Somewhat inspired by this question : Could a civilization as advanced as humans exist on a planet/dwarf planet like Pluto?

Assume two dwarf planets in tide lock orbit around one-another. Assume same mass for the two bodies. Ignore how it developed (atmosphere could be placed their artificially)..and assume tiny gravity.

Is it possible for an atmosphere to remain stable in the space between these two bodies?

I assume there is a place between these two bodies as they orbit each other where gravity is basically cancelled out by the two bodies and have an area in the center of microgravity. Could an atmosphere of sorts exist between these two bodies, suspended between the two? If yes, would the atmosphere extend to the planets surfaces? What type of orbital speed would there need to be to keep this semi stable (by stable I mean exist for a couple hundred years, artifical processes could be regenerating it...just curious if it'd simply dissipate or if the system could be stable.

As an alternative, could this setup be realistic with larger planetary bodies? Could multiple bodies (4+, or even rings) orbitting in this setup have a stable atmosphere in between them?


This isn't looking for local life on either planet...go with the vision of an artificial habitat put into the center of this planetoid setup (including the atmosphere being artificially generated when the setup was created). Would the atmosphere be stable, or would it dissipate as fast as it's created? Would it be feasible to exit the space station and wander around in this atmosphere?

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    $\begingroup$ worldbuilding.stackexchange.com/questions/4460/… ? $\endgroup$ – Vincent Dec 1 '14 at 21:34
  • $\begingroup$ @Vincent Close, but slight variation. Neither planet is capable of supporting an atmosphere on it's own. More like two (or more) planetoids supporting an atmosphere in the space between them (so side of planets facing space is complete or near complete vacuum)...kinda like a little atmosphere bubble made stable by the bodies orbiting around it. 'Bubble' doesn't need to actually be touching either planetoid (infact preferable if it doesn't) $\endgroup$ – Twelfth Dec 1 '14 at 21:43
  • $\begingroup$ Never would of thought that one of my questions would of inspired someone to ask a question $\endgroup$ – CrazySlayaNinjaBear Dec 2 '14 at 1:57
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    $\begingroup$ @CrazySlayaNinjaBear That's what happens when you get enough creative people in one place. We all trigger ideas in each other :) $\endgroup$ – Tim B Dec 2 '14 at 11:23

The short answer is yes.

However, a better answer would be "not naturally." The point you are talking about, where the gravity between two orbiting bodies balances the centrifugal force, is the first Lagrange point, or L1 for short.

There are five such points, and the three that lie on the line connecting the two bodies (L1, L2, and L3) are all unstable. Think of trying to balance a pencil on its tip: even though the pencil should balance if it is perfectly vertical, in practice balance can never be achieved. Only some sort of control system can keep the object at L1 (e.g. thrusters for spacecraft), even though the actual amount of force you need is tiny.

Even the two remaining points (L4 and L5, which lead and trail behind the smaller object's orbit) are unstable if the ratio of the two objects' masses is less than about 25, and for Charon and Pluto the ratio is around 8 or 9: too low for stable orbits.

However, even if any of the Lagrange points were stable, gas molecules simply move too fast: for room-temperature air the average speed is around 1000 mph! For the average speed to be less than escape velocity at L1 (~600 mph) the temperature would have to be around 90 K, just above the point of the air cooling into a liquid. Even at this temperature, some number of the gas molecules would be faster and able to escape, and assuming you kept the gas at the same temperature eventually all the gas would escape into space. I expect that even at the Sun-Jupiter L4 and L5 the solar wind would push away any accumulating gases.

Don't let this discourage you! If you want an atmosphere at the Pluto-Charon system L1, I would suggest using a huge inflatable spacecraft, like a giant balloon. You wouldn't need a huge amount of thrust to keep it there, just a tiny push now and then to keep it from drifting away. The mass of the air would be small enough that there would be no noticeable gravitational field, so you would float around inside---sounds kind of fun, actually!

  • $\begingroup$ It's worth noting that except for L1, "traditional" look on Lagrange points fails and you need to redo basic math behind it, if masses of bodies are too similar. OP asked for L1 so it's not a problem for this answer, just something worth noting, so others wouldn't try to use L3 or L5 without thinking. It's not only that they are unstable. It's also their positions or even their very existence. $\endgroup$ – Mołot Feb 26 '16 at 10:52

In theory, you could have a an atmosphere centered on the center of gravity between the two bodies. In practice, if there was a substantial amount of atmosphere it probably have substantial gravity of its own unless it was very thin. I would also expect the atmosphere to cause drag on the planets, slowing their mutual orbit until they crashed into each other and became one body in a cataclysmic event.

Such an arrangement would certainly not be stable.

  • $\begingroup$ If the setup was planet 1 mass = atmosphere + planet 2 mass make this more stable (not interested in naturally occuring, consider any 'technology' feasible in creating this)? I was hoping the atmosphere was not providing drag on either of the two bodies as they rotated as the atmosphere was not directly in the path of either body (atmosphere contained to center only). $\endgroup$ – Twelfth Dec 1 '14 at 22:51
  • $\begingroup$ @Twelfth: I'm a bit confused on what you envision. If a mass of gases was pulled to the center of the binary planet system, how would a creature reside on one of the planets and also reside within the atmosphere so they could breathe? $\endgroup$ – Ellesedil Dec 1 '14 at 23:04
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    $\begingroup$ @Ellesedil - Nothing natural, don't worry about creatures existing in this...think more along the lines of an advanced race putting a structure in the center of this setup (even to the point of the advanced race being the ones to setup the atmosphere in the center). Would it be stable then? I'll edit the question a bit $\endgroup$ – Twelfth Dec 2 '14 at 0:01
  • $\begingroup$ A gas cloud between the planets wouldn't be stable (there's nothing holding it together). A gas cloud containing the plants would be gravitationally bound but the planets would need something sustaining their orbits or they would lose energy to drag and crash into each other. $\endgroup$ – Nathaniel Knight Dec 2 '14 at 17:57

No. The point between the two planets is not stable, gravity is pulling away from it towards the planets as soon as you move even a tiny distance from the mid point.

The only way to get atmosphere there is if you have atmosphere on the two planets thick enough to extend up through the L1 point. The size of that atmosphere will then be constrained by their combined mass, not the mass of just one of the planets.


Robert Forward wrote a series of books starting with Rocheworld about a double planet that are so tightly coupled they aren't round, share atmosphere and at one point part of the ocean on one planet slops over to the other.

Forward has his science correct, so this might be a place to look for effects of such a double planet system.


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