Through the combination of the [[Nebular Hypothesis]] and the [[Core-Accretion Theory]], we can develop the **Standard Model of Planetary Formation**. It is primarily based from the observations of our own Solar System, and we are finding through observations of exoplanets that this model is not universal. > [! key-idea] > The composition of planets is mitigated by the stellar winds after thermonuclear ignition of the core of a collapsing nebular cloud. Due to the heat and pressure of the stellar winds, lighter elements and compounds are forced to the outer regions of the disk, leaving the denser, heavier material closer star. This leads to the composition of the planets being in their configuration today (rocky in the inside, gas on the outside). Then through migration, planets settle into stable orbits and other features of the solar system develop. Watch this YouTube Video for a Good Overview: [The Birth of the Solar System](https://www.youtube.com/watch?v=d-z0eQOEzkE&ab_channel=SEA) <iframe width="560" height="315" src="https://www.youtube.com/embed/d-z0eQOEzkE?si=LXcz2W4sqp8E-dwZ" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe> ## Step 1 - Contraction If you take some rotating fraction of gas from a [[Nebulae|nebula]] or [[Interstellar Medium#Molecular Clouds|giant molecular cloud]], and allow it to undergo gravitational collapse, the interstellar material will begin to construct a flat, spinning disk through the conservation of angular momentum. The decreasing radius of the cloud causes an increase in rotational velocity, leading to the formation of a dense, hot [[Stellar Classes#Protostar|protostar]] and an proto-planetary accretion disk. Due to the increasing density in the [[Stellar Classes#Protostar|protostar]], there is an increase of atomic interactions converting kinetic energy into heat. This leads to protostellar wind (similar to a stellar wind) that creates a temperature gradient across the accretion disk, such that the inner region is hotter than the outer regions. ## Step 2 - Ignition Once the pressure and temperature is high enough in the [[Stellar Classes#Protostar|protostar]], it will undergo thermonuclear ignition/fusion and a star is born. After it does, the stellar winds will proceed to push much of the disk materials away from the star and will begin clearing the disk. All the while, the material in the disk is undergoing the clumping process. > [!space] In the Solar System... > This material accumulation in the core will continue until $\approx 99.8\%$ of matter from the collapsing cloud has gathered in the center of the disk. ## Step 3 - Clumping ### Planets The material in the accretion disk will continue orbiting along the stellar equator. As the disk cools, the electrostatic forces causes the grains of rock, dust, and ice to begin sticking together to form loose conglomerates that eventually grow into kilometre-scale bodies known as [[Smaller Non-Planetary Bodies#Planetesimals|planetesimals]] ($\sim 10 \; {\rm km}$). At this point, gravity takes over and the planetesimals collide, fragment, mash together and grow into full-sized planets. The friction with the surrounding gas forces this planets into almost circular orbits. The types of [[Smaller Non-Planetary Bodies#Planetesimals|planetesimals]] that form determined by the composition of material available (and therefore the temperature) at that location within the disk. > [!note] The "Ice Line" > Every element has an **ice line** (also known as a **frost line** or a **snow line**) in an accretion disk. It represents the radial position ($R_{\rm ice}$) at which [[Interstellar Medium#Ice|volatiles]] are able to condense into solid grains, allowing them to accrete onto planetesimals. > > ![[ice_line.png]] > > *(The term "ice" is not purely a reference to $\rm{H_{2}O}$ in astrophysics, but instead, it refers to any solidified volatile chemical compounds)* ^ice-line - Towards the centre of the disk where the protostellar wind/stellar wind is strong and the temperature is high ($R < R_{\rm ice}$), only elements and compounds with high melting points can survive the heat (iron, rock, etc.). This material accumulates and clumps together into dense rocky, [[Smaller Non-Planetary Bodies#Planetesimals|planetesimals]] that later form terrestrial planets. - Inner Planetary System :: [[Planetary Classes#Rocky Planets]] (Terrestrial Planets) - Far away from the center of the disk where the it is cool enough ($R > R_{\rm ice}$), ice compounds are able to condense and accrete onto planetesimals. Since there is significantly more material beyond the [[Standard Model of Planetary Formation#^ice-line|ice line]], planetesimals can grow much larger. When they clump together and grow large enough that gravity can begin pulling the surrounding gas (hydrogen and helium), they develop a **gas envelope** and only stop growing when they have cleared the gas from thier orbits. In the core of the planet, it is mostly composed of a rocky-icy mix, but as the pressure grows from the gas envelope, the core condenses into being primarily rock. - Outer Planetary System :: [[Planetary Classes#Ice Giants]] & [[Planetary Classes#Gas Giants]] *Note: The stellar wind prevents inner planets from maintaining gas envelopes early in their development.* > [!note] The "Soot Line" > People also talk about a **soot line** relating to the sublimation of carbon, but the research and resources isn't great. ### Moons Planetary moons are developed from each [[Smaller Non-Planetary Bodies#Planetesimals|planetesimal]] forming its own accretion disk. ## Step 4 - Migration *(See [[Planetary Migration]] for the Types of Migration)* After the major planetary bodies are formed, they spend the next significant period of time migrating from their initial position of formation to the most stable orbit it can achieve in the planetary system. This doesn't necessarily occur before the disk is cleared of gas, dust, or [[smaller non-planetary bodies]], and in many ways, the gas and these smaller bodies are the primary contributor to planetary migration. During these migration processes, much of the material in the disk is either ejected from the system, craters into the planetary bodies, or is trapped by resonance capture. > [!space] In the Solar System... > > These processes are theorized to directly lead to the development of the [[#Asteroid Belt]], [[#Kuiper Belt]], and the [[#Oort Cloud]]. --- ## References: - [Wikipedia - Formation and Evolution of the Solar System](https://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System) - [Simulating the Solar System Formation](https://www.youtube.com/watch?v=yXq1i3HlumA&ab_channel=CaliforniaAcademyofSciences) - [Wikipeda - Frost Line](https://en.wikipedia.org/wiki/Frost_line_(astrophysics)) - [CITA - Planets in Chaos](https://www.cita.utoronto.ca/wp-content/uploads/2014/07/511022a.compressed.pdf) - [The Birth of the Solar System](https://www.youtube.com/watch?v=d-z0eQOEzkE&ab_channel=SEA)