ONE/5 Planetary Types

STEP ONE: Determine basic planetary type from table 1.5.1.

STEP TWO: Determine planetary size and density from table 1.5.2 and 1.5.3 (with optional 1.5.6). For asteroid belts, record only density and check section THREE/4 for more information on asteroid belts. For superjovians, consult 1.5.5 for determination of mass and radius.

STEP THREE: Determine surface gravity and mass by utilizing the formulae in 1.5.4

Table 1.5.1 Planetary Types

1d100  Inner Zone
1-18  Asteroid Belt
19-62  Terrestrial Planet
63-71  Chunk
72-82  Gas Giant
83-87  Superjovian
88-96  Empty Orbit
97  Interloper *
98  Trojan **
99  Double Planet ***
100  Captured Body ****
1d100  Outer Zone
1-15  Asteroid Belt
16-23  Terrestrial Planet
24-35  Chunk
36-74  Gas Giant
75-84  Superjovian
85-94  Empty Orbit
95  Interloper *
96-97  Trojan **
98-99  Double Planet ***
100  Captured Body ****

* = Reroll once on the other (Inner/Outer) table to decide what kind of interloper. Only Terrestrials, Chunks and Gas Giants allowed.

** = On 1d10 this is either a chunk (1-9) or a terrestrial planet (10). On 1d10 it is in the same orbit as either a gas giant (1-8) or a superjovian (9-10).

*** = Roll again to decide what kind of double planet. Treat all results of Asteroid Belt or Empty Orbit and up as Chunk.

**** = Roll again on the same table to decide what type. Reroll all results of Asteroid Belt or Empty Orbit and up.

Table 1.5.2 Planetary Size (equatorial radius in km)

1d10  Chunk  Terrestrial Planet  Gas Giant
1  200  (1d10 x 100) + 2,000  (1d10 x 300) + 15,000
2  400  (1d10 x 100) + 2,000  (1d10 x 300) + 18,000
3  600  (1d10 x 100) + 3,000  (1d10 x 300) + 21,000
4  800  (1d10 x 100) + 3,000  (1d10 x 300) + 24,000
5  1,000  (1d10 x 100) + 4,000  (1d10 x 300) + 27,000
6  1,200  (1d10 x 100) + 5,000  (1d10 x 1,000) + 30,000
7  1,400  (1d10 x 100) + 6,000  (1d10 x 1,000) + 40,000
8  1,600  (1d10 x 100) + 7,000  (1d10 x 1,000) + 50,000
9  1,800  (1d10 x 200) + 8,000  (1d10 x 1,000) + 60,000
10  2,000  (1d10 x 500) + 10,000  (1d10 x 1,000) + 70,000

Modification to Terrestrial Planet initial roll:
Add the Abundance Modifier

Table 1.5.3 Planetary Densities (in earths)

Zone  Chunk  Terrestrial Planet  Gas Giant
Inner  (1d10 x 0.1) + 0.3  (1d10 x 0.1) + 0.3  (1d10 x 0.025) + 0.10
Outer  (1d10 x 0.05) + 0.1  (1d10 x 0.05) + 0.1  (1d10 x 0.025) + 0.08

Modification to roll:
Add the Abundance Modifier
+1 to A and B-star inner system chunks/planets if you don't use 1.5.6 below
Rolls less than 1 become 1, rolls greater than 11 become 11.

Table 1.5.4 Mass and Gravity

Mass of the planet in earth masses:

M = (R / 6380)3 x D

... where: R is the radius of the planet (in km)
D is the density of the planet (in earths)

Surface gravity of the planet in Gs:

G = M / (R / 6380)2

... where: M is the mass of the planet (in earths)
R is the radius of the planet (in km)

Escape Velocity of the planet in earths:

V = (19600 x G x R)0.5 / 11200

... where: G is the gravity of the planet (in earths)
R is the radius of the planet (in km)

Table 1.5.5 Superjovian Size (in earth masses)

1d10  Mass
1-4  (1d10 x 50) + 500
5-7  (1d10 x 100) + 1,000
8-9  (1d10 x 100) + 2,000
10  (1d10 x 100) + 3,000

Equatorial radius (in km) = {[1d10 - (A / 2)] x 2,000} + 60,000

... where A = System Age (in Gy)

Table 1.5.6 More Precise Density Generation

For inner system chunks/terrestrials, it is reasonable to assume planets closer to the star are richer in heavy elements. To simulate this, replace the "x 0.1" factor in the density formulae with:

"x 0.127 / [0.4 + (D / L0.5)]0.67"

... where: D is the orbital distance (in AU)
L is the luminosity of the primary star

To be exacting: For main sequence stars use the luminosity of a mid-age star of the same spectral class, and for subgiants and giants use the luminosity of a main sequence star of the same mass. Densities cannot be higher than 1.5.


In systems younger than about 100 million years terrestrial planets will not yet have fully formed. These systems will be rich in random planetoids of a size somewhat like large chunks. Gas giants and superjovians form faster. Systems younger than 1Gy probably still have dust in distant orbits and a fair amount of random planetoids in the outer system.


A small airless body with trace or no atmosphere. Chunks in the inner system are rocky, those in the outer system more likely icy.


Larger than chunks, these worlds are big enough to retain an atmosphere (not all do, however) but not big enough to be gas giants.


