====== 4.3 Formation and evolution of planets ======
Planetary systems generally emerge from the gravitational collapse of massive molecular clouds, forming a central star within a rotating disk of gas and dust. Over time, microscopic grains collide and stick together, eventually building larger bodies through hierarchical accretion. Our own solar system's history serves as a primary example of this evolution, where thermal gradients and chemical transitions dictated the final placement of worlds. As shown in the provided figure, the journey from a chaotic disk to a stable system was governed by specific physical boundaries and timeframes that shaped the diverse environments we see today.

{{:bn:courses:ast100:planet-formation.webp?nolink|}}

The initial phase, occurring within the first 0.1 million years ($t < 0.1 \text{ My}$), established the chemical foundation of the solar system. The figure highlights the Protosun surrounded by a disk where temperature determined the state of matter. A sublimation line marks the specific distance where heat causes solids to turn directly into gas; for example, the silicate sublimation line at approximately $1100^{\circ}C$ represents the boundary interior to which rock cannot remain solid. Beyond this, the water ($H_2O$) and carbon monoxide ($CO$) snow-lines at $1.5 \text{ au}$ and $8 \text{ au}$ allowed ices to condense, providing the raw materials for planetesimal formation.

Between 0.5 and 1.0 million years ($t \sim 0.5-1.0 \text{ My}$), the disk split into two distinct chemical reservoirs separated by pressure gaps. These are explicitly labeled in the figure as non-carbonaceous (NC) and carbonaceous (CC) condrites. The NC region, located interior to the soot line in the terrestrial region ($\sim 0.7 \text{ au}$), consisted of rocky materials depleted in volatile carbon-rich compounds. In contrast, the CC region beyond the frost line ($\sim 4 \text{ au}$) was rich in carbon and water-ices. This isotopic segregation, reinforced by the soot and frost lines shown in the solar nebula inset, ensured that the inner rocky planets and outer gas giants would develop with fundamentally different chemical compositions.

The middle stages involved the rapid growth of massive planets in the outer "giant planets region." According to the diagram, while the inner planets were still small embryos, the abundance of ice beyond the $H_2O$ snow-line allowed large cores to form and capture vast amounts of nebular gas. The figure illustrates this outer reach extending through the Kuiper belt between $20$ and $45 \text{ au}$, where icy bodies remained relatively primitive. The gravity of these emerging giants significantly influenced the movement of smaller pebbles and gas, ultimately determining the final orbital architecture of the entire system before the nebula cleared.

During the $10-100 \text{ My}$ window, the solar system experienced a violent era of late-stage accretion and instability. The lower panel of the figure explicitly shows the growth of Earth and Mars from smaller rocky embryos located between $0.7$ and $1.5 \text{ au}$. This period was defined by high-energy collisions and the migration of giant planets across the disk. These movements were essential for the "seeding" of the inner system, as they scattered water-rich and organic-rich materials from the outer carbonaceous (CC) reservoirs into the dry, non-carbonaceous (NC) terrestrial region, providing the necessary ingredients for habitability on Earth.

By approximately 150 million years ($t \sim 150 \text{ My}$), the solar system reached its mature state with planets in stable orbits. As shown in the final stage of the diagram, the layout settled into a clear progression: a small planet (Mercury) at $0.3 \text{ au}$ (Mercury), Earth at $1 \text{ au}$, and the asteroid belt serving as a boundary between $1.8$ and $3.2 \text{ au}$. This stability marked the end of the heavy bombardment era and the clearing of remaining debris. The resulting architecture is the direct outcome of the thermal boundaries and gravitational interactions that played out over the first hundred million years of our cosmic history.