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I'm worldbuilding for a sci-fi setting with some help from old Artifexian videos, and have made a planet that's suitable for human habitation. However, as this planet is larger than Earth, to make the gravity the same (I'm doing so to have dinosaur-esque megafauna) the density drastically lowers.

Here's the necessary numbers:

  • Mass: 2.3 Earth masses (1.37x10^25 kg)
  • Radius: 1.5 Earth radii (9600 km)
  • Gravity: 1.02 Earth gravity (10.03 m/s^2)
  • Density: 0.68 Earth density (3750.00 kg/m^3)

My problem is not the low denisty in and of itself. I don't want to change any of my values, as I'm happy with how them being how they are.

My problem is that I just don't know what a density that low compared to Earth would do to the planet. If it is a problem, what would be a scientifically plausible way to explain it? The best I can come up with is having massive air pockets in the crust that are the size of cities- like the interior of a bird's bones- but the more I consider that, the more I can't help but think it's stupid.

Thanks for any answers in advance.

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    $\begingroup$ ... Ah, and get rid of the nickel in the core. It is even heavier than iron and doesn't buy you anything. $\endgroup$
    – AlexP
    Aug 31, 2022 at 0:24
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    $\begingroup$ Would getting rid of the metals kind of screw up the planet's electromagnetic field? $\endgroup$ Aug 31, 2022 at 0:32
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    $\begingroup$ Getting rid of all the metal would be a problem that way, but you could probably reduce the metal content enough to significantly reduce density without automatically making the planet uninhabitable. $\endgroup$ Aug 31, 2022 at 0:49
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    $\begingroup$ Gravity is going to be a major issue, beyond a certain depth its going to dictate density. Less than 20mi down the earth is something like 400C. It just gets hotter after that, and more and more materials will melt and become more dense. Gravity is going to make the center of your planet dense, so the rest of your planet has to be even less dense. As in, mostly atmosphere and clouds. $\endgroup$
    – cybernard
    Aug 31, 2022 at 14:55
  • $\begingroup$ @cybernard: Not really. Rocks and metals are not very compressible, and molten rocks or metals are less dense than solid rocks or metals. Only molten water is denser than solid water; water is weird. The core of the Earth is denser than the rocks above it because of what it is made of. $\endgroup$
    – AlexP
    Aug 31, 2022 at 20:39

3 Answers 3

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Ignore the crust. It's so thin compared to the rest of our planet, much less your planet, that it could be just as rich in metals as Earth and it won't matter a drop

But you will have problems.

Our core is liquid nickel-iron. That wonderful, giant ball of molten death does at least two things that could be compromised on a larger-than-Earth planet.

  1. It creates our magnetosphere. Lowering the density means you're using something less dense than all that nickel-iron. Whatever it is, it won't generate a magnetosphere as well. That means more high-energy particles getting to the surface of the planet. That's going to cause lots of grief, like various kinds of radiation poisoning, heat damage, cats and dogs living together... mass hysteria.

  2. It creates heat, which in turn warms the mantle, which in turn warms the planet. It's tough to stand in the middle of a Minnesota winter and think the planet core is helping any, but it does. Oh, it does. The sun would have a much harder time keeping our happy little jewel in space warm without it. You could compensate for this by moving the planet closer to the sun... but then there's issue #1.

And if that's not enough, there's one more issue:

  1. Gravity. Yes, you only want one Earth-G of gravity — you also want a bigger planet. Unfortunately, the funny thing about gravity... it causes things to collapse. And the lower the density, the easier it is to crush it into a higher density. In other words, the larger your one-Earth-G planet, the less likely it can exist according to the known rules of gravity. It would simply collapse to, fairly realistically, an Earth-sized globe. Something in the structure of the material that's lower density would need to keep the planet expanded to the diameter you want. But the lower the density, the less likely such a material is believable.

There are all kinds of theories about mega-earths floating around scientific and pseudo-scientific circles right now. They're educated guesses at best, speculation in the middle, and outright lies at worst. We've never seen a single habitable planet other than Earth, so there is no "scientific answer" to whether or not a mega-earth is realistic or plausible.

So, here's what you need to do...

Because we have no evidence that a planet larger than Earth can sustain life but some reasonably good reasons why it won't...

You need to ignore us and anybody who thinks you need to be "scientifically realistic" and build a great world. After all, our stated purpose (Help Center) is to be "a site for designers, writers, artists, gamers and enthusiasts to get help creating imaginary worlds." (Emphasis mine.)

