Why do the outer, gas-giant planets generally rotate much faster than the inner, terrestrial planets?

The reasons why some planets rotate as quickly as they do remain puzzling to planetary scientists. Most studies in this area have focused on the inner planets. Earth and Mars, which accumulated gradually from rocky planetesimals, most likely got spun up when they experienced glancing impacts from particularly large objects as they neared the sizes they have today.

Saturn on July 18, 2015
Saturn as imaged by Christopher Go. South is up.

But the four largest planets probably came together an entirely different way. According to planet-formation specialist Alan Boss (Carnegie Institution of Washington), these biggies must have accumulated most of their mass from gas in the surrounding solar nebula. That gas formed individual spinning disks (from which many satellites formed), and most likely it carried a lot of angular momentum as it fell onto the outer planets' cores, causing them to spin faster and faster as they coalesced.

— J. Kelly Beatty


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Randy Evangelista

December 6, 2018 at 11:54 pm

Newton’s Law of universal gravitation and Kepler’s Law of planetary motion describes the movement of planets around the sun. Equations are derived from these laws and hence the birth of Celestial Mechanics. But nowhere in the literature can we find an accepted law of Planetary Rotation because everyone is convinced that there is nothing special about the rotation of the planets. Except for the “overused” explanation…

“A long time ago in a galaxy far, far away...spinning gas and dust flattened into a protoplanetary disk and due conservation of angular momentum the planets are now rotating with RANDOM velocities”

It’s the same as saying we don’t really know how it works. We have a concept but not enough to express it in numbers.

This is a quote from Lord Kelvin (William Thomson):“I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.”

Follow the below link for the Planetary Spin Equations.


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