# Is it possible to have four moon on different orbital elevations?

Could a planet that is twice the volume of the earth, have four moons but each orbital path is at a different angle?
Eg: M1 has a max elevation of 10’N and a min of 10’S
M2 has a max elevation of 17’N and a min of 17’S
M3 and so on…

Follow up question.

These values do not apply to the previous question.

How massive would a planet need to be for it to have three moons.
One 3/4 the size of Luna
Two of equal sizes that are in a binary orbit with a combined mass of 1/2 that of Luna

A mass of 1,25 Luna.

• How large of moons are we talking here? If they're Phobos- or Deimos-like, probably. If they're like Earth's moon, that's a more dubious prospect. Oct 27 '21 at 15:02
• How old would you like the planet to be? An older planet, with older moons, will have flatter elevations.
– Pod
Oct 27 '21 at 15:42
• Would it be possible for the same planet to have one set of binary moons of roughly equal size? Oct 27 '21 at 15:49
• @PrinceThomasthe42nd for that question check out worldbuilding.stackexchange.com/questions/86456/… Oct 27 '21 at 15:58
• .. and 4 moons with a different inclination would be possible, when these moons were caught in the planet's gravitational field, in recent - geological - times ! Agree with other remarks: the 4 moons should not be too heavy and eventually, they will stabilize into the equatorial plane (rotational plane) of the planet. Oct 27 '21 at 16:02

## 3 Answers

[Added 10-28-2021

The terrestrial type planets Mercury and Venus do not have natural satellites. It is believed that they re two deep in the Sun's gravitational well to have large areas where moons could have stable orbits. Any moons formed or cputred outside their narrow range of stable orbits would have crashed into the planet or escaped into space long ago.

Earth and Mars obviously can have moons in stable orbits since they have moons. The size and number of moons does not seem to be determined by the characteristics of the planet.

The origin of Earth's moon is uncertain. But the main theory now is that it formed from a ring of debris resulting from a collision between Earth and another planet early in the history of the solar system.

I note that the orbit of the Moon is inclined by a number of degrees compared to Earth's equatorial plane.

Unlike most satellites of other planets, the Moon orbits closer to the ecliptic plane than to the planet's equatorial plane. The Moon's orbit is subtly perturbed by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbit gradually rotates once every 18.61 years,[149] which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini's laws.[150]

The Moon's axial tilt with respect to the ecliptic is only 1.5427°,[8][151] much less than the 23.44° of Earth.

https://en.wikipedia.org/wiki/Moon#Orbit

So the Moon's orbital plane seems to be tilted about 21.89 degrees compared to Earth's equatorial plane.

The origin of Mars's two tiny moons is even less certain.

Several theories are mentioned at:

https://en.wikipedia.org/wiki/Moons_of_Mars#Origin

Going by the evidence, I would say that a terrestrial planet could have zero moons, one moon, two moons, and so on, posssibly up to a few dozen moons. And the sizes of those moons could vary between tiny ones like the moons of Mars up to large moons several times more massive than Earth's Moon.

I doubt whether an Earth like terrestrial planet could have many large moons. I doubt whether the total mass of all the moons combined would exceeed 10 or 20 percent of the mass of the planet (and it would usually be much less). The mass of the Moon is 0.012300 that of the Earth, and the mass of Charon is 0.122 that of Pluto.

"Regular moons" would have formed out of a disc of tiny objects orbiting the planet above its equator. And they would continue to orbit in the same plane above the planet's equator, unless something happened, like a large astronomical body passing close to the planet and the moons and changing the orbits of the moons. Some of the moons might be ejected from orbit but some might remain in changed orbits around the planet.

After such an event which changed the orbital planes and the eccentricities of the orbits of the moons, tidal interactions with the planet would gradually circularize their orbits and bring them back into the eqatorial plane of the planet. But that would taka lot of time and might not be completed by the time of your story. The question asks for relatively minor differences in the planes of the moon's orbits.

In astronomy, an irregular moon, irregular satellite or irregular natural satellite is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They have been captured by their parent planet, unlike regular satellites, which formed in orbit around them. Irregular moons have a stable orbit, unlike temporary satellites which often have similarly irregular orbits but will eventually depart. The term does not refer to shape as Triton is a round moon, but is considered irregular due to its orbit.

https://en.wikipedia.org/wiki/Irregular_moon

And it has been suggested that the two tiny moons of Mars are "irregular moons", captued asteroids.

