On Creating Imaginary Worlds: Questions and Answers

[Culhwch]

As prosposed in the http://www.chronicles-network.com/fo...-creation.html thread, this thread is for discussion of creating imaginary worlds in our writings. The idea, hopefully, is that if we have any questions we can post them in here, and then the many knowledgeable and kind members of chronicles will, at their leisure and under no pressure of any kind, provide answers... or else. (Or else we'll create laughably implausible worlds, most like.)

One ground rule, besides the regulars - I think this will work best if this thread is kept for physical/geographical questions, so lets keep them along those lines. We'll see how this one goes, and if we have need for a metaphysical/magical/political/religious etc Q&A thread we can look into it then.

Elseways... Have at it!

[Culhwch]

First cab off the rank - not a question, but a useful link unearthed by WeThePeople... with able assistance from Lenny:

Constructed world - Wikipedia, the free encyclopedia

[mosaix]

Does the thread cover off-planet topics such as planetary orbits and the like?

BTW - great idea for a thread!

[Culhwch]

I don't see why not - that's a physical aspect that will affect your world, after all...

And as this is an SFF forum, there is no need to stick just to fantasy, either, so greater planetary/galaxial/universal questions are welcome, too...

[mosaix]

OK first question.

I have heard that many, maybe the majority, of star systems are binaries. I take it that the stars would orbit round a common centre of gravity? What type of planetary orbits can we expect?

Would there / could there be:

1) Each star having it's own set of planets?

2) Planets orbiting in a figure of eight around both stars?

3) Planets orbiting around both stars in an ellipse?

4) A combination of any of the above in the same star system?

[j. d. worthington]

Mosaix: Chris would be the one to know the more recent scientific ideas on this, but I'll take a stab at it. From my reading (admittedly several years ago), I'd say that:

1) Each star having it's own set of planets?

Is possible, depending on how far apart the stars are. In fact, I posted a thread the other day, where they have figured out how far apart they need to be in order for the preplanetary disc to form into planetary bodies.

2) Planets orbiting in a figure of eight around both stars?

is highly unlikely. The gravitational stresses of such an orbit would almost certainly preclude it; the point at which the stellar gravitational attractions would meet would tend to a) prevent planets from forming in the first place or, in the highly unlikely event of such forming (or a "rogue planet" -- should such exist -- being captured), b) cause such tidal stresses that the body would break apart.

3) Planets orbiting around both stars in an ellipse?

Depends on what you mean here. An oblong shape (like most race-tracks, for instance) is very unlikely, because the gravitational stresses mentioned above, where the forces of the two stars combine, would tend to tear such a body apart, should it ever have formed in the first place. On the other hand, an elliptical orbit similar to the solar system's, but enlarged to circle equidistantly from both stars, is -- again, from my understanding -- the most likely. This would be at such a distance that the disc of dust would be able to form a stable body to begin with, and where the gravity from the stars would jointly stabilize (more or less) a planetary (or series of planetary) orbit(s).

4) A combination of any of the above in the same star system?

That one is also highly unlikely. The gravitational and rotational stresses applied by such would be almost certain to pull any sort of body apart, from a rocky planet to a gas giant to a smaller star -- or even one larger but less dense.

Also... don't forget that there are apparently systems which are quadruple and up, as well... These, I'd say, are unlikely to develop a planetary system... but the jury is still out on that one, I think. However, remember Asimov's "Nightfall", where the system had (as I recall) seven suns....

[chrispenycate]

Actually, the combination of "each star with it's own planets" and "planets orbiting both of the stars" is quite likely. Take our solar system, and beef up Jupiter till it undergoes fusion. Whoomph; all of Jupiter's satelites become planets, quite possibly habitable. Mercury, Venus, Earth and Mars are planets of Sol, our present sun. And Saturn and the outer planets all orbit the centre of gravity of the system, somewhere between Sol and Jupiter (closer to Sol, since we didn't add enough mass to Jupiter to rival it, but probably not within Sol)
The Earth gets extra energy when it's between the two stars, so we get a suplementary summer winter cycle going periodically into and out of phase with our present one (so very hot sumers and cold winters in one hemisphere when in phase, and the other when out, and mild everything between) and this would be even more marked on Mars.
The outer planets would get a considerable boost when they were in conjunction with Jupiter.
Using Jupiter as an example this "super seasonal" cycle would be about twelve years.

[mosaix]

JD & Chris thanks for your input - I've got a lot to think about now.

