How many Earth-like worlds in a setting?

[Steve]

I'm currently working on a smaller-scale interstellar setting (a bubble of explored space less than 50 light-years across, centered on Sol), and I'm considering how many Earth-like worlds to have.

 

I realize it's my setting, and I could set it up however I please. I could have the local group of stars near Sol littered with life-supporting worlds that are Earth-like, or at least capable of sustaining humans. On the other hand, rarity makes something more special. If, within that fifty light-year wide bubble I have only two or three worlds capable of supporting humans and comparable sentient beings, that gives a different feel than if I had sixty or seventy worlds.

 

I'm just looking for opinions and discussion. How many Earth-like worlds should I have within twenty-five light-years of Sol?

[Michael Hopcroft]

Well, if you were to ask an astronomer, he would say "probably none", We've had bad luck so far finding suitable expolanets that close. There's an Earth-size planet orbiting Alpha Centarui B, but it's far away from the star and thus too cold for life.

 

Still, the fifty light-year radius surrounding Earth (double the range you posit) has some 1400 star systems, 90% of which are too dim for us to see. Those are unlikely to have a life-bearing planet. That still leaves about 133 we've observed.

 

How common you want Earth-type planets to be depends on how much travel and potential interaction with other intelligences you want in your campaign. If humans are dominant then they would probably have relatively few competitors. The planets that are out there would not have intelligent life (assuming they have life at all, which I take it is part of your definition of "earth-like") and thus open for colonization. You could build a nice little empire that way. then have someone show up from outside the sphere of unknown/unknowable intent.

[sinanju]

That's something I've thought about occasionally. This is one of those "we're living in the future!" moments. Time was when SF writers (or gamers) could realistically depict earthlike worlds within a few light-years of earth and nobody could say, "that's not right!" Sure, we might know they aren't broadcasting as much EM energy as a small star (the way earth does), but they could be inhabited by intelligent creatures with some lower (or vastly higher) tech level. Not so much anymore.

 

We're living in a world now, where we can--with increasing certainty--say that there are NO earthlike worlds within a given radius of our solar system. That we can, in fact, know what sorts of systems ARE in our neighborhood. (And this is another thing I don't remember ever seeing in science fiction--a universe where you didn't have to fly to another solar system to see what was there, because your high-tech civilization could SEE it from home, even though it was light-years away.)

[Nolgroth]

Me personally, I would at least have one other planet that is habitable. Maybe two. Is it realistic or concurrent with modern astronomy? Probably not. Would it be more fun to be able to visit other inhabited worlds? I think that it would be. I'd go with 1-3. Roll a half die if you need a nudge.

 

[Cancer]

Well, if you were to ask an astronomer, he would say "probably none", We've had bad luck so far finding suitable expolanets that close. There's an Earth-size planet orbiting Alpha Centarui B, but it's far away from the star and thus too cold for life.

 

Still, the fifty light-year radius surrounding Earth (double the range you posit) has some 1400 star systems, 90% of which are too dim for us to see. Those are unlikely to have a life-bearing planet. That still leaves about 133 we've observed.

 

How common you want Earth-type planets to be depends on how much travel and potential interaction with other intelligences you want in your campaign. If humans are dominant then they would probably have relatively few competitors. The planets that are out there would not have intelligent life (assuming they have life at all, which I take it is part of your definition of "earth-like") and thus open for colonization. You could build a nice little empire that way. then have someone show up from outside the sphere of unknown/unknowable intent.

Check places like http://www.chara.gsu.edu/~riedel/10pc2010.html and http://exoplanets.org/index.html for what is known, but the data at Exoplanets is strongly biased toward high-mass jovian planets.

 

There aren't many G-K stars in your 25 l-y radius, and I think it would be very difficult to have a truly Earthlike planet around some other kind of star.

 

Michael has the overall sense right: probably none, but you could squeeze in one if you wanted. A 1AU orbit around alpha Cen A is perhaps the most plausible, astrophysically.

 

Boost your volume up to ten parsecs in radius (65 l-y diameter) and you'll include a couple more G-K stars.

[Xavier Onassiss]

Depending on how far in the future it is, one or two (possibly more, what's the tech level) might have been terraformed. Other than that, my answer would be none.

 

Although if you're including Terra, then the answer would be one.

 

Unless....

[Clonus]

I'm currently working on a smaller-scale interstellar setting (a bubble of explored space less than 50 light-years across' date=' centered on Sol), and I'm considering how many Earth-like worlds to have. I realize it's my setting, and I could set it up however I please. I could have the local group of stars near Sol littered with life-supporting worlds that are Earth-like, or at least capable of sustaining humans. On the other hand, rarity makes something more special. If, within that fifty light-year wide bubble I have only two or three worlds capable of supporting humans and comparable sentient beings, that gives a different feel than if I had sixty or seventy worlds. I'm just looking for opinions and discussion. How many Earth-like worlds should I have within twenty-five light-years of Sol?[/quote']

 

Within 25? Ugh I've done some world generation using GURPS Space for that vicinity and once you discount the red dwarfs, you're just looking at maybe a couple of oxygen worlds, and maybe four reducing atmosphere worlds that could be earthlike with a thousand years of terraforming.

 

[Steve]

I suppose I could have a world or two in unexpected places. All I need is a planet in the life zone around a star, so I could pick one of the red dwarf stars to do this.

 

One thing I've been considering is having forerunner aliens do some biosphere tinkering and terraforming. Not enough to create a horde of life sustaining planets, but just enough to tip the balance in favor of there being some worlds for humanity to find.

