Worldbuilding and Astrophysics Feedback

[ScarletSabre]

Heya everyone, I'm ScarletSabre (Formerly Rawrbert), you may remember me from such ideas as NPCQuest, the idea for the classic hero's journey... told from the point of view of a beleagured/put upon blacksmith/merchant dealing with everything.

And now that my Troy Mclure impression is out of the way.... Don't actually go look up the chapter I posted somewhere a looooong while ago for NPCQuest, it's very very bad and rushed and was typed frantically on my phone during a moment of random enthusiasm at work. I've made a lot of notes on that idea and will hopefully be rewriting that chapter completely and plenty more soon. (Antidepressants are a wonderful thing, I must say xD)

Anyhoo, the reason I'm making this is to get a bit of feedback on a random idea I've had for a world/setting for a book. I'm not sure what I want to do with the idea, if I want to have it be sci-fi, fantasy or anything else, but the idea is for a ring of planets, probably ten, around a sun similar to Saturn's rings with their closeness, at least when looked upon from a distance.

I'm curious what people think of this idea and the potential science side of things, since the gravity of each planet would probably play havock with one another, unless they were balanced out somehow? I was thinking that they'd be locked, probably not orbiting the sun, but spinning on their own axis...es... axes? (I'm not sure how to spell the plural of axis...) and have satellites of their own in the forms of moons, debris rigns around the planet, and possibly space stations etc.

Please let me know what you think, I rather like this idea, and I hope you all do too!

Edited April 27, 2018 by ScarletSabre
A more accurate title

[DeTess]

(Disclaimer: I'm looking tat this from the point of real-world physics. As your world is a work of fiction, you are of course free to change things however you like)

Okay, I haven't done any of the complete math, but the first issue that you'd run into is that planets that would be at a similar distance from the sun as the rings of Saturn are from Saturn would be extremely hot. The sun is about 10 times the size of Saturn, and Saturn's rings go from roughly 75,000 km to 150,000 km. If you'd scale that distance up, you'd get distances from 750,000 to 1,500,000 km. For comparison, mercury orbits the sun at a distance of about 60 million kilometer.

Then there's the problem that developing solar systems don't usually develop planets in the same orbit, as usually one planet would just assimilate the other one during the process to form a bigger one.

However, if these 10 planets would just be there, and are in a life-sustaining orbits, things wouldn't be too problematic.  Tides would be really odd, and getting satellites in a stable orbit would be an interesting mathematical problem, but once the planets are there and assuming they either all share the exact same orbit or are spaced out enough that they wouldn't rip each-other apart when one passes another there wouldn't be too many physics related issue that I know of the top of my head. They would just be orbiting the sun as normal (if they didn't they'd just fall into the sun).

Edited April 25, 2018 by randuir

[ScarletSabre]

This is exactly the kind of response I was looking for, thank you so much!

The rings of Saturn comparison was perhaps a little badly explained, the size of the sun itself and the planets could be fudged to avoid the heat issue with proximity, as could their distance between the sun and each other, I essentially want an outside observer, perhaps from another planet on a much further orbit, to be able to observe the planets and get the same kind of view that we do of Saturn, with  a definitive ring-shape surrounding it. With the proximity issues of the planets itself that's probably not possible, considering the life-sustaining orbit they'd need to be in, so them simply being on the same orbit around the sun is probably the best solution.

The gravity, assimilation and ripping each other apart actually ties in neatly with an idea that I had for at least one of the planets to have a ring of it's own, a previous satellite or another planet that was ripped apart and assimilated would work well for that, and actually gives me ideas for stories, such as the planets eventually be on courses where their gravity will affect one another....

This is fascinating to read though, thank you!

Would the orbits of the planets being life-sustaining preclude varied terrain or worlds aside from our own? And how varied would the planet sizes be able to be before they'd just end up at the ripping apart point from the proximity?

I want to stick to physics fairly closely and limit the hand-wavey "This planet has an atmosphere mainly of nitrogen or a green sky or a continent made of diamond because I said so" authorial rule-breaking to a degree, haha!

Edited April 25, 2018 by ScarletSabre

[DeTess]

17 hours ago, ScarletSabre said:

Would the orbits of the planets being life-sustaining preclude varied terrain or worlds aside from our own? And how varied would the planet sizes be able to be before they'd just end up at the ripping apart point from the proximity?