The origin of the Martian moons is still controversial.[30] Phobos and Deimos both have much in common with carbonaceous C-type asteroids, with spectra, albedo, and density very similar to those of C- or D-type asteroids.[31] Based on their similarity, one hypothesis is that both moons may be captured main-belt asteroids.[6][32] Both moons have very circular orbits which lie almost exactly in Mars's equatorial plane, and hence a capture origin requires a mechanism for circularizing the initially highly eccentric orbit, and adjusting its inclination into the equatorial plane, most probably by a combination of atmospheric drag and tidal forces,[33] although it is not clear that sufficient time is available for this to occur for Deimos.[30] Capture also requires dissipation of energy. The current atmosphere of Mars is too thin to capture a Phobos-sized object by atmospheric braking.[30] Geoffrey Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated under tidal forces.[32]

https://en.wikipedia.org/wiki/Moons_of_Mars#Origin

So the orbits of Deimos and Phobos are in the equatorial plan eof Mars, and it is not certain whether there has been enough time for the orbit of Deimos, the farther moon, to be tidally dragged into the equaorial plane of Mars.

So you should put your innermost moon at the distance of Deimos, adjusted for the stronger gravity of your planet compared to Mars, and then make it a few times farther away just for safety, and put the other moons farther away still.

And if you want the moons to appear as objects instead of points of light, you should make them at least several times as large as Deimos and Phobos.

End of 10-28-2021 addition]

I question the size of the planet, described as having two times the volume of Earth.

A planet with 1.2599 times the radius of Earth will have 1.9998997 times the volume of Earth. A planet with 1.26 times the radius of Earth will have 2.000376 times the volume of Earth.

According to Habitable Planets for Man, Stephen H. Dole, 1964, humans have little tolerance for long term exposure to high g forces.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf

Chapter 2, Human Requirements has a section on gravity on pages 11-13. Dole decides on page 12 that:

On the basis of the available data, one might conclude that few people would chose to live on a planet where the surface gravity was greater than 1.25 or 1.5 g.

Chapter 4, The Astronomical Parameters, starts off with Planetary Properties on page 53, begining with the planet's mass.

For a nearly spherical, slowly rotating, terestrial planet, the relationship betweeen mass, radius, surface gravity, and velocity of escape are shown in figure 9 (see page 31). Now it will be recalled that, to be considerd habitable, a planet must have a surface gravityof les than 1.5 g.. From figure 9 it may be seen that this corresponds to a planet has a mass of 2.35 Earth masses, a radius of 1.25 Earth radii, and an escape velocity of 15.3 kilometers per second. These represent upper limits on the size and mass of a habitable planet. It is assumed that other limitations due to incresing mass have not occurred first, such as the surface being completely covered with water, or the atmospheric density being so high as to produce oxygen toxicity or nitrogen narcosis, or the atmosphere being so opaque that sunlight cannot reach the surface with intensity levels high enough for effective photosynthesis.

Note that it is the size of a planet with a surface gravity of 1.5 g, and that a planet with two times the volume of Earth would have a slightly higher size, above Dole's limit for a planet with 1.5 g surface gravity.

And note that Dole earlier wrote that the maximum surface gravity for planets which humans might want to colonize, or spend long times visiting, should be somewhere between 1.25 and 1.5 g. It is perfectly possible that in real life humans wouldn't want to live on, or even visit for long times, planets with surface gravity as high as 1.45 g, or 1.32 g, or perhaps anything higher than 1.25 g.

And of course planets with lesser surface gravity than 1.5 g would have smaller volumes than 2 times the volume of Earth.

You could always have your planet made out of slightly less dense matter on the average than Earth. But a planet which was not dense enough would not be habitable for humans. So complex calculations would be necessary to find out if a planet with twice the volume of Earth but a low enough surface gravity would have be a plausible average density.

Alien life forms that evolved on a planet would have adapted to its surface gravity, so it seems possible to me for a planet to have a surface gravity somewhat above 1.5 g and have a native civilization, evne if humans wouldn't want to live there.

But humans wouldn't want to stay there long without some type of anti gravity.

And I have found another limitation on the mass and volume of a habitable planet which I will describe later.

There has been a lot of scientific discussion about the possible habitability of planets in other star systems.

I note that the biosphere of Earth, where Earth life forms can be found, extends miles & kilometers high in the atmosphere and kilometers & miles deep beneath the sea and beneath the ground. So a human magicallly teleported to a random place in the Earth's biosphere would probably die instantly.

And if a human was magically teleported to a random location on the surface of the Earth they also would probably die swiftly, since over seventy percent of the surface of the Earth is water outside of sight of land.

And an unwarned and unprepared human magically teleported to a random spot on the surface of the Earth might also die swiftly since not all parts of Earth's surface are survivable by unprepared humans.

