# Could two planets be tidally locked to each other so close they share their atmosphere?

Could two planets be tidally locked to each other at such a small distance from one another that they share their atmosphere, and material is floating around between the two?

• There is no need for them to be tidally locked. Questions is simply: "Can two planets be so close they share they atmospheres?". Tidal lock is a bonus, allowing you to, e.g., build a skyscraper from one planet to the other, if there are no small libration. Nov 24, 2014 at 9:06
• See this answer worldbuilding.stackexchange.com/questions/1274/… and the discussion here chat.stackexchange.com/transcript/message/18052695#18052695 for some related background information. Nov 24, 2014 at 10:44
• There's a lot of discussion about roche limits, but that only applies to things like rocky planets and gas planets, that can break apart. If you had a planet which was a solid ball of titanium, not held together by gravity but rather by something much stronger like the electrostatic force, then the roche limit would barely apply.
– Rag
Mar 4, 2016 at 5:07
• @Envite: Tidal lock and a near-circular orbit will prevent a rigid body from destruction due to ever-changing forces. For both bodies being tidally locked into a very close circular orbit, the hydrostatic equilibrium is possible at least for some ratios of their masses and values of their densities; but Ī̲ expect your fixation on familiar Roche limit pictures will prevent you from understanding importance of these data. May 16, 2016 at 17:55
• @IncnisMrsi It's not just about tidal locking. Gravity from the other planet would be high enough, at the depicted distance ("close enough to share their atmospheres"), that the material in the face of each planet facing the other one would be almost weightless. Density would not matter in that situation, so rocks could float almost freely on the shared atmosphere. Not a sane place to have a planetary crust. May 16, 2016 at 21:12

The Roche Limit alluded to in other answers may not be a barrier to this.

Rigid Approximation

For two rigid planets of the same mass M and radius R, both held together by the force of gravity, the Roche Limit would be the distance between their centres D where any reduction in that distance would mean that tidal force on each planet was greater than the gravity holding the planet together.

Where the tidal force equals the gravitational force:

GM/R2 = GM(2DR - R2)/(D4 - 2D3R + R2D2)

This simplifies to:

R4 - 2DR3 + R2D2 - 2D3R + D4 = 0,

which, has a solution D = 1.8832R. The separation of surfaces is 2R less than that: -0.1168R. The Roche Limit is when the circumference of each planet is within the other planet.

Two equally-sized rigid planets whose surfaces are separated by any distance at all are well outside each other's Roche Limit.

Fluid Approximation

For fluid bodies, from https://en.wikipedia.org/wiki/Roche_limit#Fluid_satellites,

The calculation is complex and its result cannot be represented in an exact algebraic formula

but the result given is:

D = 2.44R

That, again, is the distance between their centres. When both objects have radius R then their surface to surface separation is 0.44R (2,800km for Earth). Even completely fluid Earth's could orbit each other almost completely within each other's exosphere (about 10,000km) without crossing each other's Roche Limit.

Combined

So for a rigid body the Roche Limit in surface separation is -0.1168R (overlapping)

For fluid planets the Roche Limit in surface separation is 0.44R.

A real rocky planet like the Earth would be somewhere between the two. A precise answer would take complex computer modelling of all the constituent layers. The best I can do here is guess at half way between the two (half rigid, half fluid): 0.1616R. Just over 1,000km for two earths: at about the edge of each other's thermosphere.

That would be right on the Roche Limit. I doubt planets would exist very happily at that limit but, theoretically, they would exist.

Whether that could ever arise in a stable configuration is another matter.

Capturing another object (like Neptune capturing Triton) in orbit is very rare and to do it in such a way that resulted in such minuscule separation and perfectly circular orbits would be ridiculously unlikely.

For them to condense out of gas and dust in that configuration when they would be fluid and not rigid seems even more unlikely. However, they would be growing and accreting and becoming more rigid as they do so (and then less as their cores melt) their Roche Limits would be changing up and down as they did.

It is not necessarily impossible.

• There is more hope. At just over 0.44 R both planets will be distorted (not torn apart, just elongated) [earth is basically fluid at this scale]. This distortion will reduce the 0.44R to a smaller number. Also, the iron cores are denser which will mean that the distortion weakens the gravity less, also reducing the Roche limit. Jan 30, 2018 at 2:40

No.

HDE 226868's answer explains the concept of Roche Lobe and touches that of Roche Limit, but things are a bit more complicated.

Giant stars are stars so big that their outer layers feel very weakly the gravity of the internal ones. These stars, when on close binary systems, are so big that their stellar atmospheres are bigger than their Roche lobes, so they lose mass towards the other star on the binary system, if that one is small enough (second case in the image). Usually stars are not so big so they are each in its own Roche lobe (first case), but if both stars are giant stars, they can share their stellar atmospheres. This is a real case of atmosphere sharing, but keep in mind that the distance between the star nuclei is still very big. It is just that atmospheres are big as well.

Giant planets like Jupiter have bigger gravity on their atmospheres than giant stars. This is because the radius is quite smaller, even if the mass is small as well. So it is very unlikely that a giant planet can share atmosphere with another: they would need to be very close, really close. Close enough that they will have big tidal problems. Such planets are mainly composed of gas, so they will most probably experience serious disturbances caused by the other planet, and can not be tidally locked at all.

