Viability of a mega star system

Question

So my story hinges largely upon a precursor type race centered around a single star system that went extinct tens of thousands of years prior to the modern day. Obviously, the most common way to go about making an advanced civilization of this sort would be to create a Dyson Swarm and move away from planetary living entirely in favor of constructed habitats yadda yadda yadda, stop me if you've heard this one before.
So partly out of a desire for novelty and partly out of a desire for the sick visual (comic rather than novel), I had the idea of a civilization using the standard 'space folding' FTL technology to simply transport desirable planets from their native star systems and into their own. This would eventually result in a monstrously large star system (that they perhaps feed atomic hydrogen from nebulae in order to maintain its size and gravity) with several hundred planets orbiting it in perfectly calculated and carefully controlled orbits, looking sort of like the cartoony depictions of atoms you often see but turned up to 11.

The thing is, I know just enough about astronomy and astrophysics to know that I don't know nearly enough to actually understand whether or not this is an even remotely viable strategy, or if it would all completely fall apart under conventional physics. Would it be practical, or even feasible, much less possible (provided the method of planetary transportation was a non-issue) to custom-build your own star system with hand-picked planets from around the galaxy? Additionally, what might be some unique benefits or drawbacks that could come from doing this?

Edit: suggested questions do help with some aspects (particularly using gas giants with planet-sized moons. I like that one), but the level of technology that this civ has access to makes some of the presently known limitations a non-issue, and the precision of aligning planets all into the extremely tight habitable zone may not be as big of a deal for them. I was more concerned with figuring out the orbital mechanics and what would be necessary to keep planets from crashing into each other. Keeping planets equidistant on their various axes works for that, but as I said, I'm going for the 'sick visual' to some degree (and I don't mind fudging the rules a little, as this isn't the hardest of hard sci-fi stories,) so I was hoping for multiple orbits, multiple axes. Something beyond our current capabilities by a long shot, but not something that utterly destroys conventional physics on its own: maybe just... bruises conventional physics a little.

Answer 1

TL;DR:

Assuming you have the considerable energy to get the imported planets into stable orbits, then yes.

Hundreds to thousands of Earths can fit into stable orbits in our solar systems habitable zone.

Positioning the planets:

Conversation of momentum should hold through space folding FTL - the host star systems will have different velocities relative to each other, the planets will have different relative speeds to each others suns. You'll need to apply 10's - 100's of km's per second of delta-v to a planet-sized mass within a timeframe of days to settle down the orbits... that's a lot. You may be able to optimise this by careful timing and positioning, and / or slingshot around a distant gas giant, but it's still dyson-sphere levels of energy. (Unless your space-folding can also space-bend or space-shrink/expand in such a way that this is handwaved).

The more planets in the habitable zone, with different orbital periods, the more likely 2 will eventually interact and disaster results for those people. A maths analogy would be the Lowest Common Multiple of any pair of integers in a large set.

Consider 2 planets in orbit, one in 7 month orbit, one in 8 month orbit; Eventually 7, 14, 21, 28, 35, 42, 49, 56 intersects with 8, 16, 24, 32, 40, 48, 56. When they do, both of their orbits will change. Subtly or catastrophically. You may be able to buy some time by carefully choosing the numbers, or making the orbits have different planes, or having widely elliptical orbits, but eventually it'll all fall down, with the exception of one configuration:

The most stable orbits for multiple planets in a habitable zone would be on the same ring, with the same orbital period, but equally divided in phase. These 4 planets would be stable for a very long time:

Adding a planet to the ring will also require thrust by all other planets, assuming their all the same mass, the planets should be kept equidistant from each other so that the forces the planets exert on each other cancels out, eg for 6 planets:

To get from 4 to 6 planets, the 2 incoming planets will need huge amounts of thrust to enter that orbit, but at least 3 of the existing planets will need to do manoeuvres as well. This would be 2 thrusts, one to transition to an elliptical orbit, and one to transition back to the original orbit, but "behind" where it was by a small fraction of the year, enabling room for the new planets on the ring.