You could have plenty of varied terrain based on planet size and atmosphere density and composition. A planet with a fairly thin atmosphere with few greenhouse gasses would be very cold (your typical Hoth style ice-world, with probably only the equator-regions being habitable by civilization), while one with a relatively dense atmosphere with many greenhouse gasses would be very hot (which would be more of a desert world, with the polar regions being the centers of civilization). Other archetypes you encounter in sci-fi are also perfectly possible sharing the same orbit, such as the ocean world. 

As for planet sizes, I'll try and do some actual math tomorrow. If they all share the same orbit, I don't think the issues are going to be that big, but I'd like to have done the math first.

Edit: An example of atmosphere heavily affecting a planet's temperature would be Venus. If Venus had a significantly thinner atmosphere, more similar to that of earth, it would have had surface temperatures that would actually be survivable. It's  short a couple of other features to be actually suitable for life (such as a proper magnetic field), but it wouldn't have been the pressure cooker it is now if it hadn't had that thick atmosphere. In a similar way, smaller changes in the atmosphere can create two very different planets, even if they are roughly the same distance from their star.

Another planet of interest in our own solar system you could look at for inspiration is Europa, one of Jupiter's moons. Scientists currently believe that there is liquid water below the ice covering most of the moon's surface, and that this water might contain the conditions necessary for life to exist.

Edit 2: So, I did the math. What I've done is calculate the gravitational pull of the moon on the earth, and set that as the upper limit for the allowed pull by another planet. I then calculated how far away a planet weighing 10 items as much as the earth (which would have a surface gravity of about 5g) would have to be to affect a planet with the same gravity pull as the moon exerts on our earth.

This resulted in a minimum distance of about 11 million kilometers. An orbit at a distance of 1 AU from its sun (1AU is about 150 million kilometers, which is the distance between the sun and the earth) has a circumference of more than 900 million kilometers, so you could easily fit ten planets in there without any being too close.

I can show you the exact math if you want, but I basically just abused newton's Law of universal gravitation.

Edited April 26, 2018 by randuir

[ScarletSabre]

 

9 hours ago, randuir said:

You could have plenty of varied terrain based on planet size and atmosphere density and composition.

As for planet sizes, I'll try and do some actual math tomorrow. If they all share the same orbit, I don't think the issues are going to be that big, but I'd like to have done the math first.

Another planet of interest in our own solar system you could look at for inspiration is Europa, one of Jupiter's moons. Scientists currently believe that there is liquid water below the ice covering most of the moon's surface, and that this water might contain the conditions necessary for life to exist.

This is brilliant and fascinating, and reminds me of why I always loved physics and learning about space, haha!

That's also a great example about Europa, I'd forgotten about that and it does pose for some interesting ideas... Would the planet density itself be changable without affecting the gravity issues? I'm suddenly struck by the idea of antipodes on the moon and the lower gravity being able to use tunnels to fall from one side of a satellite to another...

 

[DeTess]

1 hour ago, ScarletSabre said:

That's also a great example about Europa, I'd forgotten about that and it does pose for some interesting ideas... Would the planet density itself be changable without affecting the gravity issues? I'm suddenly struck by the idea of antipodes on the moon and the lower gravity being able to use tunnels to fall from one side of a satellite to another...

A planet of the same mass but with lower density would have a (very) slightly lower surface gravity, but it wouldn't affect any of the other planets in the system much. Using tunnels to fall from one side to the other is a fascinating idea, though those tunnels would have to be completely vacuum for someone to do it completely unassisted by anything but gravity. Planets with life on its surface generally need some kind of liquid core (which prevents direct tunnels from one side to the other) to generate a magnetic field which is required to ward off harmful radiation and keep the atmosphere where it is supposed to be, but this is less of a problem if the life is subterranean or aquatic.

[ScarletSabre]

11 hours ago, randuir said:

Edit 2: So, I did the math. What I've done is calculate the gravitational pull of the moon on the earth, and set that as the upper limit for the allowed pull by another planet. I then calculated how far away a planet weighing 10 items as much as the earth (which would have a surface gravity of about 5g) would have to be to affect a planet with the same gravity pull as the moon exerts on our earth.

This resulted in a minimum distance of about 11 million kilometers. An orbit at a distance of 1 AU from its sun (1AU is about 150 million kilometers, which is the distance between the sun and the earth) has a circumference of more than 900 million kilometers, so you could easily fit ten planets in there without any being too close.

I can show you the exact math if you want, but I basically just abused newton's Law of universal gravitation.

Holy hell that's fantastic! I'm curious how satellites such as moons or rings would be affected by the gravity of other planets now, and what the effects would be on other tides...

