Can two planets share the same atmosphere?

[wolveraptor]

Okay, I like to design alien ecosystems when I'm bored (go ahead; laugh it out). I was wondering: could there be a binary planet system in which the two planets are so close to each other, that they tug on each other's atmospheres and there's actually a transfer of gases? Would it be like an ocean of gases flowing through the space between planets, where flying life-forms could live? If such a thing is possible, would the connecting atmosphere be thick or too thin?

[Kuroneko]

Even neglecting stability issues for such configurations, which will be nontrivial because of atmospheric drag, any shared atmosphere will very quickly bleed off, particularly near the Lagrange point inbetween the planets, where there is essentially no gravity to hold it there. So, no.

[Il Saggiatore]

Okay, I like to design alien ecosystems when I'm bored (go ahead; laugh it out). I was wondering: could there be a binary planet system in which the two planets are so close to each other, that they tug on each other's atmospheres and there's actually a transfer of gases? Would it be like an ocean of gases flowing through the space between planets, where flying life-forms could live? If such a thing is possible, would the connecting atmosphere be thick or too thin?

In order to keep the planets from crashing into each other, they must be spinning around the barycenter of the system (imagine if the Moon were as large as the Earth).
Think about the tidal forces (gravitational and centrifugal): the planets would probably be both strongly deformed.
I doubt that two planets so close to pull each other's atmosphere will last long before crashing into each other.

[Sean Howard]

There's a book called Rocheworld that addresses this.

One of the planets winds up getting distorted so that it looks like an egg.

[Jarl Sven]

I forget all the specifics but in the "Integral Trees" by Larry Niven he created a "world" that was basically a torus of habitable atmosphere held in place around a star through various gravitational …..thingies
IIRC there was a yellow dwarf, a gas giant, and some rocky bodies at the Trojan points acting as shepherd moons……like I said, I forget but Niven is sometimes pretty good with the science part of the fiction

[wolveraptor]

Even neglecting stability issues for such configurations, which will be nontrivial because of atmospheric drag, any shared atmosphere will very quickly bleed off, particularly near the Lagrange point inbetween the planets, where there is essentially no gravity to hold it there. So, no.

So during the transfer, the atmosphere would drift off into space?

As for stability issues, I was thinking of the system to be spinning very fast, so that centrifugal force kept them from crashing. Of course, they'd be egg-shaped, but that doesn't really matter to potential life.

Btw, how close would they have to be anyways (just a rough estimate)? Any drifting atmosphere might be sucked quickly back into one planet or another, if it was within the planet's gravitational field. I think.

[VT-16]

Doesn´t Pluto and Charon "share" an atmosphere?

[Petrosjko]

Doesn´t Pluto and Charon "share" an atmosphere?

Nope. Pluto only has a very thin atmosphere when it hits the portion of its orbit that takes it close enough to the sun to melt off a little nitrogen and methane, then promptly loses it as it moves out again.

[Kuroneko]

So during the transfer, the atmosphere would drift off into space?

Absolutely. There is effectively no gravity at the L1 Lagrange point, making the escape velocity very low--much lower than the typical molecular velocity in gases even at low temperatures. Unless, of course, the second planet is completely immersed in the atmosphere of the first, but then atmospheric drag will force the system to collapse very quickly, even if other effects are ignored.

As for stability issues, I was thinking of the system to be spinning very fast, so that centrifugal force kept them from crashing. ... Btw, how close would they have to be anyways (just a rough estimate)?

Practically touching, at least when compared to the typical Earth-like radius. For a first-order approximation, one can use the Kepler laws. At distances reasonable enough to allow the kind of feat you have in mind, two Earth-like planets would have to orbit each other once every eight to nine hours. I hope the planets are volcanically dead and are stronger than steel, because otherwise they will be simply torn apart by the gravitational forces,

[wolveraptor]

What about planets of a lower mass, say closer to Mars in size? Would their atmospheres be too thin to share anyways? Does Mars have a thin atmosphere because of its closeness to the Sun, or can it not hang on to any atmosphere anyways?

