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

[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)

[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.

[dataweaver]

Also, the article in question doesn't say what effects the star's mass has on this: would a fairly large star push the "brown dwarf desert" outward, and by how much? Assuming that the size of the desert is proportional to the primary's mass, can the goldilocks zone catch up with the edge of the brown dwarf desert before the star is too big to be a reasonable candidate for a habitable system?

EDIT: I just ran some numbers, and sadly the answer is no. The only way you're going to have habitable brown dwarf trojan worlds is if the brown dwarf is one of those rare exceptions that exists within the brown dwarf desert. And even then, if we assume that the only way for the exceptions to get there is for a brown dwarf to migrate inward, Flyndaran's point about shaking out any habitable satellites would certainly apply to planets at the trojan points. *grumble*

Oh well. Flyndaran's right about another thing: I'm willing to do a bit of handwavery. Our understanding of system formation is still shaky, so it's possible that we're wrong about why the rare exception exists within the brown dwarf desert; it might have managed to form there in defiance of our current prevailing theory. Stranger discoveries have been made…

[Johnny1A.2]

Originally Posted by Fred Brackin

You will rapidly run into stars being too short-lived for planets to cool and evolve life,

From one table I looked at you _might_ be able to go from Sol's G2 to an F8. There's a distinctly non-linear relationship between mass and lifespan on the Main Sequence table. You can't really use stars that are much larger than Sol.

There are two zones of uncertainty in that conclusion, though. One is that if the life was introduced from outside, then the time scale compresses. If the ASBs terraformed some worlds in the star system of the big star, they don't need that vast span of deep time to become habitable, for ex.

The other is that all calculations about how long it takes for a viable biosphere to evolve are based on a sample set of one (1). That's always a shaky situation.

[dataweaver]

Originally Posted by Flyndaran

Yes, but you could always manually shove the ratios to the smallest you feel comfortable with. If that's tighter than 1.4 ratio, it doesn't take much hand waving. The data on real systems is still too sparse to make many absolute statements.

The 1.4 exclusion zone comes from the same place that the 3 exclusion zone for companion stars comes from, and it definitely varies with the mass of the planet creating the exclusion zone — though I don't recall what the relationship is. And like the companion star's forbidden zone, the inner and outer edges of the exclusion zone are also based on the shape of the planet's orbit. 1.4 is a simplification for playability.

Note also that gas giant moons often exist in resonant orbits that allow them to be much closer to each other than their gravitational influences would otherwise indicate; I suspect that the notion of resonant orbits could also be applied on the larger stage of a star system. For example, Hyperion and Titan have a 3:4 resonance, with orbital radii of 1.48 and 1.22 million km respectively. If 3:4:6 is possible, you could probably use it to fit three worlds into the habitable zone.

EDIT: As far as I can tell, a 3:4:6 ratio is possible (or is that a 4:3:2 ratio?), and you get it if you place the worlds at 0.80, 0.97, and 1.27 times the square root of L. All three of these have Blackbody Temperatures suitable for Garden Worlds (311K, 282K, and 247K, respectively). As long as you keep the ratios the same, you can vary these distances by up to 5% while maintaining habitable Blackbody Temperatures. This will vary the hottest world by up to 9K, the middle world by up to 8K, and the coldest world by up to 7K.

[dataweaver]

As a follow-up to my last post: if you make the hot world of the trio a Large Garden world and keep its atmospheric mass, planetary density, and diameter in the middle to lower sides of their respective ranges, you should be able to get something within the original constraints you asked for. The main reason for making it Large instead of Standard is to get the Very Dense atmosphere. The other two can be Standard Garden Worlds, and you'll be set.

[sir_pudding]

Originally Posted by dataweaver

EDIT: I just ran some numbers, and sadly the answer is no. The only way you're going to have habitable brown dwarf trojan worlds is if the brown dwarf is one of those rare exceptions that exists within the brown dwarf desert. And even then, if we assume that the only way for the exceptions to get there is for a brown dwarf to migrate inward, Flyndaran's point about shaking out any habitable satellites would certainly apply to planets at the trojan points. *grumble*

Recapture?