Under what conditions could a gold planet form?

Question

So, I am imagining a draconic alien species which, (don’t laugh) inhabit a planetary system that is particularly rich in heavy elements such as platinum, silver, and yes, gold. (I said don’t laugh!) However, just thinking about the physics of this, what stellar/planetary conditions would lead to a solar system becoming enriched in gold and heavy metals? I don’t necessarily mean so rich that it there are deserts of gold dust or platinum mountains; I just mean a prevalence of these metals in the crust similar to that of iron or copper on Earth.

Answer 1

Golden ring.

A metal rich asteroid like 16 Psyche got too close to your planet - inside its Roche limit and the planet tore it into a ring. This asteroid is the heart of a dead planetesimal and is rich in the metals you want.

https://thehill.com/opinion/technology/3597381-nasa-might-cancel-mission-to-massive-gold-mine-asteroid-heres-why-it-shouldnt/

Until recently, scientists thought that 16 Psyche was a solid hunk of metal, iron, nickel, gold and platinum. A recent article in Smithsonian suggested that the market price of the asteroid’s metals is 10 quintillion, hence the name “golden asteroid.” Other estimates have gone as high as 700 quintillion.

Rings don't last forever. For the last ten million years the broken bits of the golden ring have been raining down on the planet surface. The planet now is cooled and solid and so although these metal bits are dense they cannot make their way thru the solid crust to the core. They accumulate on the surface.

A lot of the falling metal melts on the way and so is dispersed widely. Not all of it. It is not at all uncommon for a hefty chunk to survive re-entry. One can sometimes gauge composition by the color of the shooting star as it comes down. Platinum makes a natty green.

People are not rich from all the precious metals, of course. They are too common. It does allow the common people to wear solid gold underwear, which is the fantastic shiny dream we all dream of.

Answer 2

Through natural processes, you won't get anything like a pure gold planet. You can get more metal than the Earth has, probably by a factor of two or maybe three. Let's unpack that.

Nuclear synthesis in the Big Bang and later in stars will produce a distribution of isotopes. The solar system distribution is probably not too far from the kind of distribution that results from such processes. It looks like so. (Taken from the wiki I just cited.)

So gold, element 79 and symbol Au, is round-about one millionth as common as silicon. The reason we see lumps of it occasionally is due to filtering processes that happen in geological processes.

Planetary collisions can produce planets with the heavier elements concentrated. If two planet sized objects collide with the right degree of glancing-blow, it can blast off the lighter rocky stuff and leave the heavier elements. The blasted-off parts can wind up as floating debris, or a large moon, or they can fall on other planets or the star in the system.

This happened to Earth and to Mercury. Earth got some extra metalic material, and the debris became our Moon.

So in the solar system, Nickel is about 1 atom per million. On Earth it is round-about 100 times as much. depending on the method of measuring. The moon has far less nickel than Earth.

The planet Mercury has a relatively large metalic core, which is probably a combinatin of iron and nickel. Probably the crust is much richer in metals than is the Earth.

So you can basically dial-up metalic content from round-about what the Moon has, to a little bit more than what Mercury has. You do it by having the planet you are interested in be hit by a glancing blow between proto-planets. This blasts away the rocky parts of both planets, and the metal parts combine. The rocky parts either become a moon or an asteroid belt or fall into the system's star.

You will still need the usual processes of pulling the gold out of ordinary background material. These include the action of water, volcanic action, and interaction between water and volcanic action. So to have pure gold around you will also want some water on your planet.

Answer 3

The younger the star system, the more enriched with heavy elements was the interstellar medium that it condensed out of, and thus the higher the proportion of heavy elements in the various planets, moons, asteroids, comets, meteroiri oids, etc. in the system.

I also note that stars with higher heavy metal conent are found closer to the galactic core, and stars with lower metal content are found farther from the galactic core.

