Planets at Lagrange points?

[Berserk Mecha]

I have an idea for a sci-fi story set in a fictional star system. For the sake of convenience, I'm considering having some planets anchored at the Lagrange points of larger bodies. By convenience, I mean having a simple way to calculate the distance between planets at different times so travel times are easier to figure out. After toying around with Universe Sandbox for a while, it seems possible for an Earth-sized planet to be anchored at the 4th and 5th Lagrange point of a star and a gas giant, although there is some libration. But is it probable that such a body could naturally form there in the first place? There's only asteroids at the Jupiter-Sun L points but would they clump into something denser over time? Also, is there a limit to the size of an object that'll remain stable at an L-point in proportion to the larger bodies?

[Mechalich]

I have an idea for a sci-fi story set in a fictional star system. For the sake of convenience, I'm considering having some planets anchored at the Lagrange points of larger bodies. By convenience, I mean having a simple way to calculate the distance between planets at different times so travel times are easier to figure out. After toying around with Universe Sandbox for a while, it seems possible for an Earth-sized planet to be anchored at the 4th and 5th Lagrange point of a star and a gas giant, although there is some libration. But is it probable that such a body could naturally form there in the first place? There's only asteroids at the Jupiter-Sun L points but would they clump into something denser over time? Also, is there a limit to the size of an object that'll remain stable at an L-point in proportion to the larger bodies?

Pretty sure the only way you can get a planet-sized body at the Lagrange points is to put/build it in that location, and even if you did that said planet would get absolutely pummeled by asteroids and other debris and would likely end up lifeless as a result - which is the only advantage a planet has over just using the asteroids that are already there for something.

Also, if you're worried about accurately calculating travel times between planets you're looking a setting that is super-duper-hard science fiction and doing something like this is going to stand out as bizarre.

[Lvl 2 Expert]

Going by our own solar system it seems like large planets are disruptive to planet formation near them. That would be a very nice explanation on why we have an asteroid belt with a total mass much lower than our moon rather than a fifth terrestrial planet. This kind of makes sense, if larger planets didn't sweep up and pull in material from a larger area they would pretty much stop growing at some pretty early point, while exoplanet surveys seem to indicate that if anything our solar system is almost an exception for how small Jupiter actually is.

So a planet forming naturally in a Lagrange point, probably not the biggest chance (it's a large galaxy, so could happen, but several in one solar system for instance may be pushing it). You could have the planet be formed elsewhere and then captured by the Lagrange point. Those Trojans near Jupiter came from somewhere. It's not too far fetched to imagine a terrestrial planet or a large moon with an unstable orbit ending up in a spot like that. If that was a likely outcome there probably was a good possibility as well that the smaller planet would just have crashed into the larger one, but that's all a risk of the past now.

And indeed, people could try to construct a planet (although they should just go for a space station at that point) in a Lagrange point, or they could go for a capture scenario on purpose. Let's say they have a gas giant closer to the habitable zone than ours, and they have a planet kind of like Venus with an extreme greenhouse gas layer, they could make the planet livable by parking it in a stable space in the giants orbit. They could also terraform the planet through a myriad of ways that don't cost nearly the same amount of energy and pack less risk of destroying their entire solar system, but it is a thing they could do. Maybe it's part of a system that let's them continuously adapt to their growing red giant of a sun: they attach all of their livable planets to a single giant in several ways and then they use a big rocket to push the giant outwards. It costs a lot of energy, but it has the advantage of not needing the rocket to push around any planet people are trying to live on.

[factotum]

The L4 and L5 Lagrange points form an equilateral triangle between the two planets and the parent star, so I think it's entirely possible to have two Earthlike worlds of similar masses in those locations. How likely it is for that to form naturally is another matter, of course.

[wumpus]

It would have to be L4 or L5 (about 30 degrees leading or trailing) as the other points are unstable. Don't put very large "planets" in these places, they are only stable for significantly reduced masses. That said, I'm fairly surprised that Jupiter hasn't trapped a Ceres (Mercury, Pluto) sized asteroid in one of those spots. I assume that plenty of large telescopes have checked both areas out and not found anything interesting (perhaps save the dog in the night).

Pedantic note: anything in a Lagrange point can't be a "planet" by the current definition, although it isn't certain that anything except Jupiter qualifies for sufficiently rigorous application of that definition.

