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

[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.

For a player in my campaign I need to concoct a star system that has THREE planets within the life zone. Thats right 3 (!!) planets in some kind of tolerable and believable orbit bands and such ...all of them plausibly habitable for humans. And is VERY "okay" if one of the 3 turns out to be a mostly "Desert World" . I also own INTERSTELLER WARS ....but I figure might as well use the more detailed system.

Any advice on this? Should I just roll dice until I get "close" to what we want? Has someone already got parameters close to this in a Playtest exercise? (and please don't send me a link to a PYRAMID article - am not a subscriber....can't afford it ..YET)

So, any planetary system designing versions of "Gearheads" out there...That can give me some tips or suggestions?

- Edmund W. Charlton

[t@nya]

I don't have GURPs Space yet, but I am interested in astronomy in a general way (read books and articles). I suggest that you roll up a system and then adjust it so that three planets are habitable. For example, Mars might very well be habitable today if the planet had greater mass and hence gravity. This would have allowed it to hold on to more of it's original atmosphere. Venus would have much more habitable (though very hot, perhaps too hot for humans) in the early history of the solar system (it started out with water which, ironically, made the greenhouse effect it suffered later that much worse). In the outer solar system, a large enough gas giant could possibly warm it's moon enough for human inhabitation to be possible (maybe a ocean world similar, but warmer, than Europa).

[MrId]

It should be possible, especially if you wave your hands real hard. Mumbling something about terraforming will help a lot too, if that fits your conception of the system. I tried to run some numbers, but couldn't quite get it, but I got pretty close. You'll end up with one hot world, one "goldilocks" world, and one that's actually a Standard(Ice) type rather than Standard(Ocean), but it's really close.

With the Basic Worldbuilding chapter, you can generate as many habitable planets as you want and put them wherever you want, so I assume you're asking about the Advanced Worldbuilding chapter. One problem that may develop is that you'll need a fairly bright star -- I'd go something like an F6 V, although you could probably get away with a G class star. This is only a problem if you want the planet to have native life, as bright stars will have shorter lifespans, and any aliens won't have long to evolve.

I would not roll dice until you get close, because you'll be rolling dice for a long time. I'd just use the rules to guide your decisions.

Let's just start with an F6 luminosity, main sequence star, on page 103. We'll also assume it's a single star, although you can add a distant companion later if you really want. Set the age to something you like (or roll dice), but it should not go over the main sequence age. For arguments sake, I'll go with 2.3 billion years for age. With an F5V star, this gives a luminosity of 3.2. The orbital zones (p. 106) are going to be: Inner Limit = .13 AU, Outer Limit = 52 AU, and a Snow Line Distance of ~8.67 AU. So your three garden planets will be within 8.6 AU of the star, and probably quite a bit closer.

Generating gas giants next, if you want three habitable planets, you'll probably want either no gas giants or a conventional gas giant arrangement. I'd go with gas giants in the conventional arrangement, but it really doesn't matter for the most part.

Now this is the tricky part -- setting up orbits to give three planets in the Standard or Large Garden Range (241-320 Kelvin). Using the formula on p. 108, we see that a temperature of 241 Kelvin give us an orbital radius of 2.3 AU, while 320 gives us a radius of 1.3 AU. Now it's possible for us to get one planet at 1.3 AU, one at 1.82 AU, and one at 2.5 AU, given the minimum ratio between orbits of 1.4. The other orbits aren't important for the moment, so we can generate those later. We'll put a standard planet in each of the three orbits, and do moons later.

What's really important is the blackbody temperature. For our luminosity (3.2) we get blackbody temperatures of 320 Kelvin at 1.35 AU, 275 Kelvin at 1.82 AU, and 235 Kelvin at 2.5 AU. Your outermost planet is technically a standard (Ice) planet in the inner most one will be uncomfortable in the summer, but it's pretty dang close. You can work out the rest of the details to fit your own setting, or fiddle with it to get it close to what you want.

Hope that helps.

[Qoltar]

Id,

That Is VERY helpful .
Funny thing is ..I used to be better at this number-crunching stuff when I was a teenager ......nowadays my brain starts rebelling and just wants to do characters and plots.

Anyone else have advice about this ? Who knows - this might help out some other GURPS GM or player out there.

