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Ok, I get it, me dumb, you smart. Only smart questions allowed. In any case I'm going to leave the question unaltered in case some others might get something out of the answers.

Thanks to everyone who's posted comments or answers. Much appreciated.

Original question: How could it be possible for an entire region of space to be filled with Earth like planets?

Based on what science tells us it's impossible. To find even one other planet that was 99% like Earth would be a miracle, but to find hundreds or thousands of already habitable worlds all fairly close to one another should be impossible.

And yet, we've all come up with stories that depict just that impossibility as if it were a given. And sci-fi from all media is filled with stories that take that impossibility as if it were a given. Star Wars and Star Trek being the two biggest culprits, but they all do it.

In movies and TV its the norm because A) we've only got one planet to shoot movies and TV on, duh... And B) it's easier for story purposes to move past that stuff. But even most novels, where you'd expect to find somewhat more reality, are filled with this trope, if you will. The Foundation and Dune series are all filled with vast Empires of Earth like planets.

Even harsh fictional worlds like Hoth, Arrakis, and the Forbidden Planet are habitable, Earth like worlds because they can support human life.

Question: How could it be possible that a region of space, say 1000 light years across, have at least one habitable, Earth like planet orbiting a star like our Sun? This is crucial because I, of course, have a story that takes place in such a universe, and I'd like to consider this as a plot point in the story.

The only things I could think of were: That region of space was artificially manipulated by god like beings to be that way, or its an alternate reality where that's the law of physics.

But how? Any thoughts? You all know my aversion to outright magic, but science/physics type handwavium is allowed.

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    $\begingroup$ We still don't know how common Earth-like planets are. The fact that we found many exoplanets that are very different still leaves a possibility that there are many Earth-like planets that we didn't find yet. $\endgroup$ – Alexander Apr 20 '18 at 17:58
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    $\begingroup$ I was told not to ever assume a question is an outright attempt at trolling, so here we go, taking this seriously. "Based on what science tells us it's impossible" What exactly does science tell us? The question itself doesn't fit with the title as well. Also that criterium is fulfilled with Earth already. 1000 light years around earth, there is at least one habitable Earth like planet, Earth. I don't think this question makes any sense currently for many reasons. Perhaps consider the sandbox in the meta $\endgroup$ – Raditz_35 Apr 20 '18 at 18:05
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    $\begingroup$ @Alexander: Also, the reason we've found many non-Earthlike planets is that some kinds of non-Earthlike planets are much easier to find, e.g. the "hot Jupiters": en.wikipedia.org/wiki/Hot_Jupiter $\endgroup$ – jamesqf Apr 20 '18 at 18:39
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    $\begingroup$ earth-like to you means that unaugmented and unaided humans can set foot on it and not die instantly? So not only comparable size, but also an atmosphere with no gases at human-poisonous levels (especially for CO2 that is a harsh requirement) and temp in the -40 to +60°C range, plus oxygen at a partial pressure for human survival? That's a pretty high bar to set. Taking into account Trappist1 (40LY distance) to mean there's ~1000 earth-size planets in goldilocks orbits in your 1000LY cube, i'd guess at least one won't kill us instantly. $\endgroup$ – bukwyrm Apr 20 '18 at 19:36
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    $\begingroup$ These universe only show a tiny fraction of all the existing planets. The Star wars universe might have a few hundred planets with a description. But the galaxy likely have millions of other planets that have never been mentioned. The know planets are important but they are all spread over the whole galaxy, light years apart. lastly, terraforming is your friend. $\endgroup$ – Vincent Apr 20 '18 at 19:55
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Your "impossibility" is considered by most planetary scientists to be not only possible but probable or even certain.

With some qualifications, of course.

Each star or multiple star system in our galaxy is separated by a vast distance from even the closest other star or star system. Parsecs are the distance units preferred by astronomers but light years are the distance units preferred by science fiction writers.

One light year is equal to about 5,878,499,810,000 miles, or 5.87849981 times 10 to the 12th power miles. And in this section of the galaxy most stars are about 5 light years from the stars closest to them.

And on the other hand planets habitable for humans have to be very close to their stars in order to have the right temperatures. Astronomers can calculate the stellar habitable zone, or range of distances in which an Earth like planet can be habitable, for any star once they know the star's luminosity, which can be calculated from its distance if known and its apparent magnitude.