[StarshipTitanic]

What about planets of a lower mass, say closer to Mars in size? Would their atmospheres be too thin to share anyways? Does Mars have a thin atmosphere because of its closeness to the Sun, or can it not hang on to any atmosphere anyways?

If one planet is far, far smaller than the other, the larger one would rip the smaller one's atmosphere away immediately (along with the planet) and there wouldn't be any sharing.

Sun-->Mercury-->Venus-->Earth-->Mars, too.

[Grandmaster Jogurt]

Does Mars have a thin atmosphere because of its closeness to the Sun, or can it not hang on to any atmosphere anyways?

Titan is smaller than Mars and has an atmosphere 1.5 times the thickness of Earth's, IIRC. Mars should be able to hold on to a substantial atmosphere if mass were the only concern.

[Kuroneko]

What about planets of a lower mass, say closer to Mars in size?

Based on the data on this page (where I should have gone in the first place), taking a rough order-of-magnitude calculation, Mars has a mass of about M ~ 6e23kg, a R < 4e6m radius (padded for planetary separation separation), so each planet will experience a force of at least F = GM²/4R² ~ 2e24N, the average "gravitational surface pressure" will be around F/(4πR²) ~ 4GPa, and this is also the tensile stress (for a ball, they should be identical, since there is no inner surface... unless my understanding of mechanical engineering is completely worthless, which isn't too unlikely).

Would their atmospheres be too thin to share anyways? Does Mars have a thin atmosphere because of its closeness to the Sun, or can it not hang on to any atmosphere anyways?

Mars is much farther from the Sun than Earth, and this fact is helpful in atmospheric retention. It's still not enough to hold it. Well, actually, atmospheric loss could be compensated if the planets are volcanically active, ala Venus, and spew out more gases periodically. What's much more worrying is the closeness of the planets and the inherent instability due to atmospheric drag. You play with the numbers yourself: for average densityρ and radius r, the planets have mass M = [4/3πr³]ρ, and thus the "average gravitational pressure" is P = F/A = [GM²/(2R+h)²]/[4πr²] < Gπ(rρ/3)². Your best bet is then to have a low-density, low-mass worlds that continuously spew new atmosphere (which makes sense, since the immense tidal forces actually help volcanic activity). Anything above 100MPa or so is unacceptably large, at least for naturaly-formed worlds. Also note that the surface gravity would have to be reasonable if you want a solid world, which constrains the possibilities rather strictly.

Erratum: I had an incorrect figure for the Earth's parameters in the previous calculation. The actual orbit time, from Kepler's Law, is T = sqrt[4π(2R+h)³/(G(M+M))], where radius R = 6.3781e6 m, M = 5.9736e24 kg, and 0 < h < 1e6m is a reasonable upper bound for the planet separation (note that for Venus, 1 bar pressure is at 50km, which would have h ~ 1e5m at this pressure for the binary planet), making it about three hours orbit time. This is even worse as far as the forces the planets experience go.

[wolveraptor]

IIRC Titan does not have significant volcanic activity, and still manages to retain a thick atmosphere. A binary planet system with sister-bodies as small as Titan could (I think) still have significant flow of gases and not be torn apart by huge gravitational forces.

[GrandMasterTerwynn]

IIRC Titan does not have significant volcanic activity, and still manages to retain a thick atmosphere. A binary planet system with sister-bodies as small as Titan could (I think) still have significant flow of gases and not be torn apart by huge gravitational forces.

No, you're not listening. It is absolutely impossible for two bodies to be close enough to share an atmosphere like that. Folks have dredged up the maths which demonstrate this. Even if they were small bodies, they'd be within their mutual Roche limit, which is the distance a large body can approach another large body before the tidal stress is sufficient to rip it apart. Also, even if your planets were made of unobtanium, the drag imposed by the common atmosphere will serve to further slow the two bodies down, resulting in them spiraling inward until they collided. Furthermore, the only reason Titan retains a sizeable atmosphere at all is because it is so far away from the Sun, it's mean surface temperature is -178 degrees Centigrade, so the gasses aren't moving all that fast.