 

17 minutes ago, randuir said:

A planet of the same mass but with lower density would have a (very) slightly lower surface gravity, but it wouldn't affect any of the other planets in the system much. Using tunnels to fall from one side to the other is a fascinating idea, though those tunnels would have to be completely vacuum for someone to do it completely unassisted by anything but gravity. Planets with life on its surface generally need some kind of liquid core (which prevents direct tunnels from one side to the other) to generate a magnetic field which is required to ward off harmful radiation and keep the atmosphere where it is supposed to be, but this is less of a problem if the life is subterranean or aquatic.

Yeah, the magnetic field was the next question I had, what would affect it, if there would be spots where it would be weaker/thinner with different densities and compositions/atmospheres etc.

Seriously though, I can't thank you enough for all of your help with this!

[DeTess]

50 minutes ago, ScarletSabre said:

Yeah, the magnetic field was the next question I had, what would affect it, if there would be spots where it would be weaker/thinner with different densities and compositions/atmospheres etc.

The origins of the magnetic field on earth isn't understood completely, but the currently most accepted theory (the Dynamo Theory) states that the magnetic field is created by interactions between the (conductive) liquid outer and solid inner core of the earth. This is something I don't know a lot about though (I'm an aerospace engineering student with a specialization in spaceflight, so I know a lot about orbital dynamics and gravity as well as some basic astrophysics, but this is outside my area of expertise).

The magnetic field of the earth (or a similar planet) would be mostly constant, with little in the way of noticeable 'weak spots' (that is: noticeable by non-advanced civilizations living on the planet. There's actually several spots where we know the magnetic field is weaker or stronger, such as the south Atlantic anomaly, but those only really matter to spacecraft orbiting through this region). However, some other planets in our solar system do not have a magnetic field generated by the dynamo effect. Mars has a very sporadic/localized and weak magnetic field that's mostly caused by areas of the crust still being magnetized (that's how I understand it anyway. As I mentioned, this is not my area of expertise). This is also one of the major reasons for the lack of atmosphere around mars.

So in summary, a planet with life on the surface and an earth-like atmosphere would need a strong magnetic field to stop solar radiation from stripping away the atmosphere and to stop harmful radiation from reaching the surface. If you planet has mostly subterranean life, with the underground areas being sealed off from the surface allowing a decent atmosphere to remain, or if life on one of your planets is completely aquatic (and with the oceans protected with a thick layer of ice to prevent evaporation) you probably can do without a magnetic field.

50 minutes ago, ScarletSabre said:

Holy hell that's fantastic! I'm curious how satellites such as moons or rings would be affected by the gravity of other planets now, and what the effects would be on other tides...

I'm curious about that too. I'm also fairly certain I could write a Master's Thesis on this topic, so I'm afraid I won't fix that curiosity . 

Edited April 26, 2018 by randuir

[ScarletSabre]

55 minutes ago, randuir said:

The origins of the magnetic field on earth isn't understood completely, but the currently most accepted theory (the Dynamo Theory) states that the magnetic field is created by interactions between the (conductive) liquid outer and solid inner core of the earth. This is something I don't know a lot about though (I'm an aerospace engineering student with a specialization in spaceflight, so I know a lot about orbital dynamics and gravity as well as some basic astrophysics, but this is outside my area of expertise).

The magnetic field of the earth (or a similar planet) would be mostly constant, with little in the way of noticeable 'weak spots' (that is: noticeable by non-advanced civilizations living on the planet. There's actually several spots where we know the magnetic field is weaker or stronger, such as the south Atlantic anomaly, but those only really matter to spacecraft orbiting through this region). However, some other planets in our solar system do not have a magnetic field generated by the dynamo effect. Mars has a very sporadic/localized and weak magnetic field that's mostly caused by areas of the crust still being magnetized (that's how I understand it anyway. As I mentioned, this is not my area of expertise). This is also one of the major reasons for the lack of atmosphere around mars.

So in summary, a planet with life on the surface and an earth-like atmosphere would need a strong magnetic field to stop solar radiation from stripping away the atmosphere and to stop harmful radiation from reaching the surface. If you planet has mostly subterranean life, with the underground areas being sealed off from the surface allowing a decent atmosphere to remain, or if life on one of your planets is completely aquatic (and with the oceans protected with a thick layer of ice to prevent evaporation) you probably can do without a magnetic field.