[Xavier Onassiss]

I've always found it odd that the forerunner aliens had such similar preference to ours when it came to habitable planets.

 

...and then conveniently went extinct. (or just plain disappeared.)

 

For all we know, those all-powerful Xenolicious-Auld-Pharts might have created something completely different, like atmospheres of nitrous oxide wafting over oceans of lime jell-O.

 

And moons of green Velveeta....

[Old Man]

About a dozen Earth-like planets, all within a few light minutes, orbiting each other in a complex arrangement that occasionally has the planets approaching so close to each other that their atmospheres touch.

[Michael Hopcroft]

I've always found it odd that the forerunner aliens had such similar preference to ours when it came to habitable planets. ...and then conveniently went extinct. (or just plain disappeared.)

 

Perhaps the original "beneficiaries" of their work proved ungrateful or rebellious. Meaning the precursors were either driven off by the peoples they planted, or they got fed up with them and decided en masse to go somewhere else. Perhaps they did what they did out of a cosmic God Complex, and it just wasn't as fun for them when the peoples started rejecting their "divinity" -- so they packed up their super-ships and left their "worshipers" to their fate, thinking they couldn't last long without their guidance.

[sinanju]

About a dozen Earth-like planets, all within a few light minutes, orbiting each other in a complex arrangement that occasionally has the planets approaching so close to each other that their atmospheres touch.

Woooo! Like the teacup ride at the fair.

 

Don't forget five earth-like worlds in a pentagonal array, all orbiting around a common point, each circled by a score of artificial suns...as the whole thing goes flying toward galactic north at a large fraction of the speed of light.

[Cancer]

Well, if you were to ask an astronomer, he would say "probably none", We've had bad luck so far finding suitable expolanets that close. There's an Earth-size planet orbiting Alpha Centarui B, but it's far away from the star and thus too cold for life.

 

Still, the fifty light-year radius surrounding Earth (double the range you posit) has some 1400 star systems, 90% of which are too dim for us to see. Those are unlikely to have a life-bearing planet. That still leaves about 133 we've observed.

 

How common you want Earth-type planets to be depends on how much travel and potential interaction with other intelligences you want in your campaign. If humans are dominant then they would probably have relatively few competitors. The planets that are out there would not have intelligent life (assuming they have life at all, which I take it is part of your definition of "earth-like") and thus open for colonization. You could build a nice little empire that way. then have someone show up from outside the sphere of unknown/unknowable intent.

There's one F star (Procyon) in your 25 light-year radius. That grows to six if you grow it to 10 parsecs (32.6 light-years).

 

There's six G-type dwarfs (alpha Cen A, tau Ceti, eta Cassiopieae, 82 Eridani, delta Pavonis, and the xi Bootis system (which is a double, G8 + K4). That goes up to 17 if you grow the limit to 10 parsecs.

 

There's about 25 K dwarfs in that 25 l-y radius, including the xi Boo system. Extending the radius to 10 parsecs gets you a total of 45 or so.

[Xavier Onassiss]

Well, if you were to ask an astronomer, he would say "probably none", We've had bad luck so far finding suitable expolanets that close. There's an Earth-size planet orbiting Alpha Centarui B, but it's far away from the star and thus too cold for life.

 

Still, the fifty light-year radius surrounding Earth (double the range you posit) has some 1400 star systems, 90% of which are too dim for us to see. Those are unlikely to have a life-bearing planet. That still leaves about 133 we've observed.

 

How common you want Earth-type planets to be depends on how much travel and potential interaction with other intelligences you want in your campaign. If humans are dominant then they would probably have relatively few competitors. The planets that are out there would not have intelligent life (assuming they have life at all, which I take it is part of your definition of "earth-like") and thus open for colonization. You could build a nice little empire that way. then have someone show up from outside the sphere of unknown/unknowable intent.

I would add that not all "sun-like" stars are created equal; even among G-type stars, some are much more similar to Sol than others. (And presumably more likely to have Earth-like worlds.) Solar Type, Solar Analog, and Solar Twin all have differing degrees of resemblance to our Sun; the nearest "Solar Twin" is 18 Scorpii, 45 light years away. Systems like this are the ones I'm inclined to bet on.

[Cancer]

Well, if you were to ask an astronomer, he would say "probably none", We've had bad luck so far finding suitable expolanets that close. There's an Earth-size planet orbiting Alpha Centarui B, but it's far away from the star and thus too cold for life.

 

Still, the fifty light-year radius surrounding Earth (double the range you posit) has some 1400 star systems, 90% of which are too dim for us to see. Those are unlikely to have a life-bearing planet. That still leaves about 133 we've observed.

 

How common you want Earth-type planets to be depends on how much travel and potential interaction with other intelligences you want in your campaign. If humans are dominant then they would probably have relatively few competitors. The planets that are out there would not have intelligent life (assuming they have life at all, which I take it is part of your definition of "earth-like") and thus open for colonization. You could build a nice little empire that way. then have someone show up from outside the sphere of unknown/unknowable intent.

Actually, back in the 90s there were folks in the tech literature proposing different stars as the best available solar twin. The guys at the place I was at were backers of 16 Cygni B. It seems like a silly thing to debate, but it's ahard question because you can't measure the colors of the Sun in the same way as you do for stars.

 

In the post-HIPPARCOS era I'm not sure what the best analog is ... you have mass and composition and age besides luminosity and color/temperature. And planetary companions, of course.