Alternate Star System Generation Rules

[Chaos]

Hi there! I´ve been away for some time, but here I am, once again, for now.

Some of you may remember that I posted alternate and expanded star system generation rules I devised some years ago. By now I´ve come up with a new set of refined rules, somewhat more complex, but hopefully they make more sense, and give the Referee some more information to work with when fleshing things out.

I´m posting these rules here because I would like some feedback from you guys about the rules... how much sense they make for you, any glaring scientific errors you spot, and so on. Of course, if you find yourself full of praise for my ideas, feel free to let me know about that as well...


Anyway, let´s get started. I´ll post the rules in several portions, because there´s quite a fews from the beginnings to the finished product.

Step 1: Stellar Configuration

Roll 2d6.

For a result of 2 to 8, the system is a solitary star.
For a result of 9 or 10, the system is a close binary.
For a result of 11 or 12, you have a close binary, and you roll 2d6 again for a second system component.
Repeat the process until you get a result of less than 11.

(For the purpose of these rules, a "close binary" is a binary system in which the smaller star occupies one of the larger stars´ planetary orbitals. The different system components are further away from each other than the outermost theoretical orbital.)


Step 2: Spectral Types

Roll 2d6 for each star.

For a result of 2 to 8, the star is a Class M red dwarf.
For a result of 9 to 11, the star is a Class F, G or K main sequence star.
For a result of 12, the star is a Class O, B or A main sequence star.

(As other kinds of stars aren´t likely to be interesting for colonization or exploitation, the rules do not generate them randomly; actually I had considered excluding Class O/B/A entirely for that reason)

For Class F/G/K or O/B/A stars, roll 2d6 again to determine spectral type:

For a result of 2 or 3, the star is Class F or O.
For a result of 4 to 6, the star is Class G or B.
For a result of 7 to 12, the star is Class K or A.

For all stars, roll 1d10 (taking a result of 10 as 0) to determine subclass.

(Having one star in 432 be a Class O is unrealistic, but where would we be if nothing required the Referee to come up with an explanation? ;-))

Climate Modifier

The Climate Modifier of a Class G4 star is +0. For every subclass "above" or "below" that, the modifier increases or decreases by 2, up to +30 for A9 and -50 for M9; for every subclass above A9, the modifier increases by 5 up to +175 for O0.

(The Climate Modifier is one component determining worlds´ climate)


Stars of different classes and subclasses also differ in their mass, which is used to determine their jump limit (which isn´t a fixed 100 diameter limit IMTU).


Step 3: Orbitals

(I´m using fixed orbitals shells with radii approximating those observed in our solar system, at least for the inner nine shells (out to Neptune). I might introduce a more variable system of orbital radii later on, as that would complicate a lot of things)

For system components that are close binaries, roll 3d6 to determine the distance between the two stars.

Result 10 or 11: Orbital 0 (any mean distance significantly closer than the 60 million km radius of Orbital 1)
Result 9 or 12: Orbital 1
Result 8 or 13: Orbital 2
Result 7 or 14: Orbital 3
Result 6 or 15: Orbital 4
Result 5 or 16: Orbital 5
Result 4 or 17: Orbital 6
Result 3 or 18: Orbital 7

The smaller star occupies the indicated orbital of the larger star.
The next orbital outside that occupied by the smaller star is always empty.

When generating planets for a close binary pair, populate the inner half of the orbitals of both stars before populating the orbitals beyond the smaller star.
(For example, if the smaller star occupies Orbital 5 of the larger star, Orbital 6 is always empty, and both stars can have planets of their own in their respectively Orbitals 1 and 2, but both stars´ orbitals 3 and 4 also remain empty. Orbitals 7 and beyond are a "common" planetary system shared by both stars.)

Stars also have a "safe" orbital, which is the innermost orbital in which the star´s size and gravitational influence allows planets to form.
Class O - Orbital 5
Class B - Orbital 3
Class A - Orbital 2
All other classes - Orbital 1
All orbitals inside this "safe orbital must remain empty regardless of all other considerations.


Each orbital has an Orbital Climate Modifier, also used to determine worlds´ climate, which is simply the orbital number multiplied by -5.