I'm curious about that too. I'm also fairly certain I could write a Master's Thesis on this topic, so I'm afraid I won't fix that curiosity . 

Well the dynamo effect requiring a liquid core and solid core pretty much rules out the antipode tunnel idea for a planet with a proper magnetic field, haha! Though it would make much more sense for a more subterranean culture on a hollower planet....

Hmmm... There are definite possibilities here, if I can figure out what the bloody hell I want to do with these planets outside of the thought experiment of figuring out how they could exist xD

Out of curiousity, why does the South Atlantic Anomaly matter to spacecraft orbiting?

And as for satellites and rings around these planets, a Master's Thesis or even just more theorising coming from you on it might not fix the curiousity, but it would definitely be fascinating to read! Haha, this entire discussion has reminded me of how cool astrophysics and our universe is (and made me ashamed that until I read your previous post that I actually forgot the word "astrophysics" xD )

[DeTess]

1 hour ago, ScarletSabre said:

Out of curiousity, why does the South Atlantic Anomaly matter to spacecraft orbiting?

The earth's magnetic field creates a couple of zones called the 'Van Allen Belts'  which contain a lot of charged particles and radiation. These belts are mostly limited to fairly high altitudes (the inner of the two belts goes from about 1000 km to about 6000 km). However, because of the weaker magnetic field at the South Atlantic anomaly, the inner Van Allen belt can begin at altitudes as low as 200 km there. This means that spacecraft passing through this area would need extra radiation shielding to prevent damage to electronics (and passengers) that they wouldn't need if they didn't pass through it.

1 hour ago, ScarletSabre said:

And as for satellites and rings around these planets, a Master's Thesis or even just more theorising coming from you on it might not fix the curiousity, but it would definitely be fascinating to read!  

No doubt, but 'third body perturbations' are probably one of the most complicated factors in orbit dynamics. You usually can ignore the effects of all other objects apart from the planet you're orbiting around and the sun, but in your proposed system, things would be close enough together that things could get a lot more complicated.

If I had to guess, I'd say that things like planetary rings, spacecraft and even moons would be in  rather unstable orbits. You probably wouldn't have any real moons around any of the planets, but debris belts are possible, provided they where created within a couple of thousand years of the moment your story is set. This is really just a slightly educated guess though, without any math to really back it up.

[ScarletSabre]

18 hours ago, randuir said:

The earth's magnetic field creates a couple of zones called the 'Van Allen Belts'  which contain a lot of charged particles and radiation. These belts are mostly limited to fairly high altitudes (the inner of the two belts goes from about 1000 km to about 6000 km). However, because of the weaker magnetic field at the South Atlantic anomaly, the inner Van Allen belt can begin at altitudes as low as 200 km there. This means that spacecraft passing through this area would need extra radiation shielding to prevent damage to electronics (and passengers) that they wouldn't need if they didn't pass through it.

Now THAT is very useful knowledge for this idea and several others I have in the back of my head... Thank you!

18 hours ago, randuir said:

No doubt, but 'third body perturbations' are probably one of the most complicated factors in orbit dynamics. You usually can ignore the effects of all other objects apart from the planet you're orbiting around and the sun, but in your proposed system, things would be close enough together that things could get a lot more complicated.

If I had to guess, I'd say that things like planetary rings, spacecraft and even moons would be in  rather unstable orbits. You probably wouldn't have any real moons around any of the planets, but debris belts are possible, provided they where created within a couple of thousand years of the moment your story is set. This is really just a slightly educated guess though, without any math to really back it up.

Hmmm.... With regards to moons/other satellites, would it be possible to have a moon shared between two planets, essentially tidally locking it in geostationary orbit between the two? As for the debris belts, that would be perfect for a few of the ideas I have in mind, though the creation of debris fields and the effects it would have on a populated planet would be something I'll have to look into, especially if they were going to be caused by one planet essentially assimilating another as you said earlier!

[DeTess]

1 hour ago, ScarletSabre said:

Hmmm.... With regards to moons/other satellites, would it be possible to have a moon shared between two planets, essentially tidally locking it in geostationary orbit between the two? As for the debris belts, that would be perfect for a few of the ideas I have in mind, though the creation of debris fields and the effects it would have on a populated planet would be something I'll have to look into, especially if they were going to be caused by one planet essentially assimilating another as you said earlier!

Most of the effects that would create a debris ring like that would probably also see the planet being hit by a lot of large asteroids, so I'd expect some serious climate change and damage to existing civilizations.

