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

Greetings, all!

I'll admit that I experimented very little with GURPS Space's solar system / worldbuilding section, and am more blondie than geeky for the purposes of figuring it out with all the fine nuances. I'm currently re-reading the section and looking at the Handbook Of The Planets, but perhaps asking a question here might give a faster and/or more accurate answer.

I'm trying to get 3 planets (not moons) into a solar system, such that it contains at least the following planets:

• Nearest of the three has a Very Dense (but not Superdense) atmosphere with 5-10% Oxygen, an polar temperature no higher than 40-50°C (313-323°K), equatorial temperature as high as needed to make the planet 'work', and a gravity in the 0.8-1.2 range.
• The middle one basically earthlike garden for most purposes (75ish hydrographics, 0.95-1.05G etc.).
• The further of the three with at least a Thin atmosphere, at least some water on it, and a temperature range that falls within -20°C to +50°C (253°K to 323°K) in at least climate (doesn't need to have to be the whole planet, a single climate zone will suffice).

The Handbook seems good for quickly placing one planet in a system, but the problem is that I want to push in three adjacent ones . . .

Thanks in advance!

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

Not even touching my books, here's my first eyeball:

Planet 1 needs either a high Atmospheric Mass or to be a Large (Garden) world to have a Dense atmosphere. I'd pick a planet size, tweak the density to get the gravity you want (remember a giant waterball would have K= ~0.2 Earths), and then adjust AM to get a Dense atmosphere.

Planet 2 sounds like Earth...not much to do there.

Planet 3 sounds like it's probably a smallish Normal (Garden) world with a potentially low Atmospheric Mass, again, follow the steps for Planet 1 until you get the atmospheric density you want.

As for the orbits themselves, they seem like they'd be too close together to be stable in the long term. This leaves a couple options:
1) They aren't stable and will shift orbits, possibly being ejected completely, in time.
2) The two smaller ones are trojans of the larger one, occupying the L4 and L5 points. How this happened to be, is beyond me, but considering that a current estimate of rocky planets orbiting red dwarf stars (estimated to be only 80% of the total number of stars in our galaxy) is within an order of magnitude of 96 billion. Even with miniscule odds, such an arrangement doesn't sound impossible. Or more interestinly, perhaps someone built the solar system . . .

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

Functionally, it takes terraforming. While you can pretty easily fit two natural worlds into the habitable zone, one will be rather cold and the other quite hot. For three, you are better off terraforming the inner and outer planets, that or making it a binary system.

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

The reason it's only one planet in a system that supports life is that in reality there seems to be a sweet spot in the orbital ranges that's just far enough not to be "too hot", just close enough not to be "too cold"... it's just right. And the distance planets have to be from each other in order to not drastically affect each others' orbits has to be pretty high too... I could be mistaken, but I think in reality it's impossible to have two planets in the sweet spot range... almost.

I also read some hypothesis about the possibility of a sister planet to Earth in exactly the opposite point of the orbit. It would be on the opposite side of the sun always. Of course it was shown that our own solar system has no such planetary body, but in terms of physics, it is possible.

There is one other thing to consider: Lagrange Points

You could possibly say that the biggest planet of the three that supports life is the "Earth", and that the L4 and L5 Lagrange Points have "planets" capable of supporting life. They'd all be at roughly the same orbital distance. The only thing you'd be fudging would be the mass of the other two planets.

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

looks like it took me too long to write that response... there were none when I started, and two when I posted. =/

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

It's pretty much not possible, absent binary stars or some very strong greenhouse effects. The basic problem is that any planet clears a zone around itself, and that forces a separation between planets which is too large for the desired temperature differences.

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

It isn't. Even without considering the perturbations from other planets, two planets of similar mass in the same orbit 180 degrees apart are unstable. The slightest disturbance from perfectly opposite - such as Earth's orbit not being perfectly circular - will produce an unbalanced attraction, and they'll drift towards each other.