So a writer might think that a good way to have inhabited systems rich with heavy metals would be to make the inhabited systems only about 10 or 20 million years old and put them at the galactic core to get the highest possible concentration of heavy metals in the planets.

But that situation has problems for being inhabited. It took a long time, tens or hudnreds of millions of years, for Earth to form, cool off, and for the first primitive lifeforms to appear. And it took billions of years for Earth lifeforms to en vnetually produce and oxygen rich atmopshere which multi-celled land organisms, such as humans, or the intelligent aliens desired to inhabit the planet, could breathe, and tens or hundreds of millions of years more until such intelligent beings did evolve.

Maybe intelligent life could evolve sooner than it did on Earth. I note that if cetaceans and proboscideans count as intleligent beings then the first members of their lineages to become intelligent might have lived 10 or 20 million years before the first proto humans.

It seems easy to believe that on some planets intelligent life might evolve when the planet is one percent (about 46 million years) or ten percent (about 460 million years) younger than the Earth. But to take billions of years off the age of a planet with naturally evolved intelligent life would require evolving intelligent life in only about 50 percent, or something, of the time it took on Earth, which may seem a lot less plausible to some people.

And of course your intelligent natives of the gold rich star system might be part of an artifical ecosystem. An advanced society might have terraformed the metal rich planets in that system to make them habitable when the planets are only a hundred million years old or so. And they might have used advanced biotechnology to create designed lifeforms to live on those terraformed planets, including the intelligent beings in the story.

Thus if the intelligent beings in the story believe they were created by by gods only 1,000, or 10,000, or 100,000, years ago, they might be correct, probably more by coincidence than by remembering the truth.

So you can have a habitable planets inhabited by intelligent beings in a very young star system in the core of the galaxy, and thus extremely rich in metals.

Except that a system in the core of the galaxy, where other stars are very close, probably wouldn't stay habitable for billions of years.

Because the stars are so close, really close encounters between stars would be many times more common than in our part of the galaxy, and would perturb planetary orbits, often causing planets to collide with each other or with their stars, or be flung into the bitter cold of interstelalr space.

And because the stars would be crowded so close togetherthe average star in the galactic core would be close to a nova, or a supernova, or a pulsar, or a gamma ray burst, muchmoreoten than on in our neighorhood of the galaxy. Thu the average planet with life might get palsted every few hundred million or every few tens of milions of years, wiping out all life on it.

So the synthetically created inhabitants of terraformed planets in the galactic core might advance scientifically enough to realize that their world may have only a hundred million more years, or maybe only a hundred more years, depending on the story, before all life on it is destroyed, and that they have to work fast to find a way to survive.

And if that star system is farther from the galactic core, the heavy metal concentration might be higher than in Earth's neighborhood and lower than in the galactic core, and the planet might have a longer future. And possibly an advanced civilization might use giant element synthasizing machines for thousands and millions of years to produce much more heavy elements than the system would naturally have.

So will the main concentrations of heavy elements be on the habitable worlds in the star system, or will be they be on uninhabitable worlds?

All the planets in the star system should have higher concentrations of heavy elements than in our soalr system, but just as the planets and other objects in our star system varying in average desnity (which largely depends on the amount of heavy elements) so too will the planets and other objects in your star system vary in their densities, which will partially be the result of variation in their proportions of heavy elements.

A geophysical planet, or a planetary mass orbject, or a planemo for short, is an astronomical object with sufficient mass for its gravity to pull its matter into an approxiamtely spheroidal shape. In our solar system not only planets, but many moons, dwarf planets, and Trans Neptunian Objects are planemos, pulled into spheroidal shape.

When an object achieves enough mass to pull itself into a spheroid shape, the pressure and temperature at its core and far above the core becomes intense. Solidmatter melts into liquid, and flows downward. So the matter in the interior of the object gradually becomes stratifified by density, the greater density material sinking to the core and the lighter density material floating toward the crust of the planemo. Thus all planemos whould have interiors statified by density of material.

