From: [a--i--n] at [acheron.amigans.gen.nz] (Ross Smith)
Newsgroups: rec.games.frp.misc
Subject: Planet generation system
Date: 2 Oct 93 18:58:15 GMT+12
This is the latest version of my planet generation system (the original
version was posted here a few weeks ago). Most of the changes are minor
-- new star type table (with all types, not just the common ones),
various bug fixes.
I've written a set of programs for generating stars and planets using
this system; if anyone's interested I can mail you the source code (in C),
or post them if there's enough interest (he says optimistically).
---------------------------------------------------------------------------
PLANET GENERATION SYSTEM
------------------------
Version 1.1 (2-Oct-93)
Copyright 1993 by Ross Smith
([a--i--n] at [acheron.amigans.gen.nz])
CONTENTS
--------
P1. Introduction
P2. Star Systems
P3. Planetary Systems
P4. Earthlike and Marginal Planets
P4.1. Physical Details
P4.2. Biological Details
P4.3. Special Features
P4.4. Examples
P1. INTRODUCTION
-----------------
This is a reasonably quick and simple system for generating complete
planetary systems in rough detail, and the most interesting planets in
somewhat greater detail. It isn't tied to any particular game mechanics,
and should be usable with any SFRP system.
I designed it because I've never been very impressed by the planet
generation schemes in most SF role-playing games, which are usually either
too unrealistic or too complicated (or, frequently, both). I wanted a
system that was simple enough to generate stars and planets fairly quickly,
while still being designed with some astronomical knowledge, by someone who
knew one end of a Hertzsprung-Russell diagram from the other. (Hint:
Neither end is likely to have anything resembling Earth orbiting it.)
You will need some six-sided and ten-sided or twenty-sided dice (or, of
course, a computer with a good random number function). I assume you're
familiar with the usual dice notation (so you know, for instance, what
"2D6+3" means). This system requires rolls on D6, D10, D20, and D100.
Unless otherwise stated, all results of less than zero should be counted as
zero, and all fractions should be rounded to the nearest whole number
(round halves upwards).
This file is copyright 1993 by Ross Smith; all rights reserved. It may
be distributed and archived freely, provided no changes are made. Comments
can be mailed to me at the address above, or posted to rec.games.frp.misc
(email preferred as our news service is unreliable).
P2. STAR SYSTEMS
-----------------
In the local region of the Galaxy, there is about one star system per
900 cubic light-years. I suggest using cube-shaped sectors either 25 or 50
light-years on a side. A 25-light-year sector has 6D6-4 star systems
(average 17); a 50-light-year sector has 16D10+50 systems (average 138;
more appropriate to generation by a computer program than by hand). Place
the systems at random within the sector (roll D100/4 or D100/2 for each of
the X, Y, and Z coordinates).
To find the number of stars in a system, roll (3D6/4)-1; round to the
nearest whole number, and count anything less than 1 as 1.
Stars are classified into seven "spectral types", based on their
temperature and colour. The types are O (blue), B (blue-white), A (white),
F (yellow-white), G (yellow), K (orange), and M (red); astronomers use the
mnemonic "Oh, Be A Fine Girl, Kiss Me" to remember the sequence. The seven
types are each divided into ten subclasses, indicated by a digit from 0 to
9 after the spectral type (except for type O, which is only divided into
five subtypes, O5 to O9). The Sun, for example, is type G2.
The vast majority of stars fall into the "main sequence", which runs
from huge, bright O-type stars to tiny, dim M-type red dwarfs, with yellow
dwarfs like the Sun falling almost exactly in the middle. Other types of
stars are above or below the main sequence.
Red giants are a late stage in the life cycle of moderately massive
stars (the Sun's mass or larger), and are denoted by a "g" prefix on the
spectral type (e.g. "gM5"). Supergiants are even brighter and more
massive, and are denoted by a "c" prefix (e.g. "cK0"). White dwarfs, much
dimmer than most main sequence stars, are the final stage in the evolution
of most stars; they are denoted by a "D" prefix, and normally no numeric
subtype (e.g. "DA"). The most massive stars will end up as neutron stars
or black holes (these have no spectral types, of course; the table below
uses "ns" and "bh" to represent them).
For each star in a system, roll D100 two or three times on the
following table, to get the general description and the specific spectral
class of the star. For supergiants, giants, and main sequence stars, roll
D10-1 for the numeric subtype (except for type O, which is only subdivided
from 5 to 9; roll (D10/2)+4).
