06-04-2014, 07:33 PM
(This post was last modified: 06-04-2014, 07:55 PM by stevebowers.)
No binary planet can exist for long if one planet is tidally locked to its primary; as you say, the other planet will migrate away and become a separately orbiting planet. Note that these two newly separated planets will still be unusually close to each other - they will continue to interact periodically until they fall into a new wider separation (or possibly hit each other, as in Pelion and Ossa in OA).
However many, if not most binary planets will be tidally locked to each other, or at least the smaller one will be locked to the larger one, as in the case of the Earth and the Moon, which is sometimes considered a binary planet. The other example of binary objects in our Solar System, Pluto and Charon, are both mutually locked.
This image, from Sean Raymond's original article, shows how two mutually locked planets would look; this arrangement would be stable for billions of years, and produce only small tides (although these two worlds are closer than most such worlds would be- they are almost contact binaries, like Harmonic Resonance in OA)
![[Image: binaryplanet.gif?w=620]](http://planetplanetdotnet.files.wordpress.com/2014/05/binaryplanet.gif?w=620)
However many, if not most binary planets will be tidally locked to each other, or at least the smaller one will be locked to the larger one, as in the case of the Earth and the Moon, which is sometimes considered a binary planet. The other example of binary objects in our Solar System, Pluto and Charon, are both mutually locked.
This image, from Sean Raymond's original article, shows how two mutually locked planets would look; this arrangement would be stable for billions of years, and produce only small tides (although these two worlds are closer than most such worlds would be- they are almost contact binaries, like Harmonic Resonance in OA)