[j. d. worthington]

Chris: Would this be the case with the latest figures on "how close is too close" for stellar bodies? Or -- because of the difference in size -- would such not matter (with Jupiter, that is)?

[chrispenycate]

Originally Posted by j. d. worthington

Chris: Would this be the case with the latest figures on "how close is too close" for stellar bodies? Or -- because of the difference in size -- would such not matter (with Jupiter, that is)?

I'm assuming an evolution roughly equivalent to our solar system, with the planetary acretion disc either a little more massive or rotating a little faster so the secondary peak contains a higher percentage of the total mass; at any rate I would expect a slightly longer year.
But we'renot talking about the sort of "ultra bright" stars whose radiation pressure could push away the dust halo required to make planets.
I don't know how much more mass Jupiter would require to ignite – twice as much, three times (without obelique assistance) but it's not going to intefere with the formation of the other planets much.

[j. d. worthington]

Originally Posted by chrispenycate

I'm assuming an evolution roughly equivalent to our solar system, with the planetary acretion disc either a little more massive or rotating a little faster so the secondary peak contains a higher percentage of the total mass; at any rate I would expect a slightly longer year.
But we'renot talking about the sort of "ultra bright" stars whose radiation pressure could push away the dust halo required to make planets.
I don't know how much more mass Jupiter would require to ignite – twice as much, three times (without obelique assistance) but it's not going to intefere with the formation of the other planets much.

Thanks, Chris! As I said, I've not looked into this sort of thing much in quite some time, so my memory on it was somewhat vague... so I appreciate the refresher/better information!

[mosaix]

Originally Posted by chrispenycate

I'm assuming an evolution roughly equivalent to our solar system, with the planetary acretion disc either a little more massive or rotating a little faster so the secondary peak contains a higher percentage of the total mass; at any rate I would expect a slightly longer year.
But we'renot talking about the sort of "ultra bright" stars whose radiation pressure could push away the dust halo required to make planets.
I don't know how much more mass Jupiter would require to ignite – twice as much, three times (without obelique assistance) but it's not going to intefere with the formation of the other planets much.

Chris, is it plausible that a Jupiter-like planet could gain the extra mass by colliding with another planetary sized body, triggering the fusion process? I know that such a collision would probably involve some changes in orbit but could a star system end up with two suns in this fashion?

[daisybee]

This thread was a great idea! A lot of interesting stuff has already been explained, but if I may ask a question?
I confess to knowing zilch about this, but have a question about moons. If a planet had twin moons, is it feasable that they could cross paths each year? Like one year, one moon is in front of the other, and then the next they cross paths so the other is in front? This may be a really obvious topic, but I have tried to search for some information and I think my rubbish explanantion is letting me down

[chrispenycate]

Normally, multiple moons will be in nested orbits (in a musical harmonic ratio, as they stabilise their distances; how's that for "music of the spheres"?)
While it would be possible for an outer moon to have an orbit sufficiently eliptical that its perigee was lower than the apogee of the next moon in, sooner or later they would…well, possibly not collide exactly, but interact gravitationally enough to destablise each other. I doubt whether the situation would hold more than a couple of million years.
I suppose that a recently captured asteroid could show that behavior for a short period, but beware the peturbations; there's a strong risk the instability would lead to an exceedingly large lump of rock colliding with the planet, and dinosaurs would tell you (if they were here to) that this is not a comfortable situation.

[chrispenycate]

Originally Posted by mosaix

Chris, is it plausible that a Jupiter-like planet could gain the extra mass by colliding with another planetary sized body, triggering the fusion process? I know that such a collision would probably involve some changes in orbit but could a star system end up with two suns in this fashion?

Or by obelisques from outer space compressing its matter into a smaller space? Mathematically, yes; the compression wave and the infall energy could trigger off a chain reaction. But why (unless you're intending to revamp our solar system) look for complicated methods of doing it, when it seems extremely likely (from the number of multiple stars we can observe) that the cosmos, given the right starting conditions, can do the job on its own. A gas-giant planet is not all that different from a small star, after all.
And, personally, I don't like the idea of gas-giant sized bodies, just failed stars, supermaxicomets or whatever you want to call them, floating around loose in the Galaxy. I would hope they'd stay in the correct star lanes, like well behaved cosmic junk.

Besides "some change in orbit"? You'd probably get something as eccentric as pluto. And the original moon system's orbits would be all smashed to b… all messed up, too.