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PostPosted: Fri Mar 31, 2017 6:23 pm 
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Migration is also a thing, remember - Planets form further out and spiral in. So either make it possible start off with a minimum orbit that is very small (e.g. 0.05 AU), or start with larger orbits and then apply a variable migration adjustment (e.g. reduce orbit size by 1-99%).

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PostPosted: Sat Apr 01, 2017 12:31 pm 
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EDG wrote:
Migration is also a thing, remember - Planets form further out and spiral in. So either make it possible start off with a minimum orbit that is very small (e.g. 0.05 AU), or start with larger orbits and then apply a variable migration adjustment (e.g. reduce orbit size by 1-99%).


Aaaargh... complications! :shock: :o

And it´s not just that, right? I seem to recall you saying that Jupiter, for example, migrated inwards, disrupting what is now the asteroid belt, then migrated back out, if perhaps not precisely to where it used to be. Messy, messy, messy...

[aprilfools]This is just too complicated for me. I think I´m going to give up on SF altogether and switch to heroic fantasy.[/aprilfools]

I think I´m starting to get a grip on how to handle this, though.

Instead of working with orbital radii in AU or million kilometers and black body temperatures expressed in Kelvin, I will use abstract orbits and BBT on a logarithmic scale - so that for every 10 orbits, orbital radius is doubled, and for every 5 orbits (and every 5 points of abstract BBT differential), energy received is halved. An orbit´s abstract BBT could then easily be calculated by taking the orbit 0 BBT and subtracting X (I´m leaning towards 1) per orbit out from orbit 0, since orbits and abstract BBT are both on the same scale.

Place the innermost planet on orbit 0, and the next planet 1d6+4 orbits out from the previous one, which on this scale corresponds to a minimum of 1.42-etc times the orbital radius and a maximum of 2 times the orbital radius. That (if we assume the asteroid belt to occupy an orbit somewhere between 2.6 and 2.8 AU radius) allows for the observed orbital radii in our solar system as possible random results - the relation of orbital radii between two neighboring planets is always somewhere between these two numbers.

Orbit 0, then, is either slightly outside the star´s Roche limit, or the point at which black body temperature allows for a solid planet to form. Probably the former for M-class dwarves and the latter for everything else. For close binaries and circumbinary orbits of near binaries, it is instead the closest stable orbit around that binary pair.

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PostPosted: Sat Apr 01, 2017 2:26 pm 
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I am not a planetary formation guy -- my interests are in asteroids and space junk. That said, as best I can tell the current thought is that planetary formation is driven by three factors:

  • Stellar nebulas form with a particular composition and density. Fluctuations in the density lead to local collapses which become planetesimals which become planets. Black body temperatures and snow lines do play into this process, but...
  • Stars get brighter with age. The lines where various volatiles condense out are closer to the primary in the early days. Planets form and grab material from their local part of the nebula via gravity. This, together with friction from the remaining gasses in the nebula, causes the planets to lose orbital energy and spiral in towards the primary. The star continues to heat up and eventually blows away the last remnants of nebular gas, halting both the accretion process and the inward spiral.
  • Meanwhile, however, these now-massive planets' gravities affect one another and the surrounding interplanetary material (leftover planetesimals -- i.e., asteroids and comets) most strongly when resonant effects come into play. Resonance can cause planets to shift around, become eccentric enough to pass one another, re-establish them in new orbits, or eject them from the system completely.

The first two take place in a very short window after stellar formation: tens of millions of years, at most. The timing is critical: how big can the planets grow, and how close can they get to the primary, before the star's increasing luminosity blows away the nebula and halts the process? The third factor can still be working a few hundred million years later, before things settle down. Planetary scientists are still working on the specifics, though, so there aren't any hard-and-fast rules to use at this time.


Last edited by thrash on Sun Apr 02, 2017 1:53 pm, edited 1 time in total.

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PostPosted: Sun Apr 02, 2017 4:54 am 
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So, err, Sir Chaos, all this talk of resonances, too much math?

;)


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PostPosted: Sun Apr 02, 2017 10:14 am 
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hiro wrote:
So, err, Sir Chaos, all this talk of resonances, too much math?

