habitable planets orbiting M dwarfs

[Malenfant]

nope. A gas giant in a habitable orbit would be so close that solar tides would strip the moons.

The gas giants themselves probably wouldn't be tidally locked to the star though, because they're more fluidic and can dissipate the tidal forces more easily than solid bodies.

 

[Jeff M. Hopper]

Originally posted by Malenfant:
nope. A gas giant in a habitable orbit would be so close that solar tides would strip the moons.

Crap, I was afraid of that.
Thanks, though.

 

[Straybow]

First, the 3:2 harmonic may be more common than just the example of Mercury would portend. The orbit of Pluto is approximately 3:2 coupled with Neptune. One could posit that it is a more natural rhythm in orbital mechanics than in spin coupling. Except we don't see it in the Gallilean satellites' spins or orbital periods, IIRC.

Hab-zone for a small M is so close that a gas giant can only be there because of perturbances that would themselves strip the moon from the GG. A moon could only be there if captured after the GG's orbit circularized. It would only be a temporary arrangement.

If the moon's orbit around the GG is coupled to the GG's orbit around the star that arrangement could be extended to a cosmologically significant duration so that it could be found in a small percentage of cases.

 

[Malenfant]

Originally posted by Straybow:
[QB] First, the 3:2 harmonic may be more common than just the example of Mercury would portend. The orbit of Pluto is approximately 3:2 coupled with Neptune. One could posit that it is a more natural rhythm in orbital mechanics than in spin coupling. Except we don't see it in the Gallilean satellites' spins or orbital periods, IIRC.

2:1 and 1:1 are the most likely resonances. And yes, we certainly do see it in the Galileans. They are in the 4:2:1 Laplace resonance - Ganymede completes 1 orbit when Europa completes 2 and Io completes 4.

If the moon's orbit around the GG is coupled to the GG's orbit around the star that arrangement could be extended to a cosmologically significant duration so that it could be found in a small percentage of cases.

No, it can't. The solar tides take angular momentum out of the pair, breaking up the planet-moon system. You can't get the moon's orbit to be 'coupled to the GG's orbit'.

 

[Straybow]

I was speaking exclusively of 3:2 coupling, which we don't see in the Galileans.

It wasn't long ago that astro/planetologists said you can't have a GG in a tight orbit. Forty years ago nobody even imagined 3:2 spin coupling for Mercury under the same assumption that solar tides are just too strong. The universe continues to amaze us.

 

[LordVince]

There's been a lot of talk about tidal-locked planets, but what about slow-spin worlds? For example, the "4 Day Planet" mentioned in H. Beam Piper's story of the same name? That was a planet in the habitable zone of its star, which was humanly habitable...more-or-less.

I remember that the Length-of-Year was mentioned as "8,000 hours", which would make a year that is very close to 1 Earth Year. In that time the planet spun on it's axis only 4 times -- 4 "days".

Just how likely is a planet with this kind of spin, located in the habitable zone?

 

[Malenfant]

Well, a slow spin planet wouldn't be impossible, but it would have habitability problems. If that planet in the book really had a 'day' that was 2000 hours long, its day side would be roasted beyond habitability and its nightside would be frozen beyonnd habitability.

 

[Fritz_Brown]

And, since it does rotate, it would alternately bake and freeze the entire planet.

 

[LordVince]

Yup, that's the way it was described. The humans lived in an underground city. As the environment was described, there were several hours just before "dawn" when a person could walk around outside without an environment suit, otherwise it was either too hot or too cold for people. Even with environment suits.

I was just wondering because, with alot of the tidal-locked talk, in various threads, recently, I begin to get the impression that tidal-lock is an all-or-nothing kinda thing. That is, a planet either slows down to a tidal-lock, or it spins fast enough for the "day" to be 20-40 hours long; there doesn't seem to be any "middle ground". And I was just wondering if that were "normal".

 

[Malenfant]

Well, Venus' day is actually longer than its year (243 days vs 224). But that's largely down to tidal braking from its atmosphere as well as solar tides.

 

[Straybow]

Except that Venus is spin coupled... to Earth. Venus always presents the same face to Earth in opposition. Another reason why I don't think a flat "can't happen" can dismiss habitable moons of GGs for M dwarfs.

 

[Malenfant]

Yes, but it had to get to that point. And a day longer than a year is bloody strange, the reason for that is the atmospheric braking, not the earth:venus coupling.

I hate to say "can't", but based on everything we know about tidal interactions the moons for ANYTHING that close to a star seem to be flat out impossible. There's no loopholes, and even if there were they'd be phenomenally rare. So rare that the chance of finding one in our galaxy would be miniscule.

 

[Straybow]

Except this is a galaxy that has been manipulated by The Ancients for their own inscrutible purposes...

In general, planetary rotation is believed to be primarily the result of collisions of planetesimals late in the formation process. As such, a single substantial impact by a body in retrograde orbit can have a seemingly disproportional effect.

If Venus' rotational period had been lengthened by such an event then Earth:Venus coupling would have dominated rather than tides from an atmosphere which would have developed up to a billion years later.

 

[Malenfant]

Well if you're invoking Ancients then what happens in reality is worthless since you can say they did anything they liked. Though it'd still be suffering from 300,000 years of natural evolution which is more than long enough to screw things up. I don't think a GG could hold onto a moon put there artificially in that period if it was within 1 AU of an M dwarf.

Your last statement is fairly meaningless - it's impossible to say what would have happened. It seems that atmospheric tides are enough to slow it down from an earthlike rotation to what it has today though.