Wikipedia has a list of potentially habitable exoplanets. The planets in that list have orbital radii ranging from 0.78 to 1.63 that of Earth. The planets in the list of exoplanets within the optimistic habitable zones of their stars includes planets with orbital radii ranging from 1.1 to 3.0 that of Earth.

https://en.wikipedia.org/wiki/List_of_potentially_habitable_exoplanets1

Earth has an orbital radius, the astronomical unit, of 92,955,902 miles, or 9.2955902 times 10 to the 7th power miles. That means that a single light year is 63,242.1 astronomical units long. So basically when travelling between habitable planets in different solar systems the size of the planet's orbits around their suns is insignificant.

Thus if a star system has one or more habitable planets the nearest habitable planets in other stars systems are likely to be hundreds of thousands, or millions, or tens of millions, or hundred of millions, of times farther away than the distances between the habitable planets in that star system.

The vastness of interstellar distances means that almost every space opera type story has the characters using faster than light (FTL) space drives to travel between stars.

So how common are habitable planets?

Nobody knows.

Back in 1964 Stephen Dole wrote Habitable Planets for Man about the probability of other star systems having habitable planets. As I remember, Dole calculated that there should be about 200,000,000 planets habitable for humans in our galaxy.

That would mean that approximately one star system out of every thousand would have a habitable planet.

In our region of the galaxy the stellar density is about 0.004 stars per cubic light year. Thus a volume of 1,000,000 cubic light years (like a cube 100 light years on each side) would have about 4,000 stars and about 4 planets habitable for humans.

Of course more recent studies have changed the calaculations. In fact they indicate there may be billions or tens of billions of habitable planets in hour galaxy.

https://en.wikipedia.org/wiki/List_of_potentially_habitable_exoplanets1

From about 1900 to 1950 the most favored theories of planetary formation involved a close encounter between the sun and another star which resulted in a lot of material being pulled out of the two stars which condensed to form the planets. Since such close encounters would be vary rare there would be very few planets in even an entire galaxy.

Modern theories of planetary formation indicate that the majority of stars have planets.

https://arstechnica.com/science/2016/05/number-of-potentially-habitable-planets-in-our-galaxy-tens-of-billions/2

https://www.thoughtco.com/counting-habitable-planets-30725963

https://www.space.com/21800-alien-planets-60-billion-habitable-exoplanets.html4

https://phys.org/news/2016-10-planets-galaxy.html5

https://www.nbcnews.com/science/space/8-8-billion-habitable-earth-size-planets-exist-milky-way-f8C115291866

So I sort of wonder if you have been reading old astronomy textbooks.

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Very possible.

How could it be possible that a region of space, say 1000 light years across, have at least one habitable, Earth like planet orbiting a star like our Sun?

Wikipedia has a list of potentially habitable planets and within a 500 light year radius of Earth there are already 11 planets (including Earth) that are possible (including Earth) and 17 more in the "optimistic" category.

Now every one of those (except Earth) could turn out to be e.g. a dead, barren world without atmosphere. Or they could all be teaming with varied and abundant plants and animals. We won't know that until either our observational tech gets unimaginably good or we figure out a way to get there and have a look.

But the odds that in a galaxy of over 100 billion stars that there isn't at least one region with at one habitable planet in every sphere 1000 light years in diameter nearby are very small (IMO). So it's not statistically unreasonable to expect to find a region with e.g. 10 habitable planets within a sphere of say 2000 light years diameter.

The only things I could think of were: That region of space was artificially manipulated by god like beings to be that way, or its an alternate reality where that's the law of physics.

Nope, I think that real life physics and reasonable statistical chance will get you what you want.

The Catch...*

A planet being in a habitable zone does not mean it can currently support life. There's an idea called the Fermi Paradox. It boils down to this : we expect life to exist wherever it can and we've found places it could. So where is everyone and why haven't we met aliens (tin foil hat brigade - go away ! :-) ) ?

The current thinking is that civilizations may well develop, but they develop at different times and they die out long before they ever get a chance to meet living civilizations elsewhere (because the speed of light and the distances makes that hard). Which may mean that even if the planets were once in an Earth-like state, they may simply no longer be (as a result of natural processes, not anything dramatic).

And that would cut the odds of finding your region of Earth-like planets down.

But it's still a big galaxy and I'd suggest that for fictional purposes you can create a region like that and feel safe enough you're not going off the deep end.

