Until the boom in exoplanet studies, we really knew nothing about how planetary systems form. In the old days, with only the Solar System as our guide, it looked simple: small rocky worlds near the Sun, big giants in the outer regions. But then we began to observe things like "hot Jupiters" orbiting almost close enough to touch the parent star, giant rocky "super-Earths" and other weirdness. So now we really have no idea if there are any hard and fast rules for how planetary systems form.
Here are some rules of thumb, which aren't laws of nature but are pretty good guidelines.
Scale: The size of a planetary system seems to be roughly in proportion to the mass of the parent star. Our Sun's planets orbit in a range from about 1/3 Astronomical Units for Mercury (45 million kilometers) out to 30 AU (or 4.5 billion kilometers) for Neptune, with small bodies comets extending out to ten thousand times that distance.
An Astronomical Unit is the distance between Earth and the Sun, and is going to come up a lot in this post. It's 148,800,000 kilometers, and for most purposes you can approximate it as 150 million km.
By contrast with our Solar System, the red dwarf TRAPPIST-1 (http://www.trappist.one) has a tenth the mass of the Sun, and its planets are all crowded into a band between 1.5 million and 9 million kilometers (0.01 to 0.06 AU) — roughly the scale of the moons of a planet like Jupiter.
So if you're creating a planetary system, multiply the mass of the parent star (in Solar masses) by about 35 AU and treat the result as the outer limit for planetary orbits in that star system. In the case of binary or multiple star systems, use that number or 1/3 of the distance between the primary star and its closest companion star, whichever is smaller.
How Many Planets?: The Sun has eight planets plus some dwarf planets and a lot of small stuff. Other stars with known exoplanets have fewer major planets, typically just two or three known. But that's probably an artifact of the methods we use to detect exoplanets: the periodic dimming of the primary star as orbiting planets pass across its disk as seen from Earth is going to be limited by the time astronomers have been watching. Distant, slow-orbiting worlds simply haven't been detected yet. We didn't know about Uranus and Neptune for most of human history.
In the absence of any other data, I'm going to use the Solar System as a baseline for how many planets a given star possesses. If you want to generate the number randomly using 6-sided dice, roll 3 dice and subtract 2 from the total, to get a number from 1 to 16 with the peak between 8 and 9.
Spacing: The planets of the Solar System follow an interesting rule of spacing known as Bode's Law (https://www.britannica.com/science/Bodes-law), but other known planetary systems don't seem to have the same orbital spacing, and apparently it's just a bit of numerology. You can basically put planets wherever you want in another star system.
It does seem to be true that the interval between planets increases with distance from the parent star, so my suggestion is to pick the distance for the innermost planet and then work outward as follows.
- Innermost Orbit: multiply the mass of the parent star (in Solar masses) by a randomly-generated number between 0.1 and 1. My suggestion is to use a 10-sided die and divide the result by 10. The result is the orbital distance of the innermost planet in Astronomical Units.
- Multiplier: determine a multiplying factor for the spacing of the next orbit. Again, I suggest using a 10-sided die roll divided by 10, and then add that to 1, so you have a number from 1.1 to 2.
- Second Orbit: Multiply the innermost orbit by the Multiplier to get the distance of the second orbit around the star.
- Recalculate the Multiplier: Roll a 6-sided die, subtract 1 from the result, and divide that by 10, to get a number from 0 to 0.5. Add that to the existing Multiplier from step 2 to get a new Multiplier. Use that to calculate the third orbit, and so on. Never use a lower Multiplier, and stop when you get a result greater than 2. If the distance exceeds the maximum size of the system, stop there. I tried it a few times and got star systems that look like this (with a parent star equal to the Sun in size). All distances are in AU.
Orbit: 1 2 3 4 5 6 7 8 9
Solar System 0.4 0.7 1 1.5 3* 5 9.5 19 30
System A 0.5 0.6 0.7 1.2 2.6 5.4 11.3 23.6
System B 0.3 0.5 0.8 1.5 3.4 7.7 17.7
System C 0.5 0.8 1.4 2.4 5.1 10.7 22.5
*Note: the 3 AU orbit in the Solar System is the Asteroid Belt, which we will consider a "failed" planet as described below.
"Failed" Planets: There's a big gap in the Solar System between Mars and Jupiter — bigger than the distance from Mars to the Sun, in fact. How come? Jupiter itself is the prime suspect. In much of the space between Mars and Jupiter, an orbiting world will "synch up" with Jupiter every few years, and experience Jupiter's distant but powerful gravitational pull. The result (according to current theories) is that much of the matter in that part of the Solar System during the planetary formation era got knocked away by Jupiter, or captured by the giant itself. There simply wasn't enough left to form a planet.
This suggests that a large gas giant — anything bigger than about 100 times Earth's mass — will disrupt the orbits between about 0.4 and 1.6 times its own orbital distance. Leave those empty or make them asteroid belts.
Migrating Planets: One of the most startling exoplanet discoveries was the category of worlds called "Hot Jupiters." These are large, massive gas giants orbiting extremely close to the parent star. Current theories of planetary formation simply don't account for that — close to a star it should be just too hot for a gas giant planet to form.
So the current explanation is that in some systems large worlds form and then migrate inward as a result of interactions with other giants. So if you swap Jupiter and Saturn in the Solar System, the smaller inner world would get kicked into an eccentric orbit, gradually dropping down over millions of years until it orbited close to the Sun.
Needless to say, such a world would probably sling away or obliterate all the worlds whose space it passed through during that period. So if you want a giant world close to the star, pick an orbit in the outer system, beyond the "Goldilocks Zone" where water stops being a rock, and erase everything between that orbit and the one you want to put it in.
Nomenclature: Back in the good old days, science fiction writers had worked out a great convention for naming planets circling other stars. Obviously they'd be numbered, from closest to farthest out. So there would be planets like Tau Ceti III or Fomalhaut IX. Nice, simple, and elegant.
Naturally astronomers had to go and mess it up. Astronomers ruin everything. Their system (which has been around for a long time, actually) is that companion bodies of stars get labeled with letters in order of discovery, beginning with "b" because A is always the primary star. It's good for their purposes, but maddening for everyone else. So Tau Ceti has eight possible planets, and going outward from the star they are b, g, c, h, d, e, f, and i. Inelegant! Infuriating!
Fortunately, even astronomers think this is kind of lame, so the International Astronomical Union has started the Named Exoworlds project to bestow actual names on stars deemed sufficiently interesting, and on their planets. Wikipedia has a list of them here: https://en.wikipedia.org/wiki/List_of_proper_names_of_stars. The IAU retains the final approval authority so we're not going to get any objects named "Planet McPlanetface" any time soon.
So the star 55 Cancri A got renamed Copernicus, and its planets b, c, d, e, and f became Galileo, Brahe, Lipperhey, Janssen, and Harriot. A vast improvement.
For a fictional world, there are several options:
- Use a real name if it's a real planet.
- Use a plausible name (without duplicating one of the ones already taken). Something a bit stodgy and uncontroversial.
- If the planet is going to have native intelligent life, use their name for it, or the closest approximation a human mouth can manage.
Next Time: Placing Planets!
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