Formation of the Solar System

Nebular-Condensation theory

• Our solar system formed about 4.6 billion years ago.
• Nebular Theory: Our solar system evolved from a contracting nebula.
  • Under the influence of its own gravity, the nebula contracts. As it contracts, it spins faster and faster, much like an ice skater who pulls in her arms. This effect is known as conservation of angular momentum. As a result of the spinning, the nebula flattens out to form a disk, much like a clump of dough which is spun into a pizza.
  • As it continues to flatten and spin, the nebula becomes denser and hotter, particularly in the center. The dense, hot center eventually forms into a star, the Sun.
  • The swirling mass destined to become our solar system is usually referred to as the solar nebula.
• Condensation Theory: An extension of Nebular Theory that incorporates interstellar dust as a key ingredient.
  • The solar nebula is initially a thin gas of hydrogen and helium strewn with tiny dust particles. These tiny grains serve as the building blocks of the planets.
  • Dust grains play an important role in the evolution of any gas cloud. These grains act as condensation nuclei, on which accretion of matter occurs.
  • Analogy#1: Snowball moving through a fierce snowstorm grows bigger as it encounters more snowflakes. Analogy#2: Raindrops form in Earth's atmosphere, as dust and soot in the air act as condensation nuclei around which water molecules cluster.
  • Clumps of matter form around the condensation nuclei. These clumps collide, stick together, and grow into moon-sized planetesimals. Planetesimals coalesce into a few protoplanets.
  • High-speed collisions also result in fragmentation, with the creation of asteroids and comets.
  • Protosun and 9 protoplanets developed after 100 million years.
• Interplanetary debris swept away after a billion years through:
  • Collisions with planets
  • Gravity-assisted ejection
  • Radiation pressure
  • Solar wind
• Evidence for Condensation Theory includes:
  • Observations of circumstellar disks within various nebulae.
    • This captivating video zooms in on the consteallation Taurus to reveal a very young star, HL Tauri, surrounded by protoplanetary disk.
  • Stars containing larger fractions of "dusty" elements, such as carbon and silicon, are more likely to have planets.
  • Computer simulations demonstrating that many orbiting particles can, after multiple collisions, form a few stable orbits.
  • This video tutorial does a nice job demonstrating all this visually.

Composition

• The ingredients of the solar nebula fell into 4 categories based on their condensation temperatures: metals, rocks, hydrogen compounds (water, methane, ammonia), and light gasses (hydrogen and helium).
• Distance of a planet from the Sun determined the temperature and thus the materials which condensed first.
  • Metals condensed first, then rocks, then the lighter compounds, then the light gasses.
  • The dense Terrestrial planets formed closer to the Sun because only the heavy elements condensed in this relatively hot region.
  • Farther from the Sun, the Jovian planets formed first by the condensation of a dense core and then by a thick outer layer of light gasses.
  • This condensation-based separation of heavy and light elements explains why the terrestrial planets are enriched in the heavier elements compared to the Universe as a whole.
• The asteroid belt region coincided with the frost line. Before the frost line, rocks and metal condense; hydrogen compounds stay vaporized. After the frost line, hydrogen compounds also condense.
• Our Moon most probably formed through a collision with a Mars-sized object.

Moons

• Most moons in our solar system probably formed together with their host planet (co-accretion).
• Some of the smaller moons, particularly those with irregular orbits ('wrong plane and/or direction), may have been captured.
• Our Moon probably resulted from a massive collision.
• Triton, Neptune's largest moon, is the only large moon with a retrograde orbit. It was probably captured from the Kuiper belt.