You wouldn't really get a moon that is shared by multiple planets unless those planets are themselves close enough together to be orbiting around each-other instead of the local star (The pair would still be orbiting the star together, of course). A sort-of example in our own solar system would be Pluto and Charon (pluto's moon). Charon is almost half the size of Pluto, so that particular pairing is closer to being a pair of dwarf planets than a planet and a moon.

Another thing you might see is the presence of a number of loose 'moons' (they wouldn't technically be called moons because they aren't orbiting a planet) that are far smaller than the planets, but also in orbit around the central star instead of a planet. these might have started around a planet, but then slowly moved away because of the influence of the gravity of other planets until they left he planets sphere of influence

Edited April 27, 2018 by randuir

[ScarletSabre]

16 minutes ago, randuir said:

Most of the effects that would create a debris ring like that would probably also see the planet being hit by a lot of large asteroids, so I'd expect some serious climate change and damage to existing civilizations.

Yeah, that's what I expected at minimum; the asteroid strikes were going to be a common occurence due to debris rings regardless, I was thinking more about how a planet/moon would be assimilated to create the ring in the first place.

18 minutes ago, randuir said:

You wouldn't really get a moon that is shared by multiple planets unless those planets are themselves close enough together to be orbiting around each-other instead of the local star (The pair would still be orbiting the star together, of course). A sort-of example in our own solar system would be Pluto and Charon (pluto's moon). Charon is almost half the size of Pluto, so that particular pairing is closer to being a pair of dwarf planets than a planet and a moon.

Another thing you might see is the presence of a number of loose 'moons' (they wouldn't technically be called moons because they aren't orbiting a planet) that are far smaller than the planets, but also in orbit around the central star instead of a planet. these might have started around a planet, but then slowly moved away because of the influence of the gravity of other planets until they left he planets sphere of influence

Ahhh, I see, so a tidally locked moon in between two planets wouldn't work like that... Though dwarf planets themselves and what effect they would have in a system like this one is something I hadn't considered, now I'm wondering what kinds of scaling I could have for the planets to vary their sizes before their gravity starts to interfere with each other... They would most likely have to be near enough the same size, I assume?

And loose moons like that are fascinating, they might be a better alternative to the tidally locked moons idea for a shared moon, one that passes between planets... Would they drift in and out of the system itself, or drift between planets as they're affected by the orbit? (I realise I'm essentially asking how they would work xD I do appreciate all this help, it's a brilliant discussion!)

[DeTess]

1 hour ago, ScarletSabre said:

Yeah, that's what I expected at minimum; the asteroid strikes were going to be a common occurence due to debris rings regardless, I was thinking more about how a planet/moon would be assimilated to create the ring in the first place.

Well, something planet size would basically just reform the planet it comes into contact with (IE: impact energies would probably melt down the surface, the planets would partially merge with the remains becoming a debris ring that could either decay, get thrown out into space or over a billion years or so form into a moon and the planet could start the whole 'becoming life-bearing' cycle all over again). I remember reading about a possible scenario of what would happen to a planet with a moon in a decaying orbit, and if I remember correctly the moon would basically get ripped apart by the planets gravity without a massive single impact event (provided the decay happened slowly enough). However, I do expect that planet to suffer from a period of intense seismic activity (and smaller asteroid impacts) as the moon approaches and is being ripped apart (disclaimer: this is me trying to remember things about an article I  read years ago and can't find again, so take it with a grain of salt).

1 hour ago, ScarletSabre said:

Ahhh, I see, so a tidally locked moon in between two planets wouldn't work like that... Though dwarf planets themselves and what effect they would have in a system like this one is something I hadn't considered, now I'm wondering what kinds of scaling I could have for the planets to vary their sizes before their gravity starts to interfere with each other... They would most likely have to be near enough the same size, I assume?

Scaling probably doesn't matter too much, but the bigger they both are, the farther apart you want them to be.

1 hour ago, ScarletSabre said:

And loose moons like that are fascinating, they might be a better alternative to the tidally locked moons idea for a shared moon, one that passes between planets... Would they drift in and out of the system itself, or drift between planets as they're affected by the orbit? (I realise I'm essentially asking how they would work xD I do appreciate all this help, it's a brilliant discussion!)

