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Thread: Earth with 1 Gravity, different radii and densities, exoplanet spin-off thread

  1. #1
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    Smile Earth with 1 Gravity, different radii and densities, exoplanet spin-off thread

    Quote Originally Posted by JCoyote View Post
    I'm more curious about how big a solid planet we could get with roughly or slightly less than 1g surface gravity. A more porous, less dense planet could have massive surface area but there would be a lot of wildly different behaviors geologically.
    I had worked this out a while ago, but I am happy to be corrected by anyone. In the attached chart, for equivalent densities, O Core means Outer Core (of Earth), L Mantle is Lower Mantle, U Mantle is Upper Mantle. Hope everyone can read the chart. All numerical terms are with Earth = 1.00 in every category.

    For example, if Earth or an exoplanet had a density overall of Granite, it would have 2.2 times our Radius with a significant increase in surface area (forgot to add it) and volume.

    Earth with an overall density of molten iron could be about the size of Venus and have 1.0 G.
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    Last edited by Roger E. Moore; 2020-Apr-07 at 09:49 PM.
    Do good work. —Virgil Ivan "Gus" Grissom

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    As for what each type of planet would be like, that's left to everyone but me.

    You might see "Big Planet" in the SF library for what a giant, metal-poor Earth might be like as a colony world.

    https://en.wikipedia.org/wiki/Big_Planet

    .
    Last edited by Roger E. Moore; 2020-Apr-07 at 10:31 PM.
    Do good work. —Virgil Ivan "Gus" Grissom

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    The biggest terrestrial planets with an Earth-like gravity would be waterworlds, with oceans hundreds of kilometers deep. At the bottom of such an ocean the pressure could be great enough to form a high-pressure ice mantle. These planets could be much larger than Earth, but still have conditions on the surface that are fairly Earth-like - apart from the absence of land.

    Going the other way; a planet with a very high iron content, and a minimal rocky crust, would be much denser than our own planet, because the core would become compressed under its own weight - so iron planets would be considerably smaller than you might think. There might be quite a few nearly pure iron planets out there- but they would be the result of planetary formation very near the star, or migration towards the star, and the resultant loss of material by evaporation.

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    Quote Originally Posted by eburacum45 View Post
    There might be quite a few nearly pure iron planets out there- but they would be the result of planetary formation very near the star, or migration towards the star, and the resultant loss of material by evaporation.
    A number of papers in arXiv.org talk about mantle loss from giant impacts, leading to Mercury-like planets with large iron cores.
    Do good work. —Virgil Ivan "Gus" Grissom

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    Yes, that's one I've used before.
    This paper suggests that planets with more than 70% iron are unlikely, but I don't really subscribe to that view.
    https://arxiv.org/pdf/0704.3454.pdf
    Note as well that this paper has some useful equations for deterimining the density of various planetary types, up to 50% water.

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    Somewhat easier to use is this graph, which gives a wide range of planetary options, while allowing for compression effects. One interesting phenomenon is that planets start getting smaller again past 300 Earths or so- about the mass of Jupiter.
    http://aleph.se/andart2/wp-content/u...anetradius.png

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