Rocky planets like Earth have a different problem: that of Roche Limit. When a solid body kept together by its own gravity, like a rocky planet, becomes near enough another solid body of the same or bigger mass, it experiences tidal effects. Tidal locking is the minor of them. But when coming closer and closer the tidal effect causes some parts of the planet to travel faster than others, causing the breakage of the minor body. Roche's limit depends on body's density. For example, Roche Limit for Moon on Earth is between 9496Km (if Moon were perfectly rigid) and 18261Km (if it were perfectly deformable). Anything nearer than that will tear apart. And since Earth's atmosphere is a fine layer of just 10000Km (for the Exosphere, that hardly can be called "an atmosphere"), two planets like those of your image would quite quickly become one unique bigger ball. More if the distance you pretend is that of Troposphere's 12Km.

• Note that Robert L. Forward was a hard-science physicist and knew what he was talking about. (He worked as a physicist and aerospace engineer and wrote books on the side). It seems likely he did the maths and knew the worlds would hang together before he wrote the books although I don't have any actual evidence to give on that front. Nov 24, 2014 at 13:51
• @TimB Second this. I would expect his physics to be sound. His biology, not so much. I see the Rocheworld as two planets that almost fill their Roche lobes (and thus are stable), the atmosphere is somewhat shared. Since the worlds aren't round they must be tidally locked. Nov 24, 2014 at 21:30
• @LorenPechtel stability does nothing to do with Roche Lobes but with Roche Limit, which is a different thing (worked out by the same person). Roche Limit deals with tidal instabilities cause by the difference on attraction and on orbiting speed due to differences in distance to the center. Nov 26, 2014 at 11:05
• @Envite Stable as in not falling apart due to going outside their Roche lobes. I didn't mean to say the orbit was stable or unstable. Nov 27, 2014 at 1:38
• @Envite But you can have two planets that are each in their own Roche lobe yet the atmosphere is outside the lobes and thus shared. Dec 2, 2014 at 4:00

Ahh, here we go.

No. But yes.

I'll leave the stuff about the Roche limit for Vincent; after all, it was his idea. Assuming he writes up an answer, you'll learn why these planets cannot be tidally locked and so near each other. This means that you won't see two planets coming really close to each other, like this image depicts. They certainly won't touch.

But atmospheric transfer is totally possible. I know, this seems counterintuitive, given that it's just been established that these planets can be so close to one another. But there's a little loophole called a Roche lobe that can make this scenario possible.

The Roche lobe (not the same as the Roche limit!) is the point in a binary star system inside of which matter is bound to the star by gravity. If matter escapes the lobe, it may be accreted by the other star, and vice versa. Mass transfer is usually in the form of gas, plasma, or other stellar material, including material from coronal mass ejections or other dramatic events. Here, though, it will be the atmosphere that is transferred.

The whole idea, by the way, is explored in Robert Forward's Rocheworld, which I managed to find out after writing the rest of this answer.

• FYI, in a search for "Roche lobe" on my browser, the Wikipedia article for the Roche limit showed up before the article for the Roche lobe. Beware the differences between the two. Nov 23, 2014 at 18:47
• That picture in the question at the top is fan-art depicting Rocheworld. Nov 24, 2014 at 9:22
• As written in the book the two planets are tidally locked and orbiting just outside the Roche limit. The planets are actually deformed into an egg shape and the atmosphere fills the Roche lobes and flows freely between them. Nov 24, 2014 at 9:25
• If the planets are outside the Roche Limit, how big their atmospheres are to overflow their Roche lobes? Nov 24, 2014 at 9:38
• It doesn't have to be massive because the gravitational gradient within the lobe means you need less energy to travel between the planets than away from them. Nov 24, 2014 at 10:42

The answer is yes and the Pluto Charon system is an example of such an arrangement. See this article from New Scientist

Two earth like planets sharing an atmosphere is explored in a fictional setting in The Ragged Astronauts trilogy by Bob Shaw

• The Ragged Astronauts series takes place in a different universe (this creeps out in a few places - for example one experiment by Toller Maraquine's brother, I think, showing pi to exactly equal 3). May 12, 2019 at 16:46

Depends what is meant by "atmosphere" Officially, the earth's atmosphere extends one third of the way to the moon, which is extraordinarily further than what is implied by the illustration provided. However, at this distance, the air certainly isn't "breathable" and probably not what is commonly meant by "atmosphere."

So certainly, if you had two earth like planets, spinning around each other at the distance that the moon orbits, gas exchange is beginning to sound likely. Also, high tide would be 8 times more significant than it is on earth, given our current moon.

perhaps someone else could speak of the probability of such a planetary system occurring, but one in place, I see no reason why it would be unstable.

• If, IF, you had two exact duplicates of Earth, and they were apart from one another the same distance as Earth is from the moon, you'd have a Rocheworld scenario? Then why are so many others saying its not possible?
– Len
Jan 26, 2018 at 14:35

If the terrestrial planet requirement can be relaxed allowing for smaller worlds with fluffier atmospheres, consider two Titan-sized planets in close orbit. With its radius of 2500km, minimal Roche distance of the surfaces is 1100km. Titan's atmosphere at 600km is at mesopause just above upper haze layers. Under the counteracting gravity of the companion, the atmosphere would stretch farther, likely connecting the stratospheres, opening the possibility of cross-planet travel by balloon.