So we all see worlds in fiction with two moons, two suns, or a large planet in the sky with rings like Saturn. My own world, Atheles, has two moons which play a part in the magic of the world. So I went to my friend and scientist Ethan Siegel to have him answer some questions to see what it would be like to live in a world with extra celestial bodies.

Multiple Moons

So Atheles has two moons, named Unsere and Lunaris. I have heard opinions that living on a planet with two moons would be too destructive to be habitable. Is that true? What would it be like living in a planet with two moons?

The first thing you have to realize is that, if you live on a rocky world that has moons at all, they likely originated way back in the earliest days of your solar system, from a giant collision between your world and a proto-planet that didn’t quite make it. The outer layers of your world — the crust and mantle — likely got kicked up into space, where parts of it fell back to your planet, other parts got blown off into space, but still other parts coalesced into one or more moons surrounding your planet. This happened in our own solar system with Earth, forming one very large moon; with Mars, forming one large moon (which has since fallen back down onto the Martian surface) and two small moons (which remain); and with Pluto, forming one very large moon (Charon) and four smaller, more distant moons.

In all of these cases, the moons you create wind up close to the planet’s surface and with relatively fast orbital period: ranging from mere hours to a few weeks. If the planet is either relatively far from the star or the moon in question is on the large size, you’ll wind up with periodic eclipses. In some cases, such as with Pluto’s moons, you’ll get the shadow of one moon falling, at times, onto another moon. Alignments can be commonplace, and if you wish for such events to play a part in the magic of your world, this could serve as an excellent trigger, perhaps by a very ancient magic that enchanted the object that collided with your planet all those billions of years ago!

From conventional physics, however, there’s very little to fear. If you can survive the effects that the largest, closest moon incites on your world — mostly in the form of tides, but possibly large enough to create earthquakes and/or volcanoes — the outer moons are only going to have small (a few percent) additional effects compared to the major moon. However, the potential effects that the planet’s gravitation would have on the innermost moon should not be understated. If that moon ventures too close to the planet’s surface, it can be broken apart by tidal forces into debris, creating a temporary “ring” of matter, before having those pieces fall down to your planet’s surface. This rain of fire could not only be deadly, but large chunks could create huge craters and potential kingdom-threatening or even world-threatening events. The closer a moon is to your planet, the faster its orbit is, so beware of (or be excited for) a world with frequent, fast, shadowy eclipses.

Eclipses, synchronized full moons, or lining up with other points in the sky plays a part in the magic of Atheles. With two moons, how often would these events happen?

The moons that are created around rocky worlds, naturally, normally all form in approximately the same orbital plane. This means that the paths of these moons and their associated eclipses can reliably be expected to line up to within one degree, as both of Mars’ moons and all five of Pluto’s moons do. Their orbital periods should also be in an approximate resonance with one another, where the amount of time it takes any two moons to orbit should be in integer ratios to one another. (For example, Phobos and Deimos orbit in a 1:4 time ratio, while Pluto’s five moons, Charon, Styx, Nix, Kerberos, and Hydra, are all in a 1:3:4:5:6 ratio.) This means, if a special alignment occurs, it should occur periodically, which is something see in our planets with multiple moons. Phobos and Deimos will line up every 31 hours or so, while all five of Pluto’s moons only line up once every 383 days.

In general, if you want any number of moons to line up, and you know the resonance ratios of the moons, find the lowest common multiple of all those moons, and then multiply by the orbital period of the fastest, innermost moon. If you add a very distant moon with a large orbital period, it’s a great, easy way to create a very rare alignment! It’s also possible to have distant, outer rings around a rocky world, so long as the rings are more distant than the outermost moon; there actually is a minor planet in our solar system between Saturn and Uranus — Chariklo — that’s been observed to have two independent faint rings!

We’re also very used to seeing the same face of our moon no matter where or when we look at it from Earth, which is an effect of tidal locking. However, if you have two large masses, like a planet and a large moon, then all the outer moons won’t be tidally locked, but rather will tumble chaotically through space. If you scaled your world system up, placing your civilization on a rocky moon orbiting a gas giant that also contained a giant, inner moon, your world would have chaotic nights, days, seasons, and axial tilt. You would be unable to predict, for example, when a given season would arrive, or how long it would last.

One of the moons, Unsere, is phasing in from another dimension. It has slowly been phasing in over a millennium. The other universe is getting closer to Atheles, and that is where the energy for magic is drawn from, slowly destroying this other universe. Is an object from another dimension able to “cross over” and if so, would a large object like a moon would cause major changes as it shifted over?

Building a fantasy world requires that you’re open-minded about placing a mix of science and magic in your world, but also consistent. The laws of physics are known and the same as they are in our Universe, but there are additional rules that apply because of the presence of magic. Designing your fantasy world means that you get to set the rules of what that magic is allowed to accomplish, but it cannot be arbitrarily powerful, the rules cannot change without a good reason, and the laws of physics will continue, unchanged, irrespective of magic.

