Subnautica Orbital Patterns Explained
chriswmackey
United States Join Date: 2018-01-15 Member: 235059Members
United States Join Date: 2018-01-15 Member: 235059Members
This post is the least I can do for a game that is hands down the best thing that I have ever played. It’s also a response to many posts throughout the internet that have been citizing what I deem to be excellent sci-fi. That is, fiction that is speculating on questions where there is not yet a clear scientific consensus. The orbital patterns of the moons in this game are not a phenomenon that we have directly observed in any existing planetary system but they are absolutely physically possible. The science of exoplanet orbits is very young and, given that the scientific community has only just come to a consensus that each star in our galaxy has at least one planet, it would be way too early to dismiss the ideas of this game as unrealistic. So this post is going to break down one physically possible scenario that explains the orbital patterns of this game’s moons and sun.
Before we dive in, I will just mention that this is only one of perhaps several physically-possible explanations for the orbits of the moons and isn’t necessarily the one endorsed by the creators. This said, it was the most plausible one that I could come up with while sitting on a bus for 4 hours. So with that, let’s break down the explanation:
I. TIDAL LOCKING
It is hard to imagine an explanation in which planet 4546b and the “white moon” are not tidally locked to one another. Tidal locking is an extremely common phenomena within our solar system. Our moon is tidally locked to the earth and we even have a example of two planetary bodies that are tidally locked with one another (Pluto and Charon), which is particularly important for the explanation here. This video has a good summary of how tidal locking happens if you want a refresher. Notably, when two bodies are tidally locked to one another, the same sides of each planet are constantly facing one another. This explains why the white moon appears at the same point in the sky throughout the game and, with this explanation, we can infer that the white moon experiences the same phenomena towards 4546b on its surface. Notably, when two equal-sized planets are tidally locked with one another, they both orbit their common center of gravity (between the two bodies) like so:
II. UNEVEN TIDAL LOCKING
This will become important in a second. When one tidally locked planetary body is slightly larger than the other, the center of gravity will be shifted towards the larger planet. This will result in concentric orbital patterns like so:
III. OTHER PLANETS ORBITING THE CENTER OF GRAVITY
Given this uneven tidally locked setup, there is nothing stopping other planets or moons from orbiting this common center of gravity. So we can introduce a red moon orbiting this center of gravity that is out of plane with the tidally locked bodies. For example, if 4546b and the white moon are tidally locked within the xy plane, we can introduce a red moon orbiting the center of gravity in the yz plane.
iV. PUTTING IT ALL TOGETHER
With the components above, we can bring the whole system together and we end up with something that is very close to what we experience in the game:
All of the planets/moons move in circular orbits, none of them collide with one another and, in the scenario I depict here, they appear to be far enough apart that they would not be broken apart by one another’s gravity. This said, I haven't tested this explanation against the exact size of the moons in the sky. Nor have I actually worked out the possible mass and densities of the moons. If anyone wants to tackle that one, I’d recommend starting by assuming 4546b has a mass that is slightly larger than Earth since this seems to be the case in the game (we move very slowly while walking on land). Also, there seems to be a general consensus among astrophysicists that a more massive earth-like planet would hold on to more water throughout its history, thereby partly explaining why 4546b is an ocean world.
V. YOU ARE HERE
Many of you may have figured it out by this point, but it would seem that our player is located close to the equator of 4546b, which explains why the sun passes directly overhead, why we see the red moon continuously on the horizon, and why we see the white moon high in the sky:
will post back soon with some suggestions for how the creators could add details to realistically depict this scenario (ie. white moon phases, rotating star maps, and a growing/shrinking size of the red moon). However, I want to emphasize that these are all minor details and they all increase the memory that the game needs so I don’t blame the creators if they’d rather leave them out. I have also found some evidence for how the creators may already be partially endorsing them (for example, the white moon is a gibbous in the game, which is the midpoint of where it should be during the night). For the time being, I just wanted to make sure that this explanation was put forward so that we have some defense for the great sci-fi aspects of this game!