That depends a lot on the orbit they enter after they leave their parent planet. I don't expect the orbit to deviate too much, so it would probably be a bit like a giant asteroid that passes the various planets once every X or so years. If the event that knocked it away from its parent planet is very traumatic though (for example: and impact between two other planets, or at the very least one planet passing dangerously close to the other), it could enter a far more elliptic orbit, which would mean it would pass planets far more rarely but its orbit might also intersect the orbit of the other planets, creating a potential impact hazard;

[ScarletSabre]

1 hour ago, randuir said:

Well, something planet size would basically just reform the planet it comes into contact with (IE: impact energies would probably melt down the surface, the planets would partially merge with the remains becoming a debris ring that could either decay, get thrown out into space or over a billion years or so form into a moon and the planet could start the whole 'becoming life-bearing' cycle all over again). I remember reading about a possible scenario of what would happen to a planet with a moon in a decaying orbit, and if I remember correctly the moon would basically get ripped apart by the planets gravity without a massive single impact event (provided the decay happened slowly enough). However, I do expect that planet to suffer from a period of intense seismic activity (and smaller asteroid impacts) as the moon approaches and is being ripped apart (disclaimer: this is me trying to remember things about an article I  read years ago and can't find again, so take it with a grain of salt).

Haha, I think I've actually read something similar, I'll have to look into that since it's a very interesting idea, and the ramifications for other planets on the same orbit is something to speculate on...

1 hour ago, randuir said:

Scaling probably doesn't matter too much, but the bigger they both are, the farther apart you want them to be.

Would that still apply if I was going to have a dwarf planet or two in the mix? I'll probably want to vary the planet sizes as much as the compositions and densities, so it'd be very good to know what kind of limit I'd be working with within the proposed space for a 1AU orbit...

1 hour ago, randuir said:

That depends a lot on the orbit they enter after they leave their parent planet. I don't expect the orbit to deviate too much, so it would probably be a bit like a giant asteroid that passes the various planets once every X or so years. If the event that knocked it away from its parent planet is very traumatic though (for example: and impact between two other planets, or at the very least one planet passing dangerously close to the other), it could enter a far more elliptic orbit, which would mean it would pass planets far more rarely but its orbit might also intersect the orbit of the other planets, creating a potential impact hazard;

Haha, both of those scenarios have potential for stories and a cool atmosphere, I think!

[DeTess]

35 minutes ago, ScarletSabre said:

 

Would that still apply if I was going to have a dwarf planet or two in the mix? I'll probably want to vary the planet sizes as much as the compositions and densities, so it'd be very good to know what kind of limit I'd be working with within the proposed space for a 1AU orbit...

Yeah, distance only really becomes an issue if you want two or more planets to be close enough together that they orbit around eachother. Otherwise you should have plentyvof room, as I'd calculated before.

[ScarletSabre]

26 minutes ago, randuir said:

Yeah, distance only really becomes an issue if you want two or more planets to be close enough together that they orbit around eachother. Otherwise you should have plentyvof room, as I'd calculated before.

That's awesome to know ^-^ What would be the dynamics of the two planets orbiting each other around a shared moon/moons that you mentioned before? It's an interesting idea that I definitely want to use either separately from this ring of planets idea, or perhaps in a separate, further orbit observing the ring from a distance.

[DeTess]

44 minutes ago, ScarletSabre said:

That's awesome to know ^-^ What would be the dynamics of the two planets orbiting each other around a shared moon/moons that you mentioned before? It's an interesting idea that I definitely want to use either separately from this ring of planets idea, or perhaps in a separate, further orbit observing the ring from a distance.

Basically, if two objects are close enough together and have the right relative velocities (so they haven't actually collided), they will orbit around a shared center point called a barycenter. The barycenter would then be orbiting around the star, with the two (or more planets) orbiting around the barycenter. For a particularly exotic set-up, you could place a single moon right at the barycenter of a constellation of two planets, which means that it would always be right in the middle. For a setup like this you'd ideally have two planets of about equal mass, as that means that the barycenter is actually in between the two with room for a third object like a moon.

[ScarletSabre]

5 minutes ago, randuir said:

Basically, if two objects are close enough together and have the right relative velocities (so they haven't actually collided), they will orbit around a shared center point called a barycenter. The barycenter would then be orbiting around the star, with the two (or more planets) orbiting around the barycenter. For a particularly exotic set-up, you could place a single moon right at the barycenter of a constellation of two planets, which means that it would always be right in the middle. For a setup like this you'd ideally have two planets of about equal mass, as that means that the barycenter is actually in between the two with room for a third object like a moon.

Haha, that's perfect! That exotic setup is exactly what I had in mind with my first question about a moon shared between planets, haha, though I didn't plan to have them orbit the barycentrer