MadSci Network: Astronomy |
A glance at a table (such as below) listing planetary densities reveals two major divisions. The inner or Terrestrial planets have a higher average density than the outer or Jovian planets. ------------------------------------------------------------------ Terrestrial Planets Specific Gravity or Radius in Density in (grams/cm^3) millions of km Mercury 5.43 2.43 Venus 5.25 6.06 Earth 5.52 6.37 Mars 3.95 3.37 Jovian Planets Jupiter 1.33 69.9 Saturn 0.69 58.5 Uranus 1.29 23.3 Neptune 1.64 22.1 Icy Planets Pluto 2.03 1.5 Densities from http://python.swan.ac.uk/~pydunbar/PH104/PH104_planets.html Radii from Serway and Beichner "Physics for Scientists and Engineers", 5th ed.ISBN:0-03-031561-1 ----------------------------------------------------------------------- It is true that the planets and Sun formed from the same cloud of gas, dust, and ice called the Solar Nebula. The Sun did not begin to shine at the time of planet formation -- that happened from 10 to 100 million years later. The main reason for the difference in planetary density is the change in temperature with distance from the center of the Solar Nebula. The inner regions were warmer than the outer regions, but not because the Sun was shining, rather because the particles in the cloud were moving faster and colliding more often near the center of the Nebula. Near the center of the dust cloud, the temperature was high enough to keep some chemicals in the cloud such as methane, ammonia, and water from becoming solid. Consequently, these chemicals were not available to build planets. Only denser chemicals such as iron, nickel, and silicates (sand) were solid and could form planets. Just inside the orbit of Jupiter, the temperature was low enough for water ice to form. Since water was abundant in the Solar Nebula, water ice provided a lot of planet-building material. Jupiter and the other outer planets therefore grew very large. Notice from the table that the planetary radii increase dramatically with distance from the Sun after Jupiter. The enormous gravitational pull of the large Jovian planets then held onto the light hydrogen and helium gas copiously present in the Solar Nebula. When the Sun did begin to burn, the strong Solar Wind swept most of the left over gas and dust from the Solar System. The smaller Terrestrial planets did not have strong gravitational fields that could hold onto large amounts of hydrogen and helium. The vast quantities of light gases trapped by the Jovian planets accounts for their relatively low densities. The planetary density is an AVERAGE over the heavy rocky-metallic-icy core and the light gaseous atmosphere. The Jovian planets have such large atmospheres composed of light gases that their overall density is low. The rocky-metallic Terrestrial planets have such thin envelopes of atmosphere that their overall density is high. Reference http://zebu.uoregon.edu/~imamura/121/lecture-5/lecture-5.html ---------------------------------------------------------------------- I don't think that anyone can answer the question concerning the minimum density necessary to support life because there could be forms of life in the Universe which are not only undiscovered but unimagined. Who knows if life (not as we know it) can exist in densities as low as one molecule per cubic meter of space? Any estimate of the number of "life"-supporting planets throughout the Universe will necessarily vary greatly with the assumptions one makes about extraterrestrial life. To demonstrate our lack of knowledge about life off the Earth, remember that we don't even know if the other planets of our own Solar System support life or not. One can, however, make the following definite statement: The number of planets capable of supporting life (whatever definition one is using for "life") increases as the total number of planets increases. This is particularly interesting in light of the relatively recent (last five years) discoveries of many extra-solar planets. There are many web sites devoted to this topic, for example http://cfa-www.harvard.edu/planets/
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