- E.W. Charlton

[copeab]

Originally Posted by Qoltar

For a player in my campaign I need to concoct a star system that has THREE planets within the life zone. Thats right 3!! planets in some kind of tolerable and believable orbit bands and such ...all of them plausibly habitable for humans. And is VERYT "okay" if one of the 3 turns out to be a mostly "Desert World" . I also own INTERSTELLER WARS ....but I figure might as well use the more detailed system.

I'd go for three very large moons around a very large gas giant that's in the habitable zone.

[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.

[Flyndaran]

I think that Space mentions a rossette, whereby your three planets share an orbital distance. This is ridiculously unlikely to form on its own, BUT will be dynamically stable once finished.
Really funny if archaeologists keep scouring them for the evidence of alien intervention they KNOW is there.... when it really is a one in a trillion shot.

[dataweaver]

I'm not sure how likely this is; but consider the Trojan points. It seems reasonable that if you get stable enough L4 and L5 points in the orbit of a brown dwarf, and enough mass accumulating there, you might get terrestial planets to form. Place one in each Trojan Point and one in orbit around the brown dwarf, and place the brown dwarf in the primary's habitable zone, and you can get three habitable worlds.

Note that this may not be feasable: it depends on the amount of matter that's likely to pool in the Trojan Points. I suspect that the more massive the brown dwarf, the more mass is likely to pool - except that the more massive the brown dwarf, the less stable its Trojan Points will be (maximum mass for satellite with stable trojan points = 4% of primary's mass). This option may be no more likely than a natural Rosetta (or it may be far more likely; I don't know); but it is an option that has the virtue of being unique: I have never seen anyone try it.

I'd design the terrestrial worlds according to the Basic Worldbuilding rules; I'd then place all three in the same orbit: one orbiting the brown dwarf, and one in each of the two Trojan Points. The main sticking point will be the worlds' climates: hold off on determining the worlds' atmospheric masses in Step 3. Instead, determine the blackbody temperature for the orbit that all three worlds will be in; divide each world's average surface temperature by this common blackbody temperature to get its Blackbody Correction Factor; then reverse-engineer the atmospheric mass from that:

atmospheric mass = (blackbody correction factor / world's absorption factor - 1) / world's greenhouse factor.

This can then be used to finish establishing the world's atmospheric properties.

Note also that the world orbiting the brown dwarf has the potential to see vast changes in climate, depending on how far out it orbits the brown dwarf: get it far enough away, and it will be swinging from the inner edge of the primary's habitable zone to the outer edge. Depending on how warm the brown dwarf is, you might even get additional heating from it, possibly enough to render the inner surface's climate Infernal while the outer surface fluctuates between Hot (during the day) and Cold (at night), or Warm and Cool.

[Pomphis]

Originally Posted by ericbsmith

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.

Or fifth: a trinary like Alpha/Proxima Centauri. You can have a habitable world for each star.

[ericbsmith]

Originally Posted by Pomphis

Or fifth: a trinary like Alpha/Proxima Centauri. You can have a habitable world for each star.

Possible, but probably less likely. Trinary systems have all sorts of problems of their own; getting 3 stable stars which are Sol-like is rather unlikely. The Alpha Centauri system consists of a G, a K, and an M class Star, only the former two are likely to privide Earthlike planets; M stars probably can't have Earthlike planets, and Proxima Centauri is a Flare Star (which is fairly common of M class), and makes it even less likely to have liveable planets.

Further, most Tinary systems have two stars which are in close orbit (meaning they may interfere with the formation of planets around the other) and a third that orbits at some distance. In the case of Proxima Centauri it's some ~13,000 AU (roughly ~0.06 parsecs) distance from the Binary pair of Alpha Centauri A/B (which orbit each other at ~11 AU at their closest).

Stellar Age can be a problem; in a Trinary system the wider orbiting star is likely to be of a vastly different age than the closer Binary pair of the system. In the case of the Alpha Centauri system at only 1 Billion years Proxima is *MUCH* younger than Alpha Centauri A/B, at 5-6 Billion years. Sol, for comparison, is about 5 Billion years old.

The arrangement of the Alpha Centauri system actually makes it possible for all three stars to have planetary systems, but the instability of Proxima Centauri makes it unlikely that all three could have Earthlike planets. It's age means even if Proxima did have a planet in the habitable zone the planet would look more like primordial Earth than modern Earth.