Spin of planets, bigger means faster?

In summary, there is no direct correlation between the size of a planet and its spin speed. Factors such as the initial size of the planet's cloud of material, tidal friction, and past collisions can all play a role in determining a planet's spin. Additionally, a more accurate way to compare spin in stars is to use the spin parameter (a) and the mass (M) to calculate a/M, which gives a unitless figure between 0 and 1. For planets, the amount of angular momentum per unit of mass (J/M) can also be considered.
  • #1
valdar
19
0
Quick question, do bigger planet spin faster?
 
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  • #2
Look up the data.
 
  • #3
valdar said:
Quick question, do bigger planet spin faster?

In general, I don't think there's any relation.
 
  • #4
You must look at planet formation to understand this issue. The short answer is yes.
 
  • #5
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
 
  • #6
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.
 
  • #7
mgb_phys said:
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
 
  • #8
A good way of comparing spin in stars is to compare angular momentum and mass in geometric units where-

[tex]a=J/mc[/tex]

[tex]M=Gm/c^2[/tex]

where

[tex]J=vmr\,k[/tex]

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.EDIT:
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:

1. What is spin of a planet?

The spin of a planet refers to its rotational motion around its own axis. This determines the length of its day and the direction of its winds and ocean currents.

2. How is the spin of a planet measured?

The spin of a planet is measured by observing the rotation of its surface features, such as mountains and valleys, over a period of time. This can also be calculated using data from satellites and spacecraft.

3. Does the size of a planet affect its spin?

Yes, the size of a planet does affect its spin. Generally, larger planets have a faster spin than smaller planets due to their greater mass and gravitational force.

4. Why do larger planets spin faster?

Larger planets have a faster spin because their greater mass and gravitational force causes them to compress and compact, leading to a faster rotation in order to maintain equilibrium.

5. Are there any exceptions to the rule that bigger planets spin faster?

Yes, there are exceptions to this rule. For example, Venus is a similar size to Earth but has a much slower spin, possibly due to its dense atmosphere and lack of large moons to influence its rotation. Additionally, some small moons and dwarf planets may have faster spins than larger planets due to their irregular shapes and varying compositions.

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