But the countless thosuands and millions of objects of smaller mass in the solar system should all have pretty much the same average density, since hteir materials shoud be mixed randomly.

But the small meteorites which are found on the surface of Earth often have widely variing averge density ies due to he differeng materials they are made of. In particular some meteorites are composed of iron nickel alloy with other heavy elements included.

But it is impossible for such tiny objects to become stratified by density. Therefore they must be fragments of planemos which were shatted into pieces. Pieces would come from different layers in the stratified planemos and thus have different densities and different elemental composiiton.

Computer simulations show that the early chaotic period of the solar system, before the orbits of objects settled down, it was common for gravitaional interactions between bodies to make bodies fall into the Sun, or collide with other bodies, or be ejected from the solar system into interstellar space.

And some of the objects which shattered into countless pieces due to collisions were planemos which were strtified by density, thus making the pieces have different densities and elemental composition.

So there could be various bodies, from terrestrial type planets to tiny bits of space dust, and every size in between, in your star system, coming from different layers of shattered planemos, just as in our star system. And you can adjust the number of objects which are the right size to be mined for their heavy elements by the civilization in your story.

The planet Mercury has an unusually large iron nickel core for its size, thus making the overall density of Mercury much higher than would be expected in such a small planet. There are three theories to explain that. One is that a massive collision ejected much of Mercury's lighter outer layers, leaving a comparatively shallow layer of lighter materials above the dense core. Presumably a different collision might possibly have ejected all of Mercury's core and mantale, leaving the metal rich core exposed. And even if one of other theories to explain Mercury's density is correct, It may be possible for a collision between worlds to knock off all a world's crust and mantle, leaving only the core, rich in heavy elements.

https://en.wikipedia.org/wiki/Mercury_(planet)#Internal_structure

M-type (aka M-class) asteroids are a spectral class of asteroids which appear to contain higher concentrations of metal phases (e.g. iron-nickel) than other asteroid classes,1 and are widely thought to be the source of iron meteorites.2

Although widely assumed to be metal-rich (the reason for use of "M" in the classification), the evidence for a high metal content in the M-type asteroids is only indirect, though highly plausible. Their spectra are similar to those of iron meteorites and enstatite chondrites,4 and radar observations have shown that their radar albedos are much higher than other asteroid classes,[5] consistent with the presence of higher density compositions like iron-nickel.1 Nearly all of the M-types have radar albedos at least twice as high as the more common S- and C-type, and roughly one-third have radar albedos ~3× higher.1

https://en.wikipedia.org/wiki/M-type_asteroid

The largest M type asteroid is 16 Psyche. For a long time it was believed to be the core of a planetesimal which had shattered in a collision.

The bulk density of Psyche (3.9±0.3 g/cm3) places constraints on its overall composition. The iron-nickel found in most iron meteorites has a bulk density of 7.9 g/cm3. If Psyche were the remnant core of an early planetesimal, it would have to have an overall porosity of 50%. Given Psyche's size, this is considered highly improbable.[8] However, there are other metal-rich meteorite types that have been suggested as Psyche analogs, each of which have bulk densities that are similar to Psyche's, including enstatite chondrites, bencubbinites, and mesosiderites.[28][21][8]

Several possible origins have been proposed for Psyche. The earliest of these was that Psyche is an exposed metallic core resulting from a collision that stripped away the crust and mantle of an originally larger differentiated parent body some 500 kilometers in diameter.[11] Other versions of this include the idea that it was not the result of a single large collision but multiple (more than three) relatively slow sideswipe collisions with bodies of comparable or larger size.[34] However, this idea has fallen out of recent favor as mass and density estimates are inconsistent with a remnant core.[8]

A second hypothesis is that Psyche was disrupted and gravitationally re-accreted into a mix of metal and silicate.[35] In this case, it may be a candidate for the parent body of the mesosiderites, a class of stony–iron meteorites.[35]

