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Why is the earth so big?

  1. Jul 19, 2011 #1
    It is bigger than all it's neighbours, which is odd.
    Obviously it is not as big as the gas giants but they are not real solid planets.

    [PLAIN]http://blrdb.com/images/BlrDb_Astronomy_Planet_Sizes.gif [Broken]
    Last edited by a moderator: May 5, 2017
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  3. Jul 19, 2011 #2


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    I don't accept your premise: it is only 5% bigger than Venus, so that's not odd.
  4. Jul 19, 2011 #3
    Well, Earth was a bit smaller until something the size of Mars made a glancing collision, donating its iron core, and their crustal debris congealed as our Moon...
  5. Jul 19, 2011 #4
    That is the diameter, the volume is 15% bigger, but nonetheless it is curious we are the biggest small planet.

    What determined our size? And indeed the sizes of the other planets?

    Somebody must know.
  6. Jul 20, 2011 #5


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    Chance determined the size. Things collide. Things collect together. Things get destroyed.

  7. Jul 20, 2011 #6
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  8. Jul 20, 2011 #7


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  9. Jul 21, 2011 #8


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  10. Jul 21, 2011 #9

    Filip Larsen

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    One may perhaps even speculate that there may be a touch of Anthropic Principle [1] at work here. For instance, it may conceivably be that it is more rare for life to evolve on a smaller planet that a large planet because smaller planets are less able to hold on to the heavier elements in an atmosphere. Or one could speculate that during solar system formation planets that form in the habitable zone [2] have a tendency to be "larger" rather than "smaller" through some mechanism.

    [1] http://en.wikipedia.org/wiki/Anthropic_principle
    [2] http://en.wikipedia.org/wiki/Habitable_zone
  11. Feb 1, 2012 #10
    As an above user said, a large Mars-sized body crashed into the Earth during its early development.

    Also, being made of iron, naturally, gravity attracts the lighter elements.
  12. Feb 5, 2012 #11


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    Gravity affects all elements almost equally. Until you get to a large fraction of the Earth's mass, the acceleration due to gravity on the surface is still about 9.8 m/s^2.
  13. Feb 5, 2012 #12
    What Filip Larsen mentioned may be interpreted as a form of natural selection. The Earth is the size it is because if it was much smaller, it could not allow us to come into existence.

    Earth organisms need liquid water to metabolize and grow in, and water will not be liquid below its triple-point pressure. So there must be something creating that pressure, and on the Earth, it's the atmosphere. If the Earth was much smaller and less massive, its escape velocity would be much less, and its atmosphere would escape into outer space.
  14. Feb 5, 2012 #13
    ^ which is what happened with the hydrogen and helium in our atmosphere during the early Earth
  15. Feb 6, 2012 #14
    I suspect that if earth was the smallest planet in the solar system, you would be saying:

    "It is smaller than all it's neighbours, which is odd.
    Obviously it is not as small as the moons but they are not real planets."

    There is nothing odd about the Earth's size. Or at least, we cannot say anything at this time with regards to it's oddness.
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