And it's a lot more likely that finding not one, but three galaxies populated almost exclusively by humans who speak English ( I love Stargate but park your brain when watching :-) ). So by these standards you are on solid ground.

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  • $\begingroup$ Good answer. However, as a Stargate fan I feel compelled to note that the humans who were found throughout multiple galaxies were originally from Earth and were taken from Earth to spread around those areas. As for English... true, and the makers explained this: in the original movie, the language barrier was portrayed more realistically, but the show developers didn't want to rehash the language barrier every episode, so they said they abandoned that for plot efficiency. Some aliens did not speak English, and presumably those that did learned it from humans. $\endgroup$ – Loduwijk Aug 2 '18 at 20:43
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What do you mean by earthlike? Obviously no other planet is going to be atom for atom identical( except in an infinite universe), but many rocky planets exist. There are many different features of earth, like mass, composition, spin rate, orbit, atmospheric composition and pressure, star type, moon size and distance, water level, asteroid bombardment frequency, volcanic activity, ect. With all these parameters, it would be unlikely for them all to be similar. They all effect life somewhat. However life could thrive despite a lot of variation in some. There is also the possibility of advanced tech being used. Steer asteroids away from a planet that would get bombarded, make a giant sunshade,... These things aren't that hard if you can get Lots of resources and equipment into space.

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Science seldom says impossible. Your question in bold type doesn't make sense because every earth like planet will be in a region of space 1000 light years across. Sometimes more than one. Planets are already more common than we had hoped. Probably there are some regions with plenty of habitable planets. If you're basing your belief on earlier estimations for finding life out there, realize that it was quite conservative calculation.

So are you after a feasible explanation for why one area is more inhabited or help getting over your belief that earths are very rare? Just within our solar system we have 3 potentials and with all the debris out there (asteroids) there could have been other contenders. Lots of possible scientific scenarios for how to arrive at more.

An explanation for densely habitable regions might be the stellar nursery they are born of, dominant stars helping in selection like Jupiter affects our solar system. Then there is terraforming and some neat ideas for creating a magnetic shield around planets like Mars that are not generating their own.

Space is big and we don't live in a particularly dense region of it, not as thick with stars as some regions not as isolated as others; pros and cons each way. Our telescopes only get a narrow view from which we extrapolate and guess. Kepler is a big step up in looking for planets but it's still akin to searching for flowers in a field while standing still at night with a tiny narrow beam flashlight. Only the ones that stand out with an unhindered view are able to be seen with any clarity and just those are going to take a long time to catalogue.

But space is big. So big that whatever is possible is probable. There undoubtable are regions of our galaxy, not to mention the 100 billion others, where there are habitable planets quite close. One in a million leaves you with a hundred thousand candidates.

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Its not that it's physically impossible for there to be systems like this is just astronomically unlikely. So there's a couple ways it could be done:

A. Somehow your universe has beat the statistics 9000 trillion to 1 and there just so happens to be an abundance of habitable planets concentrated in an area.

B.Some long lost ancient civilization thought it'd play sand castles with lower life forms so many billion years ago and after their mysterious demise the planets remained habitable and in relatively close proximity.

C.You're people could try to terraform other planets in a section of space to be habitable. Be that adding mass or water or what have you.

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  • $\begingroup$ What is "9000 trillion to 1" in reference to? Surely not nearby stars having planets. I think those odds only apply to me getting a date with Beyonce, while any possible scenario in the universe has better odds. $\endgroup$ – Hebekiah Apr 20 '18 at 18:58
  • $\begingroup$ @Hebekiah 9000 trillion to 1 is just a comedic exaggeration. What I'm trying to get across is the likelihood of there being so many human habitable planets within 1000 light years of each other. Tbh the likelyhood of it happening is probably lower than your chances of dating Beyonce. And again, the emphasis is on habitable planets, not the existence of planets. $\endgroup$ – Elazertwist Apr 21 '18 at 23:55
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  • Precursors did it for some reason (and it's possible that they did something MORE)
  • Some regular-but-advanced civilizations in this region of space can terraform mostly-earths (like Venus or Mars) to 'real' earths for their own purposes (for example: one of their colony star systems have 1 'real' earth but 2-3 'mostly-earth' and it's better to have 3-4 'real earths' in one star system.
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