Very Basic Planetary Classification System
Solar Companion: Not technically a world but not the main star or even a part of the close-binary pair. Note that close binaries don't contain planets. tidal forces which are to weak to throw a planet out of orbit can be strong enough to disturb the formation of a planet. Look at the asteroid belt. The reason why there is no planet there is Jupiter's gravity. If there was a planet there, its orbit would be stable, but it couldn't form. Now guess what a stellar companion with at least 100 times Jupiter's mass can do!
Brown Dwarf: A failed star which either didn't achieve enough mass to ignite its nuclear fires, or if it did ignite, it maintain fusion for very long. This "body" emits a good deal of infrared radiation but nothing in the visible spectrum.
Gas Giant: Fairly obvious. Great for wilderness refueling if you use hydrogen to get around. One might want to distinguish between Jupiter-type (large, low abundance of methane, metallic hydrogen mantle), and Uranus-type (smaller, higher abundance of methane, icy mantle), but this is a rather minor distinction.
Magma Ball: Ouch, this place is hot. This planet is either very close to the primary or is on its way to becoming a primordial world. It is possible that such a world is tidally locked to its sun. Then, one half would be a sea of lava, and the other half would be frozen.
Primordial: A very young world. Atmosphere poisonous (usually reducing, being composed primally of nitrogen and carbon dioxide with variable amounts of water vapor, ammonia, methane, carbon monoxide and hydrogen sulfide). Geologically instable. Lots of volcanos and magma seas. The atmosphere is usually reducing, being composed primally of nitrogen and CO2, with variable amounts of H2O, NH3, CH4, CO and H2S.
Asteroid Belt: Not a planet at all, but rather a ring of debris most likely caused by a gravitational disturbance during a planet's early formation.
Planetoid: A large chuck of rock which isn't part of an asteroid belt. Not necessarily round. Could be a captured planet.
Vacuum Ball: Like the moon, essentially. Lots of rock. No atmosphere. Too big to be called a planetoid.
Snow Ball: Like a planetoid, but composed primarily of ice. Usually found only beyond the star's biozone.
Ice Planet: May have a breathable atmosphere so long as there is the ecosystem to support it. More likely a dead planet without a breathable atmosphere.Shallow World: Has some atmosphere, but it's not something you'd want in your lungs even assuming there was enough of it to breathe, which there isn't. This place will require gobs of terraforming if you really want to live here, and it probably won't be worth the effort when you're done. Mars might be an example. Ice planets with breathable atmosphere (like HothTM) are probably very rare, and they are only a few degrees below 0 °C. Most breathable-atmosphere ice planets are probably former garden worlds which have run into trouble, and won't stay habitable for long.
Barren World: Generally a non-breathable atmosphere, but there will probably be water tied up as ice crystals in the soil. Oxygen will probably reside in oxidizing compounds such as ferric oxide or in various nitrates. World may be volcanically active. Probably on its way to becoming a shallow world. No life beyond bacteria.
Superterrestrial: a rocky, high-density-core world with a planetary mass between 2 and 10 times Earth and an extremely thick atmosphere with a share of one in the biozone. The vicinity of the star might blow away much of what would otherwise have developed into a gas giant, leaving a superterrestrial world. This may depend on how fast the star ignited, of course.
Garden World: Earth is a prime example. Not only can you breath the atmosphere without mechanical aid, but life, either native or transplanted, thrives in abundance. Not all Garden Worlds need be natural... many may be the result of terraforming projects.
Water World: Over 95% covered by water and ice. Possibly a breathable atmosphere along with a teaming native ecosystem which got evolutionarily stuck due to the total lack of any freestanding continents.
Desert World: This world isn't entirely desert, but the hydrographic percentage is relatively low. Most likely, the atmosphere is somewhat thin, and the change in temperature from day to night is fairly extreme. Assuming an ecosystem is present, however, the atmosphere may be breathable.
Marginal World: Complex lifeforms either never evolved on this planet or went extinct sometime within the past couple million years. Perhaps the sun flared and knocked out the entire ecosystem, or maybe a slew of meteors struck, churning up enough dust that the sun was blotted out for several decades. Regardless, this world is ripe for colonization. Some terraforming will be required, however, if the world is to remain habitable over the long term. Mars might have been a good example eons ago.
Belch Ball: Lifeless terrestrial planet with a poisonous, toxic, or possibly even borderline-corrosive atmosphere.
Greenhouse: greenhouse worlds are terrestrial planets which are too close to their primary. Earth has the same amount of carbonates proportional to its mass as Venus has in carbon dioxide. Venus just got cooked is all (it's really too bad that Venus and Mars weren't switched; "Minerva" [as it's known to some of us in biogeochemical circles] would have been another inhabitable world - not breathable, but easily inhabitable.) Anything with an E > 1.5 ought to get cooked, so become a greenhouse world. I recommend dropping the greenhouse shares in habitable zone to zero (if there's a greenhouse world in the habitable zone, it's clearly a specific, and artifically-caused, situation, i.e., an example of your special category) and set the total in the inner zone equal to the combined total of garden, desert, water, and marginal worlds.
Exotic World: This planet has life, but not as we know it. The atmosphere is poisonous to humans and may even contain lethal micro-organisms. The temperature, however, is probably moderate enough that an individual can walk around unscathed by the environment so long as she has a self- contained oxygen supply. Many alien homeworlds will be exotics.
Liquid World: Surface predominantly covered by ammonia or liquid methane. In the former case, the world may contain a teaming alien ecosystem. In the latter, alien life is less likely. Of course, other (non-water) liquids are also possible.
Gas Core: This world was once a gas giant, but due to some catastrophic incident in the past (most probably the death throws of what was formerly the system's primary), was cooked quite hot and thereby stripped of most of its atmosphere, leaving behind a huge (super-terrestrial) core surrounded by heavier gasses. Such a world is quite large, with a high gravity and a poisonous, possibly even corrosive, atmosphere.
Special: This world cannot be placed in any of the previously defined categories. It may be an artificial planet or perhaps a fluke of nature. GM's discretion advised.