The stars in a system should be listed in descending order of
luminosity, which you can take to be the same as the order in the table
(supergiants, giants, main sequence, white dwarfs (all in order of spectral
type), then neutron stars and black holes).
First Second Third General Specific
D100 D100 D100 type type
------ ------ ------ ------------- --------
1 1 1-10 Supergiant cB0-9
'' '' 11-20 '' cA0-9
'' '' 21-40 '' cF0-9
'' '' 41-60 '' cG0-9
'' '' 61-80 '' cK0-9
'' '' 81-100 '' cM0-9
'' 2-5 -- Giant gF0-9
'' 6-10 -- '' gG0-9
'' 11-55 -- '' gK0-9
'' 56-100 -- '' gM0-9
2-93 1 1 Main sequence O5-9
'' '' 2-100 '' B0-9
'' 2-3 -- '' A0-9
'' 4-7 -- '' F0-9
'' 8-15 -- '' G0-9
'' 16-31 -- '' K0-9
'' 32-100 -- '' M0-9
94-100 1-20 -- White dwarf DB
'' 21-40 -- '' DA
'' 41-60 -- '' DF
'' 61-80 -- '' DG
'' 81-99 -- '' DK
'' 100 1-95 Neutron star ns
'' '' 96-100 Black hole bh
P3. PLANETARY SYSTEMS
----------------------
Refer to the following table to generate the number of planets for each
star. First determine whether there are any planets; for multiple star
systems, roll only once for the whole system, using the spectral type of
the primary star. If the result is affirmative, generate the number of
planets for each star, based on the star's own spectral type.
Star Probability Number of
type of planets planets
------- ----------- ---------
c 10% D6
g 20% D6
O,B 10% D10
A 50% D10
F,G 99% 2D6+3
K 99% 2D6
M 50% D6
D,ns,bh 10% D6/2
Planetary systems can be divided into three zones; different types of
planet will tend to form in each zone. Zone A is the inner or hot zone;
zone B is the intermediate or life zone; and zone C is the outer or cold
zone (in our system, Mercury and Venus are in zone A, Earth and Mars are in
zone B, and the asteroids and outer planets are in zone C). The following
table gives the number of planets in each zone for main sequence and giant
stars; for white dwarfs, neutron stars, and black holes, all planets are in
zone C.
Total number Zone Zone Zone
of planets A B C
------------ ---- ---- ----
1-3 0 1 0-2
4-5 1 1 2-3
6-7 1 2 3-4
8+ 2 2 4+
At this point, if the star is a member of a multiple system, remove the
outermost D6+1 planets (but use the original number of planets to determine
which zones the remaining inner planets are placed in). This may leave the
star with no planets.
For each planet, three parameters are determined: Type, size, and
number of moons. Roll D100 and refer to the following table for planet
type, then generate the size. If the star is type F, G, or K (main
sequence only, not giants or white dwarfs), use the second column for zone
B; otherwise, use the first column.
Planet Code Zone Zone Zone Zone Diameter
type A B B* C (km)
------------- ---- ------ ------ ------ ------ ------------
Asteroid belt A 1-5 1-5 1-5 1-5 --
Giant G -- 6-8 6-8 6-75 3D6x10,000
Vacuum/rock R 6-60 9-40 9-40 76-80 D10x1000
Vacuum/ice J -- -- -- 81-95 D10x1000
Desert D 61-70 41-70 41-60 -- (2D6+2)x1000
Hostile H 71-100 71-100 61-80 96-100 (3D6+1)x1000
Marginal M -- -- 81-90 -- (2D6+5)x1000
Earthlike E -- -- 91-100 -- (2D6+5)x1000
Neutron stars and black holes will never have planets with atmospheres
(too cold); treat a result of H as J.
Size and number of moons are not applicable to asteroid belts. Giant
planets have 2D10 moons. For terrestrial planets (all the remaining
types), roll a D10; the planet has no moons on 1 to 4, one moon on 5 to 7,
two moons on 8 or 9, and three moons on 10.
ASTEROID BELT: A collection of rocks that never combined to form a
planet. Usually there will be a handful of large asteroids a few hundred
kilometres across, and a vast number of smaller objects.
