A hospitable planet with seven moons

Tentatively, a planet in a saga of mine is planned to have seven moons. Each represents the final resting place of a lesser deity of the story's cosmology. Also, the seven moons aligning in a certain way is a plot point one of the villains wants to utilize (something something singularity).

Now, I know moons and their gravity affect winds, seasons, and tides of the planet they orbit around. Both the size of the planet itself and the moon(s) also play a factor. For example, the more moons, the bigger the planet needs to be — big magnetic field to keep the lunar orbits intact and yadda yadda.

How drastic would seven moons theoretically affect a single planet? Would, like, 70% of the surface be inhospitable due to strong winds rubbing down land and eclipses reducing temperature? Would bodies of water be all over the place? Would the planet itself be perfectly round depending on the push-pull of the moons' gravities or look like a potato?

Anything I'm overlooking?

Assume there's one sun.

Are these seven moons the size of Earth's, or are there one or two major moons with some asteroids scattered about?

Umm... well, first, the magnetic field of a planet has nothing to do with its moons. You are thinking of its gravity. And yes, you need a planet with high gravity to hang onto a lot of moons. This is because any system with a lot of orbiting bodies of similar size is inherently unstable due to what is called the "three body problem".

You can mitigate some of these effects if the parent body is overwhelmingly dominant. For example, the Earth has one moon, but thousands of artificial satellites, launched there by us Puny Earthlings. These don't throw each other out of their orbits because they are so incredibly small compared to the planet. But if we had more than one moon of comparable size, one or both would either get flung out of orbit or crash into Earth.

Jupiter and Saturn have vast numbers of moons, but they're huge. The mutual gravitational attraction between, say, Europa and Io, is trivial compared to Jupiter's influence. Even so, they do develop orbital resonances after a long time. Mars has two moons: Phobos and Deimos, but they are tiny: probably captured asteroids that aren't even big enough to be spherical, and both of them are going to fall to the surface eventually.

For any multi-moon system to be stable, it must therefore follow one of these two rules: the moons must be so small that they exert trivial gravitational influence on each other compared to the planet's influence on them; or the moons must develop orbital resonances. These resonances become less likely the more moons there are.

Edited by Fighteer on Sep 5th 2020 at 7:02:36 AM

Honestly, to be able to hold seven Moon-sized moons, your planet would need to toe the line between Terrestrial and Neptunian. Maybe if the planet were about 7x more massive than Earth, it would be able to hold two or three.

Actually, Deimos is more on the path to escape Mars. Phobos is now speculated to be a third-generation moon, and is expected to about half-break up into a ring system within a few hundred million years before coming back out again.

Nah, I don't plan for the seven moons to be that big.

So, basically, the parent planet needs to be big enough (and/or the moons small enough) or have a strong-enough field of gravity to keep the seven lunar orbits in harmony?

Edited by erazor0707 on Sep 5th 2020 at 7:07:25 AM

Sorry, right. I just know that Mars' moons aren't in stable orbits.

Essentially, yes. For an Earth-sized planet, these moons could not be much larger than Phobos or Deimos and remain stable. You could have one big one and a bunch of minor ones, of course. In fact, the Earth occasionally captures a Trojan asteroid (the ones in weird orbits around the space between Mars and Venus) and has two or more moons for a while. They aren't noticeable except to very persistent astronomers, though.

Edited by Fighteer on Sep 5th 2020 at 7:09:46 AM

I think Mimas-like moons might be feasible.

So, let's assume the seven moons idea is a go.

Now, about how they'd interact with their planet. Like how the Earth experiences day on one side and night on the other, would this theoretical planet have similar variances?

I would presume that lunar tides on the surface of this planet would be negligible. It is too massive in comparison for the moons to have any noticeable effect. Solar tides would probably affect it more.

Not sure what you mean about variances. Smaller moons would have minimal tidal effects, and the planet would have smaller tidal effects on them as well. Earth's moon is very gradually slowing the planet's rotation... over many billions of years our day will be equal to one month, and the month itself will become longer as the Moon drifts farther away (due to conservation of angular momentum). However, this won't happen until long after the Sun turns into a red giant and (probably) eats us.

A set of smaller moons wouldn't noticeably affect the tides or the lengths of the day unless they were very close, enough that they would eventually break up.

We could assume that a few moons got close enough that they broke up and formed rings. Those moons would be more likely to affect potential rings than the water tides.

I guess I should ask how similar and different would a hospital planet with seven moons be to our Earth?

Not very different. If it were Earthlike in all other parameters, then the main difference would be... seven moons instead of one. These moons would necessarily be much smaller and therefore less visible than our one Moon. The cultural differences in any civilization evolving on that world would be interesting.