Max number of planets:

What's the upper bound of the number of planets in the ring before it all breaks down? It more likely depends on the ratio of the mass of the planets before any other factor. If the planets are equal mass, then I believe the limiting factor will be making sure the atmospheres don't touch, as that could affect their day length and create friction, which would create heat, and thus intense storms. This dense packing will also remove all moons, and any satellites not in orbit perpendicular to the planet's orbit.

Working with our solar system, and leaving a safety margin of 8 earth diameters between them, you could fit ~9000 earths onto the orbital ring of Earth in the current solar system. (940,000,000km circumference of orbit, 12,700km diameter of earth).

Some people in the comments believe you'll need more of a safety margin than this. I only have my gut feeling of "but they should cancel out" to defend this number.

More than one ring may be possible, if your habitable zone is big enough, but each ring must have an orbital period different enough so the interaction is (essentially) zero.

Interesting issues

Tides would be a pain, the more planets added to the ring, the stronger the pulls in the forward or rearward direction. As the number of planets grows, it will be like sea levels rise. Tides would synchronise with the length of the day too, high tide would occur and sunrise and sunset.

One planet would rise at midday, and at sunset would be high in the sky. Few hours later that would set. You would then get a few moments of pitch blackness, depending on latitude and geography, and then another planet would rise. When this reaches high in the sky, the sun will rise.

Your two neighbouring planets would both be perpetually in half shadow. There would be no "phases of the moon" kinda thing.

Everytime a new planet is added, the night sky will become permanently brighter. The first 50 planets will have near total darkness at night. When there's thousands, true darkness will be rare. This will make land-based telescopes useless, and mess with nocturnal creatures.

Also getting a new planet in the system would be a big deal for the entire ring. Eg it would break every planets individual calendar. It would be half a year where you couldn't rely on sunrise and sunset being their precalculated values, the day length may be subtly different. A perfectly accurate clock could appear to lose and then gain time. Given we struggle with leap seconds (Eg stock exchanges lengthen every second by a a fraction rather than risk breaking high frequency trading with a 61-second minute), this interruption could be unpopular for the ruling body. If Christians or similar are present, the calculated date of Easter would be incorrect. Festivals which occur on equinoxes and solstices might have to be moved, or worse case even skipped. Moving a public holiday with only a few weeks or months notice would cause havoc with businesses, people would have to cancel travel plans, etc.

It's possible one planet could be spared from this chaos, one single planet could keep a constant calendar through all system-wide manoeuvres and be spared this complexity - probably the ones whose citizens donated the most to the ruling parties reelection campaign.

Trade

Transport between planets "upstream" on the ring would be very costly, but "downstream" would be very cheap (daily launch windows, once you get in orbit, a tiny amount of delta V needed to get from one planet's gravity well to the nexts). Unless you're willing to expend extreme amounts of propellant, (or space-folding is available everywhere and has replaced rockets), resources and trade would always go in one direction.

The most efficient path for non-time-critical resources (eg bulk frieght) in this system, interestingly, involves them applying thrust to take them upstream. Applying thrust to start you moving towards the planet ahead of you, will speed a ship up relative to the sun, swing you out a bit, and when you rejoin the ring you'll have moved downstream. Another thrust is needed to stabilise your new orbit. See https://en.wikipedia.org/wiki/Hohmann_transfer_orbit

Renders?

Someone in the comments have asked for renders of what 9000 planets on the same ring look like:

Excuse the low quality. Here's 9000 planets on the same ring as seen from a nice vantage point

View of ring east from northern hemisphere at mid day. Will look roughly the same to the west too.

Answer 2

Sounds perfectly feasible, as long as you keep maintaining it. The planetary orbits are unlike to be stable over the long term (thousands of years), but if you can move the planets from different solar systems, you can certainly tweak their orbits from time to time to keep them stable. You’ll want to use a big star to maximise the size of the habitable zone, so it won’t be the aliens’ original home star, because big stars don’t last long enough for intelligent life to evolve.

