Assuming that there is an Earth-like planet that has 8 times the volume, 2.83 times the radius, and has the same surface gravity as Earth, and has oceans; what are potential problems concerning the density of this planet if it isn't hollow that risk it being less Earth like either composition-wise or just in how hard or soft the surface is?

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    $\begingroup$ Average density would be like 0.35 of density of Earth, right? You may be interested in biggest reasonable planet with Earth surface gravity I asked for earlier. $\endgroup$
    – Mołot
    Sep 24, 2018 at 21:11
  • $\begingroup$ When you say earthlike, can it be made out of something different? $\endgroup$ Sep 24, 2018 at 21:47
  • $\begingroup$ Based on the answers to Molot's question, your planet will need to be almost entirely ammonia and/or water. You might have to rethink your planet, or make it out of handwavium (in other words, not worry about making it scientifically accurate). $\endgroup$
    – Rob Watts
    Sep 24, 2018 at 22:10
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    $\begingroup$ Since gravity is a function of mass, no. Saturn is much less dense than water, yet it's gravity is much more than Earth's because it has that much more mass. $\endgroup$
    – pojo-guy
    Sep 25, 2018 at 3:25
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    $\begingroup$ My answer was wonky-off, but there are still issues. Go read my answer to Geology on low-density planets. You can't significantly compress a liquid metal core (needed for magnetospheres) and the larger surface area means other problems. $\endgroup$
    – JBH
    Sep 25, 2018 at 3:48

1 Answer 1


To shed mass you would have to lose your iron/molten core and the protective magnetic field it creates to shield your atmosphere from solar winds. And without a molten core you wouldn't have enough volcanic activity to replenish your atmosphere.

As a solution you could have a core of metallic hydrogen (~%7 the density of iron). It would allow you to maintain your magnetosphere while achieving your desired volume and gravity without significantly changing the surface characteristics of the mantle.


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