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What's the longest plausible orbital period for a habitable planet with a 3:2 spin-orbit resonance?

I want a planet with a 3:2 spin orbit resonance (which would experience 1/3 of a year of nighttime followed by 1/3 year daylight), but I want

  • relatively long periods of dark and night
  • liquid water possible

(As with tidally locked planets, I'm assuming a sufficiently thick atmosphere and oceanic circulation could distribute temperature enough to avoid the dark side freezing completely and the hot side completely baking.)

I thus need a planet which is

  • as far away from its star as possible (to make the orbital period longer)
  • far enough from its star for the hot side to not be destructively hot (although see the effects of thick atmosphere and clouds above)

but which is also

  • close enough to the star for the 3:2 orbit resonance to have occurred
  • close enough for liquid water (again, see the effects of thick atmosphere)

I was thinking that a cooler star than the sun (maybe K class) would allow the planet to get closer and locked into resonance without being too hot, but a star that is too cool (e.g. a red dwarf) would require the planet to orbit very close and give a very short orbital period.

What kind of star and what distance of planet would be suitable, and what would the orbital period be?

Tharaib
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    Not clear why you need a resonance. E.g. Venus has a very long day, but AFAIK is not locked to anything. Would it not be sufficient to have a late impact in planet formation that cancelled most of the angular momentum? – Sherwood Botsford Mar 28 '17 at 03:38
  • @SherwoodBotsford Good point. I could have an arbitrary distance from the star with arbitrary day/night periods. I was overthinking. Make that an answer and I'll accept it (since it answers what I want rather than what I asked for) – Tharaib Mar 28 '17 at 03:48
  • Just don't give it a moon of significant mass. Something the size of our Moon would probably have enough tidal pull to break a resonance that slow and increase the planet's rotation. – Salda007 Mar 28 '17 at 03:59
  • … with tons of volcanic activity it could be pretty far. Add wind for constant temperature around planet and you have a nice piece of rock for life. Other requirements are … too specific. – Jan Ivan Mar 28 '17 at 12:48
  • Edit the question as you(if you) no need resonance anymore, it will drastically simplify the question, and as long as there are no answers atm, you can change the question significantly. – MolbOrg Mar 30 '17 at 10:24
  • @MolbOrg If I don't need resonance for my specific planet and I change the question to reflect that, it would be deletion, not simplification or editing. I left it rather than delete it as the answer would be interesting in its own right, potentially useful to me in the future, and potentially useful as a resource for other people. – Tharaib Mar 30 '17 at 11:06
  • @JanIvan A thick atmosphere and winds to distribute the temperature were actually mentioned in the question; the specific other requirements were the point – Tharaib Mar 30 '17 at 11:10
  • @Tharaib so far, as for 2 days there is 0 answers it will be not deletion of significant part, but refining the question for it to fit it to the capabilities of the WB. It's up to you of course, but from my perspective, it would be a significant improvement of the question, because current requirements make not that much sense, as Sherwood Botsford mentioned it already. I'm not certain, but besides mercury and thus its orbital distances my guess those resonances will not play a significant role for bigger planets at bigger distances, like earth. There is some conflict between RL and u wishes. – MolbOrg Mar 30 '17 at 14:55
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    @MolbOrg I'm not sure you have understood the question - it simply says "How far from a star can a planet in 3:2 resonance be while remaining habitable?" This has an answer; if I were to remove the mention of resonance there would be nothing left to answer. Sherwood Botsford did not point out that my requirements don't make sense; he simply pointed out that if what I want is for the planet to have a very long day, I don't need an orbital resonance to get it. That doesn't mean there is a conflict between my wishes and real life; that just means there is a simpler way to achieve my wishes. – Tharaib Mar 30 '17 at 15:49
  • Yes, you right, I understood it incorrectly. "I want a planet with a 3:2 spin orbit resonance" replaced with "Let's say there is a planet with a 3:2 spin orbit resonance, how far it can be for water to be liquid, habitable etc". – MolbOrg Mar 30 '17 at 22:16

1 Answers1

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Not clear why you need a resonance. E.g. Venus has a very long day, but AFAIK is not locked to anything. Would it not be sufficient to have a late impact in planet formation that cancelled most of the angular momentum?

You could also have fun with a Roche World like setup (Novel by same name by Robert Forward)

You have a pair of planet in mutual orbit. If it were earth and twin at Luna's distance you'd have a period, I think of about 2+ weeks (about 1/sqrt(2))

The two don't have to fully locked. You could have one of them appear in the planet's rotating system of reference to take months or years to pass thorugh the sky.

I don't know how stable the climate would be. In another question I posited that if you lenghtened Earth's day to even 100 hours large parts of the planet would be un-inhabitable. Anything borderline desert right now. Places that are cold air traps in irregular terrain.

A slower rotation means smaller coriolis forces. This I think would make for larger storm systems with slower movements. That hurricane sits on top of you for 2 weeks raining 3 feet of rain a day...

Sherwood Botsford
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