MadSci Network: Astronomy
Query:

Re: 2 planets 1 atmosphere ?

Date: Tue May 19 19:08:21 2009
Posted By: Chris Peterson, Faculty, Denver Museum of Nature and Science
Area of science: Astronomy
ID: 1241052208.As
Message:

Would it be possible to have two earth-like planets, twins, orbiting each other and their star, sharing a single atmosphere (perhaps shaped like a fat 8 ?) in a stable manner? Would it be possible to aerodynamicly fly from one to the other safely?

Yes, such a system is possible. The question is really whether you can have two rocky planets orbiting close enough to each other that their atmospheres overflow their Roche lobes, but are outside their Roche limit.

The Roche limit is the distance from a body that another can get without being ripped apart by tidal forces. For a pair of Earth-like bodies of the same size and density, the Roche limit is about 2.4 times the radius of the bodies.

There is some approximation here, since the bodies wouldn't actually be spherical. They would both be deformed into sort of egg shapes, and they would be tidally locked to each other. That is, each would keep the same face towards the other, like the Moon does to the Earth.

The Roche lobe is a gravitational equipotential surface that looks like half an hourglass, with the restricted point touching the L1 Lagrange point. The L1 point is found between two bodies, and is the location where their gravitational fields are equal and opposite. Everything orbiting inside each Roche lobe is gravitationally bound to that particular body.

If you work out the math for the Roche limit and the Roche lobes, you'll find that there are reasonable cases where the atmospheres of the planets could lie outside the Roche lobe near the L1 point, and therefore the atmospheres could exchange.

Flying between them is another matter, since it seems like their might be a great deal of turbulence where the systems join.

It is unclear how long a system like that would be stable. Over geological times, I would expect frictional losses between the two atmospheres to reduce the angular velocity of the planets. In order to conserve angular momentum, that would bring the planets closer together, until their Roche limits were exceeded and they would break up.

Did you know there was a science fiction book written about just this sort of planetary system? It is called Rocheworld, written in 1985 by Robert Forward. In the book, some explorers try to fly between the worlds as you suggest, with not entirely successful results. Forward was a physicist, and made an effort to ensure that the story stayed true to real physical laws.

Roche limit: http://en.wikipedia.org/wiki/Roche_limit
Roche lobe: http://en.wikipedia.org/wiki/Roche_lobe


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