Why do all the planets orbit in the same direction around the sun?
Because they all coalesced from the same disc of gas and dust, which was originally all rotating the same direction.
The conservation of angular momentum implies that when any cloud of gas or dust is collapsing, any initial rotation, however small, will be amplified by the collapse. The same effect causes a figure skater to spin faster when she pulls her arms in toward her body, and slower when she spreads them out.
Like chroot said.
When gas and dust coalesce together, the chance of them coming together precisely at the right motions to have no rotational motion is virtually nil. The dust cloud spins, flattens, and eventually clumps together in little balls. Most of the matter winds up in one big ball in the center, but all of the balls orbit and spin in the same direction the original cloud did.
Which means a tougher question would be:
What's wrong with Venus? It orbits the right direction, but it's spinning the wrong way.
What's wrong with Uranus? Why is it lying on its side?
Why does the Sun spin so slow? If almost all of the mass winds up in the middle (about 98% of the solar system's mass lies in the Sun) and the ball spins faster as the mass gets closer to the center, the Sun should be spinning really fast (it takes 27 days for the Sun to rotate).
You answer those questions and you'll be famous.
Does the dust cloud collapse under it's own gravitational weight? I'm pretty sure it does for the Sun, but I'm not sure about the planets. I'm guessing that it has to. But I've heard that the planets form when pieces of dust collide with other pieces of dust to form larger pieces of dust and then pebbles, and then rocks, then bolders, then planeticimals, and then the largest planeticimal sucks up all the smaller ones.
But what I do not understand is how dust colliding into dust will form larger pieces of dust. A piece of dust has virtually zero gravity, and thus does not have the ability to gravitationally attract other dust. (Dusting my house would be a lot easier if the dust gravitationally clumped together :tongue2: ). And even if they did collide by the chance that their orbits just happened to make them cross paths, I imagine they'd bounce off each other or bust up into smaller dust, as their approach velocities can be in the km/s range. If things that bashed together stuck together instead of fragmenting apart, then the beach would be full of boulders instead of sand. But if a large portion of the dust cloud collapsed on its combined gravity, things would be different. I just never hear it explained this way.
IIRC, there are two stages in the formation of planets - the initial clumping, then the gravitational growth (which includes lots of collisions). The first stage does rely upon 'dust' sticking together when it collides - how can this be? First, 'dust' is a little misleading, most of it is 'ices', water ice, ammonia ice, methane ice, dry ice, ... Second, a high proportion of the collisions are at very low relative speeds. Third, once an object does grow to only a quite modest size, it can attract more dust through its gravitational attraction (and while collisions will grow more energetic, they will - net - result in the object getting bigger and more massive).
Things are a little more complicated inside the 'snow line' - inside which the proto-Sun's heat will evaporate the ices ...
I guess we're talking about very small particles travelling at very low velocities. Maybe like a snowflake forming in the atmosphere. But after they get bigger, like house-sized objects, they still have no significant gravity and I imagine they just bust each other up in their collisions. Unless their collisions were still at extremely low velocities. Maybe that's the case. Do these things have electrical charges that might help? And is it possible that a planet could form by an entire portion of the original clould collapsing under its own weight? Especially in the case of Jupiter?
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