Comment by analog31

An interesting fact is that while almost all of the Solar System has started as gas, which has then condensed here into solid bodies that have then aggregated into planets, a small part of the original matter of the Solar System has consisted of solid dust particles that have come as such from the stellar explosions that have propelled them.

So we can identify in meteorites or on the surface of other bodies not affected by weather, like the Moon or asteroids, small mineral grains that are true stardust, i.e. interstellar grains that have remained unchanged since long before the formation of the Earth and of the Solar System.

We can identify such grains by their abnormal isotopic composition, in comparison with the matter of the Solar System. While many such interstellar grains should be just silicates, those are hard to extract from the rocks formed here, which are similar chemically.

Because of that, the interstellar grains that are best known are those which come from stellar systems that chemically are unlike the Solar System. In most stellar systems, there is more oxygen than carbon and those stellar systems are like ours, with planets having iron cores covered by mantles and crusts made of silicates, covered then by a layer of ice.

In the other kind of stellar systems, there is more carbon than oxygen and there the planets would be formed from minerals that are very rare on Earth, i.e. mainly from silicon carbide and various metallic carbides and also with great amounts of graphite and diamonds.

So most of the interstellar grains (i.e. true stardust) that have been identified and studied are grains of silicon carbide, graphite, diamond or titanium carbide, which are easy to extract from the silicates formed in the Solar System.

The elements heavier than iron are not formed by fusion because of the asymmetry in the initial conditions.

The Universe that we can see has started from a mixture of equal amounts of free neutrons and protons (at a temperature of a few tens of MeV, matter has the simplest possible structure, consisting of free neutrons, free protons, free electrons, free positrons, photons and various kinds of neutrinos; upon cooling, nuclei form, then positrons annihilate, then atoms form), which have formed in the beginning hydrogen, helium and some lithium. Then, through fusion, the next elements until iron have been generated.

Iron is not the last element generated, a few elements after it have also been generated by fusion, because while they have lower binding energies than iron, their binding energies are still greater than of the lighter elements that can fuse into them.

However after the peak of the iron, the abundance of the following elements generated by fusion drops very quickly, e.g. down to germanium that is about 8 thousand times less abundant than iron.

The elements heavier than germanium are produced only in negligible amounts by fusion. They are produced mostly by neutron capture and sometimes by proton capture, and such events happen mostly during supernova explosions or neutron star collisions, because only then high concentrations of neutrons with high energies are present.

Neutron capture produces elements with Z until 100, i.e. until fermium (after that, spontaneous fission happens too fast, before beta-decay can raise the Z and enough extra neutrons can be captured to form a nucleus with long enough half-life). However the half-life of the heaviest elements decreases very quickly with Z, so the elements heavier than plutonium usually decay before reaching a stellar system from the explosion that has generated them. At its formation, it is likely that Earth also contained plutonium (244Pu has a half-life of over 80 million years, enough to survive an interstellar journey), but it has completely decayed until now, leaving uranium as the heaviest primordial element on Earth.