With that said, we can claim that a moon like Unsere phases in, and when it does, depending on its size, mass, and density, it can have powerful gravitational effects on the other moons or even the planet itself. The most major change it could cause would be in the orbit of an existing moon. In a more minimal case, that moon would simply migrate a little farther out or closer in from its current orbit. But in the case of a larger effect, a moon could either:

Be ejected from the planetary system entirely,

Be thrown into orbit around your Sun, where it risks colliding with your home world on timescales of months-to-years,

Be thrown on a collision course with the Sun, where potentially deadly flares and “solar belches” might threaten your world,

Or caused to collide with either another moon or with your home world, which could be an extinction-level threat in either case.

There are other effects that will be at play, but they will almost always take place over astronomical (i.e., million-year or more) timescales, and therefore are of reduced interest to world-builders for fantasy campaigns.

Multiple Suns

We take for granted, here on Earth, that our planet orbits the Sun while simultaneously rotating on its own axis, and that the length of a day is very different (and faster) than the length of a year. But most of the stars in the Universe are not Sun-like stars, but are the much smaller and cooler variety of star: a red dwarf. Planets around these systems become tidally locked very quickly, and always have the same side of their planet face their Sun. They’d have three climate zones: a sun-baked side where it was always day, an eternal-night side that never faced the Sun and was always frozen and dark, and a potentially habitable “ring” where it was always on the border of night-and-day. While these red dwarf stars might not make excellent homes for human beings to put a planet around, they remind us that there’s more to the Universe than just single-star systems.

In fact, our solar system may be a relative rarity. Most star systems observed have more than one star, and may be binary systems, trinary systems, or have even more stars! (Mizar and Alcor, a double star in the Big Dipper, is actually a sextuple star system.) In general, there are two ways to have planets around a multiple-star system:

Have the system be a tight binary system. This is where you have two stars — either similar in mass or with one much larger than the other — orbiting one another closely in the inner solar system, with the planets a relatively large distance away. Both suns would appear relatively close in the sky to one another at all times, and you would have a “big sunset/sunrise,” where the brighter star set or rose, and a “small sunset/sunrise,” where the dimmer one did the same. If they’re comparable in brightness, shadows would only exist when the silhouettes created by both stars overlapped. Have the system be a wide binary system. This is where your planet primarily orbits one star, but another star exists a larger distance away, perhaps where the orbit of a gas giant world is presently located in our solar system, if not further. In this case, we’d experience days and nights as normal, except there would be an additional ultra-bright light source in the sky, significantly brighter than the full moon but always much dimmer than the major sun. At certain times of the year, both stars would align in position, but at other times, they’d be anti-aligned, creating a season where the entire world was illuminated all hours of the day.

Many star systems exist that are combinations of these, where there is a central binary star system and then another one or even two wide systems, both of which may be binaries. It’s important not to have the planet be equidistant from or transfer between two members of a wide binary system; those configurations are unstable and will cause your planet to be ejected into interstellar space!

The biggest difference, from a practical perspective, would be in what day and night is like, as you’d have a “major day” governed by the brighter sun, and a “minor day” governed by the fainter one. These days will almost completely overlap for a tight binary system, but will change throughout the year in a wide binary system. It’s conceivable that different character classes, when you introduce magic or special powers into your world, could draw their power from different stars, making the time of day (or time of year) crucial in determining the success or failure of your party!

A Nearby Planet

Why settle for one planet when the universe can easily support multiple close ones? Here in our solar system, the planets are all well-separated, with all eight of them never crossing orbits, throwing one another out-of-whack, or switching places. But such a configuration is eminently possible!

First off, unlike our own solar system, many known systems exist that have gas giant worlds in the inner solar system, even in the potentially habitable zones. If these gas giants possessed one (or more) large moons, there might be multiple worlds that aren’t too distant from one another that might be teeming with similar-to-vastly-different forms of life. Rather than distances of tens of millions of kilometers, such worlds might come within just tens of thousands of kilometers. If one world can launch something into space, interplanetary messaging, transfer of materials, or even war might be possible.

Another incredible possibility is that your home world may have an orbit-swapping companion world. While this may sound unstable to you, our solar system has a classic example: the twin moons Janus and Epimetheus of Saturn. One will orbit interior to the other for a time, then they’ll switch positions, and then after another amount of time, they’ll switch back. These two worlds have been going back and forth for at least hundreds of millions of years, and are in a configuration so stable that it may last until our Sun dies. If you were simply on a rocky planet orbiting your sun (or suns), and you had another companion world that you swapped orbits with, a collision or catastrophic interaction may not be inevitable. But the threat of it, no matter how unlikely, will always be there. When a close pass occurs, your world will be at a much increased risk of natural disasters, including earthquakes, volcanoes, geysers, and other geologically-triggered catastrophes.

Finally, you may want to consider the idea of a double-planet. In the most extreme case of a large, ancient impact, one can envision a case where the original body is split into two approximately equal masses. These two masses can then orbit their mutual center of mass at distances that are perilously close to one another. The closer they are, the more distorted their shapes will be, and they will begin orbiting each other very rapidly the smaller the distances between them are. In the most extreme scenario, the two worlds will begin to touch — first atmospheres, then surfaces — as they begin the long, slow, but catastrophic process of merging together. This final scenario is not for the faint of heart, as geological disasters are not only inevitable, but the extreme tidal forces may eventually wind up coating the surface of both worlds in lava!

You can follow Ethan Siegel at his blog, It Starts with a Bang, and at Forbes. You can also support him on Patreon.