There was a recent comment to one of my posts regarding the formation of rocky planets, so I thought I should outline how I think the rocky planets formed, and why. The standard theory involves only physical forces, and is that dust accreted to planetesimals, then these collided, eventually to form embryos (Mars-sized bodies), then these collided to form planets. First, why do I think that is wrong? For me, it is difficult to see how the planetesimals form by simple collision of dust, and it is even harder to see how they stay together. One route might be through melting due to radioactivity, but if that is the case, one would need very recently formed supernova debris to get sufficient radioactivity. Then, as the objects get bigger, collisions will have greater relative velocities, which means much greater kinetic energy in impacts, and because everything is further apart, collisions become less probable and everything takes too long. The models of Moon formation generally lead to the conclusion that such massive impacts lead to a massive loss of material.
 
The difference between the standard theory and mine is that I think chemistry is involved. There are two stages involved for rocky planets. The first is during the accretion of the star, and near the star, temperatures are raised significantly. Once temperatures reach greater than 1200 ^oC, some silicates become semi-molten and sticky, and this leads to the accretion of lumps. By 1538 ^oC, iron melts, and hence lumps of iron-bodies form, while around 1500 – 1600 ^oC. calcium aluminosilicates form separate molten phases, although about 1300 ^oC a calcium silicate forms a separate phase. (The separation of phases is enhanced by polymerization.) Material at 1 A. U., say, reaches about 1550 - 1600 ^oC, while near Mars it reaches something like 1300 ^oC. Of particular relevance are the calcium aluminosilicates, as these form a range of materials that act as hydraulic cements. Also, the closer the material gets to the star, the hotter and more concentrated it gets, so bigger lumps of material form. One possibility is that Mercury is in fact essentially formed from one such accreted lump that scavenged up local lumps. Another important feature is that within this temperature range, significant other chemistry occurred, e.g. the formation of carbides, carbon, nitrides, cyanides, cyanamides, silicides, phosphides, etc.
 
When the disk cooled down, collisions between bodies formed dust, while some bodies would come together. Dust would form preferentially from the more brittle solids, which would tend to be the aluminosilicates, and when such dust accreted onto other bodies, water from the now cool disk would set the cement and make a solid body that would grow by simply accreting more dust and small bodies. Because there is a gradual movement of dust and gas towards the star, there would be a steady supply of such feed, and the bodies would grow at a rate proportional to their cross-section. Eventually, the bodies would be big enough to gravitationally attract other larger bodies, however the important point is that provided initiation is difficult, runaway growth of one body in a zone would predominate. Earth grows to be the biggest because it is in the zone most suitable for forming and setting cement, and because the iron bodies are eminently suitable for forming dust. The atmosphere and biochemical precursors form because the water of accretion reacts within the planet to form a range of chemicals from the nitrides, phosphides, carbides, etc. What is relevant here is high-pressure organic chemistry, which again is somewhat under-studied.
 
Am I right? The more detailed account, including a major literature review, took just under a quarter of a million words in the ebook, and the last chapter contains over 80 predictions, most of which are very difficult to do. Nevertheless, an encouraging sign is that the debris of a minor rocky planet around a white dwarf (what remains of an F or A type star) shows the presence of considerable amounts of water. Such water is (in my opinion) best explained by the water being involved in the initial accretion of the body, because it is extremely unlikely that such an amount of water could arrive on a minor rocky planet by collision of chondrites because the gravity of the minor planet is unlikely to be enough to hold such water. Thus this is strongly supportive of my mechanism, and it is rather difficult to see how this arose through the standard theory.