At the present time most life forms on Earth use and need the highly reactive atmospheric gas oxygen and would die without it. But some present day life forms don't need oxygen. Life on Earth existed for billions of years before various life forms produced atmospheric oxygen, and when oxygen became common in the atmosphere it poisoned and killed off most of the life forms, which were unprepared to survive such a reactive gas.

Life forms with the same environmental requirements as humans are a minority of past, present, and future life forms on the planet Earth.

So when astrobiologists discuss the possible hatability of other worlds, they usually discuss habitability for liquid water using life in general, and not for life forms with similar environmental requirements to humans in particular. If some factor makes a world uninhabitable for liquid water using life forms in general, it will not be habitable for humans in particular.

Astrobiologists, like science fiction writers before them, wonder whether hypothetical planetary sized exomoons of giant exoplanets orbiting other stars might be habitable. Habitable exomoons might possibly be a large percentage of habitable worlds in general.

One article discussing the subject is "Exomoon habitability Constrained by Illumination and Tidal Heating", Rene Heller and Roy Barnes, Astrobiology, Volume 13, Number 1, 2013.

https://faculty.washington.edu/rkb9/publications/hb13.pdf

In Section 2. Habitability of Exomoons, page 20, they discuss the possible mass range of habitable exomoons - which should probably be the same mass range as that of habitable exoplanets.

A minimum mass of an exomoon is required to drive a magnetic shield on a billion-year timescale (MsT0.1M4; Tachinami et al., 2011); to sustain a substantial, long-lived atmosphere (MsT0.12M4; Williams et al., 1997; Kaltenegger, 2000); and to drive tectonic activity (MsT0.23M4; Williams et al., 1997), which is necessary to maintain plate tectonics and to support the carbon-silicate cycle. Weak internal dynamos have been detected in Mercury and Ganymede (Gurnett et al., 1996; Kivelson et al., 1996), suggesting that satellite masses > 0.25M4 will be adequate for considerations of exomoon habitability. This lower limit, however, is not a fixed number. Further sources of energy—such as radiogenic and tidal heating, and the effect of a moon’s composition and structure—can alter the limit in either direction. An upper mass limit is given by the fact that increasing mass leads to high pressures in the planet’s interior, which will increase the mantle viscosity and depress heat transfer throughout the mantle as well as in the core. Above a critical mass, the dynamo is strongly suppressed and becomes too weak to generate a magnetic field or sustain plate tectonics. This maximum mass can be placed around 2M4 (Gaidos et al., 2010; Noack and Breuer, 2011; Stamenkovic´ et al., 2011). Summing up these conditions, we expect approximately Earth-mass moons to be habitable, and these objects could be detectable with the newly started Hunt for Exomoons with Kepler (HEK) project (Kipping et al., 2012).

The article uses the astronomical symbol for the planet Earth to express masses as fractions or multiples of the mass of Earth. However, the symbol for the planet Earth is shown as "4" in this answer for some reason. So where you see a number followed by M4 it means that number times the mass of Earth.

So they claim that a moon or planet with more than about 2 times the mass of Earth would have its internal dynamo greatly weakened, which would elimnate the world's magnetoshphere and plate tectonics, both considered to be essential for habitability.

If a world with 2.35 times the mass of Earth would have a radius of 1.25 Earth radii, and thus a volume 1.953125 times Earth's volume, a bit less than 2.0 times the volume of Earth, a world with only 2.0 times the mass of Earth would have an even lower radius and and even smaller volume, which would be much farther from being 2.0 times the volume of Earth.

And that is considered to be an upper mass limit for any world which could support liquid water using life in general, which of course includes, but is not limited to, life forms with environmental requirements similar to those of humans. And of course the majority, though not all, of the worlds in science ficiton are interesting because they are habitable for humans or for beings with similar requirements.

And thse are the articles which Heller and Barnes cited as the sources for the claim that the upper mass limit for a habitable world would be about 2 times the mass of Earth:

Gaidos, E., Conrad, C.P., Manga, M., and Hernlund, J. (2010) Thermodynamics limits on magnetodynamos in rocky exoplanets. Astrophys J 718:596–609.

http://seismo.berkeley.edu/~manga/gaidosetal2010.pdf

Noack, L. and Breuer, D. (2011) Plate tectonics on Earth-like planets [EPSC-DPS2011-890]. In EPSC-DPS Joint Meeting 2011, European Planetary Science Congress and Division for Planetary Sciences of the American Astronomical Society. Available online at http://meetings.copernicus.org/epsc-dps2011.