Before we dive in, I will just mention that this is only one of perhaps several physically-possible explanations for the orbits of the moons and isn’t necessarily the one endorsed by the creators. This said, it was the most plausible one that I could come up with while sitting on a bus for 4 hours. So with that, let’s break down the explanation:
I. TIDAL LOCKING
It is hard to imagine an explanation in which planet 4546b and the “white moon” are not tidally locked to one another. Tidal locking is an extremely common phenomena within our solar system. Our moon is tidally locked to the earth and we even have a example of two planetary bodies that are tidally locked with one another (Pluto and Charon), which is particularly important for the explanation here. This video has a good summary of how tidal locking happens if you want a refresher. Notably, when two bodies are tidally locked to one another, the same sides of each planet are constantly facing one another. This explains why the white moon appears at the same point in the sky throughout the game and, with this explanation, we can infer that the white moon experiences the same phenomena towards 4546b on its surface. Notably, when two equal-sized planets are tidally locked with one another, they both orbit their common center of gravity (between the two bodies) like so:
II. UNEVEN TIDAL LOCKING
This will become important in a second. When one tidally locked planetary body is slightly larger than the other, the center of gravity will be shifted towards the larger planet. This will result in concentric orbital patterns like so:
III. OTHER PLANETS ORBITING THE CENTER OF GRAVITY
Given this uneven tidally locked setup, there is nothing stopping other planets or moons from orbiting this common center of gravity. So we can introduce a red moon orbiting this center of gravity that is out of plane with the tidally locked bodies. For example, if 4546b and the white moon are tidally locked within the xy plane, we can introduce a red moon orbiting the center of gravity in the yz plane.
iV. PUTTING IT ALL TOGETHER
With the components above, we can bring the whole system together and we end up with something that is very close to what we experience in the game:
All of the planets/moons move in circular orbits, none of them collide with one another and, in the scenario I depict here, they appear to be far enough apart that they would not be broken apart by one another’s gravity. This said, I haven't tested this explanation against the exact size of the moons in the sky. Nor have I actually worked out the possible mass and densities of the moons. If anyone wants to tackle that one, I’d recommend starting by assuming 4546b has a mass that is slightly larger than Earth since this seems to be the case in the game (we move very slowly while walking on land). Also, there seems to be a general consensus among astrophysicists that a more massive earth-like planet would hold on to more water throughout its history, thereby partly explaining why 4546b is an ocean world.
V. YOU ARE HERE
Many of you may have figured it out by this point, but it would seem that our player is located close to the equator of 4546b, which explains why the sun passes directly overhead, why we see the red moon continuously on the horizon, and why we see the white moon high in the sky:
will post back soon with some suggestions for how the creators could add details to realistically depict this scenario (ie. white moon phases, rotating star maps, and a growing/shrinking size of the red moon). However, I want to emphasize that these are all minor details and they all increase the memory that the game needs so I don’t blame the creators if they’d rather leave them out. I have also found some evidence for how the creators may already be partially endorsing them (for example, the white moon is a gibbous in the game, which is the midpoint of where it should be during the night). For the time being, I just wanted to make sure that this explanation was put forward so that we have some defense for the great sci-fi aspects of this game!
Comments
Waaaaait I'm confused... what is the red moon orbiting if it's obviously not orbiting its own center of mass and 4546B doesn't appear to be affected by its path? What if there's a fourth moon which is doing some weird dwarf moon thing in the background which the red one rotates around or if there's a meteor belt somewhere? I'm no astrophysics buff, though (and don't speculating pretending to be one) and so I'm perfectly willing to let this one go.
Although, I do know (or think I do) that despite my instincts, density has no meaning in centrifugal force like this; that is, my science teacher has told me that its the earth's mass, not its density, which decides that the moon orbits us instead of the other way around. So I don't see why density has anything to do with your equations. Also, how do we know that the moons don't collide? What if the planet is locked in some barely asymmetrical billions-of-years long suicide mission? Although unlikely, we don't really know that much...
Also, you say that astrophysicist believe that rocky planets larger than the earth would contain more water throughout its history. Do they mean liquid water?
Does the age of the moons affect your calculations? I mean- they should take a part in precise calculations because of how they would have to have formed in order and all that. But if so, I'd have to assume that the planet and the red moon would likely have been formed before the white one because they're about the same size. However, this'd be inconsistent with the position of the white moon; as since it's obviously made of a different material as the red moon, it'd be one wacky whop of probability for two distinctly different deposits of resources to have entered a planet system at the same time and have very few overlaps. (Also, 4546b would have to have been formed before the other moons because it seems to have much more diverse resources than just being white rock or red sulfur dust). But anyhow, this doesn't make sense because the white moon seems to orbit 4546B in your model; when the red moon makes more sense to fit into that orbital niche. What with being formed about the same time and all.
Starkos,
As you see in the video I linked to on tidal locking, two planetary objects usually need to be of similar mass in order to become tidally locked to one another. Only the masses need to be similar and not the size/volume. For example, the white moon can be much smaller than 4546b but made of a denser material like iron in order to have a comparable mass to 4546b, which seems to be more of a silicate planet. I think this is what Isummon_Durt is trying to get at that, yes, you don't need the planet's density to know if the orbit is possible but you may need it to explain the size of the objects in the sky of the game.
You also bring up a very good point, Starkos, that the model that I have put forward here assumes that the red moon has a very low mass in comparison to 4546b and the white moon. Otherwise, introducing a massive red moon into the binary planet system could change the center of mass of the system, around which all of the objects are rotating.
