Dust grains are the seeds of planet formation Everything astronomers know about

Dust grains are the seeds of planet formation

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Dust grains are the ‘seeds’ of planet formation Everything astronomers know about the solar system and star formation suggests that the solar nebula was a fragment of an interstellar gas cloud. Such a cloud would have been mostly hydrogen with some helium and minor traces of the heavier elements. The original chemical composition of the solar nebula should have been roughly the same throughout the nebula. We can group the components of the solar nebula into four categories: Hydrogen/helium gases (98% of mass): do not condense. Ices (e.g. water, methane, and ammonia; 1.4% of the mass): condense at low temperatures, <-120 C. Silicates (rocky material; 0.4% of the mass): condense at temperatures ~250-1000 C Metals/metal oxides (0.2% of the mass): condense at temperatures >~1000 C The initial dust grains formed from the materials that condensed out of the solar nebula, metals, rocks, & ices. Temperature controls what planets are made of The inner nebula was hot, and only metals and rock could condense there In the cold outer nebula, ices could condense in addition to metal and rock The important factor was temperature. The inner nebula was hot, and only metals and rock could condense there. These are dense materials, corresponding to the compositions of Mercury, Venus, Earth, and Mars. The cold outer nebula could form lots of ices in addition to metals and rocks. Water vapor, methane, and ammonia were abundant in the solar nebula. So, the outer nebula was filled with a blizzard of icy particles. Those ices were low-density materials, corresponding to the compositions of Jupiter and the other outer planets.
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The frost line seems to have been between Mars and Jupiter—it separates the formation of the dense terrestrial planets from that of the low-density Jovian planets. Terrestrials Small particles of rock and metal were present inside the frost line Planetesimals of rock and metal built up as these particles collided Gravity eventually assembled these planetesimals into terrestrial planets Planetesimals were massive enough that their gravity influenced motions of other planetesimals. This increased the frequency of collisions, through which the largest bodies grew most rapidly—they had the strongest gravitational field. These planetesimals probably grew quickly to protoplanetary dimensions, sweeping up more and more material. The process of collision and accretion continued until only four large bodies remained — Mercury, Venus, Earth, and Mars, the terrestrial planets of our inner solar system. Jovians Ice could also form small particles outside the frost line. Larger planetesimals and planets were able to form. Gravity of these larger planets was able to draw in surrounding hydrogen and helium gases. According to the solar nebula theory, the Jovian planets began growing by the same processes that built the terrestrial planets. The outer solar nebula not only contained solid bits of metals and silicates—it also included abundant ices.
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