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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?

Blade Wraith
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DRY1994
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    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. – Mołot Sep 24 '18 at 21:11
  • When you say earthlike, can it be made out of something different? – PotatoLatte Sep 24 '18 at 21:47
  • 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). – Rob Watts Sep 24 '18 at 22:10
  • Molot - Yeah, the density would only be 35% that of Earth. Thanks for the suggestion, I'll have to take a look at your question. Robb Watts - Yeah, I'll have to rethink the size of the planet. PotatoLatte - The planet can be made of something different, but the qualifications for the planet I'm looking for are that it can support plant, animal and human life brought over from Earth and that it has oceans and very fertile soil (at least somewhere it does). But again, I'll have to rethink the size of my planet. – DRY1994 Sep 24 '18 at 23:57
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    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. – pojo-guy Sep 25 '18 at 03:25
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    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. – JBH Sep 25 '18 at 03:48
  • As Pojo-guy already pointed out, Gravity is directly proportional to Mass, the gravity might be the same if the size was 2.83 the radius so long as the density was a lot lower but as you've said but the mass would actually be the same – Blade Wraith Sep 25 '18 at 09:10
  • @BladeWraith surface gravity would be 0.9988 of Earth with the numbers OP provided. It would be increased by the increased mass, but decreased by the bigger distance from the center of mass. – Mołot Sep 25 '18 at 13:33
  • @pojo-guy Saturns surface gravity is 10.44 m/s2 and effective surface gravity on equator 8.96 m/s2 - Quite similar to Earth surface gravity of 9.81 m/s2. – Mołot Sep 25 '18 at 13:35
  • It is hard ot have oceans on a planet where the ground floats, On your planet the ground is less dense than water. – John Sep 26 '18 at 15:29
  • Ummm… the radius is 2.83 times that of Earth. The volume is 8 times that of Earth, so the radius must be 8^(1/3) which is 2 times that of Earth. Which one do you want?! – majestas32 Sep 26 '18 at 18:59

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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.

Skek Tek
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