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Re: CONCULTURE: dual planets

From:Andreas Johansson <andjo@...>
Date:Wednesday, November 17, 2004, 16:14
Quoting Sally Caves <scaves@...>:

> ----- Original Message ----- > From: "Andreas Johansson" <andjo@...> > > > > If two roughly Earth-sized planets were orbiting one another closely > > enough to > > have enormous tidal effects on one another, one might expect them to have > > become tidally locked to one another, that's to say, always turning the > > same > > side towards one another. If we additionally assume their mutual orbits to > > be > > close to circular, the mutual tides would be static, that's to say > > unnoticeable. You'd still get tides from the star, of course. > > > > Andreas > > Sooo, if my two roughly Earth-sized planets were tidally locked to one > another, thus causing enormous tidal effects, would their mutual orbits have > to be elliptical to cause these tides?
Yes. The tides will osciallate back and forth instead of circling around the planet as ours do.
> Because you say that if their orbits > are close to circular, the mutual tides would be unnoticeable. I want the > tidal effects. (Am I misunderstanding you, as I think I am?)
Possibly. Your questions come off as rather disparate, so I try and respond to each in turn and hope you can puzzle it back together. :)
> I also want > them to present the same face to the other, but I could dispense with that > feature.
Presenting the same side to one another is basically what being tidally locked _means_. (You could have them locked to a resonance other than 1:1, which case you'd get tides even with circular orbits, but they'd no longer present the same face towards one another all the time. I don't know if a such setup would be at all likely to occur.)
> If they present the same face, each is slowing down the other's > rotation, right?
Au contraire. That's the only configuration in which they _don't_ slow one another down. (I suppose elliptical orbits should decay towards circles over time, BTW. No idea what might be the relevant time-scale.)
> So their days and nights would be longer?
If they're tidally locked, their rotational periods would be equal to the time it takes them to revolve one time around one another (the day would be equal to the "month"). I don't know what the interval of reasonable values for this period would be, but I'd hazard that some Earth days or a couple of weeks would be possible.
> Ooh. How could > life survive in that condition?
I don't see any problem, actually. It would just have to adapt to somewhat different conditions than on Earth.
> Would any one planet seem to rise and set > for the other?
Er? Them presenting the same face to one another would mean, of course, that they were stationary in one another's skies (modulo small modifications caused by deviations from circular orbits). To have them rise and set you'd need to have them not locked to a 1:1 resonance.
> In brief, how would an elliptical orbit affect tides, > rotation, etc.?
Tides would osciallate. I don't see how rotation would be much affected.
> One planet would be slightly smaller than the other: I'm > imagining a planet earth's size, and another planet bigger than Mars, but > not as big as earth. Both support life. The bigger planet has larger seas.
I get the impression you actually do _not_ want them tidally locked. Just remove the tidal locking, and you get the tides without any need for elliptic orbits. You'd be free to chose rotational periods freely (altho they would be growing towards the tidally locked value over astronomical time-frames (like the Earth is vis-a-vis the moon)). Additionally, you'd get them to rise and set in one another's skies. You'd have to ask someone more into this than me to learn how far apart they'd need be for a lack of tidal locking not having occured to be likely. The younger the planets are, the closer they could be. (You might also use a tidally locking different from 1:1; this would give about the same effects as a lack of locking wrt tides and the like.) This would then determine how big they'd appear in one another's skies, and how long they'd need to revolve around one another. And the magnitude of the tides, of course. Andreas