[adrian_b]

In any stellar system like ours, where oxygen is the most abundant element, except for the hydrogen and helium that are contained mostly in the star and in the big planets, almost all metals are completely oxidized (with the exception of the siderophile elements submerged into the cores of the planets, because of their great density) and the amount of metallic substances with a natural origin that can be found in the accessible surface layers of the planets is negligible.

Reducing the oxidized metals requires much more energy than reducing non-metals like carbon, nitrogen and sulfur (which is what the living beings do to make their structural materials), and preventing the reduced metals to spontaneously become oxidized again is very difficult.

This is why no living beings have succeeded to use metallic materials before the humans, and the latter have succeeded to do this only after mastering the fire, which is the other thing that the non-human living beings have not succeeded to do.

There exists a second class of stellar systems, where there is more carbon than oxygen, so almost all oxygen remains bound in carbon oxides, while most other elements are present as carbides, instead of oxides, like in the Solar System. These are much more rare than the stellar systems of the Solar System type and in such stellar systems the chemical composition of the planets would be extremely different from the planets of the Solar System. Because there is no detailed information about such a stellar system (due to their distance), there is very little knowledge about whether there would be conditions in such a system for the appearance of life and how could that evolve. If there is any chance for primitive life forms to use metals in their structures, that would happen only in such stellar systems.

[airstrike]

The only possible way this comment could be more satisfying to read would be if it ended with "Until now." before the camera pans to a strange planet and the movie begins

[adrian_b]

An interesting fact is that while almost all solid objects that exist in the Solar System have their origin in the condensation of gases from which the Solar System has formed, there exist also the so-called pre-solar grains.

The pre-solar grains are microscopic crystals, i.e. particles of dust, which have come to the Solar System as already solid grains of dust, from other stellar systems, typically having been propelled by stellar explosions, e.g. those of supernovae.

Such pre-solar grains have been incorporated in the many small bodies that have been condensed from gases along with the bigger asteroids and planets at the formation of the Solar System.

Some of those small bodies have fallen on Earth as meteorites (the so-called "chondrites"). When such meteorites have been analyzed carefully, pre-solar grains have been recovered. They can usually be easily distinguished from the local objects, by having very different isotopic compositions.

Among the pre-solar grains, there are many that have come from stellar systems of the second kind, with more carbon than oxygen. Such grains, instead of being silicates, i.e. the most frequent minerals in the stellar systems of the Solar type, have chemical compositions that are unusual for the minerals of the planets of the Solar System, like diamond, graphite, silicon carbide or nitride, titanium carbide or nitride, metal grains of either platinum-group metals or iron-group metals, other carbides, nitrides, sulfides, silicides or titanides.

For now, this is the only direct evidence of the second class of stellar systems, beyond the spectroscopic observations of various stars, which provide estimations for the relative abundance of carbon and oxygen in those stellar systems.

While we have some idea about what kind of minerals might be the most abundant in such stellar systems at the time of their initial condensation from gases, I am not aware of any attempt to simulate the possible internal structure for big planets in such stellar systems, in order to determine whether in such planets there could exist some analogs of the volcanism and hydrothermal vents that can provide the energy flux necessary for the appearance of life in the planets of the terrestrial type.

[xp84]

These are two of the coolest and most fascinating comments I’ve read. Idk if you are a professional …. Astrogeologist?? or just a really smart person but I would like to subscribe to your newsletter for sure. Thanks for sharing this!

[adrian_b]

No, I am an electronics engineer. However, I happened to have some relationships with a few professional astronomers, because my father had worked for many years in an astronomical observatory, and I keep following the research publications in this domain.

There are a few such facts about the history and the diversity of the world in which we live that deserve to be known by more people.

[airstrike]

You have a talent for writing in a way that's both approachable and full of information. It comes across as super knowledgeable without losing that wonderful sense of childlike curiosity, which I think makes it particularly approachable or even inviting. It reminded me of PBS Space Time.

It's an artful balance that is so rare—especially online—that I think some of us just savor it when we do find it. I actually went through your older comments (sorry) just to keep reading what you had to say...

[djhn]

Is there any high quality sci-fi that explores the second type of solar systems?