;)


Sort of, yeah. A little. :oops:

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Man has earned the right to hold this planet against all comers, by virtue of occasionally producing someone completely bat**** insane. xkcd #556
Just like people, stars can be very important without being terribly bright. Phil Plait, "Bad Astronomy"


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PostPosted: Sun Apr 02, 2017 2:17 pm 
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thrash wrote:
I am not a planetary formation guy -- my interests are in asteroids and space junk. That said, as best I can tell the current thought is that planetary formation is driven by three factors:

  • Stellar nebulas form with a particular composition and density. Fluctuations in the density lead to local collapses which become planetesimals which become planets. Black body temperatures and snow lines do play into this process, but...
  • Stars get brighter with age. The lines where various volatiles condense out are closer to the primary in the early days. Planets form and grab material from their local part of the nebula via gravity. This, together with friction from the remaining gasses in the nebula, causes the planets to lose orbital energy and spiral in towards the primary. The star continues to heat up and eventually blows away the last remnants of nebular gas, halting both the accretion process and the inward spiral.
  • Meanwhile, however, these now-massive planets' gravities affect one another and the surrounding interplanetary material (leftover planetesimals -- i.e., asteroids and comets) most strongly when resonant effects come into play. Resonance can cause planets to shift around, become eccentric enough to pass one another, re-establish them in new orbits, or eject them from the system completely.

The first two take place in a very short window after stellar formation: tens of millions of years, at most. The timing is critical: how big can the planets grow, and how close can they get to the primary, before the star's increasing luminosity blows away the nebula and halts the process? The third factor can still be working a few hundred million years later, before things settle down. Planetary scientists are still working on the specifics, though, so there aren't any hard-and-fast rules to use at this time.


Thanks for the explanation, thrash - including the stuff originally in there which you edited out. Just because I don´t have the first idea how to handle it math-wise, doesn´t mean I don´t appreciate it, or the effort you took to explain it.

I know about Kepler´s Third Law of Planetary Motion - the square of the orbital period is proportional to the cube of the semi-major axis of the orbit. So for a X:Y orbital resonance I assume the semi-major axis of the object further out to be (cube root of (X divided by Y) squared) times that of the object closer to the star. Give me a calculator, and I can even do the math.

But do the same thing, and include a mechanism for exactly which orbital resonance to populate with a planet, for EACH AND EVERY orbit in a system? That sounds like a bitch to work out without some dedicated software or a fairly hardcore Excel spreadsheet. And that´s for a single system, never mind a whole subsector, quadrant or sector.

So... sorry. I´m pretty sure working with orbital resonances as you originally suggested would create great (and quite possibly more realistic) planetary systems than what I settled on, but the complexity of the math involved puts it beyond the scope of what I want to do.

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Space isn't remote at all. It's only an hour's drive away if your car could go straight upwards. Sir Frederick Hoyle
Earth is the cradle of humanity, but one cannot live in a cradle forever. Konstantin Tsiolkovsky
Man has earned the right to hold this planet against all comers, by virtue of occasionally producing someone completely bat**** insane. xkcd #556
Just like people, stars can be very important without being terribly bright. Phil Plait, "Bad Astronomy"


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PostPosted: Sun Apr 02, 2017 11:39 pm 
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Classic Traveller, for all its faults, did get one thing right with the world generation sequence. The planet generation sequence is quick and gets the job done. Is it realistic? Nope. Does it take into account science advances of the last forty years? Nope. However, it's playable. The thing you have to do is to find a balancing point between playability and realism. I'll be blunt: when you try to model planetary resonance and the movements of the planets prior to the snapshot in time that results from any star system generation rules you're trying to do too much.

As one of the many who has put together star system generation rules on this site, finding a balance between playability and realism was not easy. In my finished product I steered towards a more realistic setup than what exists in Traveller, but made sure the final product was still reasonably playable. To do that, I had to ignore much of the stuff you're discussing here. Orbital resonances and tracking the flight path of a gas giant during the early years of a solar system are pretty cool, and I like knowing about that stuff. But for a game, it's irrelevant. Especially if your players aren't astrophysicists. ;)

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PostPosted: Mon Apr 03, 2017 2:01 am 
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I'd say "playability" doesn't really matter. If you're a GM making worlds then you're doing that beforehand in your own time, and all you're doing in play is presenting whatever information is relevant to the players.

I've made a generation system that pretty much creates stars using stellar evolution tables and then uses some very simplified orbital dynamics to migrate worlds and throw out any that are sufficiently perturbed by more massive worlds, that is only practical to run as a computer program. And I've made a much simpler generation system that just fixes the mess that it Traveller's classic worldgen and that isn't much more complex than the CT Extended Generation System. Even the program doesn't account for resonances entirely, but either of those is "playable" and still vastly more realistic than the basic system that CT presented.

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PostPosted: Mon Apr 03, 2017 2:05 am 
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You should publish them, hint hint...

;)


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PostPosted: Mon Apr 03, 2017 2:59 am 
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The program isn't going to get released - it's in Fortran, and it's a huge sprawling mess that I don't have a hope of deciphering or translating into something else now.

As for the simpler realistic system, here it is: viewtopic.php?f=30&t=3192&p=34222

I've been sitting on that for ages and I may as well get it out there, it's not as if I'm going to do anything more with it. So enjoy, and please don't republish it (I can't be arsed with all the legalese stuff about open content or cepheus or whatever. Just use it for your own purposes, give me credit, and don't republish it!).

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