Formation of Our Solar System

Key Characteristics of the Solar System

• Orbital Direction:
  • All celestial bodies orbit the Sun in a counterclockwise direction (as viewed from above).
  • Comets can orbit in a clockwise direction occasionally.
• Ecliptic Plane:
  • Most planets orbit the Sun in the same plane known as the ecliptic.
  • Beyond Neptune, there are objects (dwarf planets, Kuiper Belt objects) with highly inclined orbits.

Solar System Formation Hypothesis

• Initial Cloud:
  • The solar system formed within a rotating disc of material around the protosun.
  • The disc underwent instabilities that caused fragmentation, leading to clumps that formed planets.
• Protoplanetary Discs:
  • Commonly seen in other star systems (at microwave wavelengths), exhibiting rings and spoke-like features.

Types of Planets

1. Giant Planets:
   • Composed mainly of lightweight materials (volatiles).
   • Examples: Hydrogen, helium, methane, frozen ammonia.
   • Relatively low density due to composition.
2. Terrestrial Planets:
   • Composed mostly of rocky or refractory materials (e.g., silicates, carbonates).
   • Found closer to the Sun and have higher density.

Frost Line

• The frost line marks the distance in the solar system where temperatures are low enough for volatiles to condense into solid forms.
• Inner Solar System:
  • Only refractory materials, leading to the formation of smaller, rocky terrestrial planets.
• Outer Solar System:
  • Temperatures allow for volatiles to condense, leading to the formation of gas and ice giants.

Planet Formation Process

Scale Transition

• Planet formation involves starting from a large circumstellar disc and zooming into the scale of individual dust grains.
• Dust in space differs from household dust:
  • Composed of silicates and chondrites.

Dust Accretion

• Dust particles are electrically charged, allowing them to clump together through gentle collisions.
• Dust can grow into larger rocks and eventually into planetesimals (about 1 km in size), which have enough mass to exert gravitational pull on one another.

Collisions and Accretion

• Planetesimals collide in a demolition derby-like process:
  • Most are destroyed and accreted onto larger ones.
  • The most massive survive to become proto-planets that clear their orbits.
• Modern asteroids and comets are leftover planetesimals from this process.

Formation of Gas Giants

• Gas giants form outside the frost line where abundant volatiles exist.
• They gain mass by colliding with additional planetesimals.
• An accretion disk forms around them, from which moons can also develop.

Atmosphere Formation

• Primary Atmospheres:
  • Formed by gas giant planets like Jupiter and Saturn from the materials present during their formation.
• Inner Planets:
  • Low mass and high temperatures make it difficult to retain primary atmospheres.
  • Stellar wind from the protosun blows lightweight gases away.
• Secondary Atmospheres:
  • Formed from volcanic activity and delivery of volatiles by comets and asteroids onto proto-Earth.
• Earth’s atmosphere today is considered a 2.0 version, shaped predominantly by volcanism and impacts.

Summary

• The solar system formed from a rotating disc of material where different compositions led to the formation of terrestrial and gas planets based on their distance from the Sun.
• Processes of dust accretion, planetesimal collision, and mass gain through impacts led to the current configuration of the solar system.