[adrian_b]

Most SF movies prefer to show only planets of the terrestrial type, in order to allow the actors to roam freely there and show their uncovered faces to the cameras.

At most there have been a few novels or movies that have attempted to describe less familiar landscapes, such as those that could be encountered on the satellites of Jupiter or Saturn.

There have been a few SF stories about planets made of some exotic materials, like diamond or some metals or some superheavy elements, but those were complete fantastic stories without any scientific base and the planets described there could not exist anywhere in the known universe.

I am not aware of any novel or movie that has tried to show a completely alien planet, of the kind that could not exist in the Solar System, but which could really exist in other stellar systems. A planet from a stellar system with a high C/O ratio might have rocks made of abrasive carborundum (i.e. silicon carbide), an atmosphere composed of methane, carbon monoxide and carbon dioxide and an ocean containing a mixture of hydrocarbons, like some kind of petroleum.

If there would be life forms there, they could have very significant differences from the life forms that can appear in the stellar systems of the Solar type.

Here on Earth, an essential chemical property for life is the distinction between hydrophobic and hydrophilic substances, i.e. the fact that water and oil do not mix, which enables the existence of the cells of all living beings, which are made of hydrophobic membranes that partition a hydrophilic solvent. Perhaps on a planet with reversed abundances, where hydrocarbons are very abundant and water is scarce, one could have reversed cell structures, with a hydrophobic solvent partitioned by hydrophilic membranes, though it is not clear if such structures can be made stable. In such a place, most metals would be present in easy to reduce compounds, so living beings with metallic skeletons might exist. (Though at least for now, the appearance of life in such stellar systems seems less likely. In the terrestrial kind of planets, the energy flux for the appearance of life has been provided mainly by the free dihydrogen generated by the oxidation of Fe(II) ions to Fe(III) ions by water, in volcanoes and in hydrothermal vents. It is not known whether some equivalent energy source can exist in a place with little water, but abundant hydrocarbons.)

An SF novel or movie with such a subject, about the exploration of a completely unfamiliar world, could be interesting, but this kind of SF novels were written only up to around a half of century ago. Most modern SF novels or movies no longer try to analyze the consequences of intriguing scientific hypotheses, but they choose the lazier way of just transposing traditional fantastic stories into a pseudo-scientific framework.

[kjkjadksj]

To be fair we are the only creatures to make much use of anything, metal or no. That being said the metals are used by other creatures. Some might eat metal rich mud for nutritional value for example.

[adrian_b]

What those other creatures eat or use are not metals, but chemical substances that contain chemical elements that would have been metals in their pure elemental state.

For instance, many living beings, from bacteria to vertebrates make and use magnetite crystals, for sensing the magnetic field of the Earth.

Magnetite contains iron ions and pure iron is a metal. Nevertheless, magnetite is not a metal, but a ionic crystal, i.e. an insulator. Your blood contains iron and your bones contain calcium, but none of that iron or calcium is in metal form, all are oxidized ions.

There are no living beings that have metallic components. There are a few bacteria that are able to reduce to metallic form the metals that are the easiest to reduce, i.e. gold and silver. However those bacteria do not use in any way the metallic gold or silver that is precipitated outside their bodies by their activity. The reducing of gold and silver is just a defense mechanism for those bacteria, because the ions of gold or silver kill bacteria, and their precipitation when they are reduced removes them from the environment.

As I have said almost all metallic elements present close to the surface of the Earth or of any other planet of this type are oxidized, i.e. they are positive ions that are bound in various ionic substances, like oxides or sulfides, and they can be found inside the bodies of the living beings in the same state as outside (unlike carbon, nitrogen and sulfur, which are oxidized outside, but reduced inside the bodies of the living beings).

Only a few metallic elements are found also as native metals, i.e. copper, silver, gold, mercury and the platinum-group metals. Even for these metallic elements, most of them are far more abundant in oxidized forms (like sulfides or arsenides) than in metallic forms. Only gold is more abundant in metallic form than in oxidized forms, like tellurides (and that is due in good part to the fact that tellurium is also a very rare element, otherwise more gold would be found combined than in metallic form; the gold ions are extremely large, so that they cannot combine well with ions smaller than the telluride ion, like the sulfide ion that combines well with the smaller silver ions).