It's a big Spiral Arm

I'm gonna do it, again... From time to time I think about the big picture, and even though I usually know better, I start trying to imagine a Spacefaring setting with aliens and a reason why we haven't seen any yet. Why are we and perhaps a few other sapient species just getting to the point where we start out on the Star Road? Well, the key phrase here, from Michael Flynn, is "It's a big Spiral Arm." I'm going to Monte Carlo the possibilities and make a grand survey, the SFRPG equivalent of Doyles' Habitable Planets for Man. I just verbed a concept from game theory, of 'rolling the dice' for all outcomes, or a useful portion of them. I need to show my method and justify some of my assumptions in world-building.

 

Note- this isn't a scientificly rigorous thang, folks- it's just a bunch of wild assumptions and guesstimates!

 

First off, stars and worlds. Mass is distributed in some fashion such that there are more objects of a smaller mass than of a larger mass, and this I usually simulate by taking the cube root of a random number from 0 to 1 (the random number function on a calculator or in a spreadsheet program), taking the inverse of that result and subtracting 1 from it. In theory, this gives e a number from 0 to infinity, and in practice it gives me, about 80% of the time, a number between 1/(.1^1/3)-1 = just over 1, and 1/(.9^1/3)-1 = ~1/30. About 80% of all the stars and worlds generated this way would be of one solar or Earth mass or less. For me, the magic range for stars and worlds is .75 to 1.25. At that mass, stars put out between 1/3 and about twice the solar flux, and planets are largely still like Earth with inert atmo of about 1/2 to a little bit more than one bar, plus the partial pressure of oxygen, which will be right around .2 for all habitable worlds, for a total atmo of ~2/3rds to ~3/2s.

 

Larger stars will have larger worlds, in general, and a bigger goldilocks zone with a better chance of something being in the GZ. Smaller and lesser is the corollary. About .1 of the population of stars and worlds will be between .75 and 1.25, or 1/(1.75)^3 = .19 less 1/(2.25)^3 = .09. If one in five star systems is like our own, rocky inner planets, gas giants and a singleton star, and one tenth have the right size stars, and one tenth have the right size worlds, that's two in a thousand. Not good odds, luckily there are billions of stars in our galaxy.

 

Then two, we're looking for planets and large moons at the right distance, and a further one tenth of star systems might have gas giants migrating into the GZ. Giant moons are likely to have brothers and sisters, increasing the chances, and because the primary helps them hang onto atmo, I'd go with 1/2 to 1 earth mass, but giant moons have less mass to grow, and tend to be smaller than worlds. Between 5 and 10, or 1/(6)^3 = .003 less 1/(11)^3 = .001 is about .002. One half of one percent, after your allow that there are more moons.

 

For the goldilocks zone, I'll go with .9 to 1.1 and call that a 20% chance (that, folks was the sound of a hundred thousand statisticians screaming out in pain!). In practice, the numbers may be very different, but for any random habitable world, I'll just roll a random number 0 to 1, divide by five, add .9 and multiply times the square root of the solar flux, from 1/3 to about twice for these stars.

 

For hydrographics, the water to land ratio, I take the square root of a random number, and 1/2^1/2 is close to 71% for Earth, so we're in the middle, for once, and humanocentrism rules! Between .25 and .75 is 50% and gives me an HR or .5 to .86, probably too conservative, but a place to start.

 

That's not all, but these are the most important factors- mass, heat and water. For regular worlds, that's 1/10^2 of the right mass, times 1/5 for the right kind, times 1/5 for the right distance, times 1/2 for the right amount of water, roughly one in five thousand stars, which is still tens of millions in the Milky Way. For giant moons of gas giants, and also, possibly, double planets, that's 1/10 of stars, 1/10 of star systems, .005 with one or more moons of the right size, 1/5 at the right distance and 1/2 with enough water, or one in 200,000. Still about a million in our galaxy.

 

If I want a setting with fifty worlds, on a flag with fifty stars, say, then I need a quarter of a million stars to sift through, a territory hundreds of light years across. This tells me things about the stardrive and human exploration. It probably also implies that there have been terraforming efforts, some possibly successful, and extensive use of habitats.

 

In a galaxy of about 200 billion stars, there would probably be about 40 million habitable worlds. Maybe an order of magnitude more worlds that some sapient species would like to settle, 400 million or so. It's a big galaxy... How many of them would have sapient life, and how many of them are out and about, for fun and profit? There's an equation for this, which we've worked part way through.

 

Optimisticly, I'm going to assume that all of these have life. We've found it everywhere on Earth, in extreme conditions, and I suspect it on Mars, in the seas under the ice on Europa, and possibly other places, as well, like the cloud-tops of Venus. I think life happens, anywhere things are reasonably stable for long periods, possibly seeded from comets, or spontaneously happening.

 

Complex life is another matter. There have been long periods on Earth where it wasn't hospitable to complex ecologies, and if we aren't careful, we may precipitate another one. Out of maybe three billion years, humanity has been around for a hundred thousand, or thirty thousand to one. So, right now, there may be sapient natives on one in 30,000 worlds out there, or about fourteen thousand in our galaxy. One in a thousand have been putting out radio signals and have an industrial civilization, a bakers dozen, including us. We've been in space for less than half that time, maybe four or five species. Maybe, just maybe, something happened in the last little while, thousands to millions of years, and there really are no other starfaring species- yet?

 

My numbers are wild-ass guesstimates, but worth working with. Even if there are starfaring civilizations spreading through the galaxy, we may still meet them on more or less equal terms, some day. If we are at the head of the class and about to bust out onto the galaxy, there are millions of planets up for grabs. I do believe I will work up yet another starfaring setting.

 

Vinnies Space Adventure rides again! Or, Big Spiral Arm. Watch this space...