Okay, that´s the stars, in the next post we´ll place planets and moons and the like. I´m looking forward to your feedback.

 

[Chaos]

Step 4: Planets

Roll 3d6 once each for solid planets and gas giants to determine the number present in a planetary system.

Result 3-7: none
Result 8-9: 1
Result 10-11: 2
Result 12-13: 3
Result 14-15: 4
Result 16-17: 5
Result 18: 6

Place the planets in consecutive orders, starting with the innermost available orbit (as determined in Step 3), solid planets first, then gas giants.

(For the sake of simplification, I am making the assumption that the arrangement in our solar system, with the solid planets nearer to the star and the gas giants further away, is the way things are as planetary systems are concerned. It just makes too much sense to me, and it is too neat and tidy not to use as a rule.)

If the system (or system component) is a close binary, roll 2d6. If the result is 9 or higher, insert an asteroid belt into the innermost shared orbital of both stars, and start placing planet in the next orbital out from that.
(i.e. if the smaller star occupies orbital 5 on the larger star, orbital 6 always remains empty, and orbital 7 may or may not contain an asteroid belt; this belt is the result of gravitational influence of the binary stars preventing the material in that orbital from accreting into a planet)

If the system has both solid planets and gas giants, roll 2d6 (again). If the result is 8 or higher, insert an asteroid belt in the closest orbital beyond the outermost solid planet; if the result is 12, insert two asteroid belts in the two closest orbitals. Start placing gas giants in the closest orbital beyond the asteroid belt(s).
(The belts result from the gravitational influence of the gas giant(s) which prevents the material in these orbitals from accreting into a planet; as I understand this is the current theory on how our asteroid belt formed)

Should the initial rolls result in neither solid planets nor gas giants, the system has a single asteroid belt in the innermost safe orbital.


If a system component is a binary, roll each star´s planets separately and fill up any orbitals which a star does not share with its binary companion; place any remaining planets of both stars into the shared outer orbits.
(In the binary example from above, each star´s orbital 3 and 4 is empty, but orbitals 1 and 2 can be occupied by planets; if either star has more than 2 planets, these go to the shared orbitals, starting with orbital 7)


Step 5: Moons8

(Again for the sake of simplicity, the rules play down the number of moons of gas giants and the number of major asteroids in a belt; it is of course possible and very easy to change this to more realistic results)

Roll 2d6 once for each solid planet and three times for each gas giants to determine the number of major moons (i.e. moons beyond a certain size); roll 2d6 twice for each asteroid belt to determine the number of major asteroids. For each roll, consult the following table:

Result 2-7: none
Result 8-9: 1
Result 10-11: 2
Result 12: 3

 

[Chaos]

Step 6: Size

Every body occupying an orbital (solid planet, asteroid belt or gas giant) has a Size value, as does every major moon (and asteroid).

For each parent body (planet or asteroid belt), roll 3d6 to determine Size.
A solid planet´s approximate diameter is its Size times 1,000 km.
A gas giant´s approximate diameter is its Size times 10,000 km.
Asteroid belts obviously don´t have a planetary diameter; the total volume of all material in them is roughly that of a planet with the same Size value.

Size calculation for moons and asteroids is a little more complicated. First, roll 1d6 to determine size category:
1-2 - small
3-5 - medium
6 - large

Roll 3d6 for each moon, multiply that with their parent body´s Size and with a factor of 5 for a small body, 20 for a medium body and 50 for a large body.

(So the theoretical maximum size for a moon is 18 times 50 times parent body Size, i.e. 900 km diameter for each 1,000 km of parent body diameter, assuming a solid planet as a parent body.)

Calculate each moon´s and asteroid´s effective Size value by dividing its diameter by 1,000 and rounding normally. (You´ll need that value later on)

Step 7: Various Parameters

(Note: from now on, all solid planets, moons and major asteroids are collectively called "worlds"; all steps starting with this one no longer apply to asteroid belts and gas giants)

A world´s Geological Activity is determined by rolling 2d6-12 and adding the Size of the world; change any result below 0 to 0.
A world´s Albedo is determined by rolling 2d6-7.
A world´s Density is determined by rolling 2d6 and adding its Geological Activity.
A word´s Surface Gravity is determined by multiplying its diameter with its density and dividing the product by 127,500; round the result to two digits (e.g. 1.09g).

