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PostPosted: Tue Apr 04, 2017 9:16 pm 
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Traveller wrote:
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.


Nor indeed science advances of the past 170 years: the Titius-Bode Law was discredited in 1846.

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PostPosted: Tue Apr 04, 2017 9:46 pm 
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Agemegos wrote:
Given this approach I would be rather inclined to do thing a bit like the way the ForeSight planet generator did. Don't make the orbitals a fixed radius: make them a fixed blackbody temperature. That is, start with shell 0 without committing to what radius it has. At the end of the process add a constant and the log of the star's bolometric luminosity to each of the shell numbers to get the log of orbital radius.


Interesting idea. Very interesting idea.

It would save math to get a shell´s black body temperature (luminosity minus shell number), which you have to do for *every* occupied orbit. Instead you have to do that calculation (luminosity PLUS shell number; call it the Modified Shell Number) then run the thing through a calculator to get to the actual orbital radius, only if or when you need an orbital radius or orbital period; if I ever publish this, I´ll probably include a table with orbital radius and orbital period for each Modified Shell Number.

And I think shell 0 should be the shell in which the black body temperature would give a planet like Earth a climate like Earth´s - a Climate value of 0.

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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
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PostPosted: Tue Apr 04, 2017 9:47 pm 
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Sir Chaos wrote:
Now... size.

Luminosity class. Referring to it as "size" presents the same sort of problems that GURPS Space 4th ed. bought into by referring to atmosphere types as "size".

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PostPosted: Tue Apr 04, 2017 11:23 pm 
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Sir Chaos wrote:
The Roche limit is a hard inner limit for orbital radius


Umm. It's one hard inner limit. I think that in the case of a rather dense planet and a very diffuse star the Roche limit might be inside the star's surface! And remember that the Roche limit is proportional to the cube root of the ratio of the density of the star to the density of the planet. The Sun's Roche limit would be about 1.56 r for a planet as dense as Earth, but 3.12 r — twice as far — for a planet as diffuse as Saturn.

Given that planets can have migrated inwards, availability of primordial material in the protoplanetary disk is not a strict limit. That is, a planet or planetismals can form out in a large and thick part of the disk and then migrate inwards to a more circumscribed or sparser region. That leaves three 'hard' inner limits that I can think of. The one that actually binds is the outermost of them.

• There's the temperature limit. Large black-body temperatures mean that the volatiles of a planet evaporate to space in a process of Jeans Escape. I guess that that means that a gas giant leaves behind a rocky-metallic residue that forms a rocky planet — it might not have formed a core in the original, but there has got to have been some dust and junk included when the gas giant coalesced, and that won't undergo Jeans Escape whereever in the planet it starts out from. However, when things get really hot even materials that aren't normally thought of as volatile (a) melt, which doesn't matter much, because they just form oceans (b) evaporate, which matters more, because they are then liable to Jeans Escape depending on their molecular mass, and ultimately (c) break down chemically into elemental gasses and simple molecules. There's an article on catastrophic evaporation of rocky planets that suggests that the limit is about 2000 K but that the evaporation takes significant time so that planets may be observed* that have formed at a cooler radius and migrated in, or have been caught as their star grows brighter with age. Such planets have a sunny face over 2000 K, metals and silicates evaporating on the dayside, blowing to the nightside as an atmosphere, and condensing on the night side. This is a transient stage, as the vapours involved don't have molecular masses all that much greater than oxygen's 32 (iron vapour is 56, silica 60) so they evaporate to space by the Jeans process; the planets lose mass, their escape velocity falls, and the Jeans process gets more rapid. I would definitely be up for featuring a system with one of these things in a hard-science exploration campaign. But they are transient and rare so I don't think I'd build a generator to recognise them. Anyway, black body temperature of about 2000 K is one hard limit.

• Then you have the tidal disruption limit, the Roche limit. Again, this is further out for gas giants than it is for rocky and especially rocky-metallic planets. If gas giants have cores then it is possible that when they reach the Roche limit they start losing hydrogen and helium from their surfaces while denser constitutent remain at the core. Such planets get denser as they get smaller, and their Roche limit moves inwards. Simultaneously they get hotter and their escape velocity falls, so they start preferentially losing hydrogen and helium to Jeans escape, which again tends to make them denser and their Roche limit move inwards. Maybe gas giants that are disrupted by tidal effects leave rocky corpses.

• And finally there's the surface of the star itself. This can be further out than the Roche limit if the planet is 14.8 times the density of the star, which is within the bounds of possibility. And Earthlike planet could orbit at the surface of a star with specific gravity of 0.373 or less, which is less dense than Sol but about right for a main-sequence star hotter than about A3 or any giant. It can only be outside the temperature limit if the star's surface is cooler than 2000K, which is only the case for L-type and late T-type brown dwarfs. The thing here is that any star that is low-density enough for the Roche limit for rocky-metallic planets to be within the surface is too hot for the temperature limit to be within the surface. So it must alway be one of the other constraints which binds.


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* E.g. KIC 12557548b

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PostPosted: Wed Apr 05, 2017 4:20 pm 
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Agemegos wrote:
(Valid and well-explained points snipped for brevity)


We have three inner limits, then: The Roche limit, black body temperature, and the radius of the star (which, as you say, is always the least constraining).

