Theoretically, could such a binary system exist?

[G'th]

Two worlds with massive and deep oceans being close enough so that the gravity wells of each planet pulled the oceans together through space? 

Saw an odd gif about some VR game where two spheres with gravity had water surrounding them and when they got close they did this. 

 

Obviously the biggest issue is that its not going to be liquid all throughout unless perhaps it develops a frozen shell that isnt somehow shattered by the planets orbit and general gravity shenanigans. And thats without getting into whether such a binary system could even support liquid water in the first place. 

 

Would be a very interesting system though if it could happen. Imagine how odd it would be if life developped separately but also together, and how that life would react to essentially space travel open to them from the start. 

[Shpaget]

No. At planetary scales rocks are pretty much just as fluid as water so the planets would tear each other apart.

The minimum distance at which two celestial objects can still hold their forms is called Roche limit.

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

 

Something similar is a plot device in the movie Upside Down.

https://en.wikipedia.org/wiki/Upside_Down_(2012_film)

[cubinator]

If it was spinning fast enough, you might get an oblate spheroid, but not two distinct spheres. That's just how gravity naturally settles things.

[Superfluous J]

Check out the book Rocheworld.

[ProtoJeb21]

Something like that with two ocean worlds is impossible — the planets would tear each other to pieces or collide, depending on the mass ratio between the two. However, this “shared fluid” binary scenario has been seen with closely orbiting stars, where they are so close to each other that they share some of their plasma envelopes. 

https://en.m.wikipedia.org/wiki/Contact_binary

[sevenperforce]

A pair of binary planets, mutually tidally locked, could be pretty darn close together. 

2 hours ago, Shpaget said:

The minimum distance at which two celestial objects can still hold their forms is called Roche limit.

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

The Roche limit is good for looking at a very massive primary and a less massive secondary, but for the case where both bodies are equal in size, the variables cancel and the Roche limit d becomes constant at 1.26 planetary radii, which would have the bodies touching already. So the Roche approach is not useful.

The limiting factor in this case is going to be a function of rotational period and centrifugal force. The closer the two bodies are, the greater their rotational rate must be in order to maintain orbit around a common barycentre. At some point, the rotational rate is going to be so immense that the ocean on the opposite sides of the worlds will get flung off into space like a slingshot.

[magnemoe]

1 hour ago, sevenperforce said:

A pair of binary planets, mutually tidally locked, could be pretty darn close together. 

The Roche limit is good for looking at a very massive primary and a less massive secondary, but for the case where both bodies are equal in size, the variables cancel and the Roche limit d becomes constant at 1.26 planetary radii, which would have the bodies touching already. So the Roche approach is not useful.

The limiting factor in this case is going to be a function of rotational period and centrifugal force. The closer the two bodies are, the greater their rotational rate must be in order to maintain orbit around a common barycentre. At some point, the rotational rate is going to be so immense that the ocean on the opposite sides of the worlds will get flung off into space like a slingshot.

Think an larger issue is that you are getting so close you have an major risk of getting some surface effects who make the siberian traps feel like somewhat bad weather. 
Things get way worse if you have other large bodies in the solar system. 
 

[sevenperforce]

49 minutes ago, magnemoe said:

Think an larger issue is that you are getting so close you have an major risk of getting some surface effects who make the siberian traps feel like somewhat bad weather. 
Things get way worse if you have other large bodies in the solar system. 
 

WAAAAAAAAAAAAAAY worse.

Also, forget orbiting.

[p1t1o]

Even without taking into account Roche limits or advanced orbital mechanics, there is so much opportunity for drag forces in the described scenario that if it ever were forced into being, it would rapidly decay and you'd eventually find yourself with one large planet.

[Superfluous J]

1 hour ago, p1t1o said:

Even without taking into account Roche limits or advanced orbital mechanics, there is so much opportunity for drag forces in the described scenario that if it ever were forced into being, it would rapidly decay and you'd eventually find yourself with one large planet.

