Reading: Chapter 23 in Beatty and Chaikin or Chapter 2 pp24-30 in Christiansen and Hamblin
As proposed originally by Kant, Laplace, and others in the 1700's, the solar system probably formed from a nebula that evolved into a disk. The initial nebula presumably looked like the molecular cloud cores in M 16, where we find stars forming today.
The presence of decay products of short lived radioisotopes such as 129Xe (coming from 129I) and 26Mg (from 26Al) in some meteoric mineral grains suggests that a supernova went off near the solar nebula. Perhaps a shock wave from the explosion initiated the collapse.
Another possibility is that the solar nebula passed through a spiral arm. These arms are known to be sights of star formation, including the massive stars responsible for nucleosynthesis. Passage through the shock associated with the spiral arm might trigger the collapse of the nebula.
The nebula contracts, but the tendency to conserve angular momentum causes the gas to spin faster and to flatten, forming a disk. Ill understood processes in the disk move most of the matter toward the center, and most of the angular momentum (and a small fraction of the matter) outward, allowing a star to form in the center. In many cases the rotation is so strong that the gas cloud breaks into a few large chunks, eventually forming a binary or multiple star system.
In the case of the solar system, the cloud formed a single disk. Click here to see HST pictures of two edge-on proto-planetary disks. In the late stages of accretion, material in the disk condenses into dust. Beta Pictoris is an example of a star with a dusty disk around it. The dust agglomerates into many small pieces known as planetesimals (asteroids and comets), most of which grow by collisions into planets. This story naturally explains why the planets are coplanar, and why their orbits are nearly circular. It also explains why most of the matter is in the Sun, and why most of the angular momentum is in the planets.
The Jovian planets formed in the cool outer parts of the disk. The lower temperature meant that ices could condense to form small bodies, which then were available for collisional accretion onto planetesimals. Large planetesimals formed early enough to allow for the gravitational capture of gas (H and He) in the disk. In contrast, the terrestrial planets formed in hotter material, so that no ices could be accreted. The planetesimals in this region did not become large enough rapidly enough to capture gas in the disk, and so remained smaller than the Jovian planets. One prediction of this picture is that the density of the planets will decrease with increasing distance from the sun.
A similar story can be told regarding the formation of the satellite and ring systems of the giant planets. As for the solar system, this picture predicts that the inner satellites will be more dense than the outer satellites, because they formed in a hotter environment, and therefore consist of more refractory, and hence denser, material.
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