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Questions Regarding an Alternate Planet

  1. Jul 13, 2010 #1
    Alright, so here's the situation: I'm a writer, and I want to write in a setting on a planet that isn't the Earth. To be more exact, I want it to be larger than the Earth. But I want to have credible writing, rather than claim that magic has made everything stable and feeling like the Earth. As such, I have a few questions. For ease of use, let's call this planet Dirt.

    1. If the circumference of Dirt is twice as long as the circumference of Earth, would Dirt need to rotate twice as fast on its axis in order to have days and nights of the same length, given equidistance to its primary light source and the same degree of rotation?

    2. If Dirt does have the doubled circumference, how much larger would its volume be compared to Earth's?

    3. If Dirt is spinning faster than Earth (which I assume it would be) would the centrifugal force at its outer crust have any tangible effect on the beings living there, e.g., "less" gravity than a planet its size would have, thus causing taller creatures to evolve?

    To be honest, I'll probably have more questions, but I'll start with these.

    Oh, and thankxs in advance!
  2. jcsd
  3. Jul 13, 2010 #2
    1) the angular rate of rotation would be the same; the rate the surface is moving would be twice as fast
    3) the change in centripetal force would be quite minimal on day to day life (the increased radius largely cancels the effect), its effect on (e.g.) large-scale weather patterns would be complex, but overall no drastic changes.
    2) if you double circumference you double radius (C = pi*r), which means you octuple (8x) the volume (V = 4/3 pi r^3), and therefore octuple the force of gravity--but keep in mind your radius is doubled, so the effective force of gravity is decreased by a factor of 4 ---> increase by 8, decrease by 4, overall your gravity is doubled which would presumably decrease average animal size. At the same time, note that the range in animal sizes on earth spans a few orders of magnitude, so the gravity change might be negligible compared to normal statistical variation.
  4. Jul 14, 2010 #3
    To paraphrase a character from "Transformers II". "Earth?! You might as well call it Dirt!"

    As the other respondent notes the angular velocity remains the same, even if the tangential speed doubles.

    Well that's little bit trickier than the other respondent implies. Higher rotational velocity means the body is more oblate than the Earth. Not by much, but the planet is a bit less spherical than the Earth and its volume is a bit less than 8 times Earth's.

    Indeed. The centrifugal force, which is a *real* force thanks to relativity, is somewhat higher than Earth's, though only twice as high with the same angular speed. Thus the gravity is reduced just a bit. Spinning the place 4 times quicker would mean a 32 times higher centrifugal force than Earth's, but that's only a reduction of 0.11 gee.

    However there's a few modifications to what I've said because of oblateness, but they're a little bit harder to compute. I'll have to get back to you on them.

    One thing to keep in mind is the compression of a planet's core means that a planet made of the same stuff, but twice as large, will be denser than the smaller planet. In the case of the Earth, the planet would mass ~13 times the Earth's mass if it was twice as big. That means the surface gravity would be ~3.25 times larger than Earth's.

    However that's natural planets. A hollow, artificial planet can be much, much larger and still have reasonable levels of gravity. The main problem is that it has to be made of something stronger than regular matter to remain stable against collapsing under its own gravity. Such "material" is described by Paul Birch in a fascinating series of articles he did years ago, the most relevant being...

    Supramundane Planets (a 2.6 Mb pdf file)

    ...worth a read. The most humungous artificial planet he describes is an immense 1.2 lightyears in radius, which means an outer surface area that's 3.2 quintillion times Earth's.
  5. Jul 15, 2010 #4


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    On a planet that spins significantly faster than Earth the wheather will be very different.

    In our atmosphere Coriolis effect arising from the Earth's rotation influences air currents and ocean currents. The Earth rotation is why the trade winds form. Also the fact that when there is a high pressure zone or a low pressure zone then air mass tends to start moving around that zone, rather than straight down the pressure gradient. This makes areas of low/high pressure exist much longer than they would on a non-rotating planet.

    Jupiter rotates faster than the Earth (at least the layers of atmosphere visible to us do.) Jupiter's red spot is a storm area that has lasted at least 300 years now. (Astronomers have observed that other storms - not as large - that arise on the same latitude tend to merge into the red spot, sustaining the red spot's energy.

    The planet's rotation reduces opportunity for air mass to migrate north or south. As evidence for little mixing between bands of latitude; Jupiter's and Saturn's atmospheres have bands along latitude lines.

    Summerizing, on a planet that spins faster than the Earth the wheather will be quite different. Exactly how is difficult to say, but the gas giants Jupiter and Saturn do give clues.
  6. Jul 15, 2010 #5
    I'd like to thank all of you, this has been very helpful in creating my new empire- I mean this world. I'm reading that paper on supramundane planets because, honestly, my day job is pretty boring. I'll try to come up with more queries with which to ply everyone later.
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