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  • $\begingroup$ To keep it warm carbon dioxide could be added to the atmosphere instead of moving the planet closer to the sun. $\endgroup$
    – Slarty
    Aug 31, 2022 at 6:50
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    $\begingroup$ While the compression aspect is true in principle I don't think it is relevant in practice with the numbers OP is talking about. Rock has a density similar to what OP needs and a planet sized ball of rock won't compress any more than a ball with an iron/nickel core like earth. $\endgroup$
    – quarague
    Aug 31, 2022 at 12:03
  • $\begingroup$ When you say "ignore the mantle" do you mean "ignore the crust". The Earth's mantle is extremely thick and makes up the majority of the volume of the Earth. $\endgroup$
    – James K
    Aug 31, 2022 at 17:18
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    $\begingroup$ @GeoffreyBrent Jupiter has a far stronger magnetosphere, generated by the liquid metallic hydrogen layer surrounding its core. So, no to your #1. $\endgroup$
    – DevSolar
    Sep 1, 2022 at 14:09
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    $\begingroup$ @GeoffreyBrent: Indeed you never should not attempt to avoid thinking in double or tripple negatives. You're unlikely to never not avoid making no mistakes that way. ;-) $\endgroup$
    – DevSolar
    Sep 2, 2022 at 20:18
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Low density normally means that a planet is lower in heavy elements e.g. iron and other metals. This isn't necessarily a problem for life, but may be a major impediment to technological development.

If the inhabitants get into spacefaring, it will be harder for them to escape your planet's gravity than it would be from Earth. Even though the surface gravity is approximately the same, it falls away more slowly, increasing the total energy required to escape.

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    $\begingroup$ The metals we use to develop our wonderful technology come from the topmost layer of the crust; and the crust, with all the metals we use, has a density lower than what the question asks for. The metals in the crust are not primordial, but were kindly brought to us by the zillion asteroids which hit Earth after all the primordial iron had sunk into the unfathomable depts of the planet. $\endgroup$
    – AlexP
    Aug 31, 2022 at 1:39
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    $\begingroup$ @AlexP Do you have a source for that second sentence? I'd have expected that the hypothesised Theia impact, and to some degree later volcanism, would've resulted in a great deal of mixing between crust and mantle, and my understanding is that there are noticeable isotopic-ratio differences between Earth's crust iron and that found in asteroids. I'd also be curious about what scenario could result in iron being absent during the initial formative stages (allowing for iron-poor core) while still forming enough iron-rich asteroids to bring the crust up to Earth-like levels later. $\endgroup$ Aug 31, 2022 at 2:16
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    $\begingroup$ Even iridium can be found primordially, it just is more abundant in asteroids. After all, Earth formed from the very same matter as the asteroids. $\endgroup$
    – DevSolar
    Aug 31, 2022 at 16:56
  • $\begingroup$ @GeoffreyBrent, the Theia impact was so energetic that it probably re-melted the planet (if it wasn't already molten at the time), with the result that the iron in Theia's core merged with the iron in the proto-Earth's core. $\endgroup$
    – Mark
    Aug 31, 2022 at 23:41
  • $\begingroup$ @DevSolar Also, if my understanding is correct: while asteroid craters at Vredefort and Sudbury are important sources of iridium and other metals, that's not because of metals contained in the asteroids themselves, but rather because they punched holes in the crust that let metal-rich magma erupt from below? $\endgroup$ Sep 1, 2022 at 6:07
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Moon style

The moon is 3.34 g/cc as opposed to Earth 5.51 g/cc; perfect for you. The moon is made mostly of light elements that on Earth are in the crust, possibly because the moon is a big chunk of crust that got knocked off the earth. Your planet is so big I am not sure what it might have gotten knocked off of though.

The moon is frozen in the middle in part because it is little. Your planet is more massive and so will cool off a lot slower than the moon or Mars. It can still be molten.

You can give your planet a molten metal core. Use aluminum! With its friends titanium and magnesium of course. Molten aluminum can generate a magnetic field as well as our iron nickel core can.

If you want something to heat up that core but you don't want any radioactive heavy metals to weigh you down you could have a big gnarly moon to tidally flex the whole planet and heat up its innards. Maybe that big moon could have all the iron and nickel in it - once it was the pit in your plum planet but it got knocked clean out by a huge impact. The reverse of what happened to Earth and Luna.

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  • $\begingroup$ Aluminum, magnesium, and titanium are all lithophile elements, and so tend to remain in the crust and mantle. The core of an Earth-like planet will form mostly from siderophile elements, which tend to be at least as dense as iron. $\endgroup$
    – Mark
    Aug 31, 2022 at 23:48

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