Explain the orbital resonance thing more, I looked it up on wikipedia but thats information overload on there. What resonances can I have/would make sense for slot #1 to #15 (thinks 9 would be a more realistic limit)
I´m no astronomer myself, so please take the following with a grain of salt:
The central star isn´t the only thing in a star system that exerts gravity; the planets also exert gravity on each other, especially the more massive ones. Over time - and we´re speaking of billions of orbital periods for inner-system planets - even the small gravitational influence two Earth-sized planets exert from each other over interplanetary distances adds up to destabilize planetary orbits. Meaning that if the orbits are unstable, planets collide, are flung out into interstellar place, or don´t even form to begin with.
For some reason that I don´t quite understand, orbits are more stable and gravitational influences of two planets on each other are not as problematic if the orbital periods of the two planets relative to each other are at certain ratios. Those are usually pretty small numbers, such as 4:3, 2:1, 5:2 etc - e.g. 4:3 means that for every four orbits of the inner planet, the outer planet completes three orbits.
Kepler´s Third Law of Planetary Motion states that the square of the orbital period of a planet is always proportional to the cube of the semi-major axis. If orbits were perfectly circular, the semi-major axis would be the orbital radius, so it´s good enough for laymen to use it as that.
That means that, if two planets have a 4:3 orbital resonance, meaning the outer planet has 4/3 times the orbital period of the inner planet, Kepler´s Third Law tells us how much greater its orbital radius must be: The cube root of the square of the resonance ratio (4/3) - about 1.21 times, in this case.
You can use this to determine the placement of planets in a newly generated system:
Start by placing the innermost planet (Planet 1) whereever you think it makes sense for the innermost planet to be.
Then, determine what the orbital resonance between Planet 1 and Planet 2 should be. Place Planet 2 as far out from Planet 1 as Kepler´s Third Law dictates it should be for its orbital period - e.g. 1.21 times Planet 1´s orbital radius if the two have a 4:3 orbital resonance.
Now, determine what the orbital resonance between Planet 2 and Planet 3 should be, and place Planet 3 accordingly.
Keep placing planets until you are as far out from the star as you want to be, or have as many planets as you want to have, or whatever other criterion you choose.
The fun thing is, orbital resonance doesn´t just work between adjacent planets; more distant planets also attract one another, and also move in some sort of resonance. For example, if Planet 1 and Planet 2 are in a 3:2 resonance to each other, and Planet 2 and Planet 3 are in a 5:3 resonance to each other, Planet 1 and Planet 3 are also in a 5:2 resonance.
(note that real-world data isn´t an exact match for this; Venus and Earth, for example, are only in an *almost* perfect 8:5 resonance; nature is never tidy and simple)
Common orbital resonances, and the corresponding ratios of orbital radii, are:
4:3 (1.21)
3:2 (1.31)
8:5 (1.37)
5:3 (1.41)
7:4 (1.45)
9:5 (1.48)
2:1 (1.59)
7:3 (1.76)
5:2 (1.84)
3:1 (2.08)
What this could mean for the slot system is that distances between slots aren´t necessarily fixed. Instead of having the slots at 0.4, 0.7, 1.0 etc AU, you could roll dice to determine orbital resonance between the planets in adjacent slots and calculate the ratio of their orbital radii from that. Say roll 1d10, with a result of 1 indicating 4:3 resonance, a 2 indicating 3:2 resonance, and so on, up to 10 indicating 3:1 resonance. Do that separately for every system generated, and you´ll have the added benefit that intervals between planets aren´t the same everywhere any more.