I wrote a story about 15 years ago - long before I could possibly have known better - that mostly took place on two Roche lobes that orbited so close they shared an atmosphere. I never said it in the book explicitly but the clues were there that the current situation was not only temporary, but were actually caused by the cataclysmic changes brought about as the two worlds slowly got closer and closer over the past few million years.

Maybe I should try to publish it so we can put it in the "Bad science in science fiction" thread.

[magnemoe]

14 hours ago, sevenperforce said:

WAAAAAAAAAAAAAAY worse.

Also, forget orbiting.

Still you would get plenty of panspermia, and sending an probe to the other planet would be pretty easy. 
This assuming separation of one planet diameter or similar. 

Might be more of an focus on rocket planes as you could land then take off and stage to get back again.

How much weaker would gravity be on the inside? 

Edited February 6, 2019 by magnemoe

[sevenperforce]

5 hours ago, p1t1o said:

Even without taking into account Roche limits or advanced orbital mechanics, there is so much opportunity for drag forces in the described scenario that if it ever were forced into being, it would rapidly decay and you'd eventually find yourself with one large planet.

Once tidal locking takes place, you'd have few sources of major drag, but you'd still have relativistic frame-dragging that would pull them together. We'd have to know the parameters in order to know what the timeline would look like.

They could potentially have a shared exosphere, if it was indeed tidally locked. Depends on a LOT of factors.

13 minutes ago, magnemoe said:

Still you would get plenty of panspermia, and sending an probe to the other planet would be pretty easy. 
This assuming separation of one planet diameter or similar. 

Might be more of an focus on rocket planes as you could land then take off and stage to get back again.

How much weaker would gravity be on the inside? 

You could certainly have places where going world-to-world was easier than going point-to-point.

The unknowns are huge. How large of worlds are we talking about? Does gravity need to be Earthlike? The lighter the worlds are, the easier it is for the whole thing to work.

I had sketched out an idea for fiction like this...Earthlike gravity, 24-hour rotation period, daily eclipses. I had the magnetic fields of the two worlds coupled, so that depending on geomagnetic activity, you could occasionally have maglev "portals" open up where ferromagnetic material would become extraordinarily lightweight, enabling pre-rocket travel.

[p1t1o]

Again, forgetting the complexities of roche limits and fancy-schmancy orbital stuff - for the water to form a "limb" joining the two bodies, it would have to be in gravitational equilibrium, meaning that the zone of space the water occupies would have to have zero net gravity. (This would also be unstable as there would be no correcting forces should something destabilise the body of fluid)

And I dont think the zone of zero gravity is that large, or that shape, when two bodies approach each other, how could it be?

Ergo - either one body sucks all of the water off the other, or each planet retains 100% of its water (assuming the orbit can be achieved in the first place).

[sevenperforce]

30 minutes ago, p1t1o said:

Again, forgetting the complexities of roche limits and fancy-schmancy orbital stuff - for the water to form a "limb" joining the two bodies, it would have to be in gravitational equilibrium, meaning that the zone of space the water occupies would have to have zero net gravity. (This would also be unstable as there would be no correcting forces should something destabilise the body of fluid)

And I dont think the zone of zero gravity is that large, or that shape, when two bodies approach each other, how could it be?

Ergo - either one body sucks all of the water off the other, or each planet retains 100% of its water (assuming the orbit can be achieved in the first place).

The gravipotential field would have a saddle point at the barycentre, so it is in unstable equilibrium at every point.

I suppose that you COULD have some sort of wacky suborbital oceanic crossflow in conjunction with orbital eccentricity or something, but there's no way it would naturally be stable.

Could possibly be constructed by a Kardashev-1.5 civilization.

Edited February 6, 2019 by sevenperforce

[DAL59]

5 hours ago, p1t1o said:

Even without taking into account Roche limits or advanced orbital mechanics, there is so much opportunity for drag forces in the described scenario that if it ever were forced into being, it would rapidly decay and you'd eventually find yourself with one large planet.

In the novel Rocheworld, the binary is on an elliptical orbit that is in a 1:3 resonance with the system's gas giant.  So, on regular intervals it gets extremely close to the giant, fixing the drag.  Also, remember that as the planets are tidally locked, and they have enormous inertia, there is not much drag.  