A third hypothesis is that Psyche may be a differentiated object (like 1 Ceres and 4 Vesta) but has experienced a type of iron volcanism, also known as ferrovolcanism, while still cooling.[36] If true, this model predicts that metal would be highly enriched only in those regions containing (relic) volcanic centers. This view has been bolstered by recent radar observations.[7]

https://en.wikipedia.org/wiki/16_Psyche

And finding out about theories of ferrovulcanism might be useful, escpecially if gold, silver, platinimu, palladium, and other heavy metals are included in the iron nickel lava.

https://arxiv.org/abs/1909.07451

And I remember reading a science fiction story about a searce for "glory rings". In the the story "glory rings" were highly valuable but extremly transient and rare. They were rings around planets (or stars) where the ring particles were composed of heavy elements or were coated with heavy elements for some reason. If the story can be identified the explaination, plausible or otherwise, for "glory rings" might be useful in a story where some worlds in a star system with an extremely high ratio of heavy elements have even higher ratios of higher elements.

Added Dec, 27, 2022 The story is "Rings of Glory by Thomas R. Dulski, in Analog july 1982.

https://scifi.stackexchange.com/questions/270581/late-20th-century-story-probably-in-analog-about-glory-rings

Answer 4

As a general purpose workaround to any natural condition you don't like: An ancient very powerful race (possibly alien, possibly just (mis)programmed robots from that race) lived here long ago. In this case they collected the heavy metals from asteroids or mined it from the planet's core. Maybe they were not from this planet but just used it as a staging point in their mining operation.

Answer 5

Earth has something like 100 billion tons of gold. Its problem is that it's mostly in the core, which is why something like 100,000 tons have been mined.

Consequently what you need is that either there was massive volcanism that brought a lot more gold to the surface, or else the bombardment of the planet with gold-rich meteors. At least one gold strike has been tied to a meteor, but you're probably better off with the volcanoes.

Answer 6

what stellar/planetary conditions would lead to a solar system becoming enriched in gold and heavy metals?

To answer that you need to know a bit more about a thing called Stellar nucleosynthesis.

Basically, Stars use Fusion reactions to produce Energy and elements: Hydrogen + Hydrogen = Helium. a Star will remain doing that fusion for Millions of years until it expends all of its fuel (Hydrogen), then it will start fusing Helium tyo produce Lithium, then Carbon, Nitrogen, Oxygen and so on till Iron, at this point the Fusion reactions completely stop and the star implodes, fast and hard, how hard depends on how big is the star, this implosion compress the atoms at the core making it possible to fusion the Iron and thus producing heavier elements, Nickel, Copper, Zinc, etc, how far in the periodic table we go depends on how big was the star. At some point the implosion will reach a limit where the compressing force will bounce back and the implosion turns into Explosion, a big Explosion, how big? you guessed, it depends on the size of the star, a Nova, SuperNova, HyperNova... and all fusioned elements in the star get expelled into the space surrounding the nova. Gold is a very heavy element; it requires a Supernova to create it.

So.. back to your query, your imaginary gold made planet would have to be in or near to a Supernova remnant, unlikely to be pure gold but surrounded by many other heavy elements, even some radioactive ones.

Heavy things get more attracted by gravity than lighter things, so lets say that the supernova gave us a planet size gold nugget, which is currently traveling the space expelled by that powerful explosion.

The shockwave of our supernova reached a big gas cloud a few dozens of light years away, this made the cloud to spin, spinning made it concentrate, then gravity showed up and a new Star System was form.

In that new Start system, lets say we had a Earth size gas proto-planet, who got hit bit our gold nugget (u didnt forgot about it right?) since the proto-planet was still 'juiced' instead of blowing up it merged with the gold nugget, all the heavy gold forms the now solid surface, radioactive elements will go to the core and keep it 'warm' for a while, and the rest, water and gas goes to form oceans and atmosphere.

I'll leave the rest you.