy..

  • $\begingroup$ Goodies I didn't ask for an assessment of habitability. I asked if the moon formation scenario is plausible, if a gas giant moon system to form like I described it. $\endgroup$ Aug 16, 2021 at 11:06
  • $\begingroup$ Ah read the question wrong ! please regard the above as an addendum.. and take into consideration that during moon formation, the orbit is also determined.. to keep your moon inhabitable, it should preferably get a wide orbit. $\endgroup$
    – Goodies
    Aug 16, 2021 at 11:29
  • $\begingroup$ Goodies What do you think about the hypothetical orbits that the moon would have? The ones I added in my addendum $\endgroup$ Aug 16, 2021 at 13:00
  • $\begingroup$ Ok, I have put the consequences of your new information, added some things about the formation of this - earth like - planet as an inhabitable moon. $\endgroup$
    – Goodies
    Aug 16, 2021 at 13:48
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    $\begingroup$ What you say about temperature is plainly wrong. The biggest factors will be the thickness of the atmospere and the distance from the star. Unless the atmospere is an order of magnitude thinner than Earths, heat will be transferred to the nightside. Oceans will health as well. The slow rotation rate means each hemisphere is a circulation cell, so the warm air can reach the high latitudes easily. Nights and days will be extremer than on Earth, more like mini-seasons, but life can adapt to such circumstances. Eocene-hothouse Earth had polar rainforests despite the long northern night. $\endgroup$ Aug 16, 2021 at 13:51

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