The best bet for solving Vicky's problem is a double planet. The larger one will have more than 1G of surface gravity, and the smaller less, but you should be able to get them both credibly habitable if your players don't look too hard.

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

Check out the scientific analysis section: "Counter-Earth"

Nothing in there saying it isn't possible... just that there isn't one there. I was wrong though... that it would "always" be on the opposite side of the sun though. Because of the elliptical orbit, there would be points where we could see it.

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

I am less than convinced. Note that section is tagged as requiring citations.
That's not what your source says:

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

I think I found that, putting the planets in the absolute minimum separation allowed in GURPS:Space, I could get as many as five habitable planets.

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

It's missing an analysis of orbital stability. Look at co-orbital objects. I believe most of them are unstable when dealing with objects of similar mass, and the only known similar-mass example is the Janus-Epimetheus system of moons.

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

I ditched the near-sun planet, and just managed to put in a Terra-like Garden at 0.79 and a standard thin-atmosphere 'garden' at 1.1 au. I get the feeling that making borderline Garden/Rock and Garden/Greenhouse planets is somewhat harder than making any of the three types (takes much more careful balancing acts).

The star is currently 0.77 Luminosity.

I was forced to change temperatures of the terran planet to 13/27/43°C. A bit on the high side, but I guess that just means I have to make the polar zones almost habitable, and the equator a somewhat uncomfortable place to live (from a human PoV, anyway). (For the record: I don't quite get how to calculate zonal temperatures from those three yearly world-average temperatures.)

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

The ratio between the innermost and outermost orbital radii that can possibly Garden planets in the Space worldbuilding section is 1.76. That's enough to have a pair of Garden planets (as small a ratio as 1.4) but not three (minimum ratio between the inner and outer is 1.96), without resorting to moons.

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

A rosette orbit could work, but that indicates deliberate placement.

That or make the furthest "inhabitable" planet a greenhouse world.

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

Yeah, I didn't clarify that I meant a problem with three. You can get three by having two co-orbital and a third further out, though that probably reduces the stability of the co-orbital pair.

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

Greenhouse worlds start at 500°K anyway, so it won't be.

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

Not sure if this is GURPS-compatible or not (don't have Space) but have you considered using a gas giant?

The goldilocks zone for a typical star seems too small to allow multiple planets. But multiple moons might be doable, if you're willing to endure some weird scenery. (A double-planet satisfies this as well).

Another way to stretch the habitable band might be to have a much larger world with a much denser atmosphere as your outer planet. The greenhouse effect could buy you a little wiggle room that way.

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

Does the goldilocks zone get wider if your star is really large and luminous? Obviously you'll have very long years, but that's bearable.

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

I don't think that will help. It will result in this zone being further from the star, and the minimum distance between planets is defined through the ratio of their orbits.

This wouldn't be much of a problem if it was easier to adjust blackbody temperatures of planets, but those seem to depend heavily on planet types.

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

Just to be clear, I was elaborating on your statement, rather than refuting.

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

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.

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

Yeah, but anything above around 2x solar luminosity doesn't live long enough.

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

I might be mistaken, but all checks so far prove that luminosity doesn't help. The orbits get bigger, but the ratios stay the same.

Oh, and I just noticed that reducing the hydrographic coverage of the distant world below the normal minimum of 50, down to 10-20, allows one to increase distance a little bit more.

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

The exclusion zone for a planet most likely does not increase as fast as the orbital radius, though Space doesn't follow that assumption.

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

Why would it? The distance the planet has to be from the sun depends on how much of the given energy it retains (depends on the properties of the planet) and how much energy is given. Energy given depends on luminosity and distance. If we adjust the star to double the energy for one planet, it will double for all other planets.

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

How about doing two suns in close orbit? Their masses combine, and their luminosity, which changes the results of the formulas.

I found that the best I could do, when playing with the numbers, is getting a warm/chilly or tropical/cool pairs... (You have to tweak, like changing the density or atmospheric mass, or having extreme high/low levels of water to change the albedo.)