https://ui.adsabs.harvard.edu/abs/2011epsc.conf..890N/abstract

Stamenkovic´, V., Breuer, D., and Spohn, T. (2011) Thermal and transport properties of mantle rock at high pressure: applications to super-Earths. Icarus 216:572–596.

https://d1wqtxts1xzle7.cloudfront.net/66412942/j.icarus.2011.09.03020210421-11730-1s4qdlh-with-cover-page-v2.pdf?Expires=1635390149&Signature=LJ5HxN2WQPOUCw5SgNGTr4x1-k4FcMHoCdffRGSbChhM2-1GlZ~5RXZZ42VP6CIFpSeZb90me37Agn3MX~cI6MotM2plH0sjql8fv5PPWTEFjyR~mNgGR6qLkBV0FRUF2FgyuBLEyJ2VKaGxGo2p5EFBfRowJ4XhJGlG8uyjM2J9U6NSPBzvGNkLPxPVc6ovyrLSkJ3peIBPXrFNB5ynVIicLp5VncITbGKXoUM91rQgJqu0iKPPACSL3pk9BcU0YtPPiqrU60SHIOiJGLaTA1t3RLeBnPXHdvEZEAkvwTj14uEZiqujVB8-ZVT-cv64LP0vjH-EkoNfNUJzsHgjTQ__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA

Thus it seems to me improbable for a planet habitable for humans and for beings with similar environmental requirements - and thus suitable for most science fictional purposes - to have a volume 2.0 times that of the Earth.

• "the size of a planet with a surface gravity of 1.5 g," Hmmmm... what if the planet has a deficit in iron? Like anemia low level of of iron and heavier metals? It may still have a smaller iron core, spinning faster; after all, so many moons with different orbital inclinations suggests a young planet. Oct 28 '21 at 1:01
• @AdrianColomitchi An iron-poor planet would probably not be a good place to develop Earth-like life. We use a lot of iron in our biochemistry, and if all (or the vast majority) of that iron is bound into the planet's core then it's not going to be available for life. Unless we're talking about an artificial planet? Oct 28 '21 at 1:26
• @Corey a good amount of trace materials arrived on Earth well after the crust solidified - otherwise we (the Earth's humans) wouldn't have tungsten, uranium, thorium at all. Besides, there's a lot less iron in human than, say, calcium. Granted, the technology there may have to do with less steel and more aluminium, titanium, vanadium and the like. Oct 28 '21 at 1:47
• @Adrian Colomitchi I don't know whether you have seen my additions to my answer, additions which I posted just a few minutes ago around 10 pM EST, 10-27-21. They suggest other problems with a mass as high as 2.0 times the mass of Earth. Making the planet less dense to have a lower mass might enable it to have 2.0 times the volume of Earth, but I don't know if that is possible or probable. As for setting a story on a young planet, that woun't work if the planet is supposed to have an oxygen rich atmopshere and thus be billions of years old. Oct 28 '21 at 2:26
• @AdrianColomitchi There could be areas with (relatively) high amounts of iron available, but subduction will deal with those pretty quickly, geologically speaking. And while the actual amount of iron in a human body is small, you'd need some ultra-efficient iron binding flora to get enough iron in the food chain if it were a trace element in the environment. How would that evolve in an effectively iron-free environment? Biology would be vastly different I think. Oct 28 '21 at 3:17

Gravitational interactions would end up flattening the orbit on a single plane, for the same reason why galaxies, accretion discs and Saturn rings end up being disc shaped.

For naturally formed moons this would happen early in their formation stage, for captured moons instead the configuration won't be stable in the long term.

• +1 agree but a question.. " for captured moons instead the configuration won't be stable" are you sure ? why ? maybe it depends more on weight ? small moons won't easily disturb eachother's orbits.. Oct 27 '21 at 16:09
• Gravitational interactions would end up flattening the orbit on a single plane, except that it may take a veeeeeery long time. Looking in our own solar system, we find that Saturn for example has a large satellite, Iapetus, at 15° inclination, while most others are at about 0°; and Neptune has a large moon, Triton, on a retrograde orbit inclined at 23°. Oct 27 '21 at 17:03

In our own solar system:

• Most large satellites of Saturn have orbits at just about 0° inclination, as expected, but the orbit of Iapetus has an inclination of 15°.

• Most large satellites of Neptune have orbits at just about 0° inclination, as expected, but the orbit of Triton is retrograde with an inclination of 23° (or 157° as the astronomers prefer to say).

• All the gas giants have numerous small satellites with orbits with all sorts of inclinations. See satellites of Jpiter, satellites of Uranus, satellites of Neptune and satellites of Saturn.

If the gas giants can, any reasonably large planet can, at least in principle.