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Two rocks placed in space with no relative motion are going to be attracted by gravity, and hit. 3 rocks, placed in space with no carefully rigged symmetry, will likely miss each other, as the gravitational attraction of the additional rock changes their course. Those near misses are the beginning of rotation. Multiply that effect by trillions, and you have ...


16

The prior gas cloud will start with some small random motions and density variations left over from its formation. As a patch starts to contract any small amount of rotation gets amplified as the cloud collapses, because of the conservation of angular momentum.


13

This questions can be split in two; for planets and satellites. The diversity of planets reflects in part the diversity in terms of chemical composition of the protoplanetary disk. We know that UV radiation from the sun can dissociate complex molecules or even very simple ones; for example, when UV rays split water molecules the result is free hydrogen and ...


13

The answer is turbulence. Stars are formed from large gas clouds that collapse under their own weight. As they do so, they become unstable to fragmentation and break up into smaller collapsing pieces. Turbulence in the original cloud means that each of these pieces has its own individual angular momentum and rotational energy, even if the total angular ...


8

Other answers good, but this question left behind: Why isn't all the disk material been swallowed by the star? Because of the excess of angular momentum carried by some particles. They insist on orbiting the star instead of falling directly to it. There is a complex process called accretion. The disk constantly transfers angular momentum to the outside, ...


7

Prelude It is now generally accepted in the planet formation community that planets form as a side-product of the star formation process in so-called protoplanetary discs. Protoplanetary discs have initial masses of few to tens of percent of their stellar host masses, are relatively cold (T<150K in about 95% or more of their mass, which is outside the ...


6

One calls Mercury a planet because it doesn't share its orbit with any other bodies of comparative size. The fact that it is in its orbit means that that orbit hasn't been cleared by another body (for if it had been cleared by another planet, then Mercury would have been cleared too.) In practice one doesn't look at "How" an orbit is cleared, only ...


4

The interactions of the Earth and its moon have slowed the Earth's rotation significantly over the billions of years since their formation. Wikipedia cites a paper that gives a figure of approximately five hours for rotational period in the past. We don't know of a similar method to slow down Mars. This implies that the Earth must have had a much faster ...


3

Such surveys are designed to detect an excess of emission over that expected from a stellar photosphere. The emission from a circumstellar disk is often modelled as a blackbody at lower temperature than the stellar photosphere. A hotter blackbody with the same emitting area emits more flux than a cooler blackbody at all wavelengths. Therefore, if you had a ...


3

Whether or not gas giants form is largely to do with whether rocky/icy cores are able to accrete large quantities of hydrogen and helium, which dominate the mass of the protostellar disc. Terrestrial planets form from the tiny fraction of the disc that is able to form solids. There is no indication that systems without gas giants have more "solids" and ...


3

Answers, in order: Why did the protoplanetary disk start rotating around the star? Star and disk form around the same time. The rotation forms because all other orbits will cross the orbit of the disk and collide. Put another way, the rotation disk is the leftover movement of particles after all symmetrical factors have cancelled each other out in collisions ...


1

The answer seems to be a clear "it depends": Homogenity of the disks depends on the age of the protoplanetary disk. Temperature is a function of the radius and the color-scale used in (old) artists' impressions is indeed correct. On Wikipedia on the topic there is a gallery of different observations, also in the visual spectrum. An impressive ...


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