This would create a shifting "gravitational mess" at the center of the system as you mention, which is much harder to model and explain. Again, this red moon can still have a low mass but occupy a relatively large volume and large part of the sky if it is made of a material that is less dense than the other planets/moons (like hydrocarbons, maybe).
Lastly, you are right, Starkos, that the red moon should, ideally, maintain a equal distance from 4546b over its orbit in order to be stable and to keep its orbit for a substantial number of revolutions without colliding with 4546b. This equal distance is definitely possible with the model I put forward here, though, admittedly, slight deviations from this delicate balance could, over time, bring about jamintheinfinite_1's suggestion that the moon collides with 4546b (or at least gets broken apart by 4546b's gravity when it gets too close, perhaps forming a ring around 4546b).
But to break my explanation down another way that might be clearer, many of us recognize that the following system, which has 4546b as much more massive than the other planets would be stable:
We also recognize that the other extreme, which has the white moon at an equal size to 4546b would also be stable, because 4546b and the white moon will pull on the red moon with equal force, which results in a net force towards their common center of mass:
The case I describe here is just in between these two extremes where the center of mass of the system does not reside within 4546b or directly in between the two planets but at some distance just outside of 4546b.
And lastly, to answer shuttlebug's question: the graphics were made using Rhino3D and the Grasshopper visual scripting interface. If anyone wants to download the free 90-day trial of Rhino and dig into what I've done, they can download my grasshopper model here and go through the cycle of the orbits in 3D.
In case you didn’t know, you can summon someone by typing an @ before their name. For example, @System .
We also have to keep in mind that in the game our character don't need to live on 4546B for too long, so it might be that some cycles just aren't witnessed.
YUCK. why did you have to add that picture. that face is messed.
Very, very much so. This is brilliantly done.
Only one question occurs to me, @chriswmackey, if you'll indulge me. Given the apparent size and range of the Red Moon, and its placement in the orbital system you're describing, wouldn't it cause perturbations in the orbit of the White Moon and destabilize its path? It just seems that with the primary bodies in this system, it would be almost inescapable that sooner or later close passes between the Red and White Moons would throw White out of its comfortable, very regular orbital track.
A possibility I would like to introduce is that the White Moon is substantially larger than hypothesized here, but orbits at a greater range, giving it a smaller apparent diameter. More mass would resist inertial changes, and greater range would reduce the effects of 4546B and the Red Moon to perturb its track.
Even with that said, though, this is magnificent work. Three-body problems are a giant headache, and this is a simple, elegant, and cohesive solution. If I could've, I would've given this a half-dozen Awesomes at least. Brilliant work, @chriswmackey!
As an afterthought, a few hours later: Perhaps the solution is closer to your equal-size posit than initially thought. Size isn't everything in these systems, and if the White Moon is substantially more dense than the average of 4546B, it could be physically smaller yet exert the same gravitational effect. This would place your stable equal-forced model squarely in the circle of plausibility, with the Red Moon orbiting the system barycenter.
Sorry for the late response and thanks for all of the nice comments + feedback. As I was thinking through the explanation I posted here, I had originally started with the same thought that the white moon might be a gas giant with a large gravitational influence and planet 4546b was more like a moon of this giant. I had reasoned that, because the moons of gas giants tend to be tidally locked with their host (for example, all 4 of the galilean moons are tidally locked with Jupiter) that this scenario would be likely. However, as I was thinking through it more, having such a massive white moon would mean that the collective center of 4546b and the white moon would be really tipped toward the white moon and so the red moon would appear like it is orbiting the poles of this white gas giant. If the white moon appeared very large in the sky and the red moon appeared very small, perhaps this explanation could have held up but, because we witness the opposite, I was left to conclude that the white moon was small, dense, and close.
And to answer the question about the stability of the orbits, the truth is that I have not evaluated this in a high degree of detail. The depiction that I have made here was built with CAD/visualization software and it isn't actually modelling gravitational physics. In fact, the way that I have depicted it here actually has a slightly changing distance between 4546b and the red moon, which is not how a solution that is stable over many cycles would behave. For this reason, I feel confident in saying that a stable solution that looks something like this exists but I am not sure how much this stability would be balanced on the edge of a knife or how likely such stability could occur naturally in the universe.
I have heard good things about the gravitational physics engine in the Universe Sandbox game on Steam. If I get the chance to mock this scenario up in there, I will post it here. And, if anyone beats me to it, please share what you find!
And, according to the compass, the rises in the northeast and sets in the southwest. I can't wrap my head around what this means. Maybe that, since we're on the equator, it's just a sign that 4546b is tilted on its axis?
And if you haven't already, check out a book called Three-Body Problem by Liu Cixin. Three-Body Problem, along with its sequel The Dark Forest, are /excellent/ science fiction.