If an earth-like planet was slowed down to very nearly match its orbital period, so that instead of being tidally-locked to its star, the planet effectively had an immensely long day. Akin to Mercury's orbital resonance, albeit a more extreme case. Alternatively, as was asked here, what if a tidally-locked planet broke out of its synchronous rotation?

I am aware that in both cases the massive amount of energy needed to either slow down or speed up the planet, would probably decimate everything on the planet. However, if this happened gradually. Would the habitable zone/belt associated with most tidally locked earth-like planets be able to migrate slowly across the planet?

If so, how wide would the belt need to be, and what would be the maximum rate at which it could potentially traverse the planet?

  • $\begingroup$ No need to migrate, it's like polar night. For some plants is ok. Although much depends on air flows, temperature difference may be to high. For marine live it will be a harder as it is not, but they will manage it. For simple plants, like moss, they may survive. Live will stay, but more precise hard to say. $\endgroup$ – MolbOrg Jul 2 '16 at 3:48
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    $\begingroup$ Don't forget about any continents separated by oceans. The oceans might act as a block to your migrating plant-life. An extra point to consider, if your day/night cycle ends up being very long...the oceans on the far side of your planet might actually freeze over during the 'night' period, unless you have strong currents moving water about. $\endgroup$ – EveryBitHelps Jul 2 '16 at 18:33

There isn't a single answer, but it's possible to make a plausible estimate, given some more details about the planet.

firstly, how wide is the habitable zone? A mile, ten miles, a hundred? This depends on what temperature range the plants can tolerate and how much "libration" the planet has. Then you need to know how fast the plants can grow to maturity, and how far they can spread their seeds at each generation. That distance, divided by the maturity time, gives you the speed at which the plants can, effectively, move.

For example, if they can get their seed to move 24 miles, and grow to maturity in a day, their effective movement - of the plant belt, not of each plant - is one mile per hour. That needs to be able to keep up with the movement of the habitable zone in the planet's very slow rotation. These plants will grow very fast, compared to earthly ones.

There might well be different varieties of plants, some that were adapted to the hotter edge of the habitable zone, some for the colder one, but that simply means you have several adjacent habitable zones.

If the required speed is too high for you to believe that the plants can grow fast enough, you need a different way of doing it. The two obvious ways are spreading by suckers, the way that strawberries do it, or just uprooting and lurching, a la trifids.

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    $\begingroup$ Here on earth we have plants whose seeds can lie dormant for decades waiting for the right conditions to germinate, and plants that race from seed to flower to seed in a few weeks. The fastest way for a plant species to move is inside a migratory bird, which is why many seeds are wrapped in berries or fruits. $\endgroup$ – nigel222 Jul 2 '16 at 11:23
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    $\begingroup$ The dormant seeds are a better bet when you have a narrow, moving, habitable zone. Bird-equivalents can't usefully carry your seeds a long way if they die before they get very far. $\endgroup$ – John Dallman Jul 2 '16 at 11:45

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