Answer 3

You could have a huge lot of planets in a system!

If you are only concerned with the stability of the system once it is formed by whatever means, you could have many habitable worlds in (relatively) stable orbits crammed relatively reasonably close together. For detailed instructions, see the Ultimate Solar System guide by Sean Raymond. The biggest one proposed contains around 400 habitable planets within 1000AU, see Building the ultimate Solar System part 6: a system with multiple stars for detailed explanation. Even if left unchecked after the formation, it would still take around a billion years for it to fall apart.

You could make it even better by introducing a supermassive black hole.

Answer 4

Your scenario is that of the Captive Systems maintained by Carontians in Roger McBride Allen's Hunted Earth series.

These systems are unstable; you need to get your planets in resonant orbits, and this almost automatically puts most of them outside the Goldilocks zone.

Even just two planets in the same orbit would be in an unstable equilibrium, and this is the exact scenario that is thought to have given rise to the Theian impact. Stability ensues if the Trojan mass is less than 1/25th of the planet's, but this prevents it from being habitable.

You can manage for a short while with specific configurations known as Klemperer's Rosettes, but this too has restrictions on the planet sizes.

So, your only "real solution" is to use handwaving. The system must be actively kept stable, and this can be done by using Klemperer configurations with disappearing planets. The small "stabilizing" masses in the Klemperer configurations are periodically phased out via wormholes to a distant companion of the megastar, where they are put in stable orbits; this is done in such a way that the orbital instability in the megastar system compensates for the growing instability from the large planets.

In practice, as soon as two Earth-like planets in an orbit start drifting too much apart (which means they drift towards some other Earth-homologue), a Moon-sized lump of mass is FTL-gatewayed in the empty space between them, attracting them together. If the mass appears soon enough after the instability manifests, the mass requirements are low.

Of course, if you happened to land on the uninhabited "holding" binary system and land on on of the many Moon-sized planetoids that are there, just before it is phased out, this could be a nasty surprise for the rest of the crew.

Answer 5

You might want to read the answers to this question.

And there is a blog called PlanetPlanet about planetary formation. It has some sections about science fiction worlds.

It has a section called Ultimate Solar System with posts designing solar systems with successively more habitable planets.

And the more habitable planets there are in one of those solar systems, it less likely it would be for such a solar system to form naturally, and the more likely it would be that such a solar system would have been constructed or engineered by a highly advanced civilization.

So we can be pretty certain that systems like the Ultimate Retrograde Solar System, the Ultimate Engineered Solar System, the Black Hole Ultimate Solar System, and the Million Earth Solar System would have been deliberately constructed by advanced civilizations.

And don't just blow away the idea of space habitats for the central star system of a space empire instead of or in addition to many planets in that system.

You should read "Bigger than Worlds" by Larry Niven, Analog science Fiction/Science Fact March, 1974, which has been reprinted many times.

Answer 6

Start with a brown dwarf or other large "dim" planet orbiting a star. Your first set of planets are now moons around that brown dwarf.

Then stick a much smaller gas giant at the leading and trailing Lagrange points of this brown dwarf. Stick more planets in orbit (moons) of these smaller gas giants.

Most other "Goldilocks" configurations result in dynamic instability; as the bodies drift away from the stable rosetta pattern (or other stable pattern), they drift further away from the stable pattern, until the system collapses.

The leading/trailing points are far more stable, in that perturbation leads to the body orbiting the point instead of being rapidly ejected.

Small bodies orbiting a large one (moons) can be stable, and the distance to the brown dwarf (or gas giant) doesn't matter much to the temperature of the moon, which is determined by the distance to the star (which is relatively constant).

The gas giant or brown dwarf would evaporate as its atmosphere was shredded by the star, and the moon's orbit might decay slowly, but this should be much slower than any rosetta configuration and require less intervention.

The primary dwarf must be much lighter than the star, and the secondary gas giants much lighter than the primary dwarf, for the math to work out here.