[AMX]

It is absolutely impossible for two bodies to be close enough to share an atmosphere like that. Folks have dredged up the maths which demonstrate this. Even if they were small bodies, they'd be within their mutual Roche limit, which is the distance a large body can approach another large body before the tidal stress is sufficient to rip it apart. Also, even if your planets were made of unobtanium, the drag imposed by the common atmosphere will serve to further slow the two bodies down, resulting in them spiraling inward until they collided.

Are you absolutely sure about that?
I mean, the problem with tidal forces exists only when they are not tide-locked (which does mean that the system can't form - but it can exist, after it's been artificially created).
And shouldn't atmospheric drag be a nonissue, since the atmosphere would rotate along with the planets?

[GrandMasterTerwynn]

It is absolutely impossible for two bodies to be close enough to share an atmosphere like that. Folks have dredged up the maths which demonstrate this. Even if they were small bodies, they'd be within their mutual Roche limit, which is the distance a large body can approach another large body before the tidal stress is sufficient to rip it apart. Also, even if your planets were made of unobtanium, the drag imposed by the common atmosphere will serve to further slow the two bodies down, resulting in them spiraling inward until they collided.

Are you absolutely sure about that?
I mean, the problem with tidal forces exists only when they are not tide-locked (which does mean that the system can't form - but it can exist, after it's been artificially created).
And shouldn't atmospheric drag be a nonissue, since the atmosphere would rotate along with the planets?

No, such a system is entirely unstable. And no, that atmosphere doesn't rotate at the exact same speed as the planet, and the terrains of the planets will be slightly different, so over long periods of time, you will have slight increases or decreases in the planets' rotations, leading to oscillations known as libations. Worse still, both planets are not going to be perfectly homogenous, and will be subject to solar tides, which will provide slight gravitational peturbations of their own, as well as shifting about the mass of the planets' oceans.

If you had the ability to create this sort of system, and then left it alone, the end result will invariably be either a mutual collision, or one of the planets being ejected.

[CaptainChewbacca]

Titan keeps its atmosphere because the hydrocarbons are very highly concentrated in solids and liquids, and will slowly sublimate to replace lost gasses due to gravity and solar wind.

Mars doesn't have its atmosphere because it has nothing to maintain it in the way of active volcanics, a biosphere, or a hydrosphere.

Your two-planet system can't exist in any form. Tidal stresses get to be too much when your objects pass the Roche limit. "I'll spin them really fast" means you don't understand the mathematics of why it won't work. Bodies don't stay in close orbit, not even stars. They spin off or collide, but binary systems are almost universally far apart.

[Zero]

If two planets were close enough to share atmospheres, they would tear each other to pieces with tidal forces, or collide. It's just not a stable situation. Interesting idea, though.

[Beowulf]

To tell you exactly how thick the atmosphere of Earth is, take a globe and a sheet of paper. Put the sheet ontop of the globe. That sheet of paper is compared to the globe is the same relative thickness as the atmosphere is to earth.

[CaptainChewbacca]

The Earth's functional atmosphere is about 6 kilometers from sea-level up. Beyond that, its too diffuse.

[wolveraptor]

Damn. That was the only way I could think of to have complex life travel between planets. It would've also served as a starting point for inter-planetary life. Ah, well.

[Sean Howard]

Damn. That was the only way I could think of to have complex life travel between planets. It would've also served as a starting point for inter-planetary life. Ah, well.

Dude trust me, read Rocheworld. Robert Forward is known as Captain Hard Science of that scene.

[wolveraptor]

Is it online anywhere? Or do I have to do this the old-school way?

[Sean Howard]

Is it online anywhere? Or do I have to do this the old-school way?

Yeah you'll have to buy them. Its actually a series, but the initial one explains all the physics of the shared atmosphere and orbital situation. In fact, there is an appendix in the back that backs up the physics in even more detail with references, etc.

Its been a few years, I can't remember the details of the physics behind it, but it has to do with Lagrange points and density of the planets involved.