Spin of planets, bigger means faster?

  1. Quick question, do bigger planet spin faster?
  2. jcsd
  3. Look up the data.
  4. Nabeshin

    Nabeshin 2,202
    Science Advisor

    In general, I don't think there's any relation.
  5. Chronos

    Chronos 10,348
    Science Advisor
    Gold Member

    You must look at planet formation to understand this issue. The short answer is yes.
  6. I always thought that the smaller planets would rotate more rapidly than larger ones, under identical conditions of course. Much like a figure skater draws in his or her figure and seemingly rotates faster. However, I may be (and more than likely) am wrong. If someone could address this I would be interested to learn
  7. mgb_phys

    mgb_phys 8,809
    Science Advisor
    Homework Helper

    Planet Speed at equator (km/h)
    Mercury 10.9
    Venus 6.5
    Earth 1670
    Mars 867
    Jupiter 45600
    Saturn 37000
    Uranus 10900
    Neptune 8460
    Pluto 47

    You would expect small planets to spin faster - from conservation of angular momentum

    But larger planets were formed from larger clouds of stuff.
    As stuff contracted, then because of conservation of angular momentum it speeded up, the bigger/faster the original cloud the faster it ended up. Of course if you took the existing planets and made each of them smaller - they would speed up even more.

    Then there are effects that have happened since. Mercury's rotation is slowed by tidal friction with the sun so has a very slow speed (long day), the Earth's is slowed a little by friction with the moon.
    Uranus probably got hit by something in the past - which is why it has a weird axis tilt.
    Mars might also have been affected by whatever caused the asteroid belt.

  8. Very interesting. Thank you for providing this
  9. A good way of comparing spin in stars is to compare angular momentum and mass in geometric units where-





    where [itex]a[/itex] is the spin parameter in metres, v is the equatorial rotation velocity, m is mass, r is the equatorial radius and k is the moment of inertia coefficient (0.4 for an idealized sphere of uniform density).

    a/M produces a unitless figure between 0 and 1, the higher the number, the higher the spin. For the Sun (k=0.06), a/M=~0.188, for a 2.2 sol neutron star with a frequency of 1500 Hz (k=0.35), a/M=~0.488.

    This doesn't appear so straightforward with planets as M works out considerably smaller than [itex]a[/itex] but there should still be a way of comparing spin geometrically.

    In the case of planets, you could probably get away with just considering the results of [itex]a[/itex] which is considered to be the amount of angular momentum per unit of mass (sometimes expressed as J/M). In this case, Jupiter is the clear winner and Mercury has the least 'spin' per unit of mass.
    Last edited: May 2, 2009
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