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Our planet has a surface full of silicates and a core made of iron. It's due to this that Earth is the way it mostly is - most rocks and many minerals contain some form of silica. But what about a planet with an iron surface and a core of, let's say, mercury (to have an element denser than iron)? This isn't to say that this planet is purely iron and mercury, but it has the ratios of what we have of silica and iron, respectively. (This would mean that ~90% of this planet's crust would be iron.)

Is such a planet feasible? And if so, how would that affect plate tectonics and development of life, if life is even possible on this kind of world?

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  • $\begingroup$ why not? after a supernova debris gathers under the influence of gravity and as pressure build up the denser element such as iron will concentrate in the core then a couple of catastrophic events shave away majority of the crust and throw the planet into oblivion(void where matter density is super low) there you have it a giant cannon ball in space ;) $\endgroup$ – user6760 Oct 9 '15 at 2:08
  • $\begingroup$ Not feasible--where are you going to get that much mercury? I'll accept a planet made of metal--the leftover after the outer layers have been stripped by an encounter with something very energetic but I won't accept that much in the way of heavy metals. $\endgroup$ – Loren Pechtel Oct 9 '15 at 4:52
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A very close variation to such a planet already exists in our own solar system. It's the first planet Mercury (not the chemical element mercury, but the planet). Wikipedia states:

Although Earth's high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not as compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.

Mercury's core is 42% of its entire volume. In contrast, Earth's core is a paltry 17% of its entire volume. I cannot say anything about the presence of element mercury on this planet however. Iron appears to be far more common in our solar system than mercury. The supernova that created the debris from which our solar system was formed, had much larger amounts of iron as compared to mercury. Iron has atomic number 26 and it is far far more likely to be formed in the core of a red giant than mercury that is more than twice heavier (with an atomic number of 80).

Basically you expect high-iron planets close to their parent stars. The simplest reason being that solar wind sweeps the planet's atmosphere and some lighter elements away, leaving only the heavy ones behind. In our own solar system, the inner belt contains 4 terrestrial planets while the outer belt contains the gas giants. It is very likely that a sizable portion of the inner planets was swept away by solar winds and was later captured by the outer planets, making them truly monstrous planets.

So all in all, yes, planets containing very high ratios of iron are possible. However planets containing an equally high ratio of mercury (element) are very unlikely.

This being said, the question of life on such planets is purely opinion bases. I would not expect any earth-like life on those planets. We, being carbon based organisms and being used to seeing life of only one type, have no idea what really defines life and how can it form. For us, life is impossible without water (and lots of it). And then you need nutrients and a long array of essential chemicals to kickstart the evolutionary history the way it happened on earth. We cannot say how life can evolve on other planets and under what conditions. We also don't know what type of life it would be.

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Start from a stellar remnant, it is theoretically possible that a planet composed largely of iron and heavier elements could be formed as the result of the destruction of a star. A fragment of the core of a Sol-like star near the end of it's life would be mostly Iron from stellar fusion mixed with the heavy elements that were pulled into the star from the original Molecular Cloud. Such a world would be extremely radioactive and would start out white hot but the surface would eventually cool and solidify.

I would expect that the Crust of such a world would be largely Iron, the way the Earth's crust is largely Basalt with the equivalent of Continental Crust being composed of the lighter Transition Metals like Titanium and Chromium, and some lower density compounds formed by heavier elements. The mantle would indeed have a good percentage of Mercury but would almost certainly contain a lot more Lead than any other single component formed by the decay of many heavier radioactive elements that were originally in the stellar core and will also have a lot of still radioactive isotopes that haven't yet decayed.

I would expect this world to be extremely geologically active, the mantle is going to be much hotter than Earth's and the world as a whole will have far greater heat reserves for far longer. It will be hotter because it's composed of denser minerals with higher heat capacities and because more heat is constantly being pumped in by far more radioactive decay than in the Earth. The world will have more heat reserves because of three mechanisms; higher energy crystallisation reactions, repeated crystallisation and dissolution due to radioactive decay destabilising crystal structures, and, because of the relatively high levels of radioactive isotopes (as a percentage of the whole), more radiogenic heating, including more long half-life isotopes.

