Planetary formation

[Carlobrand]

An issue I've found with the Traveller star system generation rules is the way it puts the planets in the same Bode's Law arrangement as Solar planets irrespective of the star they orbit. I'm curious as to the science behind that. Certainly there's a reason the planets fell into that pattern around good ol' Sol, but does that reason apply in exactly the same manner around a B-class giant or an M-class main sequence star?

It occurs to me that material orbiting in the same relative planetary orbits during planet formation would be orbiting at different velocities due to the larger or smaller mass at center, and they might not interact in quite the same way. Wouldn't that have an impact on the likely orbital loci of the various planetary nuclei? (May I never have to use those plurals in the same sentence again.)

 

[BlackBat242]

They had to base the game on some kind of science, or it would have been fantasy rather than science fiction.

And since our own solar system is the only one we had hard data on in the mid-1970s, the physics of our system was the only science they could use!


There is still nothing to contradict that physics (although there are more questions now, due to RECENT discoveries)... and since the laws of physics do not change from solar system to solar system (except in really bad fiction), then there really is little to complain about.


Besides... this is a system generation system for a game (that has to be understandable by the average high-school student/graduate)...not a university-level astrophysics text!

 

[BytePro]

Titius-Bode's relationship (Law/Rule - but I think relationship fits better) is simply an equation derived from optical measurements... There is no scientific theory behind it.

Hence, when it didn't really match our system closely enough, it invoked a bit of derision in certain scientific communities. Other relationships have been observed and later a theory found and accepted in the scientific community - so a lot probably had to do with egos and politics and poor naming convention.

Theories as to why a good portion of planets are a fair fit to the equation are typically based on harmonics. Personally, I would suspect such is based mostly on initial state - i.e. masses, velocities, heat, etc. of a pre-system mix.

As for its use in game - it is really just the basis for a whole integer orbit numbering. Within the limits of 2D6 for random generation, a desire to easily refer to things with whole numbers (just like PC stats, UWPs, etc.), it provides just as good a basis as any other. And do note that it does not refer to Titius-Bode (at least in CT) nor does it match it 100% - it simply matches our system (mostly). Personally, I would have come up with something different (but related) based on star type, but my preferences for crunchy are different than most...

 

[mike wightman]

Try GURPS: Space and adapt it to your needs.

 

[BytePro]

o:
Yeah, that's really helpful, ...

Answered the question you posted - re: 'the science'. So you are welcome.

Apparently you meant to ask for an alternative random generation game mechanic. Also answered this - 'it provides just as good a basis as any other'. That is an 'informed' answer.

As there is no scientific basis for determining planetary orbits - Titus-Bode is all that even came close - it really is whatever you want. There are highly complex programs used in the scientific community for modeling planetary formation from accretion disks - but they are just models based on pseudo random number inputs and interaction algorithms.

Any sized planet could appear in any orbit it doesn't melt away in - age of the system and star type making some setups highly unusual, but not impossible, as a planet could always be captured (with 'temporary' being aeons).

Re: 'square root of the mass of the stellar object' - <shrug>, ok. Scientifically, not any better or worse than Titius-Bode.

However, regardless of the fact that Titius-Bode is not 'realistic' (a common complaint) and nothing to do with whatever generation frequency is chosen - Traveller (CT) orbit numbers are based on our solar system. That readily supports making statements like 'a Mars to Jupiter distance', without doing any real math.

 

[BytePro]

I should add - planetary orbits aren't a carved in stone thing. Planets shift over time - given the influence of other objects. Large extrasolar planets are generally believed to have migrated to their positions (as they are not believed to be able to form at their current orbits given their star's gravity and energy) - which also supports the theory that our systems planets migrated as well.

The Kirkwood gaps in our main asteroid belt are believed to be formed from the shifting of orbits - notably of Jupiter and Saturn - which in turn are believed to have settled down some 4 billion years ago when most of the larger Kuiper belt masses stopped being ejected. (As modeled by computer programs).

The upshot of all this is it would be quite reasonable to have any size planet in any orbit - at least for a time.