Geological Activity is a stand-in for several things, mostly the presence of a liquid core than generates a planetary magnetic field, thus protecting the world´s atmosphere from solar wind, and the presence of geological processes that generate at least some heat in the interior. Both are beneficial, from the point of view of habitability, but they also generate earthquakes, tsunamis, volcanism and the like on the planet, which can make living there "interesting" (in the ancient Chinese sense). Earth´s Geological Activity would be about an 8 on the scale used here (roughly average, given Earth´s Size of 13).
Albedo, as mentioned earlier, is a modifier rather than the fraction as which it is usually expressed, in order to fit into the generation process; I have no fixed scale for which "real" albedo corresponds to which modifier.
Density, again, is a relative term rather than a concrete "g/cm^3" value. Geological Activity influences Density because the liquid cores that it represents are metallic and thus usually quite dense. Earth´s density is about 15 on this scale.

 

[Andrew M]

I like the idea of a minimal math system generator. You might want to put your system in a document and put it in the file library. It doesn't matter if it's provisional. You should be able to update it later.

The fixed orbits used in Book 6 are a falacy, though you probably knew this. If helps keep things simple, which is a good thing. Fixing the order of rock and gas planets does seem too rigid though.

Rather than giving the stars luminosity factors and the orbits distance factors, it would be easier just to give the Goldilocks orbit. This is the single most important part of the system afterall. For your temperature scales, try to select values that have a distinct physical or physiological value. Freezing & boiling points, limits of human comfort and tolerance. The temperatures where different gasses can boil off into space. Remember that there is a minimum ambient temperature, even in deep space, so the scale will stop somewhere.

It is good to see you using albedo as a fudge factor. Do the serious maths and physics so that you know what the realistic values are, but then when you apply these don't worry too much about making everything fit. Using a logarithmic scale (such as orders of magnitude) are a handy way to turn multiplications into additions, which makes things simpler in the end.

Do this to satisfy your own needs first of all. The chances are that others will find it useful too. It is looking good so far.

 

[Chaos]

I like the idea of a minimal math system generator. You might want to put your system in a document and put it in the file library. It doesn't matter if it's provisional. You should be able to update it later.

The fixed orbits used in Book 6 are a falacy, though you probably knew this. If helps keep things simple, which is a good thing. Fixing the order of rock and gas planets does seem too rigid though.

I´m using a modified Titius-Bode rule here - I stick to it up to the point where it describes our solar system reasonably well (orbital 8), then move to a more incremental model - 10 AU from orbital 8 to orbital 9, 15 AU from 9 to 10, 20 from 10 to 11, and so on. A binary system can theoretically have 30+ orbitals, and with Titius-Bode, orbital 20 is already about 2.5 light-years across...

Rather than giving the stars luminosity factors and the orbits distance factors, it would be easier just to give the Goldilocks orbit. This is the single most important part of the system afterall. For your temperature scales, try to select values that have a distinct physical or physiological value. Freezing & boiling points, limits of human comfort and tolerance. The temperatures where different gasses can boil off into space. Remember that there is a minimum ambient temperature, even in deep space, so the scale will stop somewhere.

I have some reference points already. 0 is the average temperature on Earth, which is around 15°C. From +10 to -10, there´ll always be at least some region on the world that´s reasonably comfortable for humans. Absolute zero is at -90 on the scale, but world climate cannot be less than -90 + Geological Activity.

It is good to see you using albedo as a fudge factor. Do the serious maths and physics so that you know what the realistic values are, but then when you apply these don't worry too much about making everything fit. Using a logarithmic scale (such as orders of magnitude) are a handy way to turn multiplications into additions, which makes things simpler in the end.

Logarithmic scales are an interesting idea. I had thought of making Climate more or less linear (using the equivalents for 0 and -90, 3° on the Kelvin scale per point of Climate seemed good), but that would make for ridiculously low temperatures close to a star. Logarithmic scales would be a solution.