The mantle is probably going to have a similar viscosity to that of the Earth, if not being even more fluid due to the high percentage of Lead and other low melting point heavy metals. If those assumptions hold then tectonics will be almost identical to the Earth but with higher temperatures and different "rock" forming minerals/alloys.

Life as we know it, i.e. carbon-based and water soluble, wouldn't stand a chance on such a world, at least not initially. There's too much ionising radiation, too many heavy metals in the crust, and too little of the elements we need to function, Oxygen, Carbon, Hydrogen, Nitrogen, Calcium, and Phosphorus account for 99% of the mass of the Human body (Source) and all of them, with the possible exception of Hydrogen, will be vanishingly rare on this remnant world.

Now it is possible that such a world might be able to accumulate large volumes of lighter elements later in it's evolution through collision with debris from the planetary system, if any, of the star system that centred on the star it was part of. This material would initially coat the world forming a, geologically, short lived "life layer" that could support Carbon-based life for a time before tectonic activity mixed this material into the mantle.

Let me know if you want me to go in depth on any of this stuff, I've gone pretty lightly over a lot of bits.

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Life isn't possible according to any model we have but the planet is fascinating.

Since this planet (IronForge) has such a high concentration of very heavy metals it probably formed in the remnants of a very heavy supernova or series of supernovas. Earth has only trace amounts of mercury compared with the terrestrial ratio of silica to iron.

The mass of the Earth is composed mostly of iron (32.1%), oxygen (30.1%), silicon(15.1%), magnesium (13.9%), sulfur (2.9%), nickel(1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements.

So if we take all those elements and multiply their atomic mass by three, while keeping the ratios the same, we get the following list:

  • Mercury
  • Chromium
  • Technetium
  • Krypton
  • Indium
  • Radon
  • Promethium
  • Zirconium

Any free oxygen has been consumed a long time ago. Carbon based life and silicon based life will have a very hard time finding resources to form cell structures.

Is life possible? Extremely unlikely and if there is life it won't look anything like life on Earth, obviously. By shifting the weight of IronForge so much higher compared to the silicon-iron ratio of earth, the amount of industrial heavy metals in the crust and general environment will be far far higher than on Earth. With a radon-krypton atmosphere, any life will need to fight very strongly against the radiation coming from radon and radon decay products.

This is in no way a pleasant place to visit for humans. The air itself is radioactive and there's heavy metals everywhere.

Estimating which elements will be in the crust or in the core is beyond my ability to estimate. Having a mercury core would be interesting because it's conductivity is so high.

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16 Psyche has a surface that's estimated to be 90% metallic (iron), as it has an extremely high radar albedo. It's presumed that it's the former core of a protoplanet wherein gravity started to separate metals from minerals but then the rocky exterior was blasted apart, leaving just the metallic core. However, despite being described as a "metal world", the density is lower than Earth's, so I'm not sure what the discrepancy is from.

An orbiter is planned to launch in 2023.

Psyche orbiter logo with tagline: "Journey to a metal world"

The atmosphere of the rocky planets largely came from volcanic mantle outgassing. That was Earth's "second atmosphere", the first one after the hydrogen/helium nebulous atmosphere. If the planet was impacted violently enough to rip off large portions of the mantle, the source of much of that outgassing would be gone, so the atmosphere around it would be thinner than normal.

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  • $\begingroup$ Psyche's only 200 km in diameter, so its core isn't compressed nearly as much as Earth's. $\endgroup$ – Spencer Mar 3 '18 at 23:58
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Iron is the last element that forms in large quantities before a star collapses into either a neutron star or a black hole. All heavier elements have much less abundance in the universe as they are mostly produced only in Neutron star mergers. These are extremely violent astronomical events which are also extremely rare, limiting the abundance of heavy elements that can end up in planets.

So, a close to full iron planet is about the heaviest you can get. I wouldn't think that anything like a mercury planet would be possible.

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