[Space] Making a solar system with 3 (semi-) habitable planets?

[dataweaver]

Looking over GURPS Space, putting planets at a Brown Dwarf's Trojan Points would require GM intervention. It would also be fairly rare: realistically, a brown dwarf with an orbital eccentricity of even 0.1 probably wouldn't have stable Trojan Points. Fortunately, putting it in a Close orbit around the primary (necessary if it's going to be in the habitable zone of a reasonably-sized Primary) improves the odds of an essentially circular orbit; but even so, you'd need to roll a 7 or less on 3d6 for that to happen. And that's not counting the rolls needed to put it in the primary's habitable zone or to ensure that its mass is low enough to allow for stable Trojan Points (for reasons I don't understand, the ratio of the two bodies needs to be roughly 25 to 1 or more in order for the trojan points to be stable; thus my talk about brown dwarfs rather than red dwarfs).

Still, there are a lot of stars out there; and if brown dwarfs are truly more numerous than even red dwarfs, this sort of arrangement might be surprisingly common (as in, "rare rather than exceedingly rare").

Another mechanical issue to consider: for planets orbiting a brown dwarf that has Moderate or closer separation from its primary, you should determine their average Blackbody Temperature by determining it separately for the brown dwarf and for the primary, and then adding them together. That's another reason to prefer a brown dwarf over a red dwarf: a given planet probably won't be close enough to it to get overheated by the combined luminosities of its two "suns". Also disregard the "no Garden Worlds" restriction stated in the Brown Dwarf rules: that assumes that the primary source of radiation for the planet is coming from the brown dwarf, which wouldn't be the case here.

But that does mean that for your three-world system, you probably want to put the hottest world in orbit around the brown dwarf; that alone will be sufficient to keep its temperature closer to the high end of the scale. Meanwhile, the other two worlds are going to have to end up with different climates due to differences in Albedo, since they'll each essentially be the same distance away from everything in the system as the other is.

Finally, placing one world in orbit around the brown dwarf and the other two at its trojan points means that the distances between the three worlds is fixed: in a relatively low-tech setting, transfer orbits will be tricky because of this (and if they can be done, they won't have "launch windows"), and travel times will not have seasonal variations as the relative positions of the worlds shift.

[Langy]

Originally Posted by dataweaver

Looking over GURPS Space, putting planets at a Brown Dwarf's Trojan Points would require GM intervention. It would also be fairly rare: realistically, a brown dwarf with an orbital eccentricity of even 0.1 probably wouldn't have stable Trojan Points. Fortunately, putting it in a Close orbit around the primary (necessary if it's going to be in the habitable zone of a reasonably-sized Primary) improves the odds of an essentially circular orbit; but even so, you'd need to roll a 7 or less on 3d6 for that to happen. And that's not counting the rolls needed to put it in the primary's habitable zone or to ensure that its mass is low enough to allow for stable Trojan Points (for reasons I don't understand, the ratio of the two bodies needs to be roughly 25 to 1 or more in order for the trojan points to be stable; thus my talk about brown dwarfs rather than red dwarfs).

The problem with this idea is that a brown dwarf in a close orbit with a star isn't a stable orbit; it falls directly into the star. It's extremely rare to find a brown dwarf that's closer to its star than 5 AUs for stars roughly the size of our sun.

Check out Wikipedia's page on the Brown-dwarf desert.

[Flyndaran]

Originally Posted by Langy

The problem with this idea is that a brown dwarf in a close orbit with a star isn't a stable orbit; it falls directly into the star. It's extremely rare to find a brown dwarf that's closer to its star than 5 AUs for stars roughly the size of our sun.

Check out Wikipedia's page on the Brown-dwarf desert.

Migrating from beyond the snow line would almost certainly shake out any habitable moons.
But for gaming purposes, most GMs would care about impossible/possible rather than likely/unlikely.

[BaHalus]

Originally Posted by Daigoro

Would it be acceptable for the outermost planet to get most of its heat from volcanism rather than insolation?

In the last estimatives I knew about life on earth, about 2/3 of the life in our planet gets its energy from the earth, not from the sun. Of course, it is mostly archaea and bacterias, but there are ecosystems based in geothermal energy that includes macroscopic beings as well. The greenhouse effect can give a huge contribution too, to warm things up. Not sure, but I think that if you exchange mars and venus places you could get more pleasant planets.

Anyway. I've heard that venus could have life in its clouds and mars in its underground. Many satelites are good candidates for life, despite distance from the sun (gravitational tides produces a good amount of heat in some of them)