I'm helping a friend construct a fictional planet for a project, and we've run across a problem. He's looking to create a planet nearly identical to Earth, except only larger. It would need to have the same gravity, similar rock content, etc. However, I watched Artifexian's terrestrial planet creation video and used the resources in the video description. ---That's where the issue comes in. According to the graph that was linked, a planet larger than Earth that maintains the same mass falls under the "waterworld" category. We're looking for a world with continents, so that obviously won't work. Is it possible to create an Earth clone that is the same in all aspects except maybe 25%-50% larger?
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1$\begingroup$ This is a good question! Just to clarify, are you keeping the mass of the planet the same but expanding the radius by 25-50%? I just want to make sure I'm not misunderstanding. $\endgroup$– HDE 226868 ♦Commented Jul 20, 2021 at 0:23
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$\begingroup$ Re: "25%-50% larger", does that refer to surface area or radius of the planet? $\endgroup$– GrumpyYoungManCommented Jul 20, 2021 at 0:28
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$\begingroup$ Similar how to build a planet larger than earth, but has the same gravity?, Could there be a planet bigger than Earth, but with less gravity? // comment not necessarly to point out it a duplicate, but to help ai overlords to link those things together based on similarity $\endgroup$– MolbOrgCommented Jul 20, 2021 at 5:17
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2 Answers
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The video's right; a radius increase of ~25-50% would imply a lower density than that of Earth, which would be likely achievable only if the planet had a substantial water component (I'd cite Seager et al. 2007, but you can look at a lots of different models; beware that there are plenty of uncertainties). However, if you're willing to fudge the mass a little, you can increase that radius to the extent you want.
A nice example is Proxima Centauri b, which is believed to be a bit more massive than Earth and a bit larger. The uncertainties on its mass and radius are substantial, and depend on which observations you trust, but some have argued for median values of 1.30 times the radius of Earth and 1.60 times its mass (Tasker et al. 2019). This would yield a surface gravity of about $0.95g$ - perfectly enjoyable - and would imply a larger amount of water than Earth but still a largely silicate composition.
Whether Proxima Centauri b actually has this mass and radius will require a better understanding of its orbital inclination (which does affect our measurement of its mass) and other parameters, but a planet with $R=1.3R_{\oplus}$ and $M=1.6M_{\oplus}$ would fit what you're looking for.
(As an addendum: "Super Earth" does have a specific meaning in astronomy, but it's typically applied to planets weighing in at somewhere between a couple Earth masses and 10 Earth masses, many of which have hydrogen-helium envelopes and are far from Earth-like, though they may still have surface gravities near $g$ - see Gliese 163 c and Gliese 1214 b, among others.)
answered Jul 20, 2021 at 0:32
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$\begingroup$ It depends on the composition of the planet as well. A heavy planetary core relative to the rest of the planet's mantle and crust mass would also contribute to the lowering G effect .. An entity on the planet's surface would experience less gravity, because the heavy core is relatively far away.. while the core contributes significantly to the total mass M. $\endgroup$– GoodiesCommented Jul 20, 2021 at 2:01
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2$\begingroup$ @Goodies The mass distribution shouldn't really make a difference, so long as it's spherically symmetric - the shell theorem says that the only important quantity when calculating the surface gravity is the total mass contained within the planet. If you increase the mass by some small amount, it would increase the surface gravity to the same extent regardless of which layer it's in. $\endgroup$– HDE 226868 ♦Commented Jul 20, 2021 at 2:11
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$\begingroup$ Ah science is right of course.. If Newton already said it.. but I find that shell theorem difficult to grasp, actually.. there's a lot of math in that article ! intuitively I'd say density should have some effect.. suppose the center of the planet is a miniature black hole, earth mass.. Total planet radius comparable to earth.. On the surface, I'd feel a gravitational force larger than earth's, because total mass is larger. But I'm quite far away from that miniature black hole ! 6.371 km.. If I'd go to space on that distance from the planet surface, I would not feel any gravity (??) $\endgroup$– GoodiesCommented Jul 20, 2021 at 2:30
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$\begingroup$ @Goodies density can have an effect but it needs to be non spherically symmetric $\endgroup$– jk.Commented Jul 20, 2021 at 9:09
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2$\begingroup$ @Goodies If you took a Black Hole of the mass of the Earth and then put it in the centre of a balloon the size of the Earth, the gravity on the surface of the balloon would be exactly the same as the gravity you are experiencing now. As long as you keep it spherically symmetrical, you can move the mass around inside the surface as much as you like - it won't make any difference. $\endgroup$ Commented Jul 20, 2021 at 15:06
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You could create such a world. Just create a planet with two times the radius of Earth and make a hole in the middle. To prevent the world from collapsing put some construction on the inside surrface to resist the collapse. A huge magnetic field generator could be placed in the middle to give the world an appriate magnetic field.
