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PostPosted: Fri May 21, 2010 1:45 am 
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Ok, I've crunched some numbers and here's how Tidelocking works, and the practical upshot of it.

Tidelocking depends on several factors all at once: the mass of the star, the size of the planet, the age of the system, the initial rotation period of the planet, the tidal dissipation factor (Q) of the planet, and the orbital distance of the planet. Thus it's a bit hard to predict when it occurs, so we have to set a few things constant to get something predictable out of it.

e.g. We want to see if an earthlike planet is tidelocked around an M V star. So I put in a planet exactly like the Earth (initial rotation 10 hours, Q=20) in the habitable zone of stars with a variety of (low) masses and checked to see how old the system had to be before they got tidelocked. And here's what I got (hab zone distances are for initial stellar luminosities):

Code:
Mass         Hab Zone              Age to tidelock
0.1-0.6      within 0.29 AU      within 0.1 Ga
0.7          0.39 AU             0.27 Ga
0.8          0.52 AU             1.20 Ga
0.9          0.67 AU             4.16 Ga (100 hrs at 3.75 Ga)
1.0          0.86 AU             longer than age of star


0.6 solar masses (Ms) is K6 V, so anything in the habitable zone around stars between K6 V and M9 V will be tidelocked - no ifs, no buts.
0.7 Ms is K4 V.
0.8 Ms is K2 V.
0.9 Ms is G9 V.

So in practical terms, anything redder than about K2 V that is old enough to potentially allow life more complex than bacteria to form is going to have tidelocked planets in its habitable zone.

Smaller planets take longer to tidelock, but then as you get smaller you get less able to hold onto breathable atmosphere - however, a size 5 world (which can hold onto water, N2 and O2) basically tidelocks in about the same time as shown above - it seems you have to get a lot smaller before you can escape being tidelocked in that time.

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PostPosted: Wed Jun 23, 2010 4:47 am 
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G'Day,

Have you read the paper "Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability" by M. M. Joshi, R. M. Haberle, and R. T. Reynolds 1997?

What did you think of it?


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PostPosted: Wed Jun 23, 2010 4:56 am 
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A while back, yes :). I thought it was er... good? (I mean, it's a science paper. The authors probably know what they're talking about :) ). IIRC that's one of the classic papers about tidelocked planets.

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PostPosted: Thu Jun 24, 2010 6:08 am 
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The thing that struck me most about it was that the whole "The atmosphere will freeze out" was a boogeyman. That and with an ocean of any sort, there would be al lot of heat transfer. Exactly what is at the substellar point makes a big difference. If it is water, there would be a hell of a lot of evaporation. If it was land, there would be some very dry winds blowing.

Lastly that a tide-locked world would be a very windy place.


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PostPosted: Thu Jun 24, 2010 6:39 am 
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Yeah. TBH even with those papers, I'm still uncertain as to how one is supposed to guesstimate the temperature on any given tidelocked planets... how hot would it get at the subsolar point? How cold would it get at the antisolar point? Would water really just concentrate on the darkside? Could you get situations where the subsolar point is actually quite pleasant (say, about 20°C?) And so on.... climate/atmospheres was never my strong point. I can possibly answer questions like that on a qualitative level, but for the numbers more often than not I just go by what GURPS Space says and take their word for it.

I guess I can say that thinner atms = bigger temperature difference between day and night sides. And bigger temperature difference = stronger winds to try to balance it out (I think they're called katabatic winds? May be wrong there...).

A slightly eccentric orbit would make the star wobble in the sky, which means that if you're right on the twilight zone, you could have sunrises and sunsets over the course of an orbit - especially if you have mountains on the horizon that can block the sun out for part of its wobble. This sort of thing means it's not going to be a nice clean, straight line between light and dark side.

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PostPosted: Thu Jun 24, 2010 2:18 pm 
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Wouldn't orbital eccentricity also cause tidal heating of the core? Possibly enough to allow an otherwise solid planet to have a small molten core?

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PostPosted: Thu Jun 24, 2010 8:57 pm 
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Cyborg IM1 wrote:
Wouldn't orbital eccentricity also cause tidal heating of the core? Possibly enough to allow an otherwise solid planet to have a small molten core?

A close orbit with a high eccentricity around a massive star (or gas giant)
should lead to tidal heating, which is discussed as the reason for the liquid
water oceans on some of Jupiter's moons - but whether this could be pro-
duce enough heat to keep a small planet's core liquid ...


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PostPosted: Thu Jun 24, 2010 9:12 pm 
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Tidal heating certainly keeps Io's rocky interior molten and makes its surface volcanically active. A tide-locked planet orbiting an M V star could be close enough for it to be heated up significantly by tides, especially if there are other planets in the system that are keeping its orbit very eccentric.

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PostPosted: Fri Jun 25, 2010 12:55 pm 
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Also there is another form of Tidal Locking that is rarely mentioned in SF but exists in our own solar system and could be a really cool setting.

Resonance Locked worlds like Mercury. Mercury is locked in a 3:2 resonance with the sun. It turns 3 times for each 2 orbits, so every orbit and a half, it changes which face is sunward.

This situation makes for REALLY long days, but not permanent days. Resonance locked orbits are probably a more common situation when the planet starts out with a fairly high eccentricity, while true tidal locking (1:1 resonance if you will), will occur for planets with almost circular orbits.

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PostPosted: Fri Jun 25, 2010 1:56 pm 
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Cyborg IM1 wrote:
Resonance Locked worlds like Mercury. Mercury is locked in a 3:2 resonance with the sun. It turns 3 times for each 2 orbits, so every orbit and a half, it changes which face is sunward.

I once tried to design such a planet for a setting, but the details (e.g. the
weather patterns) turned out to be a true nightmare.


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