[kerbiloid]

Even if such biplanet was possible, the water column would consist of separated blobs. The surface tension, after all.

Edited February 6, 2019 by kerbiloid

[sevenperforce]

2 hours ago, kerbiloid said:

Even if such biplanet was possible, the water column would consist of separated blobs. The surface tension, after all.

It could be like a river. A space river. Spiraling flows or whatnot.

[YNM]

I suppose such conditions would be incredibly transient in the overall scheme of things. It's like counting on a drop of water just touching the surface vs. the whole history of a water drop, all the way until it splashes.

 

How transient is it ? I don't know.

[kerbiloid]

6 hours ago, sevenperforce said:

It could be like a river. A space river. Spiraling flows or whatnot.

Yes, a river of blobs. And stones.

Edited February 7, 2019 by kerbiloid

[Cunjo Carl]

I don't think it would be stable over astronomical timescales, but it seems to me it should be able to exist hypothetically for long enough to make a story happen. After all, such a system would require divine intervention to set up anyways right? No one says it has to last forever! Admittedly, lasting long enough for life to develop is almost definitely a no. However, it might even be a fun plot point that citizens already on these planets would need to pump water around to help keep it all stable for as long as possible. Anyways, I decided to take the idea and see how far I could run with it.

Assuming the bodies are rigid, we can get a little idea of some limits on the system, and then maybe use those to test the sanity of an (insane) example case. For a pair of orbiting bodies, the period of rotation is (Kepler's third) T = 2*pi*sqrt(a^3/GM) where 'T' is the orbital period, 'a' is the sum of the planets' semi-major axes to the mutual center of gravity, G is the gravitational parameter and M is the combined masses. If we assume 2 equal planets (rigid for starters) with radii r and mass m, this would become . T = 4*pi*sqrt(r^3/Gm) We could assume our planets have super dense cores for simplicity, but maybe let's assume they're water-density through and through just to see what happens. In that case the mass would be m = (4/3)*pi*r^3*Rho where r is the planet radius and Rho is the density of water (1000kg/m^3). Bringing it all together, we get T = sqrt(12*pi/GRho) .

For the planets to be grazing, they'll have a specific period that depends on their densities, but not their size! That's interesting, so perhaps we can have them test this up on the ISS  

For water the orbital period T = 6.5hours regardless of planet radii (as long as they're the same). Sounds nice! The actual number should be a bit larger because the planets will deform towards eachother.

Of course in real life, the surface of the planets will be pulled into an oblate shape due to centrifugal forces. Also, let's assume everything's tidally locked (it should be!). We can say for any drop of water, it will feel 3 forces: gravity from the 2 planets' centers of mass, and the centrifugal force orbiting around the planets' combined center of mass. Now we can just solve for the shape of the potential well, and thank our lucky stars some people on the internet have done this for us already!  Copyright Hale Bradt '09, Section 8 in https://www.cambridge.org/us/files/1913/6681/8626/7708_Tidal_distortion.pdf

Though these potential wells are for tidally locked stars, they can be stand-ins for our tidally locked planets just as easily. The potential wells in the photo on the right are just 3d interpretations of the one on the left. Here we can imagine our planets are as though we filled the potential wells up with water to the level it just bridges between the two, and makes a channel of some thickness. Notice in the picture on the right, we can fill it up to make a stable channel that won't leak out to space.

The very center of the channel won't experience any net force, but it will have a net pressure, which is a good sign. The outer surface of the water channel bridging between the worlds will feel a net inwards force towards the center of the channel due to gravity from the two planets. At the outer surface of the bridge, the gravity of the planets will cancel eachother in the back-and-forth direction, but will still pull towards the point that's directly between them in the 'radial' direction. It's the same force that allows people to orbit satellites around certain Lagrange points. As for the stability in the back-and-forth direction, the water in the very center of the channel will be supported by the water pressure of the attached worlds. If you were to draw a line between the planets' centers, the middle of the bridge would be the lowest pressure point, but could still be submerged and have a pressure if there was enough water. We can imagine the supporting pressure towards the middle of the channel as being provided by the inwards pull from all other points on the planets' surfaces.

