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Planetary Formation Based on Matter Condensation and Accretion

  1. Nov 4, 2011 #1
    The understanding of the origins of our planet (as well as the other planets in the Universe) has always remained a question to astronomers. Inherently, the application of Newton’s known concepts of gravity has granted us the ability to make educated assumptions concerning planetary formation.
    We are well aware that hydrogen and helium are the most common elements in the Universe. Despite their abundant nature, their abundance is not so great on the inner terrestrial planets. This is due to the heat energy from the Sun. The reason being, invoking thermal physics, that temperature is the measure of the kinetic energy of molecules and atoms of the measured system. The higher the temperature, the greater the velocity of the molecules and atoms and this will give the needed velocity to exceed a planet’s gravitational force (however, this is dependent on the mass of the planet). This is the case with planets that are close to a star. Planets that are farther away from the star (such as our Jovian planets) will have lower temperatures and so they maintain the ability to trap in the gases.

    The issue of temperature’s role in the planet formation process is addressed by the behavior of matter found on various planets. For instance, compounds of iron and silicon are solids up to the temperature of 1000K. Elements such as hydrogen and helium are gases except in environments of extreme low temperatures and high pressures. Compounds such as CO2, CH4, NH3 and H2O will solidify at temperatures ranging from 200K to 300K (the solids formed from the compounds are known as ices). So the composition of a planet will depend on the matter present at the time of formation and distance from heat sources (such as a star).

    At the initial state, the solar nebula is roughly less than 50 K (which is less than the condensation temperature of most compounds excluding hydrogen and helium). Within the solar nebula, ice coated dust grain particles are scattered and soon the gravitational attraction begin to attract all the particles toward the center of the solar nebula. As the matter is condensed, the pressure and density of the center begins to increase. This concentration of matter is referred to as the protostar (or protosun). The gravitational attraction will increase the internal temperature of the protostar and the protostar will possess an overall angular momentum. The angular momentum is significant because it maintains enough matter for planetary formation. In a sense, the solar nebula is contracted to a flattened disk with rotation. This description is supported by astrophysicists since planetary orbits within our own solar system are in similar planes. Temperature-wise, the center of the solar nebula (the protostar) is roughly 2000 K while the outermost regions of the solar nebula remain at around 50 K or less. As mentioned before, the composition of planets themselves are dependent on their location. Near the protostar, ices will be vaporized by the high temperature and a rocky solid portion will remain. However, the ice covered dust grains are able to sustain further away from the protostar due to a decreased absorption in thermal energy.

    Dust particles within the solar nebula will often coalesce into matter conglomerates referred to as planetesimals. The formation of the planetisimals is a process that requires roughly a few million years. In the case of planetisimal formation, the dust particles are held together by gravitational and electrostatic forces. Gravitational forces allowed planetisimals collided and formed larger masses called protoplanets. This gravity-induced accumulation of matter is referred to as accretion. Inner protoplanets will accumulate material by means of accretion and this material will tend to have high condensation temperature. The impact energy and the natural decay of elements will melt the solid material and so inner planets will start their existence as spherical accumulations of molten rock. Due to a disparity in density, more dense material will sink into the center of the protoplanet. This will inevitably coerce less dense material to the surface.

    Outer protoplanets are formed in a similar manner. The only main distinction is the location of the protoplanet will affect the material. The cores of protoplanets can be rocky but due to the accumulation of dust particles, the net energy would be enough to vaporize excess ices, therefore the outer planets would be composed of mainly gases.

    During the duration of planetary formation, the protostar’s center will reach temperatures that are high enough to initiate thermonuclear reactions. Once the nuclear reaction has started, the protostar becomes a star. Once a newborn star starts its life, the force of the nuclear reactions from within the stellar core will be enough to expel the outer layers of the star out into space. This loss of mass in initial star stages is referred to as the T Tauri Wind (sidenote: the name is in reference to the star found in Taurus in which this event was first observed). The T Tauri Wind is important in the fact this will sweep the rest of the solar system area of excess materials so that the accretion of material will be stopped. Matter loss (such as the T Tauri Wind) is common in stars even after the initial T Tauri Wind. Usually, matter loss will take place in the form of solar wind which consists of high speed protons and electrons emancipated from the star’s outer layers.
     
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  3. Jan 17, 2013 #2
    I don't quite understand why the sun is so completely made of hydrogen and helium, with only roughly 0.15% other elements.

    I assume that the composition of the "cloud" that ultimately formed the solar system was fairly homogeneous. I also get, from the above article, how proximity to the protostar causes variation in temperature and energy, so leads to formation of the rocky inner planets and gaseous outer planets.

    I don't understand what happened to the rocky material within the area that became the sun. When the protostar formed, and heat and pressure increased, what process pushed out the silicon, aluminum, oxygen, etc.? Why is the sun so seemingly pure?
     
  4. Jan 18, 2013 #3

    Chronos

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    The composition of the sun is determined by the gas cloud from which it formed, which tells us the solar nebula was almost entirely composed of hydrogen and helium [~98.2% by mass]. The balance [~1.8% by mass] was all the remaining elements. The material that ultimately became the planets was similarly composed, but, unevenly distributed. This is mainly because hydrogen and helium liquify and freeze at extremely low temperatures, allowing them to drift further from the sun before becoming vulnerable to gravitational capture. See http://lasp.colorado.edu/~bagenal/1010/SESSIONS/11.Formation.html for discussion.
     
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