Level 6: Globes — 5 \(\text{billion}\) years
Macromolecules
Size: \(1 \text{nm}\)
Initial state: Heavy molecules
Production process: Water-based chemistry
Meta-stable equilibrium: Macromolecules
Stable future: Organic monomers
Planetary systems
Size: \(50 \text{AU}\)
Initial state: Galaxies
Production process: Dust accretion
Meta-stable equilibrium: Planetary system
Stable future: Post-chaotic system of geologically inert planets
The emergent concept at Level 6 is that of Globes. Perhaps an expected heading, since we will ostensibly introduce planets here. But as previous concepts, this doesn’t refer to a physical object. Before humans had a sense of what the earth really was in its physical characteristics, its insides or its composition, there was a clear notion that this thing under ouf feet was "the world". Note that I’ve refrained from using the term "world" for this emergent concept, since I have used it in very foundational contexts around our metaphysics sections; I want to avoid any semantic uncertainty, so I may use "world" liberally after that caveat, but will stick to "globes" for the official heading, even though the spherical shape is not exactly the key point here.
Conceptually, a galaxy is a system and a "cocoon" in many ways; it then matches our definition of a mesosphere. But it is not a world. In a sense, a "world", or globe, is another (albeit symbolic) compactification. To small enough beings, it provides a board for the game of life that could seem infinite in its two-dimensional extension, with only a slice of the third dimension available. That illusion is enabled by the presence of a solid layer of materials to serve as a ground.
It might be anthropocentric, or at least geocentric, that we’re insisting on this feature as a necessary stepping stone to what sentient beings can consider conceptually as "a world". It sure is, and everything will be from that point on. It is a very solid hypothesis that physico-chemical structures as complex as living beings could not emerge outside of a planetary environment, but by no means one that I could prove.
It seems that the only objects that could fit the bill are the ones in planetary systems. Dust grains are solid, but too small to act a "solid ground" for anything, since even a hefty macromolecule could "circumnavigate" it in a moment. Most stellar objects and media are not solid: they can be gaseous (clouds, brown dwarfs...), plasma (stars) or degenerate matter (white and black dwarfs, neutron stars, exotic stars...).
The concept of globe is then associated here, on the small scale, to the kind of materials that make up such solids, and on the large scales, to a planetary system as a whole: a system that includes a collection of globes. From where I stand, this relates more specifically to our Solar system, which indeed has been understood as a system of worlds for quite some time. In ancient Greece already, cosmological pluralists assume there were infinite worlds, a thesis whose statement by Anaxarchus supposedly reduced Alexander the Great to tears. But it’s only Galileo’s telescope that helped confirm without the shadow of a doubt that planets, up close, looked similar to our moon, and then probably to our earth. The moon, of course, much more visible, offered a prime example of an alternate world on which, perhaps, fantastic creatures roamed a strange land...
But let’s get back to it. The one factor that led our universe to harbor trillions of planets is the continued chemical enrichment stellar activity. Primordial stars were just surrounded by an ambient gas, similar to their native molecular cloud environment or to the interstellar medium (depending on their age). However, like our sun, they must have generated a heliosphere: that is the volume swept by stellar wind (plasma ejecta from the star). The heliosphere is likely shaped like the tail of a comet, since most stars are rotating around the center of their galaxies. The space within the heliosphere is already like a cocoon, preserved from the bulk of cosmic rays by the mechanical and magnetic effects of the stellar ejecta.
That space is where the future planetary system will be nested. But we’re still far from that state. We’ll place ourselves at the very start of a protostar’s life, when a molecular cloud starts its collapse. This has been happening continuously since the cosmic dawn, but the material composition of that cloud that would lead to planetary genesis has only been generalized since two billion years after the Big Bang. Our solar system, in comparison, was born 9.1 billion years after the Big Bang.
In what follows, for legilibity, we will periodize the different epochs in terms of the Solar Era (SE), starting from the sun’s birth, when an undistinguished volume from a molecular cloud starts to densify.
The nebular epoch : \(0\) to \(1\text{M}\) years SE
The transition from nebula to protostar to star has been discussed very quickly before. That is because in a largely non-metallic star, that stage is not extremely consequential. Now, the presence of elements heavier than helium and of cosmic dust is bound to change the dynamics at play. Every nebula starts with a certain angular momentum, which is due to density gradients and disturbances from the rest of the cloud. That angular momentum is conserved, so around the protostar, a protoplanetary disk will form, that gets faster and thinner over time. It represents about 1% of the protostar’s mass.
Initially, neither the protostar nor the disk would be visible, since they are surrounded by a wide envelope, one to ten thousand AU (Astronomical Units, the average sun-earth distance), which continuously discharge failling gas onto the center. The disk’s diameter at that point is a few hundreds AU. The envelope only becomes transparent after \(10^5\) years.
