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What would the consequences be of a high number of solar systems being within close proximity to one another? I'm mainly interested in the consequences for life on multiple planets. When I say 'close proximity' I mean the stars all being between 1000-100,000 AU apart from one another, and roughly all being G-category stars?

I'm trying to create a setting where the distances between other exo-planets is not as vast as our own relative position in the galaxy, due to the issues limiting light-speed space travel.

The effects I am taking note of are:

  • Gravitational effects (how much the stars will be attracting one another, and how it will affect planetary orbits)
  • Stars heating planets
  • The amount of light being received by close stars

Would habitable planets be able to survive with such a dense amount of stars nearby? If so, what are other variables to consider that would change the planets features?

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Your environment is quite similar to that in a globular cluster. At its densest, a globular cluster may see peak stellar number densities of $\sim1000$ stars per cubic parsec, which implies a mean separation of about 20,000 AU. This leads us to conclude that many, if not most, planets will be stripped away through encounters with other stars, leading to a large population of free-floating planets.

Your systems will experience the same problems. However, $N$-body simulations have revealed some characteristics of the planetary systems that will survive intact:

With mean distances of a few tens of thousands of AU, light from other stars will not affect habitability, thanks to the inverse-square law. A star 20,000 AU away should contribute a bit more than one billionth the flux of the Sun, if the Sun was 1 AU away.

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    $\begingroup$ Given the high density of stars in the question would flares and CMEs cross between systems? If so could such events adversely effect habitability? $\endgroup$
    – Ash
    Commented Jun 10, 2019 at 18:19
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    $\begingroup$ @Ash I would be extremely surprised if that was an issue. While some Sun-like stars do exhibit superflares, separations of thousands of AU should be enough to greatly reduce the flux at a nearby star of energy or particles from such an event. $\endgroup$
    – HDE 226868 ♦
    Commented Jun 10, 2019 at 18:25
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    $\begingroup$ @Ash you'd probably have more to worry about with ageing neighbours turning into novae or supernovae; that's the sort of thing that would hit the reset switch on the evolution of local life. $\endgroup$
    – Starfish Prime
    Commented Jun 10, 2019 at 19:08
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    $\begingroup$ But on a related note, how plausible is the existence of a mostly G (or smaller and cooler) cluster with sufficient metallicity to be worth colonising, or to have produced its own life? All those interesting elements have to come from somewhere, and those grandparent-generation stars may still be around in one for or another, right? $\endgroup$
    – Starfish Prime
    Commented Jun 10, 2019 at 20:34
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    $\begingroup$ Regarding cluster metallicity: The stars that produce those heavy elements are quite massive, and likely only live for a few tens of millions of years (maybe a hundred million years if we're being generous) at the most. By the time life arises, they'll be long gone. $\endgroup$
    – HDE 226868 ♦
    Commented Jun 10, 2019 at 20:37
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I'd say that the central stars would have lifeless small rocks as the planets would lose their original orbits. The inner most stars would get hotter from the output of the ones surrounding them. The central region would be high in radioactivity from all the solar winds streaming in. The perimeter stars would be richer in planets from having captured the wanderers.

Starlight intensity would not vary much given the nearest star is 200 times as distant as Pluto is from the sun.

No comets. They can't survive even a single pass without falling into one star or another.

Life's chances for any peripheral system that's more than a few thousand AU's from nearest neighbor are pretty much the same as on singleton systems.

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    $\begingroup$ Do you have any references to support this? These comments sound more like guesses than factual statements. You state that starlight intensity wouldn't vary much, but also state that stars would be hotter from output of the surrounding stars. These statements seem to directly contradict eachother. Moreover, why should perimeter stars preferentially capture wandering planets? The orbital mechanics of capturing free-floating planets is very unlikely under many circumstances, IMO, and I see no reason why the outer stars would preferentially capture them. Also, solar winds aren't radioactive. $\endgroup$
    – conman
    Commented Jun 11, 2019 at 13:05
  • $\begingroup$ Why no comets? What do you mean by "pass"? Is that a comet's orbit or is it two stars passing close by each other (how close?)? Or something else? $\endgroup$
    – 8bittree
    Commented Jun 11, 2019 at 15:34
  • $\begingroup$ I agree with what @conman says, and Hussain, I definitely think you should edit your answer to address those concerns. That said, I do agree that some comets - in particular extreme long-period comets, given the extent of the Oort Cloud - will likely be perturbed by passing stars. The Oort Cloud's outer radius is certainly much larger than the mean stellar distances we're talking about here, so I think you're right about this disruption. $\endgroup$
    – HDE 226868 ♦
    Commented Jun 11, 2019 at 23:02
  • $\begingroup$ The Oort cloud theoretically ranges from 2000AU to 200,000AU. Given the OP's parameters, the orbits of the comets would overlap multiple systems. Any comets formed therein wouldn't be able to complete a handful of orbits before being captured in close orbit and melting and falling into a gravity well. $\endgroup$
    – Hussain Akbar
    Commented Jun 12, 2019 at 4:46
  • $\begingroup$ @conman You're completely correct, it's not. Would the density increase cause higher temperatures? One would imagine so. With the Oort cloud being theoretically 2000AU to 200,000AU no comet would be able to complete more than a few orbits without falling into a gravity well. In this N-body system, larger planets would either not form, or escape inner orbits to fall into outer ones. Or, would they? My brain aches trying to visualize this cluster. $\endgroup$
    – Hussain Akbar
    Commented Jun 12, 2019 at 5:45

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