Twin planets

[Moontanman]

Is it possible for planets to orbit each other over an extended period of time, i am thinking billions of years, as described by Robert L Forward in his book Roche world?

 

 

http://en.wikipedia.org/wiki/Rocheworld

[D H]

Sure. Pluto and Charon, and closer to home, Earth and its Moon (were the Moon just a bit more massive it would have to be classified as a twin planet rather than a moon).

[Moontanman]

Sure. Pluto and Charon, and closer to home, Earth and its Moon (were the Moon just a bit more massive it would have to be classified as a twin planet rather than a moon).

 

 

I understand that but in the book the planets were so close together they were egg shaped with the pointed ends always facing each other and they were so close they shared an atmosphere... they orbited each other in hours...

Edited January 25, 2012 by Moontanman

[CaptainPanic]

I understand that but in the book the planets were so close together they were egg shaped with the pointed ends always facing each other and they were so close they shared an atmosphere... they orbited each other in hours...

I'm not an expert (<-- disclaimer)

 

I think it could be stable. It should be in a balance. If one of the two planets is heavier, it will accumulate a thicker atmosphere than the smaller planet... and the atmospheres will find an equilibrium.

 

There's a few additional comments:

- The tides and also the difference in density of the atmosphere would be huge along the surface.

- The rotation of the planets would be very fast.

- The weather patterns of the planets would be linked (and I wouldn't want to have to make the forecast)

- You need one hell of an atmosphere though... If you look at Earth, our atmosphere is only 200 km, with a planet diameter of 12,756 km.

- I have no idea about the water balance... would water be able to completely accumulate on one planet? If one planet is larger than the other, it would have a thicker atmosphere. But at the same time, it's the same distance from the sun as the other planet, so temperatures could be roughly the same. The thicker atmosphere would condense more water and the water might all move towards the bigger planet. Just an idea - I haven't thought this through in detail.

 

Reading the description of how a supernova can form (obviously not a stable situation): a binary star system, with one star growing to become a red giant. When they "share a common envelope", their mutual orbit is suggested to shrink. Why? I can see tidal forces slowing their rotations, but why would it reduce the orbit? Is that because a significant part of the mass of the larger star moves towards the smaller star (significantly changing the mass of both centers of gravity)?

 

In case of the planets, if they cannot exchange too much mass, then I guess this could become a stable situation, even though it wouldn't be stagnant (with for example the weather patterns influencing each other).

[Widdekind]

If a satellite orbits closer to a parent body, than the "geo-synchronous" radius; then the satellite will orbit faster than the host planet spins, so that the satellite will "orbit ahead" of the tides that it generates into the central planet. Thereby, i.e. by "tugging spin-forward" on the tidal bulge it generates into the planet, the satellite will deposit its orbital energy, into "spinning up" the planet, even as the satellite gradually de-orbits itself. Er go, if the Roche radius, is less than the synchronous radius, then the bodies will gradually converge & coalesce.

 

For two equal-density planets, the Roche radius is 1.25 - 2.50 R. Meanwhile, the synchronous radius is

 

[math]\omega_{orbit} = \omega_{planet}[/math]

 

[math]\frac{G M}{R^3} \approx \omega^2[/math]

Naively, the Roche radius is so close to the planet that said planet would have to be spinning, at near break-up rotation rate, for the synchronous radius to lie within the Roche "disruption radius". If so, then "by the time" some body was orbiting close enough to some planet, to be tidally distorted; then it would also be "orbiting ahead of its tides", and gradually spiraling in towards the other body. Thus, the "Roche world" scenario seems plausible -- except that large planets are not "rigid", and would not "bump into each other", and simply touch at the point of initial contact. Rather, they would "smush" & "squish" together.

[emilievak]

If two planets were so close they shared an atmosphere there would be a loss of heat energy and planets would get closer until they fell upon one another.