Big planets which are mostly gas and usually have systems of moons. Detailed along with superjovians in THREE/5.


The stage between gas giants and brown dwarves, these massive planets provide significant heat from gravitational contraction. Superjovians tend to disrupt other close planetary orbits.


The division into Chunks, Planets, and Gas Giants is a somewhat nebulous one. Indeed, a large chunk in the outer regions might have a significant atmosphere, and a small planet may be like a chunk.


Lots of small chunks in a more or less defined orbital area. All systems have stray chunks, but systems with large belts tend to have more. Also, younger systems are typically more rich in stray asteroids. If a massive planet (50-100 Earth masses or more) orbits just outside or inside an asteroid belt, the planet's LaGrange points typically contain a lot of asteroids, indeed a significant percentage of the actual belt's mass. Skip sections TWO/1 to TWO/3 when detailing asteroid belts.


An orbit which is empty. Perhaps you understood that?


A planet of a type and density typical of the inner system but orbiting in the outer system, or typical of the outer system but orbiting in the inner system. This may be a high-density or low-density planet thrown into a distant orbit, or an anomaly in system creation. Interlopers are likely to have large eccentricities. Optionally, an interloper may be a planet with a much greater orbital distance than the rest of the system - treat the original orbit as an Empty Orbit and place the world 1d10+1 times the outermost orbit's average distance. Or, the interloper could be in the same but retrograde orbit (moving clockwise).


Typically a sizable chunk but rarely (1 on 1d10) a terrestrial planet which orbits in the LaGrange point of another planet - a gas giant, very massive terrestrial planet or superjovian.


This is two planets which are so close in size that the term "moon" is no longer very descriptive. Double planets are tidally locked to each other - thus their rotation period is equal to their orbital period. Pluto and Charon are a double planet system. Due to tidal stress double planets are more likely in the outer system. Another option possible with chunk-sized bodies close to the star is that the chunks are in shared orbits and exchange them periodically. To generate a double planet roll twice on the same column, and generate distance by checking the distance for lunar objects (TWO/3).


This is a planet (chunk, terrestrial planet, gas giant, or superjovian) which didn't originate with the system. It may be older or younger, and is typically in an inclined eccentric orbit, possibly even retrograde.


Not a part of regular system generation, but most systems have such clouds of distant comets, remnants of the gas which formed the system. Oort Clouds typically lie up to 1 LY away.


A simple way to identify planets is to take the stellar name and add a roman numeral to it, in order of distance from the star. Empty orbits and asteroid belts are usually not numbered. In this way, Earth would be "Sol III". Moons maycan be named with a letter in order of their distance from the planet, very small moons may be omitted. Thus, Luna would be Sol IIIa. This is just one way. Moons in our solar system is given roman numerals in order they were discovered and rings are given letters, sometimes in Greek.


Red giants:A star which has evolved to a red giant will have several effects on the system. First of all, worlds close to the star will be engulfed by it, and thus they spiral inwards to be destroyed. It is possible that a world can survive if it does not endure this process for too long especially if the red giant is cool and very big. Second, other worlds in the system will be heated. For worlds this can lead to a significant amount of mass is lost by outgassing the world "boils" away and leaves a remnant core of silicates. Another problem is that red giants seldom are very stable in luminosity they often vary. This would also have large effects on worlds. Red giants also undergo mass loss from stellar wind.

White dwarves:The disastrous formation of a white dwarf is likely to influence the system. Atmospheres may be blown away, the mass change of the star initiate orbit changes, gas giants may lose much of their atmosphere only leaving the dense core etc. Thus, close worlds to white dwarves are unlikely to exist. The red giant phase would have destroyed such worlds. In the same way, icy worlds may have been significantly affected by the previous stage.

Neutron stars:Neutron stars are formed by supernovae, and supernovae will destroy the system planets will be torn apart. However, neutron stars may have planetary systems though very strange ones. A blasted companion star may provide material for new "planets". These worlds are not habitable the radiation from a neutron star is very high.


Although planets probably can't form on their own (except for superjovians and maybe the largest gas giants) it is likely to be a large amount of rogue worlds out between the stars, worlds that have been lost. Some superjovians may have formed on their own, but most will have been ejected from their systems by close encounters and gravitational action.

Chunks: Most of the chunks are likely to have been ejected from systems early in formation. As all systems must lose some planetesimals in this way these must be fairly common in interstellar space but very hard to detect.

Planets: Planets can also have been ejected, of course. These worlds will be very cold, though interior heat sources may still be active of course.

Gas giants: These will also have been ejected from systems. It is quite possible that at least some of their moon systems survive, and moons heated by tidal action may stay active.

Superjovians: These can have been ejected or formed separately. Old superjovians don't generate much heat, but the younger ones may provide some heating though not enough to heat a moon it its own to life zone status.

Abundance: Any estimate of how common these rogue worlds are is bound to be very hypothetical. Still, if we don't count the smallest chunks but only consider the larger chunks, planets, gas giants and superjovians, we can have several such bodies per 10LY cube, perhaps 1D5-1. (Remember, a lot of these worlds will end up in the much sparser halo).

1-4  Large chunk (1000km+)
5-6  Planet
7-8  Gas Giant
9-10  Superjovian

These bodies will be very cold but otherwise much as normal worlds. Finding them will be very hard, though.