 

[Carlobrand]

Answered the question you posted - re: 'the science'. So you are welcome.

That bit was actually directed at BlackBat. Your post was informative.

 

[far-trader]

That bit was actually directed at BlackBat. Your post was informative.

And you Carlobrand apparently need to check your PM box about that.

 

[BytePro]

Sorry Carlobrand - I got that and meant to put a smiley at the end of my first line, wasn't meant to sound so snarky.

Felt BlackBat242 was on the right track in that science doesn't offer any useful insight for figuring orbits. Though it sounded like he was calling Titius-Bode physics and stating it hadn't been contradicted or that physics has an answer to why our system is the way it is, so I was trying to clarify such in a useful way.

There is the physics of Orbital Resonance that relates to orbits (and planetary rings), but it quickly becomes so complex I'd agree it doesn't belong in a game, though it could inform things in a computer program. Anything else I came up with over the years just seemed too complex to not be full of logical holes and thus be even less useful than Titus-Bode. But, hey, maybe something will pop up in this thread!

Looking at some of my 20 year old notes (CT based) - I ended up using the 'Captured Planet', 'deviation' method of 2D-7, just extending it to 2 decimals. It tends to be nearer the whole integer, of course, but is random enough and with the +/- deviation, can even end up with multiple planets in the habitable zone. In the outer zone, planets still end up with large spreads, but that feels right.

I preferred to stay close to the originals, despite the lack of validity of Tidius-Bode as a Law/Rule - and everything else I came up with required changing a lot of LBB generation rules (as they use orbit number). And, ultimately, was no more scientific, nor realistic, than just using Titius-Bode.

Going through those notes - found mention of the Oort cloud as 'potentially useful for misjumps and hiding out', but no calcs. No mention of Kuiper Belt or Hills Cloud (inner Oort disk) - which probably would be more useful to have in those roles. (Hmmm... maybe because the notes are so old.)

We don't know much about these - though Kuiper Belt will actually be visited by New Horizons probe after Pluto (mission headed by my dad's old boss - so hopefully I'll get some inside scoop! ).

IIRC, we have viewed Kuiper Belt objects around other stars (Oort structures are so rarefied, I'm not even sure we are positive our system has one yet?). So maybe there is enough RW data to add these to star gen?

 

[77topaz]

Wouldn't a big O or B class star only be able to get asteroid belts, or at most gas giants, around it, (in general, of course), because of photoevaporation and other effects? (I'm using that for my "Orion OB1" sectors)

 

[BytePro]

Suspect you are referring to theories of planetary formation and generalizing that to the possibility of planets existing in a given orbit.

Photoevap would strip atmos and disrupt protoplanetary accretion - which could hinder or help planetary formation (I believe that is still a subject of speculation) - and the effects would naturally depend on distance (with size and density of O and B varying greatly, so no pat answer is likely to suffice).

The aspect that was often neglected in past lines of thought was that planets can, and most certainly do, change orbits. Especially in the system formation stages where lots of mass is moving into the star or finding their own (often 'temporary') orbits and perturbing the orbits of others. Like the migration of large exoplanets that have been found in close orbits.

 

[Hemdian]

Back in 2000, Tyge Sjöstrand was working on an alternative world generation system that has a different method of calculating orbits. It may be of interest. Available as HTML or PDF from here (scroll down for links).

 

[far-trader]

Carlobrand

You've left me little choice. You've ignored or failed to see the issued Warning and instructions to edit your offending post.

You've ignored the notice in the thread above from me (I can't credit you failed to see it since you have been in the thread at least twice since I posted it, it is in Bold Red, and has your name in it).

You've also since ignored or failed to see a followup PM to the above.

That's three strikes in my book and to insure you get the message I've had to resort to a public notice here.

I've deleted the offending post you failed to edit.

 

[Carlobrand]

Carlobrand

You've left me little choice. You've ignored or failed to see the issued Warning and instructions to edit your offending post.

You've ignored the notice in the thread above from me (I can't credit you failed to see it since you have been in the thread at least twice since I posted it, it is in Bold Red, and has your name in it).

You've also since ignored or failed to see a followup PM to the above.

