4.1 Solar system formation models

Our solar system's birth is a cosmic drama of collapse and creation. A molecular cloud fragments, forming the protosun and a swirling disk. From this disk, planets emerge through collisions and gravity, shaping the celestial neighborhood we call home.

The process unfolds over millions of years, with inner rocky worlds and outer gas giants taking shape. Evidence of this epic formation lies in planetary orbits, compositions, and remnant objects like asteroids and comets, preserving clues to our cosmic origins.

Solar System Formation

Stages of solar system formation

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• Gravitational collapse of a molecular cloud
  • Triggered by a shock wave from a nearby supernova or stellar wind, causing the cloud to compress and fragment
  • Cloud fragments into smaller, denser regions called clumps, which are the sites of future star and planet formation
• Formation of the protosun
  • Clumps with sufficient mass collapse under their own gravity, forming a central condensation
  • Central region becomes the protosun, a young, growing star surrounded by a swirling disk of gas and dust called the protoplanetary disk
• Accretion of planetesimals
  • Dust grains in the protoplanetary disk collide and stick together through electrostatic forces, forming larger particles
  • Particles continue to grow into kilometer-sized planetesimals through further collisions and gravitational interactions
• Formation of terrestrial planets
  • Planetesimals in the inner solar system, where temperatures are higher and volatile compounds cannot condense, collide and merge
  • Larger bodies, called protoplanets, form through the process of accretion, as planetesimals gravitationally attract one another
  • Protoplanets differentiate into layered structures, with dense metallic cores, rocky mantles, and thin crusts (Mercury, Venus, Earth, Mars)
• Formation of giant planets
  • Planetesimals in the cooler outer solar system accrete into solid cores of ice and rock
  • Cores that reach masses around 10 Earth masses can capture significant amounts of hydrogen and helium gas from the protoplanetary disk
  • Gas accretion leads to the formation of gas giants with massive atmospheres (Jupiter, Saturn) and ice giants with smaller gas envelopes (Uranus, Neptune)
• Dispersal of the solar nebula
  • Intense solar wind from the young Sun gradually clears away the remaining gas and dust in the protoplanetary disk
  • Planetesimals not incorporated into planets during the formation process become the building blocks of asteroids, comets, and Kuiper Belt objects

Core accretion vs disk instability

• Core accretion model
  • Planetesimals in the outer solar system accrete into solid cores composed of ice and rock
  • Cores with masses around 10 Earth masses can capture significant amounts of hydrogen and helium gas from the protoplanetary disk
  • Gas accretion leads to the formation of gas giants with massive atmospheres (Jupiter, Saturn)
  • Explains the presence of solid cores in giant planets, as inferred from their gravitational fields and interior models
  • Requires a relatively long formation timescale of several million years, as the core must grow large enough to accrete gas efficiently
• Disk instability model
  • Protoplanetary disk fragments directly into massive, self-gravitating clumps due to gravitational instabilities
  • Clumps cool and contract, forming giant planets with masses similar to those of Jupiter and Saturn
  • Can form giant planets more rapidly than the core accretion model, on timescales of a few thousand years
  • Does not necessarily result in the presence of solid cores, as the planets form directly from the gravitational collapse of gas
  • May have difficulty explaining the formation of less massive giant planets like Uranus and Neptune, which have smaller gas envelopes

Formation of terrestrial planets

• Gravitational collapse
  • Clumps within the protoplanetary disk collapse under their own gravity, increasing the local density of gas and dust
  • Leads to the formation of the protosun at the center and the concentration of material in the protoplanetary disk
• Accretion
  • Dust grains in the inner solar system collide and stick together through electrostatic forces, forming larger particles
  • Particles grow into kilometer-sized planetesimals through further collisions and gravitational interactions
  • Planetesimals collide and merge, forming larger protoplanets with masses similar to the Moon or Mars
  • Protoplanets continue to grow through impacts and gravitational interactions, eventually forming the terrestrial planets (Mercury, Venus, Earth, Mars)
• Differentiation
  • As protoplanets grow, they generate heat through the energy of accretion and the decay of radioactive elements
  • Heat causes the interior of the protoplanet to melt and separate into layers based on density
  • Denser materials, such as iron and nickel, sink to the center, forming the planet's core
  • Less dense materials, like silicates and oxides, rise to the surface, forming the planet's mantle and crust
  • Results in the layered structure observed in terrestrial planets, with metallic cores, rocky mantles, and thin crusts

Evidence for formation models

• Planetary orbits
  • Planets orbit the Sun in nearly circular, coplanar orbits, with low eccentricities and inclinations
  • Consistent with the formation of planets from a flattened protoplanetary disk that rotated around the protosun
• Planetary compositions
  • Terrestrial planets are composed primarily of rock and metal, with densities ranging from 3.9 to 5.5 g/cm³
  • Giant planets have substantial gaseous envelopes, with densities ranging from 0.7 to 1.6 g/cm³
  • Consistent with the temperature gradient in the protoplanetary disk and the condensation of different materials at varying distances from the protosun (metals and silicates in the inner solar system, ices in the outer solar system)
• Asteroids and meteorites
  • Asteroids are remnants of the early solar system that did not form planets, providing insight into the composition and conditions during planetary formation
  • Meteorites, fragments of asteroids that fall to Earth, provide direct samples of the materials present during solar system formation
  • Chondritic meteorites contain chondrules (small, spherical grains formed by rapid heating and cooling) and calcium-aluminum-rich inclusions (CAIs), which are among the oldest known solar system materials, dating back to 4.567 billion years ago
• Kuiper Belt and Oort Cloud
  • The Kuiper Belt is a region beyond the orbit of Neptune containing numerous icy bodies, such as dwarf planets (Pluto, Eris) and comets
  • The Oort Cloud is a spherical shell of icy objects at the edge of the solar system, extending up to a light-year from the Sun
  • These regions are thought to be reservoirs of material left over from solar system formation, containing pristine samples of the original protoplanetary disk
• Exoplanetary systems
  • Observations of other planetary systems reveal a diversity of architectures, suggesting that the specific outcome of planetary formation depends on the initial conditions in the protoplanetary disk
  • Some systems have giant planets orbiting close to their stars (hot Jupiters), indicating that planetary migration can occur due to interactions with the protoplanetary disk or other planets
  • The variety of exoplanetary systems, including systems with super-Earths and mini-Neptunes, demonstrates that the solar system is just one possible outcome of the planetary formation process