GIANT: A huge planet consisting mainly of hydrogen, ranging from ten
to a thousand times the Earth's mass. Example: Jupiter.
VACUUM/ROCK: A small, rocky planet with no atmosphere. Example:
Mercury.
VACUUM/ICE: Similar to vacuum/rock, but composed mainly of ice rather
than rock; found only in the outer part of a system. Example: Pluto.
DESERT: Comparable in size to Earth, but with a barren surface and a
thin, inert atmosphere. Example: Mars.
HOSTILE: Comparable in size to Earth, with an atmosphere containing no
oxygen but with some other active component, or having some other feature
which makes the planet's surface unpleasant, such as extreme volcanic
activity. Hostile planets may occasionally have native life forms, but
they will be based on a very non-Earthlike biochemistry. Example: Venus.
MARGINAL: Almost Earthlike, with oxygen in the atmosphere, but with
some serious problem that makes it an unpleasant place to live, such as
very high or low temperature, atmospheric pressure, or oxygen level; traces
of some toxic gas in the atmosphere; dangerous volcanic activity; or high
radiation level. No examples in our solar system.
EARTHLIKE: Oxygen atmosphere, temperatures not too far outside the
human comfort zone, and a reasonably stable surface. Note that the
presence of oxygen in the atmosphere necessarily implies the presence of
life (oxygen is too reactive to occur in large amounts from geological
processes alone, and would disappear in a relatively short time if life
were to become extinct). Example: Earth.
The details of a system's planets can be conveniently written using the
code letter for the type, the size in thousands of kilometres, and a dot
followed by the number of moons, if any. Asteroid belts are simply
indicated by an "A" with no numbers (and are not counted towards the
"official" number of planets). Use slashes to mark the zone boundaries.
Here are some examples, using our solar system and an imaginary one
generated for Alpha Centauri.
Sun (G2)
9 planets: R5 H12 / E13.1 D7.2 / A G140.16 G120.18 G50.15 G50.8 J2.1
Alpha Centauri (G2,K0,M5)
(A) 6 planets: R9.3 H8 / H10 M15.2 / J5.1 G110.9
(B) 2 planets: R10.2 / H8 /
(C) 1 planet: H17.1 / /
P4. EARTHLIKE AND MARGINAL PLANETS
-----------------------------------
P4.1. PHYSICAL DETAILS
Generate a planet's orbit period using the table below, dependent on
the spectral type of its sun. If there are two Earthlike or marginal
planets orbiting the same star, generate two periods and assign them in the
correct order (the inner planet, obviously, gets the shorter period). If
the outer period is less than 25% more than the inner one, generate both of
them again.
Spectral type Orbit period
of star of planet
------------- ----------------
F0-F4 600+2D100*3 days
F5-F9 400+2D100*2 days
G0-G4 270+2D100 days
G5-G9 150+2D100 days
K0-K4 120+D100 days
K5-K9 70+D100 days
To find the planet's rotation period, roll D100, add 10 for each moon,
and refer to the following table. Note that the orbit periods (above) are
given in Earth days, not local days.
Result Rotation
------ ------------
1-65 D20+9 hours
66-90 D20+20 hours
91-98 D10 days
99-103 D100 days
104+ D10 days
Roll (D6+6)/2 (keep fractions, don't round) for the planet's density in
grams per cubic centimetre. Multiply diameter by density and divide by
70,000 to calculate the surface gravity in Earth gravities (round to the
nearest 0.1).
Roll 2D6-4 for Earthlike planets, or D10+D6-6 for marginal planets, and
multiply by 5 to find the average surface temperature in degrees Celsius
(of course negative numbers should be retained here, not counted as zero).
If a star has two planets in zone B and both of them are Earthlike or
marginal, make sure the temperatures are in the right order; the outer
planet should have a temperature no higher than the inner one (greenhouse
effects and similar phenomena can affect a planet's temperature, of course,
but not by all that much on a planet with an approximately Earthlike
atmosphere). If the outer planet's temperature comes out higher, generate
both temperatures again.
Roll 2D6-2 and multiply by 10 to find the percentage of the planet's
surface covered with liquid water. Subtract 20 if the temperature is zero;
subtract 40 if the temperature is below zero.
