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Could there be contact binary planets?


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We know there are such things as contact binary stars, where pairs of stars orbit so closely together around their centre of mass that they're actually warped out of shape, touching, sharing stuff. We also expect there is such a thing as a binary planet, where two bodies of similar mass orbit a point outside the body of both objects, an example (if only they were classified as planets...) is Pluto and Charon.

This got me thinking, would it be possible for there to be a contact binary planets? If they were gas giants they would share an atmosphere, and probably have crazy weather and electro magnetic interaction. Or two water worlds could share an ocean. Or two rocky planets that are warped out of shape and touching, maybe with crazy volcanic activity.

Would it be possible or is my imagination getting carried away? What effects would it have on different types of planets?

Barycentric_view_of_Pluto_and_Charon_29_May-3_June_by_Ralph_in_near-true_colours.gif

w-uma-binary-1.jpg

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My guess is that gravity would want to make that thing spherical (a single sphere, that is). I think there's a rather low upper limit on the possible mass of a binary contact.

Edited by Frida Space
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It may be possible for tidal locked planets, but this condition will not last much time, they lose a lot of kinetic energy.

I guess some will said that the planets will tear apart, but I think it should be a case where this is possible.

PD: rocheworld, yeah good book, they use beamed sail, forward was the first who come out with that idea.

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No wait, I thought contact binaries and binary systems were two different things? As in, binary systems = two bodies orbiting around a common barycenter, contact binary = two separate objects which have merged into a single, two-headed object (see 67P or several asteroids and comets)

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Could you actually argue that Pluto and Charon are contact binary since there's evidence of them sharing atmosphere?http://www.space.com/30089-pluto-moon-charon-red-pole.html

New Horizons so far has found no atmosphere whatsoever around Charon. Also, even if there was one, the term "shared atmosphere" can be misleading: Charon would simply collect the N2 molecules escaping from Pluto (so not part of Pluto's atmosphere) and forming a very transient layer of gas which would be lost to space quite quickly.

EDIT: Think of it this way; it's like finding a Martian meteorite on Earth. It escaped from Mars and landed on Earth, but (I hope) that wouldn't lead you to argue that Mars and Earth are a binary system :)

Edited by Frida Space
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Could it form? Extremely unlikely, basically impossible. Planetary formation time is longer than such system's hypothetical durability.

Anything other than two large boulders (small asteroids) would be so ephemeral and would result in massive geological disturbances. Energy would be shed in friction and eventually you'd get them fragmented, lava covered, then one body, possibly with a temporary ring.

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It depends on the density ratio and the radi of the planets. The problem is the roche limit. The roche limit of the primary body needs to be smaller than the combined radi of the planets.

So you need to solve the function 1.26*Rprimary*(rhoprimary/rhosecondary)^(1/3) < Rprimary+Rsecondary.

Say we want 2 planets made of the same stuff. So rhoprimary = rhosecondary.

1.26*Rprimary < Rprimary+Rsecondary

0.26*Rprimary < Rsecondary

So a contact binary between 2 bodies with the same density is possible provided that the secondary body is at least a quarter of the radius of the primary. It becomes harder if the secondary is less dense than the primary. The moon could never be a contact binary with earth because the density of the moon is only 60% that of earth.

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Yeah, the problem will be with Roche limit and not friction (I mean, they should tidal lock each other anyway). Though for really small bodies it can be hand-waved (look at 67P), not the case at all for a body with surface gravity of Earth.

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Yeah, another day I will play a bit with the math of roche limit to find the best example on proxity, density, atmosphere and gravity.

Not sure what is the conclusion of Relathon because I did a quick read. Where the 1.26 comes?

But I guess is possible, There are many different links in internet with examples with different configuations, even with 2 earths at 2.4 radius of distance, they can share an atmosphere due the roche lobes.

http://www.madsci.org/posts/archives/2009-05/1242825020.As.r.html

They found a binary star like that:

http://phys.org/news/2009-12-imaging-young-binary.html

I imagine that Forward did many calculations before write his novel Rocheworlds, the same as any hard science fiction writer.

