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

[vicky_molokh]

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!

[Humabout]

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 . . .

[David Johnston2]

Originally Posted by vicky_molokh

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 equatorial temperature no higher than 40-50°C (313-323°K), 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!

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.

[kdtipa]

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.

[kdtipa]

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

[Anthony]

Originally Posted by vicky_molokh

I'm trying to get 3 planets (not moons) into a solar system

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.

[johndallman]

Originally Posted by kdtipa

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.

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.

[kdtipa]

Originally Posted by johndallman

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.

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.

[johndallman]

Originally Posted by kdtipa

Check out the scientific analysis section: "Counter-Earth". Nothing in there saying it isn't possible... just that there isn't one there.

I am less than convinced. Note that section is tagged as requiring citations.

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.

That's not what your source says:

Originally Posted by Wikipedia, unsourced

"If such a planet actually existed, according to present scientific cosmology, it would be permanently hidden behind the sun ..."

[Diomedes]

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.