A planet's mineral resources are measured on an arbitrary scale,
running from 1 (worst) to 10 (best). The Earth is rated 8, fairly high,
because of its high density (implying a relatively high ratio of metal to
rock), and its active volcanic and tectonic processes (which carry minerals
from deep in the interior to the surface). A planet's mineral resources
rating is generated by the formula:
Minerals = Density + (Diameter/10,000) + D6 - 4
P4.2. BIOLOGICAL DETAILS
Roll D100 on each of the following tables to determine the evolutionary
level of the local life forms, and their chemical basis.
D100 Life present
Earthlike Marginal
--------- -------- -----------------------------------------------
-- 1 No life
1-10 2-30 Single-celled organisms only
11-20 31-45 Simple invertebrates and plants (in seas)
21-30 46-60 Advanced invertebrates and plants (on land)
31-40 61-75 Simple vertebrates (fish, amphibians)
41-100 76-100 Advanced vertebrates (reptiles, birds, mammals)
D100 Biochemistry
Earthlike Marginal
--------- -------- ----------------
1-20 1-5 Earthlike
21-80 6-60 Protein-based
81-100 61-99 Carbon-based
-- 100 Non-carbon-based
For simplicity, the many possible varieties of biochemistry are divided
into four broad classes.
EARTHLIKE: Essentially the same chemical basis as Earth's life forms,
with only small differences, if any. Plants and animals with Earthlike
biochemistry can usually be safely eaten by humans (and vice versa).
PROTEIN-BASED: Similar to Earth life in its basic chemical components,
but with enough differences to make the biochemistries incompatible. Life
of this type will nearly always be inedible, and frequently poisonous, to
humans.
CARBON-BASED: Based on the same elements as Earth life, but arranged
in very different compounds. Always inedible, and usually extremely
poisonous.
NON-CARBON-BASED: Completely different to Earth's biochemistry in
every way. Invariably poisonous. Very rare, at least on anything
resembling an Earthlike planet.
P4.3. SPECIAL FEATURES
P4.3.1. GENERATING SPECIAL FEATURES
There is no such thing as a typical Earthlike planet; every world is
unique. To reflect this, some earthlike or marginal planets will have one
or more "special features" that are not covered by the usual planetary
parameters. Roll a D100 three times on the following table to determine
what the features are. If you get the same result (other than "No special
features") twice, roll again.
Most features are restricted to planets with certain properties (for
example, the "Freshwater oceans" feature requires a non-zero ocean
percentage), or are incompatible with certain other features (for example,
a planet can't have both "High inclination" and "No seasons"). See the
detailed descriptions below for explanations of the incompatibilities and
prerequisites. If you get an incompatible result, roll again.
GMs are encouraged to let their imaginations run riot in expanding this
table!
D100 Special feature
------ -------------------------
Astronomical features
1 Eccentric orbit
2 High inclination
3 High tides
4 Meteor storms
5 No seasons
6 Rings
Geological features
7 Radiation hazard
8 Rugged terrain
9 Volcanic activity
Hydrographic features
10 Freshwater oceans
11 Many islands
12 Poisonous oceans
13 Swampy surface
Atmospheric features
14 Cloud cover
15 Toxic gas
16 Unstable climate
17 Very dense atmosphere
18 Very thin atmosphere
Biological features
19 Intelligent life
20 Semi-intelligent life
21 World forest
Archaeological features
22 Ancient artifacts
23-100 No special features
P4.3.2. ASTRONOMICAL FEATURES
ECCENTRIC ORBIT: The planet's orbit is highly elliptical, producing
extremes of temperature at closest approach (periastron) and furthest
distance (apastron). Incompatible with "No seasons".
HIGH INCLINATION: The planet's axis is tilted by more than 45 degrees,
producing extreme temperature differences between summer and winter.
Incompatible with "No seasons".
HIGH TIDES: A large, close moon or sun produces extremely high tides.
Requires either a K-type sun or at least one moon.
METEOR STORMS: The planet suffers frequent meteor storms, often
including large asteroid or comet impacts, making the planet's surface a
dangerous place. Marginal planets only.
NO SEASONS: The planet has close to zero axial inclination and orbital
eccentricity, so there are no seasonal effects. Requires a single star;
incompatible with "Eccentric orbit" or "High inclination".
RINGS: The planet has a natural ring system.
P4.3.3. GEOLOGICAL FEATURES
RADIATION HAZARD: Radioactive minerals, or fallout from a nuclear war,
create a significant radiation hazard over parts of the planet. Marginal
planets only.