That's three strikes in my book and to insure you get the message I've had to resort to a public notice here.

I've deleted the offending post you failed to edit.

For the record: whether you choose to credit me or not, I have not seen any notice. I recall Hemdian's post about an alternate world generating system, but I do not recall seeing any of the Page-1 entries following my 4:16 pm response until your announcement here prompted me to go back and see what the issue was. I'm not sure what PM I was intended to see or where I'm intended to find it; I am frankly finding site navigation to be a bit challenging. You are free to accept or reject my assertions as you see fit, but it is nonetheless the case.

I have no objection to inappropriate posts being deleted. I am as guilty as anyone else of the occasional lapse in judgment. I remain quite baffled that the piece in question was considered to have risen to that level, the more so after reviewing the FAQ, but I allow that my own opinions of my own actions are inherently biased and I respect the opinion of the moderator as final word on such issues. I will try to be more circumspect in my response in future.

I have no objection to public notice. I don't tend to go much beyond reading the public posts at this site. I will make an effort to locate the PM but, as I am still relatively new and have little reason to expect PMs from others here, I can't guarantee that I will give that feature daily attention. That being the case, I take no offense at any action you deem necessary to get my attention on an issue of concern. Clearly, this example shows that there are times I am perhaps less attentive than I should be.

I do object most strenuously to the assertion that I deliberately ignored you. With all due respect to your systems, this is the first inkling I've had that you had any objection to anything I've written anywhere in this forum. I am deeply embarrassed to have put you in the position of having to take what is clearly a rare and unusual step here. I value your input and your opinions, and it was not my intention on coming here to be a source of trouble for you.

 

[far-trader]

Carlobrand I have not seen a more reasonable reply so well stated in a very long time, if ever. I am convinced on all counts and apologize for my accusation that I was ignored. It is late (way too late here for me, I hope I somewhat coherent).

I will follow up with a more detailed explanation in the next day, via PM (aka Private Messages, I see you have found them). You can set up your account here to have PMs forwarded automatically to your email if that is helpful. I know the little number count is easy to overlook in the header.

Thank you, sincerely, for taking the time to respond and explain.

 

[mike wightman]

If you can't find it - private messages (pm) are in the top right hand corner of the screen just above the log out button.

 

[Carlobrand]

Thanks, Mike, I found it. Apologies to all for creating a disruption - and apologies to Blackbat for my acting like a jerk.

 

[Fritz_Brown]

I ended up using the 'Captured Planet', 'deviation' method of 2D-7, just extending it to 2 decimals. It tends to be nearer the whole integer, of course, but is random enough and with the +/- deviation, can even end up with multiple planets in the habitable zone. In the outer zone, planets still end up with large spreads, but that feels right.

Yeah, the only problem with 2D-7 is how often a captured planet ends up in a regular orbit - which it would share with a regular planet. I generally 'adjust' those out of the regular orbit. (Unless you're planning a Gor-like setup - which would have to ignore the other planets in the system to actually work.)

I use the regular LBB6 generation as it provides enough information to create a usable system. (I have a spreadsheet that will generate a subsector using LBB6 - but it is a little unwieldy at 50MB.) It would be nice if it didn't generate quite so many airless rocks in the habitable zone along with so many garden planets out past mars. But, a few manual tweaks can fix that.

 

[BytePro]

Yeah, the only problem with 2D-7 is how often a captured planet ends up in a regular orbit - which it would share with a regular planet. I generally 'adjust' those out of the regular orbit. (Unless you're planning a Gor-like setup - which would have to ignore the other planets in the system to actually work.)

The odds of rolling a 7 and having a planet already there aren't really that high - and of course, one can just re-roll. My 'just extending it to 2 decimals' makes for even lower odds (esp. as I did it for normal as well as captured planets). For orbit numbers, even 0.01 difference is generally quite a separation - though if they are close, then they may have more interesting effects on each other.

Yeah, LBB 6 gen could use some tweaking for even more playable systems - though other than orbit numbers, I've found it a good enough baseline for the random gen parts (the rest I do by Ref hand).