There are other potential consequences. The lack of tides might well lead to a very different evolutionary path, as well as affecting erosion, water cycles, and other parameters. I am not nearly knowledgeable enough to give a detailed dissertation on that topic, however.

Edited by Fighteer on Sep 5th 2020 at 7:40:35 AM

Hm. Might need to do a bit more research on front then. Sounds like I'd need to look solar tides to compensate.

But, it sounds like year/month/day (and thus age and time) won't be terribly affected.

For one, there are seven quick-moving objects in the sky instead of one. Such a planet would have to be much larger than Earth—but not just the size of an ice giant—to be able to hold onto those seven moons. They would also be much smaller in the sky than Earth's moon, due to all of them being much smaller than Earth's moon. There wouldn't be significant tides, and lunar lineups would be extremely rare. Some of the moons may not even be discovered until astronomers look to the skies if they're small enough. And who's to say that these Magnificent Seven couldn't possibly capture an eighth moon, probably a retrograde one?

If solar tides are that much more significant, then you'll have to deal with tidal locking from the parent star. Also, it implies that the star is large enough or close enough to perturb these moons. There's a reason Venus and Mercury don't have any. The Sun would steal them in short order.

However, assuming this planet is a Super-Earth, it likely has company. Planets like this usually tend to form in tandem—if you have a Neptune-sized planet, expect to see a bunch of Super-Earths. Also, compact systems like TRAPPIST-1 and Kepler-90 are more common than spread-out systems like our Solar System. So it is entirely possible that, assuming the star is completely sunlike, there could be tons of Super-Earths and Neptunes in the region, all with their own moons.

Sure, but if these things have double Earth's mass (hypothetically), that's also nearly double the gravity (radius is higher, so not quite double) and so they would be very different places. The predicate for the topic was that this is a relatively Earthlike planet.

I would think holding onto these multitudes of moons would require at least 3 Earth masses, assuming a logarithmic configuration.

I also think that a Super Earth would need different orbital parameters. At the same distance from a Sun-like star, such a planet would have a lot more internal heat and a much thicker atmosphere. It would probably be more like a Super Venus.

Wouldn't that burn up everyone on it though?

Not if it suffered a giant collision in its past that stripped it of a chunk of its atmosphere, which could have formed its moons.

In a nutshell, seven large moons around a terrestrial planet is simply impossible. Simulations have shown that they cannot hold on to even two moons before orbital instability causes them to be ejected or crash into the planet. Seven moons would just be a chaotic fireworks frenzy on a Detonation Moon scale.

Well, since you said you want the moons to be final resting places of gods, you can just Hand Wave it with A Wizard Did It anyway and no one will bat an eye. Fantasy works don't have to be hardwired to real life science to work, just add magic.

Ha, two world building threads at once? Seems like it's my lucky month. Anyhow, a few comments:

• I think the biggest issue with seven moons is that their orbits might destabilize themselves through tidal/resonance interactions. That would be a function of their mass (the heavier, the less stable) and whether they enter resonances (hard to predict).
• I don't have the math at hand but I am pretty certain that the effect of eclipses on the planet's heat budget are negligible, certainly in comparison to tidal heating. Remember, the shadow of Earth's comparatively large moon only covers very small areas of Earth.
• On the flip side, I am not sure that tides would appreciably alter atmospheric circulation and temperatures. If memory serves, tidal heating decreases with decreasing viscosity, so the atmosphere will always receive much less tidal heating than the planet's interior. Tides may have effects on atmospheric circulation but since (see below) insolation is much more than tidal heating they won't be very strong.
• Size alone isn't going to increase heat flow of a planet appreciably and certainly not to the point of cooking the planet. According to this publication heat flow increases linearly with mass but on Earth it's less than 100 mW/m2. Even on Io the surface heat flow is only about 2W/m2, which is less than 1% of Earth's insolation. As I said in TheWhistleTropes's worldbuilding thread, even a heatflow 8 times larger than Io's would only match the radiative forcing of the Permian-Triassic mass extinction but would fall well short of "Tidal Venus" levels (that's the technical term for planets/moons that are cooked by geothermal heating).
• The big concern of a planet w/o moons such as a Moonless Earth is that the axial tilt of this planet may fluctuate wildly. There isn't a scientific consensus on this question, though.

Regarding the size of the moons, and building on one Divine Flame 100 said, above, if there are supernatural elements at play then one potential idea might be for the moons to be hollow.

This might then allow for low-density—and thus low -mass and -gravity—moons that are nevertheless large enough to be easily spotted in the sky.