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Comment by hef19898 | Hacker News Reader

hef19898

Or one with three planet sharing the same orbit, perfectly spaced!

3 planets orbiting a common barycentre would be very cool. I think something like that was mentioned in the Peter F Hamilton "Night's Dawn" trilogy of sci-fi books.

Larry Niven's Known Space features a series of planets orbiting a common center like that; it's a grand engineer project, and a lifeboat of sorts.

A pentagonal form of a Klemperer rosette (first thing I thought of also): https://en.wikipedia.org/wiki/Klemperer_rosette

This is dynamically unstable; Any miniscule imperfection ends up being magnified by the forces involved. The angular momentum of the system is conserved, but resonances build chaotically and are likely to eventually concentrate enough in a smaller body to throw it off past escape velocity.

Spaced-out resonant triplets of bodies in the same plane are often dynamically stable - an imperfect ratio is damped by various orbital forces until it approximates a perfect ratio.

That's why it would require alien technology to keep it perfect.

Why they'd do this, though, would be a mystery.

seems like it'd be a primitive way (in one way, obviously not in the tech sense) of displaying power

Perhaps it's just a giant advertising billboard.

So, like, the aliens elected a lizard version of Donald Trump? Make the planets gold and it all tracks.

Well, perfectly spaced is the only way 3 planets can share the same orbit.

And if we go for a gas giant and two small planets, there are probably many of those out there. We almost got one such trio.

Tell that to Janus & Epimetheus: https://en.wikipedia.org/wiki/Janus_(moon)

Perfectly spaced is not a stable orbit.

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

You want L4 and L5 for that.

L4 and L5 are unstable unless there is an extreme, as in many orders of magnitude, difference in the masses involved.

At equal mass the separation increases until they are all equidistant from each other assuming a completely circular orbit.

That applies to a small body with two large ones, not three similarly sized planets.

Ah, you are correct.

Is there any statically stable shared orbit for planets though? The first wobble and they start accelerating toward each other don't they?

Or a planet with 13 moons evenly-spaced on the same orbit? C.f. Ilus IV / New Terra in The Expanse.

I just wonder so, if there are multiple planets in equal distance between them, would we be able to tell the difference between three planets and one on a fast orbit?

That sounds like aliasing issue in signals; we should be able to distinguish between them as long as we sample more often than half the actual orbital period (i.e. with a sampling frequency larger than the Nyquist frequency).

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

Another question, is there something like a max. rotation speed for a planet in a certain distance to a sun of a certain size?

Edit as a general response: Question answered, multiple times, thank you! Also, there my basic physics knowledge resurfaces, thank you for that as well!

There is only one stable speed at which a planet can orbit in a circle; any slower, and it'll start falling in toward the star, and any faster and it'll start to move away from the star.

The only way to vary the speed is a powered orbit, and that's not likely to happen with a planet.

I respect that you reserved some room to be surprised.

For a given stellar mass and orbital radius (assuming a circular orbit), there's not really any wiggle room on how long the planet's orbital period is. Speeding up or slowing down the planet requires it to orbit at a different distance. If you meant the speed of a planet's rotation about its own axis, I guess the limit would basically be the point at which it tears itself apart by spinning so fast that its gravity no longer holds it together.

Yes, but that would be dictated by how fast can it spin before shattering into pieces. So mostly gravity/composition thing, I'd imagine. Many things in solar systems are (postulated to) derive from the rotation of the protoplanetary disk, via conservation of angular momentum - however, planets can also get spun up or down after forming by e.g. collisions with other objects, including extra-system objects.

Just realized, I meant orbit speed and not rotation aeound the planets axis. Shouldn't write in parallel to meetings... Your answer was very interesting so, thank you!

I see! In the other case, the answer is: velocity vector determines the orbit. For any given point at any given orbit, there's only one valid velocity vector relative to the star (direction and magnitude) - tweaking it tweaks the shape of the orbit.

One of the best way to get an intuition for orbits is to play Kerbal Space Program for a few hours :).

Going back to my buried physics knowledge, that is quite logic. Thanks again!

I am just afraid to touch Kerbal Space Program, I really cannot afford another time sink at the moment!

As one time ovner of a Star Wars RPG PC whos secret super weapon was his tremendous Astrogation skill, I really should so I guess!

Not convinced until they find a Dyson sphere...

They built a ring world in order to harness enough solar power to continue and sustain their proof of work economy. Alas, when even that was not enough their world collapsed and the successors to their race returned to the trees.

Dyson spheres are probably really rare. It's just bad strategy, everyone within your galaxy knows you're there pretty much immediately when a star just up and disappears one day. Granted, the non-K2s just stare in awe maybe, but the other K2s will fuck your shit up. Can't exactly pack it up and run either, not with a medium-sized star in your suitcase.

Anyone with only mildly better tech than we have now, can see your ecosystem changing over the course of the seasons before you invented fire, let alone built a Dyson swarm. We're just starting to have this capacity already in special cases, though we've not found any sign of an ecosystem, just "boring" diamond rain etc.

