Is it plausible for a planet that is positioned in the habitable area of a solar system to have little extractable metals?

Question

I am big fan of Asimov's Foundation and I am wondering about the plausibility of some of Foundation's imagined words.

According to this article Terminus was a special world with little metals:

It was the sole planet orbiting its isolated star and had almost no metals. The nearest planet was Anacreon, 8 parsecs (26 light years) away. Being on the fringe of the galaxy, there are almost no stars in the sky.

Since the planet hosted the first Foundation it is clearly habitable (i.e. atmosphere, no extreme temperatures). However, I am wondering how could it have so little metals if it seems so similar to Earth (habitability-wise) which has lots of metals. Is this possible in our Universe?

I am interested in answers that use current discovered exoplanets information or some articles that deal with planets come to be.

Question: Is it plausible for a planet that is positioned in the habitable area of a solar system to have little extractable metals?

Answer 1

Interesting question, though probably a better fit for world-building.

Firstly "metals" is often used by astronomers to refer to any element other than hydrogen or helium. That is clearly not the intended use here. Terminus has a solid surface at temperatures that humans can stand, and a breathable atmosphere (perhaps after terraforming). So, what can it be made of?

The most obvious option is silicate rock (mainly aluminium and magnesium silicates) similar to what most of the outer layers of the Earth are made of. It is certainly imaginable that a planet might be more thoroughly differentiated than Earth, so that all the iron etc. ended up in the core, and there were no, or almost no, useful ore bodies in the crust.

On the one hand, this clearly fails, since magnesium and aluminium are metals. On the other hand, it is quite possible in context that what is meant is "easily extractable metals" and/or "the metals required for a specific range of technologies", which might include iron, copper, etc. (given that Asimov was writing in the '30s that is probably what he was thinking of).

In fact, with enough energy you can extract aluminium or magnesium from these rocks. We don't do this, because it's cheaper to find rare compounds of those elements from which the metal is more easily extracted, but the Foundation could certainly have had aluminium and magnesium in this scenario assuming they had enough power.

A stranger possibility would be carbon. Several gas giants are believed to have carbon-rich cores. Suppose one of those lost its outer layers in some cataclysm, the resulting smaller planet could differentiate to have a thick diamondoid crust over a rocky mantle and iron core. Although if diamonds were literally as common as dirt on Terminus, I can't help feeling someone would have remarked on it.

A similar but slightly less exotic option would be silica -- so a planet with a mainly or entirely quartz crust. I can't see a scenario for that to form -- one where you'd have silicon and oxygen and not much else around when a planet was forming.

Now we're definitely into worldbuilding territory, but I wonder is ice is a contender? If you found a world with an icy crust a few hundred km thick, is there a way to terraform it without melting all (or even most) of the ice? YOu could imagine covering the ice in a layer of vacuum-filled diamond honeycomb and then dumping a few meters of imported or manufactured soil over it, creating an atmosphere somehow and then putting a heavy greenhouse layer (CO2 and SF6 and so on) to warm the surface. Could the first-empire scientists have done something like that? Taking this back to the original question -- an ice world could exist at the outer edge of the habitable zone (or just beyond it, if you like) and then be made habitable by adding a greenhouse layer and changing its albedo. If you can avoid melting the ice, that would meet the requirements.

Answer 2

I'm going to approach this question in two steps: what metals are you talking about, and could you have a planet where those metals are not easily extractable.

What metals?

I get the sense that you're specifically referring to the non-lithophile metals, which include the d-block transition metals iron, nickel, copper and gold, and the chalcophile metals zinc, tin, lead, arsenic, mercury and silver.

This is an important distinction, since the advent of metallurgy was a critical step in human development in taking us out of the Stone Age, and it might be reasonable to posit that any advanced civilisation would follow a similar trajectory.

The final phase of the Stone Age, as we transitioned from the Neolithic to the Bronze Age, is called the Chalcolithic because it's marked by the first use of copper (although lead-smelting may have slightly preceded copper-working in some places). The Bronze Age initially alloyed copper with arsenic, but bronze made from copper and tin turned out to be both less toxic and more durable; bronze, in turn, was replaced by superior steel tools. Steel requires a sophisticated ferrous metallurgy involving iron alloys with a carbon content, and knowing how to make it is the mark of a culture's entry into the Iron Age.