About 2 planets sharing an atmosphere.. in that case Larry Nivel was beyond that in his book "The Smoke Ring"

220px-The_smoke_ring.jpg

https://vimeo.com/40676197

http://www.larryniven.net/physics/img27.shtml

But yeah, many incredible things may be possible in the universe if you match all the exact conditions that you need, the universe is so big, that any small chance becomes reality.

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Yeah, another day I will play a bit with the math of roche limit to find the best example on proxity, density, atmosphere and gravity.

Not sure what is the conclusion of Relathon because I did a quick read. Where the 1.26 comes?

That's 2^(1/3). The 2 was inside the density ratio brackets.

Derivation can be found here.

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It does not look like they are really touching stars. Only those circumprimary disks are really touching.

Yes, the black parts are the actual stars, if that was earth and a twin it would be around GTO.

Now a real contact binary would probably not be habitable, if they touch tides from sun and other planets would have them move a bit crunching the crust, you would get insane volcanism with effects far worse than an dinosaur killer regularly.

Sharing atmosphere would be easier, now this might generate other issues, how would air distribute itself? if the planets was 100-1000 km from each others?

How would gravity behave, low gravity would keep the air pressure down and the air column high so you might be able to fly between them even if 1000 km between each other. If not an small rocket engine might be enough to take you to the other planet, gravity would be lower at higher attitudes.

The smoke ring is something different and even weirder, theoretically its plausible but you would need an uranus sized venus like planet to provide the air.

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It does not look like they are really touching stars. Only those circumprimary disks are really touching.

Take a look to the graphic, it also show the gas circulation, but well that is what is call roche lobe.

That's 2^(1/3). The 2 was inside the density ratio brackets.

Derivation can be found here.

Thanks Ralathon, I will take a look when I have time and see if I found a better case.

- - - Updated - - -

Yes, the black parts are the actual stars, if that was earth and a twin it would be around GTO.

Now a real contact binary would probably not be habitable, if they touch tides from sun and other planets would have them move a bit crunching the crust, you would get insane volcanism with effects far worse than an dinosaur killer regularly.

You will have tides only if they are not tidal lock. If the both planets form at the same time (which is just a gas cloud with 2 mass centers), then there is almost a 100% chance they will be tide lock, also in the way that is not a capture event, the gravity forces will be lower, so they might be close to its roche limit.

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Take a look to the graphic, it also show the gas circulation, but well that is what is call roche lobe.

Thanks Ralathon, I will take a look when I have time and see if I found a better case.

- - - Updated - - -

You will have tides only if they are not tidal lock. If the both planets form at the same time (which is just a gas cloud with 2 mass centers), then there is almost a 100% chance they will be tide lock, also in the way that is not a capture event, the gravity forces will be lower, so they might be close to its roche limit.

No tides between the two planets, you, you would still get tides from the sun, this should cause an difference between planet a agains sun equal distance and planet b closest as they rotates.

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Which is less than half than the moon on earth, that is nothing on planet deformation. Yeah we can notice with the moon 1 or 2 meter difference, but earth has 12000000 meters in diameter.

Now in this system you may have 2 or 4 times that "virtual diameter", which may increase in 2 or 4 times the tide difference.

But like this is half of moon, then it will be equal to have 2 or 4 meters of difference between both planets.

Which is not enoght to change the gravity between both.

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Which is less than half than the moon on earth, that is nothing on planet deformation. Yeah we can notice with the moon 1 or 2 meter difference, but earth has 12000000 meters in diameter.

Now in this system you may have 2 or 4 times that "virtual diameter", which may increase in 2 or 4 times the tide difference.

But like this is half of moon, then it will be equal to have 2 or 4 meters of difference between both planets.

Which is not enoght to change the gravity between both.

The san andreas fault moves about 40mm a year. Have fun with the earthquakes when the fault moves 2 meters back and forth every day (And days will be a lot shorter on a contact binary).

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The san andreas fault moves about 40mm a year. Have fun with the earthquakes when the fault moves 2 meters back and forth every day (And days will be a lot shorter on a contact binary).

Huge earthquaskes can move much further than that. Anyway, that 2m are the values from real life earth (by the moon) and obviously we are not under permanent super-earthquakes.

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