[Berserk Mecha]

Thanks for the replies. Seems like an Earth-size planet naturally forming at an L point would be far-fetched. The technology of the setting isn’t advanced enough to the point that people can construct Earth-size planets, but space stations like O’Neil cylinders exist. I might have it so that a planet was pulled into an L point billions of years ago through natural means. Having two planets at L points is probably stretching it. As for being an asteroid magnet, most asteroids at the L point would have been drawn to the new planet early on when it arrived. A few would be drawn in later but those could be taken care of by settlers with ballistic missiles and a good detection system. As for size, I might have it so that the two largest bodies in the system are a large star and a red dwarf. That might make a stable planet more reasonable.

[halfeye]

... I might have it so that the two largest bodies in the system are a large star and a red dwarf. That might make a stable planet more reasonable.

Um, nope

http://www.syfy.com/syfywire/vacuumi...oxima-centauri

I'm sure there's a way to make a system with an Earth-like object in it, but it seems that red dwarfs aren't it (unless you're prepared to fudge it a bit).

[Leewei]

There is a very good reasoning to artificially place bodies into these configurations. (Maximizing capture and use of solar power in the inhabitable zone.)

It is very unlikely to happen in nature since even small displacements of the bodies leads to them falling together, drifting apart, or a combination of this.

That is to say, in your background, there either is a known progenitor who set this up, or else scientific minds would strongly suspect this. It may even figure into the plot of your game.

[Manga Shoggoth]

This sounds a little like the Puppeteer's Homeworld/Fleet of Worlds in Ringworld (Larry Niven), except they put their planets in a Kemplerer Rosette (Well, sort of...).

According to the article, it is unstable, so wouldn't form naturally (or for very long).

[Berserk Mecha]

Well, seems like red dwarfs are nastier than I thought. That’d be pretty savage for any planet orbiting its habitable zone, but not necessarily for a planet further away at an L point. If I have a binary star system, I’d want the stars to be pretty far apart, like 5 to 10 AU apart. That means that a planet anchored at the 4th or 5th L point would be the same distance away from both stars. I imagine that’d be a safe distance from stellar flares.

As for any planets orbiting the red dwarf, maybe I could have them be tidally locked so that only one side faces the dwarf. The other side would be shielded from the magnetic rays. Supposing that the larger star’s habitable zone is wide enough, the planets orbiting the dwarf would have a day / night cycle with the larger star providing light. The planet at the L point would also be in the habitable zone if this were the case.

This is all a little contrived, but hey, I’m having fun here.

[factotum]

As for any planets orbiting the red dwarf, maybe I could have them be tidally locked so that only one side faces the dwarf.

A planet close enough to a red dwarf to be in its habitable zone would almost certainly be tidally locked, because they have to be really close in for that to be the case. If it was also in the habitable zone of a larger star then I would say that it would (a) get too hot and (b) this would imply that the two stars are much closer together than your 5-10AU estimate. (Note that stars significantly larger than the Sun burn out much faster, so it would be very unlikely for a habitable planet to have time to form around such a star, and the habitable zone for a Sun-sized star is obviously around 1AU out!).

[Lvl 2 Expert]

So maybe a brown dwarf? Those start at around 13 Jupiter masses and harbor just the faintest traces of nuclear fusion. " Hot Jupiters" (large planets close to stars) and double stars are pretty common, so a brown dwarf inside the habitable zone of an orange or yellow dwarf would be doable. The planet/moon around the brown dwarf would have to stick relatively close to still be a moon and not start independently orbiting the larger star, so it would probably be tidally locked, but if its rotation around the brown dwarf is fast enough the planet would as Berserk Mecha suggested still have a day night cycle. There are just going to be places who get their day when they're closest to the star and places with their day furthest from the star. Those last places also get a night lit by some extra spectacular moonlight from the brown dwarf since it both produces its own light and reflects that of the sun, and depending on their exact orbit they might get a solar eclipse (almost) every day. The "day when closest to the star" folks meanwhile never see the brown dwarf. The planet doesn't have a lunar cycle, where the brown dwarf sits in the sky depends entirely on your location. There is a whole ring of placed around the globe where they enjoy eternal "sunsets" (which look more like probably oversized moonsets).

If you combine this with an axial tilt of the dwarf system compared to their orbit around the main star you lose the eclipses, but you do get seasons.

[hamishspence]

Um, nope

http://www.syfy.com/syfywire/vacuumi...oxima-centauri

I'm sure there's a way to make a system with an Earth-like object in it, but it seems that red dwarfs aren't it (unless you're prepared to fudge it a bit).

It's worth noting that Proxima Centauri is very small and very violent - bigger red dwarf stars might be much more stable.

Also, Proxima might be atypically violent even for its size. TRAPPIST-1 is somewhat less violent and has lots of Earth-size (roughly) planets:

https://en.wikipedia.org/wiki/TRAPPIST-1