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Could a planet get heavier without needing to grow in size?

  1. Apr 25, 2014 #1
    Could we (at least) in theory change a planet's weight with denser core material
    so that it could be as large as it is, but have increased the gravity to the level like earth?
  2. jcsd
  3. Apr 25, 2014 #2
    the term you want is the objects mass, and yes you could replace less dense material with a higher density material, to increase the mass without increasing the volume. Though probably never practical lol

    see the wiki link on mass vs weight

    Last edited: Apr 25, 2014
  4. Apr 28, 2014 #3
    There is a secondary issue. I'm guessing that you are looking for a smaller planet but with the same surface gravity. Reducing the diameter and keeping the mass the same would actually increase surface gravity. So increasing the density is more effective than you might expect.
  5. Apr 30, 2014 #4
    "Size" should not be used to mean "diameter" or "mass" or "length" or "area" or "volume" or any other such far less ambiguous term. IOW, using the word "size" makes your meaning more difficult to understand.
    As a matter of fact, once planets (specifically Gas Giants) get to be the diameter of Jupiter, their diameters increase very little as their mass increases - accumulation of more hydrogen increases their density and has a very small effect on their diameter. That is why if you compare the diameters of Saturn and Jupiter they differ very little while the mass of the two is significantly different.
    One of the neat examples of integral calculus is using it to prove that the force of gravity depends on only two things: the distance you are from the center of mass and the amount of mass closer to that center (picture a sphere with you on its surface). This means that if the density of the Earth were constant (it is not), that you would substantially decrease your weight as you dug a hold towards the center of the Earth. In reality, the gravity of Earth increases at first (see Wikipedia) before it drops to zero at the center.
    "Theoretically" is another one of those terms which need be used with caution and awareness. The density of the hydrogen in the center of the Sun is (estimated) to be about 110 g/cc and yet we all know that we can use hydrogen gas to lift zepplins because it is so light. Iron's density is about 12 g/cc, so "theoretically" replacing the hydrogen in the Sun's core with iron would decrease the density (and mass) of the Sun! So, the word 'theoretically' can and often does mislead unless you clearly understand what assumptions are being made about the system being discussed and the theory (theories) being used. If we could magically (and isothermally) replace all of the iron, nickel and silicon in the Earth's mantle and core with iridium (one of the densest of the elements) keeping the volume the same, then there would be a large increase in the gravity at the surface of the Earth. Such a magical change would have serious (probably fatal) consequences for us including to the length of the day, our orbit around the Sun, our magnetic field (which protects us from radiation from the Sun and Cosmic Rays), as well as the chemistry of the Earth and the toxicity of lava. Since iron is one sink for oxygen, and silicon is another, such a change could have dramatic long term effects on our atmosphere as well. The Sun is 99.9% of the mass of the Solar System, finding enough denser material to replace the less dense stuff composing some fictional planet would be "theoretically" impossible given our current technology (or any foreseeable future technology).
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