The disk epoch : \(1\) to \(10\text{M}\) years SE
Around the million-year mark, the envelope is cleared out and there’s an "empty" area surrounding the protostar, which then enters its "Tauri" phase. There’s still no nuclear burning, but a violent collapse leading to continous bipolar outflows perpendicular to the disk.
Meanwhile, in the disk, the accretion of dust starts. That process will ultimately give birth to the planets, but from that simple assertion it’s important to acknowledge the order of magnitude: sub-micron dust particle have gathered to constitute the earth with its \(6,000\text{km}\) diameter, a \(10^12\) factor.
The spinning disk is a fertile environment for this accretion. First of all, the physics favor a differentiation by mass whereby the biggest dust grain progressively migrate to the midplane of the disk. Huddled together, the grains are pushed against each other in frequent collisions by the thermal agitation and the variability in the disk’s spinning. At the same time, the fact they’re all spinning at almost the same speed means that the intensity of the shock stays moderate and rarely lead to the solid particles being crushed.
Initially, the rocky or icy dust grains aggregate atoms from the ambient gas one at a time via electrostatic forces. When the resulting pebbles are large enough, they start the accretion process and stick to each other. An object is called a planetesimal when it reaches a diameter around \(1\text{km}\). That is crucial threshold, as the moment when the object’s gravitational pull starts to be the dominant effect in attracting and holding on to smaller pebbles. As it keeps growing, the effect of gravitation increases, and the impact of aerodynamic forces to diminish.
By the time a planetesimal is \(100\text{km}\) wide, it is totally insensitive to aerodynamic forces and is able to move freely within the disk under the sole influence of gravitation and mechanical constraints. We now call it a protoplanet. Since protoplanets are not part of the aerodynamical flow anymore, they tend to be faster than the disk itself, and tend to migrate closer to the star.
The protoplanetary epoch : \(1\) to \(10\text{M}\) years SE
An important radial border within the protoplanetary disk is the "snow line". Beyond that line, the disk is far enough from the star that CNO ices (methane, ammonia and water) can coalesce into icy dust, whilst within the "inner system", it stays diffuse as a part of the cloudy disk. Beyond the snow line, we also find silicates and metals, although in relatively low concentrations, since those tend to gravitate closer to the star. Be that as it may, most of the non-star mass of the system is in that area. Currently, the mass beyond the snow line is equivalent to \(350M_oplus\) (earth masses), versus only \(2M_oplus\) within.
The growth of protoplanets continues via oligarchic growth, meaning that a subset of large objects are the only one growing, and start to clear up their surroundings by absorbing shocks from planetesimals and smaller protoplanets, or by ejecting them from the orbit. When a critical mass of solid metal, rocks and ices of \(10 M_\oplus\) is attained, the object is officially a giant planet, large enough to attract surrounding gas (hydrogen and helium). The giant planets in our solar system are Jupiter, Saturn, Uranus and Neptune. A prominent hypothesis at the moment is that they formed on the same orbit, but then migrated according to their mass differential.
Five to ten million years after the protostar first appeared, the Tauri phase ended and the stellar object at the center of the system became a proper star, entering its main sequence of hydrogen burning. But before that, the strong solar winds characteristic of the Tauri phase had fully cleared out the disk from gas, debris and dust. Due to their mass, the giant planets held on to their gas (part of which had been compressed into a mantle of liquid and solid hydrogen/helium, surrounding the ice/rock/metal core). They also kept a ring of dust and pebbles, although it’s unclear if that’s a universal feature of giant planets.
As to the rest of the planetesimal-sized objects, they were cleared out into the asteroid belt that marks the border of the inner solar system (between Mars an Jupiter, 1.5 to 5 AU from the sun), into the Kuiper belt (beyond Neptune, 30 to 50 AU from the sun), or into the Oort cloud (a collection of icy, up-to-planetesimal-sized objects that have been thrown out by giant planets onto very eccentric orbits, way beyond the heliosphere, thousands to hundred thousands AU away, well into interstellar space when they approach the sun, we call them comets).
The planetary epoch : \(10\) to \(100\text{M}\) years SE
The formation of terrestrial (or "rocky" planets) follows a similar path. Icy grains are absent within the snow line, so the protoplanets in that area are entirely made of metals and rocks. The timing and nature of the formation process is still a subject of active research. It is generally assumed, at the time of writing, that rocky planet formation took much longer than giant planet formation, up to 100 million years, although the full extent is uncertain.
One frequent idea is that the inner solar system underwent an additional "chaotic phase", when it ended up hosting about 100 protoplanets of moon-to-mars size. Because of the more constrained space (at least 100 times smaller than the comparable band in the outer solar system), collisions are much more frequent, leading to catastrophic events, like the complete shattering of objects, potentially followed by re-assembly. It is likely that the already-formed giant planets, as well as the asteroid belt that their orbital "cleanup" had nourished around the frost line, were a major influence on rocky planet formation.