Do this to satisfy your own needs first of all. The chances are that others will find it useful too. It is looking good so far.

Thanks.

 

[Chaos]

Okay, let´s put some (figurative) meat on those planetary bones... (I´ll finish posting the current version of the rules before I start revising it and possibly integrating things like logarithmic scale.)

Step 8: Atmospheric Pressure

To determine Atmospheric Pressure, roll 4d6-14 and add the world´s Size.

Change all results below 0 to 0. Atmospheric Pressure cannot be greater than the sum of Size and Geological Activity. If the world is a moon orbiting a gas giant, add 1/3 of the gas giant´s Size (rounding down) to the upper limit.

(Here, Geological Activity is a stand-in for the planetary magnetic field, which protects the atmosphere from being blown away by solar wind; a gas giant parent body´s magnetic field has the same effect.)

On this scale, Earth´s Atmospheric Pressure is about 13. (You´ll see that in many steps I have assumed Earth to be fairly average for its size.)

Step 9: Climate

To determine a world´s Climate, add the following values:
- the parent star´s Climate Modifier
- the world´s orbital´s Climate Modifier (i.e. orbital number times -5)
- the world´s Size
- the world´s Geological Activity
- the world´s Albedo (i.e. a positive Albedo value actually corresponds to a fairly low albedo)

If the world is a moon orbiting a gas giant, also add 1/2 of that gas giant´s Size.
If the total of the parent star´s and orbital´s Climate Modifier is below -90, use -90 instead.
Change any result below -85 to -85.

A Climate value of -90 corresponds to 0 K, or as close to 0 K as things can get in a planetary system. Earth´s Climate is 0, or approximately 287 K.

Step 10: Hydrographics

To determine Hydrographics, roll 4d6-14 and add the world´s Atmospheric Pressure value. For each point that Climate is above +5, and for point that Atmospheric Pressure is below 6, reduce Hydrographics by 2.

Change any result below 0 to 0, and any result above 20 to 20.

Each point of Hydrographics means that approximately 5% of the surface are covered by water (liquid or frozen).

 

[Chaos]

Here´s the last random part, and also the one which is the most speculative:

Step 11: Biodiversity

To determine Biodiversity, roll 4d6 and apply the following modifiers:

Class A or hotter star: -16 (i.e. not a Class F, G, K or M main sequence star)
Hydrographics 0: -8
Hydrographics 20: -4 (no significant land masses means far less varied land-based life)
Atmospheric Pressure 5 or less: - -3 per point below 6
Climate 8 or higher: -1 per point about 7
Climate -8 or lower: -1 per point below -7
Geological Activity 1 or 2: -4
Geological Activity 0: -8

Biodiversity measures not just diversity itself, but also the complexity of the life forms present.

Step 12: Atmospheric Composition

To determine Atmospheric Composition, roll 4d6-14 and add Biodiversity.

Less than 0: Hostile
1-10: Toxic
11-15: Marginal
16-20: Suboptimal
21+: Optimal

(Hostile is more or less equivalent to the Traveller designations "corrosive" and "insidious", Toxic is equivalent to "exotic" and Marginal to the various "tainted" results)

Step 13: Habitability

A world is habitable if it meets the following prerequisites:

Atmospheric Pressure from 6 to 20
Climate from -10 to 10
Atmospheric Composition 11+

If Atmospheric Pressure is outside the 10 to 16 range, then at least at some altitudes have a pressure to high or too low for human habitation. Likewise, if Climate is outside the -4 to 4 range, some latitudes are too hot or too cold for human habitation.


Step 14: Habitability Category

To determine a habitable world´s Habitability Category, add up the following:

- the deviation of Atmospheric Pressure from the optimum of 13
- the deviation Climate from the optimum of 0
- the shortfall of Atmospheric Composition below 21
- any points of Geological Activity above 5
- 1 point per full 0.05g that Surface Gravity deviates from 1.00g

0-4 points: Category A
5-9 points: Category B
10-14 points: Category C
15-19 points: Category D
20-24 points: Category E
25+ points: Category F