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Increasing Planetary Rotational Speed

  1. Sep 14, 2014 #1
    Hey there folks,

    As a forward: I'm trying to build a "high fantasy" universe in which to write stories, but within its historical timeline of events there are some cosmological happenings that I'd prefer to not just wave a wand at, so to speak. I'd like there to be at least SOME solid grounding in science so the world still retains some shred of believability.

    The planet I'm focused on developing has gone through an era where its axial rotation has been very slow. It's locked in an orbit where one side always faces the sun. During this time, sentient races have evolved and progressed on the planet. Now... that perpetual day/night environment is great for designing cool fantasy races from the extreme polarities, but... that's about where that dynamic's usefulness ends. The planet is way too static to hold much interest, what with no day/night change, seasons, etc...

    So finally, my questions:

    1.) What kinds of physical cosmological events could occur that might speed up this planet's rotation so that it might at least have a more earth-like day/night cycle, without (here's the tricky part) killing off all sentient life on the planet?

    2.) Is it possible that this event could happen on a time-scale in which the changes could be experienced in the length of... let's say maybe a century long window?

    So far my best guesses have been:

    - somehow pull the moon in closer to the planet
    - have a massive comet/asteroid fly by incredibly close using its gravitational influence to speed it up and/or graze the planet with a "small", indirect impact

    Other than that, I got nothin'.
    Any help or suggestions would be immensely appreciated! Thanks!
  2. jcsd
  3. Sep 14, 2014 #2

    Simon Bridge

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    - Impact or close pass with another body: this would be bad for the inhabitants but need not kill off everyone
    - super-Huge eruption/ ejected matter: basically think a rocket strapped to the planet

    2. The time-frame depends on the total angular momentum change and how noticeable you want it to be.

    Whenever you want a sciency feel foundation, always turn to conservation of momentum and energy.
    Here you are adding a Lot of energy and momentum to the planet - you can work out how much to ball-park figures yourself - where does it come from?

    Note: a quick calculation (you should check) using the Earth as a model suggests that the energy exchange would be equivalent to 1013 MT TNT. This is roughly 100,000 times the energy of the impact supposed to have wiped out the Dinosaurs.... spread over time, that is still an extinction-level event every day for 100 years.
    I suspect someone would notice - but not for long.
    Last edited: Sep 15, 2014
  4. Sep 15, 2014 #3
    Oh boy... looks like my options are pretty bleak. Any idea if the moon's proximity would make much difference? I know that as it drifts away from earth our days get longer (albeit VERY slowy). Aside from tidal and tectonic factors, would a closer moon mean shorter days?
  5. Sep 16, 2014 #4
    Would Mercury-like 3:2 lock be acceptable?
  6. Sep 16, 2014 #5

    Simon Bridge

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    ... A closer moon does not automatically mean shorter days - this is where the mechanism is important. The Moon retreats due to tidal effects - so the gravitational field between the earth and the Moon allow angular momentum to be exchanged. Reversing the process would involve figuring out how to get the same mechanism to drag the Moon closer.

    Since this is SF - then you could pay around with a planet with the other situation - where it's moon is being dragged in. This means that it's moon would eventually hit the planet though.

    Your main problem is that any time-scale less than 10000yr would result in some drastic effects .... all that energy has to come from somewhere and it will do something.

    I think you need to work out what role the various situations play in your story - how do they contribute to or advance the plot? Why is it important that the Planet is tide-locked to it's star? Is it good enough just that the Sun is immobile in the sky for some geographically limited people?
  7. Sep 19, 2014 #6
    Got it!

    Your planet orbits a very small red dwarf once per 24 hrs. (It is easy to forget that a tidally locked planet is, in fact rotating.) That system drifts into a solar system with a much larger star. In THAT system, there is a jovian in the habitable zone. It captures the planet, separating it from the original red dwarf. The dwarf then flies away, taking up a distant orbit.

    The rotation of the planet never really changes, but the views from the surface would change radically. You'd see a new sun (and then the jovian) grow and "dethrone" the stationary star, which would then grow small, distant, and mobile. The new sun would rise and set every day, while the old one would behave like a planet as seen from earth.

    Of course now the planet has tides. Expect lots of earthquakes, and perhaps a world-ending flood and atmosphere thickening when the ice side of the planet melts.

    BTW: My understanding is that the same forces that could tidally lock a planet would also make moons impossible. Hence Mercury and Venus are the only planets in our system without moons.
    Last edited: Sep 19, 2014
  8. Sep 19, 2014 #7

    Simon Bridge

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  9. Sep 19, 2014 #8
    Would it? You'd see the new star growing for hundreds of years, but it would only take about a decade between getting noticeable warmth from the new star, and capture by the jovian. Then the old star would fly away in a few weeks. You'd only have about one earth year of double solar input.

    Also, the old star would likely disrupt all the Jovian's previous moons, and change the jovian's orbit as well. (Presumably in to the habitable zone for the story to work.)
  10. Sep 19, 2014 #9
    BTW Simon, thanks for the links. It looks like a 24 hr year would make the star a brown dwarf, rather than a red one. If Bubchubb2's concept can work with a much longer day, that would be more probable. Or there could be a reason for the planet to be cooler than current models suggest: Perhaps the subsolar storm creates a really high albedo - reflecting much of the red dwarf's heat away.
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