Overview
This lecture explains the origin of the solar system, focusing on the formation of the Sun, planets, and the materials involved, based on observational evidence and theoretical models.
Features and Composition of the Solar System
- Images of young stars with gas and dust discs suggest our Sun once had a similar disc from which planets formed.
- Terrestrial planets are small due to the rarity of heavy elements, while Jovian planets and the Sun consist mainly of abundant hydrogen and helium.
- Hydrogen and helium make up about 98% of the mass in the solar system; all other elements combined account for only about 2%.
Formation of Elements and the Solar System
- Only hydrogen, helium, and small amounts of lithium and beryllium formed during the Big Bang (~13.7 billion years ago).
- Heavier elements are produced inside stars by fusion or during supernova explosions and returned to the interstellar medium.
- New stars and planetary systems, including our solar system, form from this recycled, enriched material.
Abundance and Rarity of Elements
- Elements like oxygen, carbon, silicon, and iron are produced in massive stars, while gold, silver, platinum, and uranium are rarer and formed under special conditions.
- Oxygen is abundant but much less than hydrogen and helium; gold is extremely rare in the universe.
Radioactive Dating and Age of the Solar System
- Radioactive dating measures the age of rocks by comparing the ratio of radioactive isotopes to their decay products.
- The age of a rock is the time since its atoms became fixed in their current arrangement.
- Oldest Earth rocks are about 4 billion years old; oldest moon rocks are ~4.4 billion years; meteorites provide an age of 4.54 billion years for the solar system.
The Nebular Theory of Solar System Formation
- The nebular theory proposes the solar system formed from the gravitational collapse of an interstellar cloud called the solar nebula.
- A shock wave, possibly from a supernova, likely triggered the collapse of the nebula.
- Most matter concentrated at the center formed the proto-Sun; remaining material formed a rotating, flattening disc (protoplanetary disc).
- Conservation of angular momentum caused the disc to spin faster and flatten as it contracted.
- The proto-Sun's core heated up, eventually igniting nuclear fusion and forming the Sun.
- Observational evidence (e.g., Hubble images, Orion Nebula) confirms the existence of protoplanetary discs today.
Key Terms & Definitions
- Solar Nebula — The cloud of gas and dust from which the solar system formed.
- Protoplanetary Disc — A rotating disc of gas and dust surrounding a young star, where planets form.
- Radioactive Decay — The process in which an unstable atomic nucleus loses energy by emitting particles.
- Radioactive Dating — Technique to determine the age of rocks using the decay rates of radioactive isotopes.
- Conservation of Angular Momentum — Principle stating that a spinning object's rotation speed increases as its size decreases if no external torque acts.
- Supernova — A powerful stellar explosion enriching the interstellar medium with heavy elements.
Action Items / Next Steps
- Review concepts of element formation, nebular theory, and radioactive dating.
- Prepare for the next lecture on the detailed process of planet formation from the protoplanetary disc.