It's also useful for compulsive completists (like me) who want to know what the home-system of various Saucer-Men looks like.
There's been a whole lot of discoveries and science involving exoplanets the past decade or so and some of that is incorporated here on this chart. Some of my definitions come from The Planetary Habitability Laboratory of the University of Puerto Rico at Arecibo webstie.
Dwarf Planet, barely a planet at all.
Mercurian, under 0.1 Earth Masses
SubTerran, 0.1 to 0.5 Earth Masses
Terran, 0.5 to 2.0 Earth Masses
SuperTerran, 2-10 Earth Masses
Neptunian, 10-50 Earth Masses
Jovian,50-200 earth masses.
SuperJovian, over 200 Earth Masses up to 80 times the mass of Jupiter.
They can be Hot (typically near their star but some jovians produce their own energy), Warm (habitable zone) and Cold (outer zone)
Habitable worlds (and moons) are defined by a mean thermo-class with ranges that could support life we'd be compatible with (or recognize) on some level: Hyperthermo 100 C to 150 C, Thermo 50 C to 100 C, Meso 0 C to 50 C, Psychro -50 C to 0 C, and Hyposychro -100 C to -50 C.
The chance for a world to be populated depends on the sci-fi universe. For soft Space Opera or Saucer-Men universe I'd say any of the 5 picky Thermo classes for a habitable world means the world has a 90% or higher chance of having life, a warm world should have a 33% chance in such a universe and all other shoudl have a 2% chance (Space Opera worlds are strange). For a trek-o-verse any of the terran worlds has a 99% chance of having life, all others should be about 5%. For a "hard" sci-fi universe terran worlds with the 5 thermo-class zones should have a 30% chance of life, other Terran, SubTerran and SuperTerran worlds shoudl likely be 3% (or less), and others are probably lifeless unless the GM thinks that would be no fun.
Number of Planets
How many planets in a system? In real-life it seems to depend on star size but research is showing us previous schemes of classification are not holding up to reality. So I’m going to go abstract and simple here.
Each star has three zones: Inner, Mid, and Outer. Inner planets will tend to be Hot, Outer ones will tend to be cold.
In this generation method each star has 1d8 inner zone worlds, 1d6 Mid zone worlds, and 2d6-2 outer Zone worlds.
Inner Zone World Table
1-2 Hot Dwarf
3-15 Hot Mercurian
16-25 Hot SubTerran
26-34 Hot Terran, 1d6-4 Moons
35 Hyperthermo-Terran, 1d6-4 Moons
36-45 Hot SuperTerran, 1d6-3 Moons
46-60 Hot Neptunian, 1d6-3 Moons
61-90 Hot Jovian, 1d6-4 Moons
91-100 Hot SuperJovian, 1d6-4 Moons
Mid “Habitable”-Zone World Table
3-4 Warm Dwarf, 1d6-4 Moons
5-10 Warm Mercurian, 1d6-4 Moons
11-16 Warm SubTerran, 1d6-4 Moons
17-19 HyperthermoSubTerran,1d6-4 Moons
20-21 ThermoSubTerran, 1d6-4 Moons
22 MesoSubTerran, 1d6-4 Moons
23-25 Hot Terran, , 1d6-4 Moons
26-29 HyperthermoTerran, 1d6-3 Moons
30-33 ThermoTerran, 1d6-3 Moons
34- 38 MesoTerran, 1d6-3 Moons
39- 40 PsychroTerran , 1d6-3 Moons
41 HypoPsychroTerran , 1d6-3 Moons
42 Warm SuperTerran, 1d6-2 Moons
43-44 HyperthermoSuperTerran, 1d6-2 Moons
45-47 ThermoSuperTerran, 1d6-2 Moons
48-50 MesoSuperTerran, 1d6-2 Moons
51-52 PsychroSuperTerran, 1d6-2 Moons
53 HypoPsychroSuperTerran, 1d6-2 Moons
54-60 Warm Neptunian, 1d6 Moons
61-65 Hot Jovian, 1d10 Moons
66-80 Warm Jovian, 1d10 Moons
81-85 Hot Superjovian, 1d6
86-100 Warm SuperJovian,1d6
Outer-Zone World Table
4-10 Cold SubDwarf, 1d6-4 Moons
11-15 Cold Dwarf, 1d6-4 Moons
16-20 Cold Mercurian, 1d6-3 Moons
21-24 Cold Subterran, 1d6-2
25 HypoPsychroSubTerran, 1d6-4 Moons
26-29 Cold Terran, 1d6-3 Moons
30 HypoPsychroTerran, 1d6-4 Moons
31-34 Cold SuperTerran, 1d6-1 Moons
35 PyshcroSuperTerran, 1d6-2 Moons
36 HypoPsychroSuperTerran, 1d6-3 Moons
37-75 Cold Neptunian, 1d12-1 Moons
76 Warm Jovian, 2d12 Moons
77-90 Cold Jovian, 3d12 Moons
91 Hot SuperJovian, 1d8Moons
92 Warm SuperJovian, 1d8 Moons
93-100 Cold SuperJovian, 1d8 Moons
Moon sizes for SubTerran and Smaller Worlds(1d10)
Moon sizes for Terran and SuperTerran Worlds (1d20)
Moon Sizes for Neptunian and Larger Worlds (1d100)
Moons can be same size class as a world but not larger.
Moons will typically be same temperature class as appropriate for zone.
In Mid Zone there is a 10% chance a moon of a super-terran or larger world will have an atmosphere allowing for Thermic variance if mercurian or larger.
In the Outer Zones there is a 10% chance of a moon of a super-terran or larger world with Thermic variance if dwarf sized or larger.
2-4 Thermo Moon
There you go a basic system to generate a system for a sci-fi game universe.
I have an old pickier then Traveller world generation system compatible with Traveller I'm considering renovating after recent reading about what scientists have found and think could be out there, exo-planet theory and exploration is an amazing and growing field.