There are global issues to consider in creating a planet, such as moons, climates, the sun, and the rest of the solar system. Unless we’re planning to do something truly unique with any of these, like two suns or moons, we’ll probably want to go with an Earth-like situation, which is the focus of this chapter.
However constant celestial bodies appear to us in our limited lifetimes, their relationships are always in flux. We can invent a scenario that isn’t likely in a four-billion-year-old solar system, but which is possible in a two-billion-year-old one. The reason is that gravity slowly stabilizes relative positions. For example, gravity causes satellites to eventually orbit in the same plane. If we have them in two different planes, we’re implying the solar system’s youth. Either that, or one of those moons was recently captured by the planet and hasn’t stabilized with it yet. By “recently,” we’re talking in billions of years, meaning this happened well before our species crawled out of the ocean.
This may give us some leeway to invent a situation lasting long enough to tell our story, at the risk of eye-rolling by physicists. One goal of this chapter is to reduce such reactions and make reasonably informed decisions. Fortunately, physicists make up a relatively small percentage of Earth’s inhabitants. Unfortunately, they make a far higher percentage of science fiction readers!
Some ideas and terms apply to more than one section of this chapter. The alignment of three or more celestial bodies into a straight line is a called a syzygy. An eclipse is one type. Another type, called a conjunction, is when celestial bodies appear to be lined up from a low point in the sky to a higher one, as seen from another location (like Earth’s surface). The term conjunction might be familiar to fantasy fans, as this has been used in stories, such as the films The Dark Crystal and Pitch Black, to indicate a rare aligning in the heavens, one that portends a great event. This is usually when some poor schlep is sacrificed, for example.
Appendix 1 is a template for creating a solar system and its planets, moons, and other satellites. It includes more comments and advice, and an editable Microsoft Word file can be downloaded for free by signing up for the newsletter at https://artofworldbuilding.com/newsletter.
Our planet is either rotating clockwise or counterclockwise. The Earth does the latter, which is why the sun rises in the east and sets in the west. We can reverse this for a quick way to thrust our audience into a different perspective. Of course, the stars, planets, and constellations will also travel across the sky in the opposite direction. Be aware that this affects ocean currents, covered later in this chapter. For world builders, this affect is largely that of which coast of a continent has warmer or colder waters. Prevailing winds will also be reversed in each hemisphere.
The Sun’s Impact
Our lone sun is technically a yellow dwarf star of medium age. Most world builders, particularly in fantasy, will just want to go with the same and call their work done. More extreme situations might be better suited for worlds we’ll only use for one story. In SF, with characters traveling across the cosmos to various planets, the viability of life in other suns’ solar systems is debatable.
For example, a planet in the habitable zone of a red dwarf star would experience tidal locking (the planet’s same side always faces the sun, just as one side of the Moon always faces Earth). This causes perpetual night on one side and perpetual day on the other. This sounds cool at first until we consider the huge temperature variations that would make Earth-like life difficult or impossible to form.
A moon orbiting such a red dwarf’s planet could sustain life if it was also tidally locked to its planet, because that would cause the moon to not always have the same side face the sun. Even so, the lack of ultraviolet light also makes life unlikely, as does the variable energy output from the red dwarf (our sun is relatively stable). Maybe life can’t originate there, but there’s no reason we can’t put a mining or research colony there. We can use such worlds but maybe not have much life originate there unless that life is extremely different from Earth’s.