At the end of the process, we end up with the four planets known today: Mercury, Venus, the Earth and Mars. Given their physical features and they orbital dynamics, the end stages of their formation may have been very dramatic, for instance with a major protoplanet collision creating what is today the earth-moon system from two distinct objects of a similar size.
The Hadean epoch : \(100\) to \(500\text{M}\) years SE
When a rocky planet is formed, the Hadean ("hellish") epoch starts. Although a planet can be geologically active for billions of years, the features that we think of as constant are not in place before the Hadean ends. We could still talk of a planetary embryo at this stage.
Most of what follows is grounded in the young planet’s thermal conditions. Earth today has three sources of energy:
One external source: solar energy, the radiant light and heat from the sun.
Two internal sources:
Radioactive heat, from the decay of radioactive isotopes.
Primordial heat, converted from incoming meteorites’ kinetic energy.
Today, the earth’s internal energy is tiny compared to its solar energy (the ratio is around 0.03%). It was always the case, although internal sources used to be much more powerful. Radioactive heat has undergone a predictable decline since the density of radionuclides follows a negative exponential law (this doesn’t account for atmospheric isotopes that are constantly replenished by cosmic rays, but only represent a negligible part of the earth’s overall natural radioactivity). Primordial heat’s used to be much higher than today during the earth’s embryo phase remember that to clean up its surroundings, a rocky planet needs to sustain impacts scores of planetesimals and perhaps dozens of protoplanets but then fell off a cliff due to the changes we’ll describe. Since then it has followed a regular decline, as the heat dissipates.
With a rocky planet being so hot in that phase, it’s not so much like a rock as like a big ball of molten lava. Materials inside are then free to move around, and under gravity’s influence, settle into layers based on their density. This is called the differentiation phase, and has given rise to the following structure for earth, which is thought to be fairly representative of rocky planets:
The inner core is a solid ball of mostly iron (some have suggested it was a pure, single iron crystal).
The outer core is a fluid made of iron and nickel (with fairly low viscosity, due to its high temperature).
The mantle is a solid layer of silicate rocks, although on geological timescales, it behaves like a viscous fluid. Silicates are salts that associate silicon dioxide (or silica: \(\text{SiO\_2}\)) with metallic oxides. Silica represents about 45% of the mantle mass, with magnesium, ferrous and aluminium oxides respectively at 39%, 8% and 4%.
The lower mantle is made of post-perovskite (closest to the core) and bridgmanite.
The transition zone is made of ringwoodite and wadsleyite.
The upper mantle is mostly olivine and pyroxenes.
The crust is the brittle, rocky, outermost layer of the earth.
Differentiation in itself leads to heat transfer that contribute a lot to cooling. It also initiates a number of key processes.
First, the existence of the magnetosphere is grounded in that of the outer core. Remember that the outer core ends up being a liquid, surrounded by two solid layers, one hotter (the inner core), one cooler (the mantle). This generates convection currents: the cooler, denser material sinks, while the warmer, less dense material rises. In addition the rotation of the earth plays a significant role in having these currents twist and loop in a specific fashion. Since the outer core is metallic, its electrons are free and shared between atoms, so the currents induce a magnetic field. That field ends up acting as a shield for the planet, deflecting most of the sun’s radiation.
Convection also started in the mantle, for similar reasons. Because of its temperature at the time, viscosity was much lower than today, leading to much stronger convection current. Progressively, minerals with the highest melting-point temperatures made their way towards the surface, and enough heat dissipated for those to crystallize and solidify, forming the crust. However, they were still unstable slabs, soon to melt all over again, when magma (molten rock) finds its way from deeper in the mantle to the surface.
All of this activity also gave rise to the earth’s atmosphere. Like other sizable rocky planets, the earth had a first atmosphere of dihydrogen and helium captured from the protoplanetary disk, but as we’ve seen, that was blown away by solar winds. Earth’s second atmosphere comes from the inside: the constant magma cooling and rock melting leads to a continuous emission of vapor and steam, that builds up to a proper atmospheric layer. Why wasn’t that one blown away? First of all, solar winds were not as powerful as during the Tauri phase; but more crucially, the magnetic shield around the earth offers such a degree of protection that the atmosphere has been very resilient to solar radiation. It is still being peeled off, but at a vaninshingly low pace. The early atmosphere had a different composition from the modern one, with a 89/10 split between nitrogen and carbon dioxide (today it is 78/21 between nitrogen and oxygen).
Lastly, it is possible that the Hadean earth has water oceans already. Indeed, the temperature was much higher, but pressure was too (around 27 times today’s pressure): those are conditions under which water can exist in a liquid state. That water may have come from internal sources (dissipation from the mantle), or from external ones (ice deposits from countless comet impacts during that era).