A Dyson swarm will keep you safe from any threat smaller than another Dyson swarm — and while you may not be able to "pack it up", you can use one to run to other galaxies… in fact, almost all of them… at close enough to the same time that light cones matter… and get the settlers moving at a significant fraction of the speed of light… and have a lot of redundancy.

I would assume it takes more than a day to build a Dyson sphere. But more to the point from a distant observer's viewpoint the star isn't going to just blink out. The Dyson sphere has to radiate just as much energy as the star produces, so it would probably appear just as a red dwarf. Unless the alien civilization has some way to destroy energy it will be in a constant battle to avoid cooking the inhabitants of the sphere.

> Unless the alien civilization has some way to destroy energy (...)

If the point of a Dyson sphere is to collect energy, wouldn't it be enough to just use it or store it?

To use, not to collect. And thermodynamics appears to say you can't just store it.

A stellar mass black hole might be an interesting "cold end" in this regard… if you can find or make one, but to do that you'd need to start with a Dyson swarm.

> To use, not to collect. And thermodynamics says you can't just store it.

I don't think these semantic games are productive. Thermodynamics says you can transform energy. "Collect" in this context means using energy in a way that allows you to retrieve it in the future. For example, charging a battery or condenser with light with a PV panel, powering a motor that accelerates a flywheel, coiling a spring, heating a material, etc.

If you have a Dyson swarm, the best way to "store" the power output of a star for later use is to perform star lifting, making the star itself smaller and lighter so it doesn't burn as hot in the first place, with the extra mass being used to construct a collection of gas giants in the same system.

If this kind of thing appeals to a group with meaningful control over a Dyson swarm, given that Dyson swarms and the capacity to make them can also be used to build and power a rapid, direct, and near-simultaneous colonisation of all galaxies in our future particle horizon (with significant levels of redundancy, though obviously we can't determine if "significant" is "sufficient"), you should anticipate the first such group turning almost every star in the universe into a red dwarf (or brown dwarf, for those they don't care to colonise just yet) in very short order.

I don't know if anyone's been looking for signatures of this specific thing; though I am told that "1 AU sphere of room-temperature metal" would be quite easy to spot, if you downsize the stars first then the size of the corresponding room-temperature sphere gets smaller, and I don't have a quantifiable number for "quite easy" nor any sense of scale for how much effort is going into such searches.

Unless your civilization has ways of infinitely storing energy or exporting it somehow you'll need to be in equilibrium over the long term. This means radiating away the waste heat, at stellar scale.

> Can't exactly pack it up and run either, not with a medium-sized star in your suitcase.

If you're K2 just take the star and solar system with you. Stellar engines can be used to move stars. https://en.wikipedia.org/wiki/Stellar_engine

Eventually.

Shkadov thruster:

> After a period of one million years this would yield an imparted speed of 20 m/s, with a displacement from the original position of 0.03 light-years. After one billion years, the speed would be 20 km/s and the displacement 34,000 light-years, a little over a third of the estimated width of the Milky Way galaxy.

Caplan thruster:

> Caplan estimates that the Bussard engine would use 10^12 kg of solar material per second to produce a maximum acceleration of 10^−9 m/s2, yielding a velocity of 200 km/s after 5 million years.

Svoronos Star Tug:

> The Svoronos Star Tug can, in principle (assuming perfect efficiency), accelerate the Sun to ~27% the speed of light (after burning enough of the Sun's mass to transition it to a brown dwarf).

Temporal displacement fields: Wrap you star system in one of those and remove from the normal universe with a time shift of a couple of seconds! Let those pesky K2 Dyson Sphere civilisation figure that out!

Tried that, it didn't work; that's how I ended up stuck in this insane reality where JavaScript ate the world, and my nickname is all I have to show for it.

Ah, sucks when this happens, doesn't it?

>but the other K2s will fuck your shit up.

[Citation Needed]

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

If aliens have vacuum cleaners, I hope they're better than ours tbh.

Mandatory mention of Spaceballs incoming... I just don't want the reverse function on my home vacuum.

IDK, I miss those old vacuums that could run in reverse - they're perfect for building hovercrafts for kids.

Make a big disk, punch the vacuum cleaner's pipe through it, put a blanket over the whole thing, add a chair on top. Turn power on, you have a hovercraft. A staple of city science fairs where I live.

THAT is a great idea! Would one of those leave blowers work? Ideally a battery powered one?

I think Culture style Orbitals are more elegant - no need for shadow squares to create day/night cycles.

Orbitals - The Tiny Houses of solar scale structures

Every megastructure is an invisible dot compared to the next size up.

https://www.youtube.com/watch?v=JvKSRKT0nzM

There is also The Ring in the Xeelee universe - which is millions of lightyears across:

https://xeelee.fandom.com/wiki/The_Ring

Mind you it might not count as it's a means of escape, not a place to live.

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