[SPACE 4e] System Design help? (star systems that is...)

[ericbsmith]

Originally Posted by Qoltar

Alright I bought GURPS:SPACE 4/e and I am looking for a little bit of advice or at least someone to point me in the right direction.

Not speaking for the rules in Space (I've read Space 4e, but not really digested the info), but it's going to be mostly impossible to place more than two worlds within a habitable zone of a single star without one of three unlikely things happening:

• 1) Two of the planets will share the same orbit, rotating around each other. For two Earth-sized planets they will be tide-locked towards each other revolving around a common axis. This will likely result in a day length measured in weeks (much like our Moon, which is tide-locked to the Earth and orbits once every 28 days). The third planet may end up at another orbital position within the habitable zone.

• 2) Some serious Terraforming on at least one of the worlds. Remember that Mars probably once had water, but it was not stable over billions of years. It would be possible to Terraform worlds to be Earthlike but which would be unstable without maintenence. Perhaps the campaign is set some 100,000 years after the "Ancients" left, and their Terraformed worlds are starting to become unstable.

• 3) An outer planet is a Brown Dwarf, radiating enough heat to warm one of it's moons. The world will be rather dim, and likely subject to severe tidal effects making it geologically unstable.
  3a) A Large Gas Giant or Brown Dwarf is located in the habitable zone and has a large host of moons, several of which may be habitable.

And, of course, there's a variation on #1 in which two planets share the same orbit, but orbit on opposite sides of the sun, or (maybe) residing in each others' Lagrangian Points relative to the Sun. This kind of configuration is likely to be unstable since slight variations in orbital velocity would eventually draw the two planets closer to each other. Again, though, this is another chance to draw "Ancients" into the background, and any Ancient Race capable of moving or building entire worlds is kind of scarry.

There is also a fourth possibility: Two stars orbit each other at a distance of some dozens or hundreds of AU. Each star can sustain it's own planetary system, including habitable worlds. One may have two worlds in the habitable zone while the other has one, but on an interstellar scale these stars are definately a binary star system.

In fact, for the space campaign I've been cooking up I planned on having the capital world located in just such a binary system, figuring that any Solar system which has 3 habitable worlds would be so valuable it would quickly grow to become the center of the region economically and politically. At any rate, any system with 3 habitable worlds is going to be so rare (possibly once in a campaign) that there's no good way to randomly generate it; you really need to design such a system.