Astro-QA #8 : Why do the outer, gas-giant planets rotate faster than the inner, terrestrial planets?
- March 09, 2018
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To understand this, we need to think about how the planets formed. When the sun was formed and fusion started the lighter gas were swift far out in the solar system and heavier rockier material and metalics with high melting point stay closer to the Sun.
Molten metal and dust particles stuck together to form rocks. Then gravity brings then together to form planets. So, in the hot inner region of the solar system the rocky planets with metal cores are formed. Earth and Mars formed in this region. Their cores were only made from rocky material and they never grew very large. With relatively low masses, they were only able to pull in material from areas that were relatively close.
The situation is different though in the outer region. Their proto-cores were accreted from both rock and ice, and so they could grow quite a bit larger. Once those cores reached a critical mass (somewhere around 10 Earth-masses) they had enough gravitational pull to start accreting hydrogen gas, as well. That starts a positive feedback - accrete more hydrogen, mass gets bigger, gravitational pull is stronger, accrete more hydrogen, etc.
Now, a little about conservation of momentum: if you have a spinning mass and start pulling it inwards, it spins up. The more you pull it inwards, the faster it goes. The classic example here is an ice skater pulling in his arms and speeding up.
So, imagine the material in the proto-planetary disc all has a uniform angular momentum. You have two planets, one that only grabbed material close by, while the other pulled in material from much farther away. Since the second planet's material had much farther to travel towards the center of rotation, you'd expect it to spin faster once formation finished.
It's for this reason that Jupiter and Saturn spin the fastest at around 10 hours per rotation while the inner planets rotate slow.