At the lower end, M9V stars have something like 8% the mass and radius of Sol, and 0.015% of the luminosity.
At 12.5 times or so the radius of an M9V star, Sol has something like 2,000 times the volume, but still only 12.5 times or so the mass, meaning the density of the M9V stars is about 160 times higher. Sounds weird to me, so I´m very open to the relevation that I made a stupid mistake here.
Plugging these numbers into the Roche limit formula, I get the result that a M9V star´s Roche limit with any object should be about 0.43 times Sol´s Roche limit with that same object.
On the assumption that results for luminosity classes between M9V and G2V lie somewhere in between their values, I make the assumption that the Roche limit does not scale very strongly between luminosity classes. An Earth-like planet has a Roche limit of about 240,000 km for a M9V star, and about 560,000 km for Sol, so my wild estimate would be "less than a million km" for a F0V star and "a few million km" for an O class star. So that´s the Roche limit.

Black body temperature (BBT)... the coolest M class stars have a surface temperature so low I kind of doubt that, whatever the maximum BBT for a planet is, a planet outside such a star´s Roche limit probably doesn´t receive enough energy to reach that BBT. But for simplicity let´s assume that a planet at an M9V star´s Roche limit has *exactly* the highest permissible BBT.
Now, the luminosity of the O class stars is 30,000 times that of Sol, or more, compared to 1/7000 or so for an M9V star; that means the "BBT limit" for closest planet to the star is about 84 times as far for Sol as for a M9V star, and about 173 times as far for the least luminous O class stars as for Sol. Assuming the BBT limit for the M9V is the same as the Roche limit, 240,000 km), that would work out to just about 20 million km for Sol and 3.4 billion km or so for an O9V class star.
The limit for Sol works for me. The limit for the O9V looks harsh, but then, O class stars are kinda extreme; it´d be 45 to 100 million km for AxV stars, which sounds much better to me.

I think we´ve got this covered, then. All it takes now is some research (mine, not yours) into how luminosity scales from subclass to subclass, and we have our innermost orbital limit nailed down.

_________________
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: Thu Apr 06, 2017 9:36 am 
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A secret admirer (Feel free to out yourself if you like!) has graciously provided me with his collection of data on the stars in our neighbourhood, allowing me to use some real world data to "anchor" what I´m doing here.

Right now I think I will put the "BBT limit" for planets at 40 shells in from the "sweet spot", the one Earth occupies around Sol; this means the hottest planets receive 16 times as much energy as Earth does, and a somewhat more than 4 times as Mercury does. That´s around 9.35 million km orbital radius around a G2V star, and about 112,000 km around a M9V star - perilously close to the surface, and also well inside the Roche limit. In fact, with these settings, the Roche limit is a more constraining limit than the BBT limit for some of the least least luminous M stars - up to about M6V or M7V or so. Most likely, a special rule "increase the inner limit (i.e. move it OUT from the star) by 3 shells per subclass below M5" should do the trick.

That leaves one more question here: How come the innermost occupied orbit around Sol is so far from the BBT limit? Certainly Mercury´s temperature is well below the maximum that a planet should be able to survive.
Rather than put the BBT limit relatively close to Mercury´s orbit (which is about 18 shells in from our "sweet spot"), I think the best solution is to have a random roll about how close to the inner limit to start rolling for occupied orbits - 0, 5, 10 or 15 shells. Call it the "inner gap" roll for now.
There won´t be an occupied orbit *exactly* on the inner limit, so using that as the basis for the first "orbital separation" roll (as specified several posts up), the innermost planet can be anywhere from 3 to 9 shells out from the inner limit - or from whatever the "inner gap" roll above determined.

Mercury, as I stated, is 18 shells in from the sweet spot, and thus 22 shells out from the BBT limit. That means its placement could be duplicated in system generation by rolling an "inner gap" of 15 shells, and an "orbital separation" of 7 shells.

_________________
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: Thu Apr 06, 2017 5:35 pm 
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Agemegos wrote:
Traveller wrote:
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.


Nor indeed science advances of the past 170 years: the Titius-Bode Law was discredited in 1846.

Titus-Bode. However, I honestly wasn't thinking of Book 6 when I made my comment, but Basic Traveller itself. Basic doesn't care about anything but the parameters of the mainworld.

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PostPosted: Thu Apr 06, 2017 8:38 pm 
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Traveller wrote:
Agemegos wrote:
the Titius-Bode Law was discredited in 1846.

Titus-Bode.

Titius-Bode.

Like a crater on the Moon, it is named after a German astronomer, Johann Daniel Titius.

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© My posts on SFRPG must not be reproduced beyond the board except with explicit permission from me.


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PostPosted: Fri Apr 07, 2017 12:54 am 
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Why in the heck did I think you misspelled it?

Damn.

Sorry.

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PostPosted: Fri Apr 07, 2017 2:31 am 
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Traveller wrote:
Why in the heck did I think you misspelled it?

Damn.

Sorry.

None of us is as young as we used to be.

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— Brett Evill

My SFRPG setting, Flat Black

© My posts on SFRPG must not be reproduced beyond the board except with explicit permission from me.


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