Our solar system's formation is a cosmic tale of gravity, motion, and composition. It all started with a rotating disk of gas and dust that collapsed, giving birth to the Sun and planets. The inner planets formed rocky and small, while the outer giants grew massive and gaseous.

The story doesn't end there. After formation, our cosmic neighborhood experienced a tumultuous period called the Late Heavy Bombardment. Giant planets migrated, stirring up asteroids and comets. These events shaped the solar system we see today, leaving clues in the form of craters and planetary compositions.

Key Constraints and the Solar Nebula

Constraints on solar system formation

• Motion
  • Orbits nearly circular and in the same plane indicate a common origin from a rotating disk of gas and dust
    • Planets orbit the Sun in the same direction (prograde) suggests they formed from material orbiting in the same direction
    • Most planetary orbits are nearly circular (low eccentricity) and in the same plane (low inclination) implies a flattened disk structure
  • Rotation in the same direction as orbits supports formation from a rotating disk
    • The Sun rotates in the same direction as the planetary orbits (prograde) indicates a shared angular momentum from the solar nebula
    • Most planets rotate in the same direction as they orbit (prograde) with the exception of Venus and Uranus which may have undergone collisions or other events
• Composition
  • Inner planets (terrestrial) are rocky and small compared to outer planets (gas giants) which are gaseous and large
    • Rocky composition of inner planets (metals and silicates) suggests they formed in a higher temperature region where volatiles could not condense
    • Relatively small size and mass of terrestrial planets compared to gas giants indicates a smaller supply of material in the inner solar system
  • Outer planets (gas giants) are gaseous and similar in composition to the Sun
    • Gaseous composition of outer planets (hydrogen and helium) matches the composition of the Sun and suggests they formed from the same material
    • Relatively large size and mass of gas giants implies a greater abundance of material in the outer solar system
  • Solar composition is mostly hydrogen and helium like the gas giants
    • Sun is composed primarily of hydrogen and helium which matches the composition of the gas giants and supports a common origin
    • Similar composition between the Sun and gas giants indicates they formed from the same source material in the solar nebula
• Age
  • Radiometric dating of meteorites provides a lower age limit for the solar system
    • Meteorites are the oldest known solid objects in the solar system with ages around 4.6 billion years from radiometric dating
    • Age of meteorites provides a lower limit on the age of the solar system since they formed early in its history
  • Solar system formation occurred relatively quickly based on the age of meteorites and the Sun
    • Quick formation within ~100 million years is inferred from the ancient age of meteorites at 4.6 billion years
    • Rapid formation is also supported by models of star formation and the estimated age of the Sun

Changes in solar nebula

• Gravitational collapse of the initial cloud of gas and dust (solar nebula) formed a flattened rotating disk
  • Nebula began to collapse under its own gravity which increased its density and triggered further collapse
  • Rotation of the nebula caused it to flatten into a disk shape (protoplanetary disk) due to conservation of angular momentum
• Heating and cooling of the nebula occurred as it collapsed and the Sun formed
  • Gravitational energy was converted to heat as the nebula collapsed causing the temperature to increase
  • Center of the nebula became hot enough for nuclear fusion to begin which marked the birth of the Sun
  • Outer regions of the disk cooled allowing solid particles to condense from the gas
• Particle growth proceeded as solid particles collided and stuck together
  • Solid particles in the disk collided and formed larger particles through electrostatic forces and gravitational attraction
  • Continued growth of particles led to the formation of planetesimals which are the building blocks of planets
• Clearing of the disk occurred as planets formed and removed gas and dust
  • Newly formed planets gravitationally interacted with the remaining gas and dust in the disk
  • Gas was either accreted onto the planets or expelled from the system by solar radiation and winds
  • Dust was incorporated into planets or removed from the system leaving behind the planets and other objects

Planet Formation and Evolution

Terrestrial vs giant planet formation

• Terrestrial planet formation occurred in the inner solar system where high temperatures prevented volatile condensation
  • Inner solar system had higher temperatures that only allowed metals and silicates to condense into solid particles
  • Lack of volatile compounds (water, methane) in the inner solar system limited the size of terrestrial planets
  • Solid particles collided and merged to form planetesimals which continued to grow through accretion
• Giant planet formation occurred in the outer solar system where volatiles could condense
  • Lower temperatures in the outer solar system allowed volatile compounds to condense in addition to metals and silicates
  • Abundance of icy particles and planetesimals provided more material for planet growth
  • Core accretion model suggests giant planets formed in two stages:
    1. Icy planetesimals collided to form solid cores of about 10 Earth masses
    2. Cores then rapidly accreted gas from the surrounding disk to form massive atmospheres
  • Gravitational instability model proposes an alternative formation mechanism for gas giants
• Differences in composition reflect the formation conditions in the inner and outer solar system
  • Terrestrial planets are composed mainly of metals and silicates due to high temperature formation
  • Giant planets have a gaseous composition (hydrogen and helium) similar to the Sun with presumed rocky/icy cores

Post-formation solar system events

• Late Heavy Bombardment (LHB) was a period of intense asteroid and comet impacts on the terrestrial planets
  • LHB occurred approximately 4.1 to 3.8 billion years ago based on the dating of lunar impact basins
  • Intense bombardment may have been caused by the migration of the giant planets which disrupted asteroid and comet orbits
  • LHB had significant effects on the terrestrial planets including crater formation and delivery of water and organic molecules to Earth
• Giant planet migration is thought to have occurred after their formation
  • Giant planets likely formed closer to the Sun and then migrated outward to their current positions
  • Migration was driven by gravitational interactions with the remaining gas and dust in the protoplanetary disk
  • Outward migration of Jupiter and Saturn may have triggered the LHB by disturbing the orbits of asteroids and comets
• Formation of the Oort Cloud and Kuiper Belt from scattered remnants of the protoplanetary disk
  • Oort Cloud is a spherical region of icy objects (comets) at the outer edge of the solar system up to a light-year from the Sun
  • Kuiper Belt is a disk-shaped region of icy objects (dwarf planets, comets) beyond the orbit of Neptune
  • Both regions formed from icy planetesimals that were gravitationally scattered outward by the giant planets
• Planetary differentiation into distinct layers (core, mantle, crust) driven by internal heat
  • Differentiation is the separation of a planet's interior into layers based on density
  • Heat from radioactive decay and kinetic energy of accretion drove differentiation
  • Denser materials (metals) sank to the center to form the core while lighter materials (silicates) rose to form the mantle and crust
  • Differentiation led to the formation of Earth's magnetic field and onset of plate tectonics

Solar System Formation Theory

• The nebular hypothesis provides a comprehensive framework for understanding solar system formation
• Angular momentum conservation explains the flattening of the solar nebula into a disk and the prograde rotation of most planets
• The condensation sequence describes how different materials solidified at varying distances from the Sun, influencing planet composition
• Planetary migration played a crucial role in shaping the final architecture of the solar system