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Old 08-24-2018, 07:18 PM   #11
Agemegos
 
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Default Re: The Stars Our Destination

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Originally Posted by AlexanderHowl View Post
Within the Milky Way Galaxy, there may be as many as 1 trillion stars (recent research suggests that the old figures vastly underestimate the actual number of stars) and there may be an average of ten planets per star (though that depends of the individual researcher). In addition, there are likely millions of lesser objects of interest per star system. So, how do you deal with that wealth of locations in your space campaigns?
First, by recognising that nearly all that profusion is spurious. There are perhaps a trillion stars in the Milky Way, but that's counting red dwarfs, late K orange dwarfs, brown dwarfs, blue-white or blue main-sequence dwarfs hotter than about A5, white dwarfs, sub-giants, giants, supergiants, and hypergiants, none of which are of any more interest than individual seeds on the grasses on the prairie when I am running a Western. All that matters about them is that there are so many of them that whatever resource any one could supply would be ubiquitous and cheap.There are maybe 30 million stars in the Milky Way the right size to plausibly have a human-habitable planet, and half of those are too young or too old. And the average number of habitable planets per candidate star is more like 0.05 than 10. All that's without even considering plausible limits to the Galactic habitable zone.

We don't have ten trillion planets to worry about. 1.5 million is much more like it. The other 9,999,998,500,000 can be ignored like seeds of grass.

Now, 1.5 million human-habitable planets is still too many for some purposes.
  • If I want to run something in a setting like Lensmen or Star Wars, with a galaxy full of diverse intelligences and cultures, with a star drive fast enough that you can get across it in a day or a week or so, I treat the individual planets as you might treat, say, individual dwellings in North America. That is, I make up anything plausible I want when I want it, and let character-players do the same within the bounds of taste and genre.
  • If I don't want to deal with a degree of diversity indistinguishable from a continuum then I don't use the whole Galaxy for my setting. I specify a comparatively slow means of interstellar travel (in my usual SF setting, FLAT BLACK, ships travel at only about 1,000 times the speed of light, so it would take 170 years to cross the Galaxy) and a comparatively short history (FLAT BLACK is only 950 years in the future).

In designing FLAT BLACK I decided that I wanted inhabited planets to have a certain cognitive weight — I wanted them to be more numerous and harder to keep track of than countries on Earth, but not so profusely numerous that they must blur together. I decided on a count of one thousand inhabited worlds, which makes colonies in FLAT BLACK less numerous than, but on the close order of magnitude of, county-equivalents in the USA (of which there are 3,142, or 3,242 if you include territories).

So I coded a program that would use linear congruent pseudo-random numbers to generate a GURPS Space random star system around a star, reading its known stellar characteristics from the Extended Hipparcos compilation of astronomical data. Then I formed an extract of the XHIP that excluded unlikely and very distant stars (to reduce the run time). And then I wrote a loop that would generate a system around each star in the extract and record salient data. Which I ran two hundred times. I looked at the results, decided that run number 169 suited my requirements best, and settled on that as my setting. Now I have a list of the habitable worlds within 175 light-years of Sol in my setting that I can sort any way I want, and I can re-generate a detailed system sheet for any system, and a detailed planet sheet for any world in any system, within 175 light-years of Sol and get the same results every time. Then I did a little historical modelling to determine which worlds would be settled when, and chose the date when the number of settlements reached one thousand — it turns out to be AD 2959. That's my game date. Now knowing how large a volume my inhabited planets are scattered through (it turns out to be a sphere 155 light-years in radius) I can choose the speed for my space travel that gives the outer worlds the degree of remoteness I want them to have.

That's how I got the thousand worlds I wanted, and how I established that people haven't got around to populating the other 1.499 million habitable worlds in the Galaxy yet.
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Old 08-24-2018, 08:03 PM   #12
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Default Re: The Stars Our Destination

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Fi

We don't have ten trillion planets to worry about. 1.5 million is much more like it.
First I want to give you applause and great thanks!

Then I want to see if I can go a bit farther in providing a little context.

