Ykhjuy

I would like to know how close two planetary systems could be without having an adverse effect on each other. My goal is to drastically improve the chances of having habitable planets near enough that humans having the technology of today could realistically send manned spacecraft to other planetary systems and even have useful communication and trade.

I'm thinking specifically about our own solar system, and to reduce the guessing, let's assume the other system is identical to ours: same sun, same planets. Their "Earth" could be lifeless, I don't think it affects anything.

More difficult to define is what it means to "adversely affect" our own system. I have a few ideas, but these are not 100% set in stone:

• The other sun should not be brighter in our sky than Venus at its brightest.


• It should not be possible to detect any gravitational effects on our outer planets due to the presence of the other system using the best instruments to date.¹ - No stealing of planets.


• There should be no weird electromagnetic effects like auroras from the other sun other than the visible light and whatever else is expected from observing another star.


• I do not care about the Oort cloud, simply because I hope that the distance will be less than 1 ly.


• I also don't care about any effect on human mythology. Early astronomers would likely find that this star is special and much brighter than the others, which is fine.

A bonus question: Would it change if we imagine a number of these systems packed in a sphere-packing formation? If I understand correctly, it would amount to 12 neighbors equidistant from the Sun.

^{1. I admit having no clue how much we can detect, so this is one requirement that may have to go.}

The other sun should not be brighter in our sky than Venus at its brightest.

This is your tightest constraint. Astronomers are even now doing searches for Planet X and even for brown dwarfs out in the Oort cloud, and have not yet concluded that nothing is there. This shows that current technology can't with certainty even detect the gravitational effects of a Sun-like star at lightyear distances. There are certainly not large gravitation effects (other than the discounted Oort cloud disruptions.)

OTOH, Venus peaks out at -4.9 magnitudes while Alpha Centauri i at -0.3. So Alpha could get 4.6 magnitudes brighter before it was brighter than Venus. So Alpha could get about 73 times brighter which is 8.5 times closer, which is about a half a light-year distant.

So a half light year is probably the limiting distance for a Sun-like star. Dimmer stars (which are still capable of having life-bearing planets) could get closer and still not be brighter than Venus.

I very much doubt that having twelve of them would be significantly different than having one -- though the sky would be pretty spectacular!

Not difficult at all.

Here is an artist's concept of what noon look on Pluto (about 30 AU away)

If you go out a few light weeks away (1 light week is ~ 1200 AU) you can easily have a Sun-like star appear as bright as Venus.

As gravitational effects go - the force of gravity is inverse of the square of the distance, so it's fades very quickly. Sun's gravitational pull at 1 AU (the average Sun-Earth distance) is tiny portion of Earth's gravity ( https://van.physics.illinois.edu/qa/listing.php?id=184 ) but obviously that's enough to keep Earth orbiting around it. Stealing of planets could still happen, but if it does, it will at galactic speed - over hundreds of millions of years. We'll still be able to detect it - we use the gravitational effects to detect exoplanets hundreds of light years away, but it won't have any practical effects on our planet.

This far out there won't be any auroras, the solar wind of the other sun will be way too dispersed for that and the solar wind of our Sun will keep it out of our solar system.

This all assumes that the neighboring star is similar to the Sun. However, the Sun is bigger than your average star, it's G2V star, so you can easily postulate that the other star is smaller, less luminous star.

So in short - a few light weeks will probably be enough to keep the visible effects to a minimum.

As for having a cluster of such solar systems close by - also not a problem. The Solar system is in a region of the Milky Way known as the Orion arm, which is not very dense - the solar systems are on average 3 - 4 light years away.
If we were in the galactic center, we would have a lot other solar systems that are a lot closer to us. If you want a bunch of neighbors that are close, just place the Solar system in a denser part of the galaxy.

https://en.wikipedia.org/wiki/Orion_Arm#/media/File:OrionSpur.png