[NB: There are plenty of iron relics that predate the Iron Age, but these were made from meteoric iron, an iron-nickel alloy that requires no prior smelting of ores; terrestrial iron's high melting point is well beyond the temperatures that could be achieved in Stone Age pottery kilns.]

Easily extractable?

Wikipedia describes the early stage of the Earth's formation:

The proto-Earth grew by accretion until its interior was hot enough to melt the heavy, siderophile metals. Having higher densities than the silicates, these metals sank. This so-called iron catastrophe resulted in the separation of a primitive mantle and a (metallic) core only 10 million years after the Earth began to form, producing the layered structure of Earth and setting up the formation of Earth's magnetic field.

If we assume that rocky exoplanets would follow a similar trajectory, we can certainly postulate that the non-lithophile metals (see my first section) would be depleted in the lithophile mantle. However, I'd expect there would still be ore bodies of even the most heavily depleted metals (e.g. iron), due to volcanism or as a relic of the earliest crust before the "iron catastrophe".

The giant impact hypothesis provides one mechanism for an exoplanet to form with a much higher concentration of lithophile elements. It's proposed that a solid object (Theia) crashed into the early Earth, ejecting a significant proportion of the two mantles (mostly lighter lithophile elements) into space. Much of this ejecta ended up forming the Moon. A similar scenario but with one or both bodies more massive could certainly result in an even bigger body than the Moon forming out of this lighter ejecta.

However, it's also suggested that the giant impact not only re-liquified the Earth's mantle but caused significant remixing of the mantle with the outer core. It's not clear to me whether, in the absence of a giant impact, there would have been an even stronger differentiation of elements: in other words, even less residual iron in the mantle.

This is all supposition, of course, and there are many questions still to be answered in the Theia model. Nonetheless, it seems reasonable to say that it is indeed plausible for an exoplanet in the habitable zone to have a significant deficiency of non-lithophile elements in the planet's crust.

Answer 3

Confirming the absence of an element on an exoplanet is not within our current means. Given the absence of any context for the lacks metals (does it have to be present in the crust? elemental form? How scare counts as absent?) its hard to conjecture what level of oddity would be require to achieve "absence of metal". However, I think its safe to say its not super likely that rocky planet was completely devoid of metal.

Answer 4

It depends a bit on whether you are talking about extractable metals or about extractable metals. Which is to say:

1. Iron is pretty universal, so if you are thinking about iron and its cousins then your concern will be whether it has all sunk down into the core, whether there is a thick layer of ice between you and the ground – in a word, extractability.
2. If you mean more precious metals such as gold and uranium, the question is not extractability, it is about the things being there in the first place. It is no longer thought that most of these r-process elements are created in an exploding supernova, because a supernova explosion is too slow and too gentle. The current trend is to see them as made of "splashed neutronium": either the débris of colliding neutron stars or in the wreck of a neutron star which has been disrupted by a black hole. The reason the question then becomes more interesting is that these things do not happen very often.

The key paper to read is A nearby neutron-star merger explains the actinide abundances in the early Solar System by Imre Bartos & Szabolcs Marka in the May 1 issue of Nature: the link has an abstract. The key points are:

• Neutron star collisions are rare. They mention something like dozens of collisions per 100 million years (as I understand it, this rate is for our galaxy taken as a whole).
• Their paper specifically identifies, by isotopic analysis, that our solar system was within a reasonable radius of such a collision shortly before it came into being, where "reasonable radius" is something quite small, of the order of 1,000 light years. (Very small, in the context of the size of the galaxy).
• It follows that where freshly created r-process matter is concerned, our galaxy is pretty spotty. When your solar system is born, you are either near a recent neutron star merger or you are not; but you are much more likely not to be.

The question that needs looking into further is how spotty the galaxy is if you are willing to accept stale r-process matter as well as fresh. The staler matter will be depleted in radioactive elements (for instance, there will be less uranium and thorium and more lead) but of course the abundance of stable elements such as gold will be unchanged. If you include the staler matter, is the galaxy still blotchy, at least? If it is, you can easily have exoplanets which were born outside a blotch and so have no heavy elements.

By the way, since a bit under half of the Earth's internal heat (no reference, so please check!) comes from uranium and thorium, a planet which was born far away from a neutron star merger would not only lack uranium mines but might also have a much colder core and a different system of tectonics to deal with the smaller heat flow.