Excuses to explain lack of tidal lock in a red dwarf system

In a story of mine, I have a six-planet system orbiting an M-type red dwarf star. The planet in question is the fourth one, a Goldilocks superearth of slightly less than 3.5 Earth masses^note plus 1.1g surface gravity and a nitrogen/helium/CO 2 atmosphere of pressure where humans only need EVA gear instead of just a breathing mask with oxygen tank if there's a dust storm, based on Gliese 581g.

The problem is, the Planets page says such a system is always tidally locked, yet I already mentioned in the story that the planet has a day-night cycle. Is there a way to explain away why the planet isn't tidally locked?

One possible excuse I can think of is that the planet was discovered to be very slightly elliptical at the equator, hinting that it used to rotate much faster but slowed due to tidal effects; it currently has a fifty-something hour day and is expected to achieve tidal lock in a few more billion years, as evidenced by all three inner planets (two terrestrials and a Hot Neptune) being tidally locked. Would that work?

edited 11th Jun '14 12:58:15 PM by amitakartok

Two explanations.

One, you may use a moon to a planet instead of a planet.

Two, tidal locking takes time. Here is a formula. If your planet was the product of a giant impact that hit just at the right angle, it may spin extremely quickly at the start.

edited 11th Jun '14 1:25:57 PM by SeptimusHeap

Also, a planet having a moon helps mitigate tidal locking.

I saw that formula but the article also says that it's inaccurate to several orders of magnitude due to requiring estimations of unknowable parameters.

The impact theory sounds good; the Gliese 581 system does have a very large comet belt with 10+ times more content than ours'.

I was thinking of the planet having several moons but the Planets page says it's not possible (plus there's the fact that the local FTL requires a stable Lagrange point; if there's no planet-moon point, a planet-star point must be used but it's quite a bit farther). Maybe it originally had one moon that broke up from tidal forces?

edited 11th Jun '14 1:54:17 PM by amitakartok

A double planet might work as well. Also, a comet (unless very large) probably isn't enough.

...like, double planet that ended up in a collision, with the smaller breaking up? Kinda like how the Moon was created, except the bigger planet's larger size pushing out the Roche limit far enough that the leftovers couldn't assemble but fell back down to create the current planetary body?

Wait if I read the planets page...

I'm not sure I understand. The Other Wiki says red dwarfs would likely be tidally locked but there's no certainty of it.

That's precisely why I originally decided against the lock. It says "predicted" and not only can predictions be wrong, one of the very foundations of modern science is not accepting anything as absolute truth unless it's proven to be true beyond any reasonable doubt.

The formula posted above says it's impossible to calculate the time needed to achieve tidal lock with perfect accuracy because two parameters are just educated guesses the astronomers have no way of obtaining without knowing the exact material composition of the body in question. Errors can be as much as several orders of magnitude.

edited 11th Jun '14 5:09:48 PM by amitakartok

Sorry, I'm not much help in this, but I have a question (although maybe the answer to this question will help you with your One Big Lie, should you choose to employ one): I was under the impression that a tidally locked planet could exist around any star. In our own solar system, for example, IIRC Mercury is tidally locked. A red dwarf should differ from our own sun only in mass, size, brightness/energy output, and gravity, the latter of which causes tidal locking. Gravity is inversely proportional to distance, so couldn't you "solve" the tidal locking problem by moving the planet farther away? (Unless this screws up the Goldilocks zone).

The problem with tidal locking happens because a red dwarf is dim. A planet orbiting a dim star has to be close in order for it to recevive substantial insolation (—> be inside the habitable zone). Being close means it's more likely to get tidally locked. So no, unless you want to have an Ice Planet moving the planet out will just trade one problem for another.

As far as I know, Mercury is not tidally locked, it's got a 3:2 (?) orbital resonance.

When I was in high school years ago we were told Mercury was tidally locked but more recently I learned it's got orbital resonance resulting in a day that's longer than its year.

IIRC, an early Larry Niven story is based on a tidally locked Mercury.

In this case, the planet is orbiting at 0.146 AU, well within Mercury's orbit if it were around the Sun; the outward-neighboring planet^note another superearth with over 9 Earth masses; simulations predict that it could be an ocean planet but I propose it to be a "snowball" planet due to an anti-greenhouse effect similar to the one on Titan is in a Mercury-esque spin-orbit resonance, according to current simulations. That's why tidal locking is a problem.

Climate-wise, I decided it has no surface water and thus, no life. Well, at least until several icy comets were deliberately dropped onto the equatorial regions to form several lakes large enough to be visible from orbit. Said lakes were then seeded with algae, although this terraforming project is expected to not pay off until well after humanity's disappearance via extinction/evolution. On the other hand, wouldn't terraforming the atmospheric carbon dioxide into oxygen reduce greenhouse effect and potentially cool the planet beyond habitability?

As evident, I'm trying to avoid All Planets Are Earth-Like.

edited 12th Jun '14 5:52:13 AM by amitakartok

Climate-wise, I decided it has no surface water and thus, no life. Well, at least until several icy comets were deliberately dropped onto the equatorial regions to form several lakes large enough to be visible from orbit. Said lakes were then seeded with algae, although this terraforming project is expected to not pay off until well after humanity's disappearance via extinction/evolution. On the other hand, wouldn't terraforming the atmospheric carbon dioxide into oxygen reduce greenhouse effect and potentially cool the planet beyond habitability?

Actually, surface waters aren't required for life; see also the Wikipedia article for desert planets.

As for the CO₂, that depends upon how much of it and other greenhouse gasses there is in the atmosphere. Even a substantial drawdown may not be enough to freeze the planet.

I did read the Wikipedia article about other possible chemistries for life that do not involve stuff we use; over half of the article was about listing off why none will work and why ours is the only way to go. It just smells too much like Humans Are Special to me.

I don't want to delve into realistic alien life too much, though I have ideas regarding other planets. Just next door to the planet being discussed above one is the ice world Eidothea, covered in a mixture of water ice, dry ice and various hydrocarbon ices; there's enough stuff frozen in there that melting it via terraforming would rapidly turn the planet into a Venusian pressure cooker from the methane alone, let alone the CO2. Tidal heating from the star is fueling some cryovolcanoes and geysers, providing liquid water that spawned some primitive microbial life; this discovery would start a debate whether it's morally right to commit deliberate panspermia if your homeworld can no longer support life, or even whether it's right to exploit life-bearing planets for material goods (in this case, hydrocarbon mining for the system's chemical industry).

edited 12th Jun '14 6:25:48 AM by amitakartok