The Formation of Planets

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Frost Line

Different gases have different condensation temperatures. Hydrogen compounds condense at temperatures less than 150K while rocks' condensation temperatures are between 500-1300K and metals are at 1000-1600K11. The frost line is the distance in the solar nebula from the protostar to where ices like hydrogen compounds can condense, while only metals and rocks can condense inside this line. The center of the protostar is too hot for any material to condense. But the further away you go from the center of the nebula the cooler it is because the flux, which is inversely related to the area, decreases. As the radius from the protostar increases, the cooler the temperature and the more materials can condense10. The frost line marks a transition from the warm inner regions where terrestrial planets form to the cool outer regions where jovian planets form. In our solar system, metals and rocks could condense starting from the radius where Mercury is currently located. Ices could condense at 2.7 AU, or where the asteroid belt is located.8,12

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Frost Line

Formation of Terrestrial Planets

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Terrestrial planets are formed though a process called accretion. Accretion is when microscopic solid particles that condensed grow into planets. These microscopic particles are able to stick together through electrostatic forces and not through gravitational attraction because these particles are too small. But as they grow in mass, their gravitational forces will increase and accelerate their growth. Soon they will be large enough to be considered as planetesimals. These planetesimals will keep growing in size until they reach a limiting size where gravity cannot keep them together. Soon gravitational encounters between planetesimals will result either in a change of their orbits or collisions. The largest ones will survive and grow into terrestrial planets and clear their orbits of small planetesimals8.

Formation of Jovian Planets

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Accretion also occurs in the formation of jovian planets. The difference in the formation of jovian planets is that they are created beyond the frost line, meaning that ices are available to create them. Hydrogen compounds are 1.4% of the material present in a solar nebula while rock and metal are only 0.6% of the material. As a result, planetesimals that accreted in the outer solar system contain large amount of ices and are larger compared to the terrestrial planetesimals due to all the material available. Soon, these planetesimals will grow large enough that their gravity is strong enough to capture hydrogen and helium gas, which make up 98% of the solar nebula. The gas that is being captured by the planetesimals will become a spinning disk around the planetesimal that looks like a mini-solar system8.


Young Jovian Planet surrounded by disk of gas11

End of Planet Formation

When the young star starts fusing hydrogen to helium in it core, it will have a strong solar wind. A solar wind is a flow of neutral or charged gas that blows away from a star. These solar winds will blow all the hydrogen and helium gas in the solar nebula into interstellar space and seal the fate of the planets8.


Aurora caused by a solar wind13

Summary11

Last Updated: November 10,2011