If we consider the Milky Way galaxy as a disc 100,000 light-years in diameter and 1000 ly thick we can see how much space we end up with between the average pair of worlds.

Someone may want to point out that the Galactic core is much thicker than that and what about all the globular clusters? But those places are where a great many of the wrong kind of stars live. It's really only the spiral arms that are likely to have Earth-like planets.

Properly we should probably punch out a hole in the center of our disc but these are only rough calculations so we'll just round down at the end to cover that.

100,000 ly across and 1000 ly thick and we invoke the Pi r squared thingy and get....7.85 trillion cubic light years. My but that is a very large number.

Then we divide that by 1.5 million and get 5.23 million cubic ly per human habitable world. We'll start our rounding down and call it 5 million.

Then we have to do the Pi r cubed thing and get a sphere with a radius of c. 115 ly for each world with an average of 230 ly between such worlds but let's round down to 200 ly (or c. 60 parsecs).

That happens to be almoat twice the number I got when I took the system in Gurps Space 1e and did a brute force calculation of what its' system would predict. So human-habitable worlds have gotten about 2x as scarce in the last 30 years. :)

Increase these numbers/decrease the distances however much you want to account for terraforming projects and anything else you want but you may see why I never bother with creating a maximally accurate stellar map of all the stars within x distance of Earth. Even 1 habitable world wihtin a 2 digit number of ly is probably beating the odds.

....and no, i don't go with ftl speeds on the lower end of the scale either. If you want that average trip to take a week you're in excess of 1LY per _hour_. Cosmic supers probably need speeds imeasured in LY per minute rather than the default Hyper Warp of 1 LY per day we see in the Warp Power in Basic..
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Old 08-24-2018, 08:42 PM   #13
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Default Re: The Stars Our Destination

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....and no, i don't go with ftl speeds on the lower end of the scale either. If you want that average trip to take a week you're in excess of 1LY per _hour_. Cosmic supers probably need speeds imeasured in LY per minute rather than the default Hyper Warp of 1 LY per day we see in the Warp Power in Basic..
I seem to recall that in Lensman the spaceships travelled at about 90 parsecs per hour in interstellar space. That's roughly 2.6 million times the speed of light.
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Old 08-25-2018, 02:53 AM   #14
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Default Re: The Stars Our Destination

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I seem to recall that in Lensman the spaceships ravelled at about 90 parsecs per hour in interstellar space. That's roughly 2.6 million times the speed of light.
And here I am working at speeds of 5-10 lightyears per week in my setting. :) Man do I feel slow!

(And a few ships travel at 1.5 to 2 lightyears per year, mostly sleeper ships developed a TL earlier and headed out beyond the frontier....)
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Old 08-25-2018, 08:50 AM   #15
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Actually, any A-type through M-type main sequence star may have habitable planets, so we are talking about 400 billion potential candidates outside of the Core. Out of those, each star probably possesses an average of two orbits within the habitable zone, meaning that there are 800 billion orbits within habitable zones. While it is difficult to calculate the exact number of orbits with potentially habitable planets or moons, I would suggest that 20% of them having potentially habitable planets and 5% of them having potentially habitable moons, meaning that there would be 200 billion potentially habitable planets or moons.

Of that number, they are probably equally divided between Garden, Greenhouse, Ice, and Ocean. Garden worlds already have indigenous life and, since the likelihood that they use compatible amino acids is phenomenally low, each one of them will be uninhabitable. Greenhouse worlds probably can be terraformed, but they offer such challenges that an FTL capable civilization would likely not bother terraforming them. It would be the Ice and Ocean worlds that would be terraformed by an FTL capable civilization, so there would be 100 billion terraforming candidates available (the indigenous life of Ice and Ocean worlds are likely single-celled and would be overwhelmed by any terraforming process or driven into any extreme ecosystems that resembled the original climate of the world before terraforming).