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

End result is that the luminosity sum is the same for both planets, unless the stars are far apart.

By making the further planet only have 20ish hydrographics, I think I can make the earthlike planet totally Earthlike in climate, if I want to. Though at this point, I'm getting worried about deriving regional climates from the planetary data. Currently looking into one of the older threads on the topic.

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

Because the exclusion zone (not the life zone) is the zone where the planet's gravity is sufficient to eject all other objects from its orbit.

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

Aren't exclusion zones roughly comparable to to Orbit×1.4 and Orbit/1.4? (I.e. vaguely less, such that at a ratio of 1.4 is more-or-less safe from the other orbital body's gravity.) I thought those numbers come from laws of astrophysics, not made up in G:Space. Are they real-life or not?

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

I thought the exclusion zone would be about the no closer than .14 A.U. no matter the ratios.

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

Isn't the exclusion zone the greater of the two? (I think by now I'm too sleepy to remember stuff and follow the discussion. Goodnight all.)

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

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.

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

They are empirically observed ratios, and scale in some way with the mass of the planet, since Ceres fails to clear its orbit.

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

Calling Ceres a planet is a bit of a leap. When you compare extraplanetary bodies to states in size it shouldn't be called a planet, in my humble opinion.

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

One possibility to consider is to put a brown dwarf in the primary's bio-zone (make it no more that 4% of the primary's mass, and aim for the upper limit of star sizes that have a decent chance of habitable worlds). This would create a set of five libration points that could potentially have planets (because a planet's mass is insignificant compared to a brown dwarf), and they would potentially be habitable (because a brown dwarf isn't hot enough to cause problems). A case might even be made that planets could naturally form in such spots, since the whole point of libration points is that matter found there doesn't get swept away by the two bodies' gravitational influences.

The L4 and L5 points are your best bet in this regard, since any other objects in the system that might interfere will most likely be dealt with by the brown dwarf; the L1, L2, and L3 points have a much greater risk of having their orbits perturbed by other objects in the system, so you'd have to make sure that no other worlds are close enough to cause them problems.

As well, the L1, L2, and L3 points are likely to have heating problems: the L1 and L3 points will be closer to the primary than the brown dwarf; and while I haven't worked the numbers, I suspect that the reduced distance is likely to make any planets found there too hot to be habitable.

And an L2 world would face the opposite dilemma: not only would it most likely be outside of the primary's bio-zone, but it would very likely be perpetually in the brown dwarf's shadow. This would make for an interesting world; but not a habitable one.

And finally, the brown dwarf itself would be in the primary's biozone, and so would any of its satellites. I know you didn't want moons; but the difference between "moon" and "planet" is a somewhat fuzzy one, based in part on whether the thing you're orbiting is a planet or a star: a Jupiter-sized object isn't going to have any planets orbiting it simply because it isn't big enough to manage it. A brown dwarf, being somewhere between a planet and a star, is potentially massive enough that its larger satellites might qualify as planets. I wouldn't put more than one such satellite there; even that much strains credulity. But between that one and the L4/L5 planets, you probably could manage three habitable worlds in one system.

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

Habitable brown dwarf Trojans? Pretty cool idea, and one I wouldn't have thought of.

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

Those aren't actually from the laws of physics or anything, just how our own solar system turned up. It's called the Titius-Bode Law or Dermott's Law. As you can see on the 'actual data' section of the page, it's not perfect, and isn't really an accepted rule for people in that scientific field - the planetary science journal Icarus doesn't even allow people to publish articles trying to improve it any more.

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

Some form of harmonics should apply to other systems. But what numbers those have to be is anyone's guess.

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

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

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

I don't think Garden worlds can get a noticeable part of their heat from volcanism. AFAIK only hydrographics meaningfully affect BBT of Garden worlds.

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

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)

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

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.

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

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.

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

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.

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

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…

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

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.

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

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.

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

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.

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

Recapture?