Why do bigger planets have shorter days?

[dstebbins]

There are a few exceptions. For example, Venus has the longest day, although Mercury is the smallest planet (ever since Pluto got demoted). However, for the most part, large planets have short days.

 

Planets that are farther from the sun have longer years. That makes sense... they have a much greater circumference to orbit around.

 

But... you'd expect bigger planets to have longer days, for the exact same reason... more distance the top clouds have to go around.

 

What exactly causes bigger planets to actually have shorter days?

[pantheory]

This seems like a reasonable question for speculation because I know of no consensus answer. Stars and planets with a large torus type vortexes which started their formation, can extend their gravitational influence farther outward away from the star/ planet itself because of their larger mass. By doing so it is able to capture more material in stellar or planetary vortexes which are generally traveling on the same plane. Each body it captures, its orbital motions is transferred into stellar or planetary rotation as the star or planet absorbs the matter which is orbiting it adding to the increased rotation of the central absorbing body. Mercury is close to the sun so tidal locking influences have likely slowed down its original rotation period which still might have been slower than the Earth because of its smaller size. Venus is farther out from the sun and tidal forces of gravity alone seemingly could not have caused its retrograde orbital motion. It is likely that a proto-planet collision of some kind could explain its slow retrograde rotation rate, something like a Mars sized body that is presently thought to have created our moon.

 

The orbital axis of Uranus also seems to indicate that it iteracted with a proto-planetary body in its early history because of its greatly inclined axis of rotation relative to the plane of the solar system.

[baric]

What exactly causes bigger planets to actually have shorter days?

 

I think you need to establish that this is true, first. For starters, you simply cannot compare the observed rotation rate of the gas giants to terrestrial planets, like this:

 

Jupiter - 10h

Saturn - 11h

Neptune - 16h

Uranus - 17h

Earth - 24h

Venus - 243d

Mars - 25h

 

Why is this invalid? Because you are measuring the atmospheric rotation rate of gas giants against the surface rotation rate of terrestrials. These are completely different measurements, as evidenced the difference between the surface and atmospheric rotation rates of Venus. Since gas giants have no surface, you have to use atmospheric rates only if you want to add terrestrials to the comparison:

 

Jupiter - 10h

Saturn - 11h

Neptune - 16h

Uranus - 17h

Venus - 96h

 

Think about what might drive the rotation rate of an atmosphere... Larger gas giants generate more internal energy (from gravitational contraction) than smaller gas giants. Therefore, it is plausible that their atmospheres will be more turbulent as a result.

 

But there is solar energy as well. In our system, the larger giants gain more solar energy than the smaller because they happen to be closer to the Sun. We have no "hot Jupiters" in our system, but there are known extra-solar examples with wildly turbulent atmospheres. This would be especially true for tidally-locked giants that have large temperature differentials between their hot and cold sides.

 

Finally, gas giants have no true surface to slow down atmospheric winds. That will also make their atmospheres rotate much faster than on terrestrial planets.

[khaled]

I think that it depends on both the planet's distance from its star (the gravitation point that keeps it in orbit), and its size ...