With 100 billion terraforming candidates divided between the 500 billion stars outside of the core, every 1:5 stars within an FTL capable polity would likely have a terraforming candidate. Such systems would attract settlement and trade as people will want to be in on the ground floor for such a massive endeavor. In addition, 1:10 stars within an FTL capable polity would possess Garden plants, where scientists would research the native life for exotic drugs for medicinal and/or recreational purposes. Of the other star systems, they would probably be inhabited by any group that wanted some isolation in return for extra risk (an FTL capable polity might even make such areas reduced tax zones in order to encourage development).
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Old 08-25-2018, 09:11 AM   #16
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Actually, any A-type through M-type main sequence star may have habitable planets,
This is very unlikely. Check Gurps Space p.103.

Even an A9 only spends 2.1 billion years on the Main Sequence. A quarter of that time any Earth-like worlds didn't even have a solid surface. An oxygen atmosphere doesn't seem to have formed until about 2.5 bilions years ago.

So no even vaguely Earth-like worlds arounbd A types unless the evolution of the atmosphere is greatly accelerated over our history.

https://en.wikipedia.org/wiki/Timeli...istory_of_life

You'd rather wait til after the 1000 ft high tides have significantly abated too. :)

Back to Gurps Space the largest stars that would give their plantes the 6 billion years we've had is an F8.
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Old 08-25-2018, 09:30 AM   #17
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Default Re: The Stars Our Destination

In my setting, I chose about 20 main worlds to describe in depth. There are another 40 or so worlds that are briefly described minor colonies or stop-over points of interest between the main worlds, and another dozen briefly described worlds in the closed-off, repressive alien empire that it would be difficult for people to travel to. I purposely don't specify the geometry of these worlds, only their connections - by leaving the positions and distances vague and just mentioning that it takes about maybe half an hour on wormhole trams to go between worlds (with an additional 2 hours for decompression when going from Zhiroom to Whum) I can just bypass all those boring non-inhabitable places. Then I give lots of hints that there are still plenty of habitable worlds out there to discover or re-contact to let the characters be part of expeditions to explore strange new worlds, meet strange new people and new civilizations, and boldly go etc. There are just enough briefly described minor worlds that have very deadly poisonous atmospheres or which are regularly blasted by lethal levels of radiation from nearby pulsars or are in the throws of massive tectonic upheaval with continent-sized lava lakes to give the idea that the universe is a large and hostile place and be grateful for the few habitable worlds you can get.

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Old 08-25-2018, 09:39 AM   #18
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Default Re: The Stars Our Destination

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Actually, any A-type through M-type main sequence star may have habitable planets
In addition to what Fred Bracklin said, there's now the very real question as to how habitable worlds around M-class dwarfs can be. These stars are prone to some pretty ferocious space weather, what with the extreme solar flares and burping off massive coronal mass ejections. It is an open question whether all this solar activity will erode away the atmospheres of any planets it has, leaving only airless balls of rock. We are finding out there is a lot that we don't know about the interaction of the solar space plasma and its magnetic fields with the planetary magnetic field and extreme upper atmosphere.

In my setting, I chose to allow habitable planets around higher-end M-class stars, and put several of my main worlds in orbit around such stars. This leads to wonderfully alien worlds with all the quirks we've found out about what living on a tide-locked world might be like (here's an example).

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Old 08-25-2018, 01:36 PM   #19
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Default Re: The Stars Our Destination

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In my setting, I chose to allow habitable planets around higher-end M-class stars, and put several of my main worlds in orbit around such stars. This leads to wonderfully alien worlds with all the quirks we've found out about what living on a tide-locked world might be like (here's an example).
In my own setting, M2 is the coolest I'm going for habitable worlds, though any M could have planets. I haven't put an upper limit on the F type stars having habitable planets yet, though I'm angling for F5 at present.

This still gives me about 40 possible systems within a 30 lightyear radius to play with for my setting.
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Old 08-25-2018, 02:15 PM   #20
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I generally use a 50 ly radius from Earth for FTL capable campaigns. With ~2,000 stars, it is easy enough to know the particulars of each star, including known planets (though we have probably only found 1% of the planets within 50 ly, as we could not have found a system like the Sol System with our technology unless it was on the same orbital plane as the Sol System). As our detection technology improves, we will likely find stuff that invalidates our own fictional systems, but that is just life.
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