RUGGED TERRAIN: The planet's land surface is almost entirely covered
with mountains and rough, rocky terrain, with practically no flat plains or
rounded hills. Requires an ocean percentage less than 100; incompatible
with "Swampy surface".
VOLCANIC ACTIVITY: The planet has many active volcanos, enough to make
life dangerous over a large part of its surface. Marginal planets only.
P4.3.4. HYDROGRAPHIC FEATURES
FRESHWATER OCEANS: The planet's oceans have very little salt in them,
either because they're geologically young or because some life form has
affected their composition. Requires a non-zero ocean percentage.
MANY ISLANDS: The planet's land surface is broken up into a large
number of small islands, with no large continents. Requires an ocean
percentage greater than zero but less than 100.
POISONOUS OCEANS: The oceans contain some contaminant or (more likely)
life form that makes the planet's sea water poisonous to humans. Marginal
planets only; requires a non-zero ocean percentage.
SWAMPY SURFACE: Most of the planet's land surface is low-lying and
covered with swamps, marshes, mud, quicksand, and so on. Requires an ocean
percentage less than 100; incompatible with "Rugged terrain".
P4.3.5. ATMOSPHERIC FEATURES
CLOUD COVER: The planet has a permanent cloud cover over its entire
surface; it probably rains most of the time. Requires a temperature
greater than zero degrees Celsius; incompatible with "Very thin
atmosphere".
TOXIC GAS: The atmosphere contains some gas which is poisonous to
humans, making filter masks necessary. Marginal planets only.
UNSTABLE CLIMATE: The climate undergoes severe and unpredictable
changes from year to year.
VERY DENSE ATMOSPHERE: The atmosphere is dense enough to be
unbreathable without special respirator equipment, except perhaps on very
high mountains. Marginal planets only; incompatible with "Very thin
atmosphere".
VERY THIN ATMOSPHERE: The atmosphere is too thin to breathe without
special respirator equipment, except perhaps in very deep valleys.
Marginal planets only; incompatible with "Cloud cover" or "Very dense
atmosphere".
P4.3.6. BIOLOGICAL FEATURES
INTELLIGENT LIFE: The planet has a native sentient life form, which
may or may not have advanced technology. Requires advanced invertebrates
or higher life; incompatible with "Semi-intelligent life". Less likely
with life other than advanced vertebrates (25% chance, otherwise roll
again).
SEMI-INTELLIGENT LIFE: The planet has a native life form which is not
fully sentient yet, but comes close, and may develop further. Requires
advanced invertebrates or higher life; incompatible with "Intelligent
life". Less likely with life other than advanced vertebrates (25% chance,
otherwise roll again).
WORLD FOREST: The entire land surface (except perhaps ice caps) is
covered by a single huge forest. Requires an ocean percentage less than
100.
P4.3.7. ARCHAEOLOGICAL FEATURES
ANCIENT ARTIFACTS: Artifacts left behind by aliens, thousands or
millions of years ago, exist on the planet.
P4.4. EXAMPLES
Sun (G2)
G2, 9 planets: R5 H12 / E13.1 D7.2 / A G140.16 G120.18 G50.15 G50.8 J2.1
(3) Earth: Earthlike, diameter 13000 km, density 5.5 g/cm³,
gravity 1.0 G, rotation 24 h, period 365 d (365 local days),
temperature 15°C, oceans 70%, mineral rating 8, 1 moon;
advanced vertebrates, Earthlike biochemistry;
intelligent life.
Alpha Centauri (G2,K0,M5)
(A) G2, 6 planets: R9.3 H8 / H10 M15.2 / J5.1 G110.9
(A.4): Marginal, diameter 15000 km, density 5.0 g/cm³,
gravity 1.1 G, rotation 8 d, period 324 d (40.5 local days),
temperature -5°C, oceans 10%, mineral rating 6, 2 moons;
advanced invertebrates, protein-based biochemistry;
eccentric orbit, volcanic activity.
(B) K0, 2 planets: R10.2 / H8 /
(C) M5, 1 planet: H17.1 / /
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--
... Ross Smith (Wanganui, New Zealand) ... [a--i--n] at [acheron.amigans.gen.nz] ...
"A Real Cat's aim is to get through life peacefully, with as little
interference from human beings as possible. Very much like real humans, in
fact." (Terry Pratchett)