This is a bit outside my expertise, so anyone with a good knowledge of the appropriate physics would be nice. Just thought of an unusual world concept to go along with all those people who dream of ringworlds, Dyson spheres, and such.My idea: A binary planet (a pair of binary planets?), er, two masses that orbit each other, have roughly the same size and mass, and keep the same face towards each other. The potentially interesting aspect: The inhabitants of one planet build a space elevator connecting to the other one.
It's my understanding that our moon keeps the same face towards us because of its origin as a torn-out chunk of Earth. Seems to me the right circumstances could divide a planet evenly. Of course, I realize this, along with the chance of making both halves life-sustaining would be wildly improbable, but it's a big universe, and a potentially cool setting helps.
So, any difficulties to discuss? Stuff that comes to mind: 1. Even if life develops on both places, the two would be quite different. 2. The two faces might not be synchronized enough to support an elevator for long. 3. Routine eclipses might make for interesting, though potentially nasty climates.
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I might have to run a couple simulations to check it, but I don't believe an even mass split is actually possible if you want the planets to be in a stable orbit around a star. This is what's known as the Three-body problem. It's very difficult to work out, and you only tend to get stable solutions when one of the masses is negligible (the closest we've seen is with Sun-Pluto-Charon, where Charon has 11% of Pluto's mass). At an even split, the system would be incredibly unstable, and probably couldn't exist.
Now, even if it turns out there's some small range of values for which the system is stable, getting the two planets to constantly face each other would require ridiculously long days. You would probably end up with days on the order of one to two weeks, by our measurements. It's not necessarily a problem, but something to be aware of.
Synchronization itself actually isn't too hard to pull off. As long as the bodies aren't perfectly uniform and immutable (which they won't be in reality), gravity will slowly drag on the two to bring them closer to synchronicity over time. Eventually they'll reach a perfect alignment and stick there. An elevator still may not be quite feasible just because of the massive distances involved. Even minute disturbances would cause huge shifts in where the elevator is pointing.
As long as the bodies aren't perfectly uniform and immutable (which they won't be in reality), gravity will slowly drag on the two to bring them closer to synchronicity over time.
Even if they are perfectly uniform, tidal forces should do the job. But yeah, getting a stable orbit is the main problem.
Wouldn't the orbit be more elliptical than spherical? Even if facing can be maintained, wouldn't that alone totally ruin anything but a really stretchy space elevator?
Of course, then there's the upcoming Infinite "I have chained the moon!" Undiscovery.
A wizard did it! But really, I just can't see how that works.
It's my understanding that our moon keeps the same face towards us because of its origin as a torn-out chunk of Earth.
The moon shows the same face to Earth because it is tidally locked. A binary planet system with both of the same size and mass would likely experience this as well.
Well, what about a space elevator that's not fixed at either end, stretching perhaps 95% of the distance? Each side could then have a 2.5% tower, or at worst, regular rocket launches to their end of the worldbridge.
Robert Forward wrote a sci-fi book in 1984 called Rocheworld (earlier published as "The Flight of the Dragonfly") which featured an equal-sized double planet. One was dry rock (Roche) and the other a water world (Eau). The name is also an intentional allusion to a Roche lobe, a figure-eight astronomical shape.
The planets were close enough together so that, on rare occasions, a giant fountain of water would splash from Eau to Roche. I don't know if it's possible, but it was cool in the book!
Forward was a physicist, and he strove for scientific accuracy in his fiction, while still having fun with some wild ideas. The laser/light-sail propulsion system in the book is particularly well thought out and described. The book is still a great read 20+ years on.
Second to last season of LEXX featured two planets (Fire and Water) locked in just such an orbit, although quite a bit closer such that their atmospheres just barely touched.
While Lexx was generally quite silly, that particular season, with the introduction of the character "Prince", was actually pretty good.
I was thinking about this again last night for some reason, and I wonder if it might be possible if the two planets are situated at a Trojan or Lagrange point of a gas giant (or second star)?
Why do you need the two planets to be roughly the same size? Could be interesting with the change in gravity from one world to the other.
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