 

 

Shares of Likelihood by Orbital Zone

Contact

Inner

Biozone

Outer

A: Solar Companion

-

-

-

-

B: Gas Giant

-

1

1

20

C: Magma Ball

5

3

-

-

D: Primordial

-

3

2

-

E: Asteroid

-

2

3

4

F: Planetoid

-

2

2

2

G: Vacuum Ball

-

2

2

2

H: Snow Ball

-

-

1

3

I: Ice Planet

-

-

1

3

J: Shallow World

-

2

2

2

K: Barren World

-

-

2

2

L: Marginal World

-

-

2

-

M: Garden World

-

-

2

-

N: Desert World

-

-

2

-

O: Water World

-

-

2

-

P: Belch Ball

-

2

2

2

Q: Greenhouse

-

3

2

-

R: Exotic

-

-

2

-

S: Liquid World

-

-

2

1

Z: Special

-

-

-

-

: Empty

1

4

6

8

Total Shares/Zone

6

24

38

49

Zones: Inner, Biozone, and Outer orbital zones are pretty self-explanitory. The contact zone is that which actually makes contact with the stellar material but is still outside the vaporization radius (this zone can only occur in red giants and some orange supergiants).

Factors of Planet Type: So far, according to this system, the only influencing factor of what sort of planet you end up with in a given orbit is the planet's orbital zone. Granted, O, B, and A stars aren't supposed to produce planets due to their ridiculous level of luminosity (and as corroborated by their high angular momentum), however, we only have a few type A stars in the near- star data we're working with, and I think they are all of the smallest type, so I may overlook this consideration.

Explanation of Shares: Under this system, the program will first determine the orbital zone of the world it is generating, then it will go to the chart above, map out a probability spectrum, roll the dice, and see which world type gets chosen. So for an outer zone orbit, there is only a 20/49 chance of a gas giant being chosen (this may seem low to you, I dunno).

I've been fooling around a little with the probability data given in GURPS:Space. Here's a summary of the chances of various stars having worlds (the estimates are a bit high since I didn't take empty orbits into consideration and interpreted the "+2" rule on the margin of p99 to refer to the total roll, rather than per die). Let me know if you think these total figures are screwy or not.

I may use this data as a basis for the program I'm working on depending on the responses I get from you folks.

OIa: Planets not possible

OIb: Planets not possible

OV: Planets not possible

BIa: Planets not possible

BIb: Planets not possible

solo binary

BII: 0.5% / 0.0% chance of 4 to 19 planets

BIII: 0.5% / 0.0% chance of 4 to 19 planets

BIV: 0.5% / 0.0% chance of 4 to 19 planets

BV: 1.9% / 0.0% chance of 3 to 18 planets

AIa: 0.5% / 0.0% chance of 6 to 21 planets

AIb: 0.5% / 0.0% chance of 5 to 20 planets

AII: 0.5% / 0.0% chance of 5 to 20 planets

AIII: 0.5% / 0.0% chance of 4 to 19 planets

AIV: 1.9% / 0.0% chance of 3 to 18 planets

AV: 4.6% / 0.5% chance of 2 to 17 planets

FIa: 1.9% / 0.0% chance of 6 to 21 planets

FIb: 1.9% / 0.0% chance of 5 to 20 planets

FII: 1.9% / 0.0% chance of 4 to 19 planets

FIII: 1.9% / 0.0% chance of 3 to 18 planets

FIV: 9.3% / 1.9% chance of 3 to 18 planets

FV: 83.8% / 62.5% chance of 2 to 17 planets

GIa: 9.3% / 1.9% chance of 6 to 21 planets

GIb: 9.3% / 1.9% chance of 5 to 20 planets

GII: 9.3% / 1.9% chance of 4 to 19 planets

GIII: 9.3% / 1.9% chance of 3 to 18 planets

GIV: 16.2% / 4.6% chance of 2 to 17 planets

GV: 98.1% / 90.7% chance of 1 to 16 planets

GVI: 98.1% / 90.7% chance of 3 to 13 planets

KIa: 50.0% / 25.9% chance of 5 to 20 planets

KIb: 98.1% / 90.7% chance of 5 to 20 planets

KII: 98.1% / 90.7% chance of 4 to 19 planets

KIII: 98.1% / 90.7% chance of 3 to 18 planets

KIV: 98.1% / 90.7% chance of 2 to 17 planets

KV: 98.1% / 90.7% chance of 1 to 16 planets

KVI: 98.1% / 90.7% chance of 3 to 13 planets

MIa: 98.1% / 90.7% chance of 3 to 18 planets

MIb: 98.1% / 90.7% chance of 3 to 18 planets

MII: 98.1% / 90.7% chance of 3 to 18 planets

MIII: 98.1% / 90.7% chance of 3 to 18 planets

MV: 98.1% / 90.7% chance of 1 to 16 planets

MVI: 98.1% / 90.7% chance of 4 to 14 planets