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I am writing a story where a secret organization in the present day controls a portal between Earth and a different Earth-like planet. The organization secretly ships resources between planets to make money. Natural resources common on Earth but rare on the other planet are resold on the other side and vice versa.

What metal or mineral commonly found on Earth could be extremely rare on a different Earth-like planet (assuming that this planet formed naturally)? Why would this geologically occur?

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    $\begingroup$ I think the answer depends mostly on what you mean by "Earth-like." If being Earth-like means that iron isn't rare, then iron isn't an answer; otherwise, iron is an answer. As far as I know, the same goes for every element. $\endgroup$ Nov 26, 2022 at 10:15
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    $\begingroup$ How come that Earth has so much coal and petroleum? Is there any reason to believe that another planet, even if it has life, would have experienced the same evolutionary history which created the conditions for Earth to have its ample reserves of coal and petroleum? Moreover, why doesn't comparative advantage apply? That is, even if gold is still rare on the other planet, but nevertheless is less rare than on Earth, why cannot they sell gold? $\endgroup$
    – AlexP
    Nov 26, 2022 at 13:16
  • $\begingroup$ is this planet in the Milky Way Galaxy, or outside it? $\endgroup$
    – Alastor
    Nov 26, 2022 at 15:01
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    $\begingroup$ @AlexP the comment re petrochemicals is a good one. I picture the future me, resplendent in 5 oclock shadow, reading your musings on why the Earth has coal and oil and how circumstances on a different planet might not have produced any. Help that future come to be! $\endgroup$
    – Willk
    Nov 26, 2022 at 16:26
  • $\begingroup$ It's not a "metal or mineral", but the obvious answer here is "water", actually. Earth has, comparatively speaking, a huge amount of the stuff just lying around (contrast with, for example, Mars), and it's also incredibly useful in so many ways (not least of which, if the inhabitants of the other planet are biologically anything like us, kinda essential for life itself).. $\endgroup$
    – Foogod
    Nov 26, 2022 at 20:04

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It's a napkin stretch, so not enough information

Where in terms of location were you placing your different Earth-like planet? More specifically, where in the Milky Way, was Earth 2.0 placed? Location matters here.

Our Earth is located about 26,000 ly away from the Galactic Center. This is where the average interstellar metallicity is moderate, i.e. neither too high, nor too low. Note that "metallicity" here in the context of astronomy doesn't mean "metals", it refers to elements with a atomic no. higher than hydrogen/helium. The moderate metallicity in the area of the Milky Way, where the Solar System is located, is the reason why the Earth has an adequate amount of heavy elements. Uranium and lead, both elements with extremely high atomic numbers, are actually fairly common elements. In fact, uranium can be found in nearly every rock, river, ocean (in minute quantities though) on Earth. Lead is basically dirt common, ranging from 50-400 ppm naturally..

If you get close enough to the Galactic Center, heavy elements become more and more abundant. That is because there are more and more stars per light year present there. That region is so compact, that the supernovas from the stars here tended to mix together really easily, due to the close distances involved here. This means that the radiation and temperature present here would transmute elements like silicon, iron and nickel into heavy elements like gold, lead, uranium etc. In fact, if your Earth 2.0 resided there, despite the metric f**kton of radiation pouring onto you every single second there, then Gold and Platinum would be dirt common there, compared to our Earth. The extreme conditions in the Galactic Center can cook up some really, really strange stuff. The metallicity can become really, really high there, upto the point that gold and platinum would be abundant resources and easily procurable.

However, if Earth 2.0 were to be located on the outskirts of our galaxy, then things are the exact opposite of what happened back in the Galactic Center. The metallicity there is so pathetically low, that forget gold, even oxygen, the third most abundant element in the universe (and therefore the Milky Way), is about as rare there, as gold is here on Earth. Practically speaking, there are basically lesser and lesser stars per light year, than it is near the Solar System. This means that there are lesser supernovas and neutron-star collisions (which are the two primary processes that created most of the gold in our universe) per light year. This results in a extremely huge deficit of heavy elements there. This means that the even the very oxygen in our atmosphere and crust *(Oxygen makes up about 50% of the Earth's crust), would extremely rare, and a valuable commodity for any civilization living in this reason.

TL;DR It depends on location, inside the Center of the Milky Way, gold is dirt cheap, whereas on the outskirts of the galaxy, even simple light elements like oxygen are extremely rare.

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    $\begingroup$ Upvote for metric f**kton. I have not seen one yet but they are common in Europe, I understand. $\endgroup$
    – Willk
    Nov 27, 2022 at 17:38

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