We all know the tale of Jack and the Beanstalk--a farmboy sells his cow for some "magic beans", which in turn grow into a mountainously tall stalk that led him to the land of a giant. Now, scientifically speaking, the only way for Jack to meet the giant is if the beanstalk led him to a habitable world with lower gravity.

Which was the sort of thing that "Jack", a member of the mad scientist illegal terraforming community, wanted to take advantage of when the real scientific community discovered that one of the alternate Earths has an iron-rich core the exact same size as ours is, but hidden beneath a smaller crust, which compresses the mantle. This combination of smaller crust and same-size core means that the planet would be warm enough to support liquid water, therefore life. For him, it is the perfect place to turn the story of "Jack and the Beanstalk" into an actual ecosystem, complete with smuggled stores of fabacean seeds, or "beans", just to see if the lower gravity would be enough to turn these small herbs into giant plants that anyone named "Jack" could actually climb up.

But here is the thing--if the crustal diameter is too small, therefore compressing the mantle too tightly, then the crust would liquefy, turning the surface into a volcanic hellscape. So in an alternate Earth where the core is the same size, how much smaller would the crustal diameter be for this world to be livable instead of hell?

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    $\begingroup$ Free-Wi-Fi-fun, are we doing a fairy tale theme this season? If so I'll smash that delta button... $\endgroup$ – user6760 Dec 4 '19 at 5:10
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    $\begingroup$ So the giants cloud castle becomes a habitable moon in geosynchronous orbit & the beanstalk is a bio engineered space elevator cable (cars not included) you can grow from seed? $\endgroup$ – Pelinore Dec 4 '19 at 14:43
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    $\begingroup$ You might want to consider a separate question on it's effect on tides & how large you can plausibly make the giants world without serious ecological impact on the earth .. if you're keeping the real moon you might want to know what orbit for the giants world is best to avoid collision or throw it's orbit too far out of whack. $\endgroup$ – Pelinore Dec 4 '19 at 15:03
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    $\begingroup$ You may be looking for something like a Pluto Charon binary system. $\endgroup$ – Pelinore Dec 4 '19 at 16:00
  • $\begingroup$ @Pelinore I did not ask for the moon. $\endgroup$ – JohnWDailey Dec 6 '19 at 2:01

I'm going to be honest, I think this sounds a little too far outside of science for this to work.

Base requirement is that the Earth and moon would have to be completely different because they would need to be closer and tidally locked so that the moon doesn't immediately rip the beanstock out of the ground. Pluto and Charon are a great candidate for that, but they are frozen because of the distance from the sun, and are too small to hold an atmosphere. Of course, if you increase the size and bring them closer to the sun, they end moving too far away to be feasible.

The tidal locking would also cause the Earth to be dramatically colder on the moon side because it would experience much colder nights, while the other side would be warmer and drier because the moon produces the tides, so oceans may be likely to accumulate more on the moon side. When I say warmer and colder I don't mean like greater difference than the Sahara and Antarctica.

I guess it might be feasible with those parameters, but that's a hell of a lot of handwavium. There's a reason fantasy and scifi don't usually overlap well unless you just kind of drop real science.

I know you were primarily asking about the crustal diameter, but I think you have a few more bigger issues before that really comes up. Is the mantle size different in anyway? That would definitely have a greater affect, because the crust is so thin around the edges of the Earth, that changing that alone would essentially have next to no affect except make the surface molten as suggested. A much smaller mantle, which would likely be necessary for this scenario would however have drastic affects on the Earths internal heating. I guess this is just more of a few things to consider.

  • $\begingroup$ I did say that the core is the same size, and the diameter of the crustal shell is smaller, so that should give you an idea as to how much smaller the mantle is. But how small would be "too small"? That's the question in focus here. The moon has no part in this particular question. $\endgroup$ – JohnWDailey Dec 5 '19 at 0:42
  • $\begingroup$ @JohnWDailey Oh, I mistook one of the comments under the question as having come from you. If the moon plays no part in this, am I now understanding correctly by saying that Jack is from Regular Earth, the beanstock is taking him to smaller Earth? If so, is there some indication that the beanstock is some sort of metaphor for a method of FTL, or is the name of the method, or a literal beanstock somehow transports him to another planet? $\endgroup$ – TitaniumTurtle Dec 5 '19 at 17:02

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