# Spin of planets, bigger means faster?

by valdar
Tags: bigger, faster, means, planets, spin
 P: 19 Quick question, do bigger planet spin faster?
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 Quote by valdar Quick question, do bigger planet spin faster?
In general, I don't think there's any relation.

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## Spin of planets, bigger means faster?

You must look at planet formation to understand this issue. The short answer is yes.
 P: 78 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
 Sci Advisor HW Helper P: 8,961 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.
P: 78
 Quote by mgb_phys 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.

Very interesting. Thank you for providing this
 P: 602 A good way of comparing spin in stars is to compare angular momentum and mass in geometric units where- $$a=J/mc$$ $$M=Gm/c^2$$ where $$J=vmr\,k$$ where $a$ 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 $a$ but there should still be a way of comparing spin geometrically. EDIT: In the case of planets, you could probably get away with just considering the results of $a$ 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.

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