So, if you were sailing along the surface of the channel bridging worlds and dropped a penny, it would sink 'quickly' to the inside of the channel, and then slowly towards whichever world was closer. Meanwhile neutrally buoyant things would just happily float like the rest of the water.

Stability... We're imagining the planets are the same size, and in this case it all works out nicely. We should doublecheck the system won't destabilize (have the channel grow) from small perturbations though. For example, what would happen for a small perturbation like water flowing from one planet to the other? Since a planet's radius is dependent on the cuberoot of volume, the receiving planet would grow less than the losing planet shrinks, causing the channel to narrow. This is a stabilizing factor then! Also, gravitationally, the receiving planet would pull towards the combined center of mass less than the losing planet would pull away, which is also stabilizing. Over astronomical time scales, there's no doubt the system would work its way towards becoming a single oblate sphere, but over short time scales I think it could be stable as two planets with a connecting channel!

... Or I'm full of hooey, that's always a possibility, too! Thoughts?

 

Edited February 7, 2019 by Cunjo Carl
added a bit of stuff.

[KerikBalm]

Well, we see lots of bilobed small objects that seem to be "loose rubble" without much mechanical strength, so this would just be scaling it up, and replacing dust/rubble/sand with water...

It would require a system with a lot of angular momentum, and wouldn't tidal locking move the bodies farther apart (as the moon slowly retreats from the Earth, as the Earth ever so slowly moves towards tidally locking to the moon). So... if they were formed, initially not tidally locked, they should separate over time. If you put them closer together to compensate, then before tidal locking happens and pushes them apart, I guess  they collapse inward (releasing a lot of heat and potential energy).

If they formed (via divine/super advanced technological intervention) already tidally locked and orbiting their barycenter such that they just touch... I guessssss it could be stable.

I find the idea of joining atmosphere's but not oceans to be more interesting.

Joining oceans means that with a boat you can go from 1 lobe to the other, life on both worlds oceans would be in constant contact, only really diverging once land animals evolve - likely early tetrapod like animals would be independently evolved from a recent oceangoing ancestor... animals living in the shallow coastal areas wouldn't get from lobe to lobe.

Joining atmospheres means that the sea life and land life would likely be completely independent. Likely bird-like animals wouldn't fly high enough to reach the other lobe. Microbial spores would likely cross, and both lobes would likely have common microorganisms and a common ancestor from way way back (maybe Eukaryote like life would be independently evolved on each world).

A society at the tech level of 1950's earth would then essentially have an entirely new world to colonize, without any space travel. Imagine the implications for the first civilization to make a plane capable of flying to the other lobe... and the challenges...

[kerbiloid]

What if introduce a chunk of dark matter stuck between the planets.

Say, these two planets have been attracted to the iinvisible lump of dark matter.
Or maybe have formed on its opposite sides from dust.
They are aside of it, orbiting it and touching it.

The water is also attracted to the dm between the planets, it forms an column-like ocean around and inside of it.
The interplanetary column-looking ocean touches the planets, and connects to their own oceans.

The dm-chunk also creates a gravity field holding air around the column ocean and allowing to breathe while one is travelling in a boat.

[sevenperforce]

33 minutes ago, kerbiloid said:

What if introduce a chunk of dark matter stuck between the planets.

Then the planets are pulled together and collide.

You also run back into the Roche limit issues from before.

[kerbiloid]

8 minutes ago, sevenperforce said:

Then the planets are pulled together and collide.

They rotate enough fast to stay aside.

P.S.
The Roche limit was mentioned to explain the water flow from one planet to another.

In "my" version there is no Roche limit condition.

Edited February 7, 2019 by kerbiloid

[sevenperforce]

2 hours ago, kerbiloid said:

They rotate enough fast to stay aside.

If they are both co-orbiting a dark matter "chunk" at those distances, then you end up with a chaotic three-body problem with no stable solution and the whole thing goes to hell.