The Moon(s) Impact
Earth has a stable moon with no atmosphere, but other moons in both our solar system and beyond are volcanic, icy, or have toxic atmospheres. While our moon is relatively large in relation to the planet it orbits, and has a steady, close orbit, satellites orbiting other planets are often smaller, farther away, and have eccentric orbits. Most solar system objects orbit in the same direction, but some are backwards. This retrograde motion usually means a moon (or a planet) formed elsewhere and was captured by the planet (or sun). By contrast, a moon that formed when the planet did will orbit in the same direction. Large moons tend to match the direction while smaller ones can go either way.
Tidal Locking
The term “tidal locking” will make many of us think of tides, but these are unrelated phenomenon. Our moon is tidally locked to the Earth. The same side is always facing us because the moon rotates on its axis in the same number of days it takes to orbit us. This might seem coincidental and unique, but most significant moons in our solar system are tidally locked to their planet; those nearest experience this first. Tidal locking is an eventual result caused by gravity. Early in a moon’s orbiting, it might not be tidally locked, but ours may have become locked in as few as a hundred days (it’s proximity and size having much to do with this). A moon that is not tidally locked may have recently formed or been captured by the planet. Either way, the stabilization process hasn’t completed.
As world builders, we have some leeway to claim a satellite is locked or not. Most people are unfamiliar with the concept and we should only mention it if locking has occurred, as readers will assume the opposite without being told. Note that a close, large moon like ours will almost certainly be locked; during the brief period when ours was not, it and the Earth were molten and devoid of life.
Normally, only the satellite is locked to the planet, but they can become mutually tidally locked, as happened with Pluto and its moon, Charon. This means that each of them only sees one side of the other. If we stood on our moon, we’d see all sides of Earth as it rotates, but from Earth, we see only one side of the moon because they are not mutually tidally locked. If they were, the moon would stay in the exact same spot in the sky. About half the planet would see it, while the other half wouldn’t even know it existed unless traveling to the far side of the world. This would eliminate most tides (see next section) except those caused by the sun.
We can create a planet that orbits the sun in the opposite direction from other planets, a fact which would likely be noticed. In a less technological setting, supernatural significance might be attributed to this. In SF, perhaps inhabitants of that planet realize it originated from another solar system and wonder what it’s doing here. Where did they come from? We can also create a captured moon that orbits in a different direction than the planet or its other moons; these are typically farther away because gravity will eventually change that for nearer moons.
