Solar System Formation, Lecture notes of Astrophysics

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Solar System Formation

Question: How did our solar system and other planetary systems form?

“Comparative planetology” has helped us understand

• Compare the differences and similarities among the objects in our solar system
• Figure out what physical processes could have led to them

• Then construct a model of how our solar system formed based on this

• This model must explain our own solar system…
• …but might or might not explain other planetary systems
• If not, modify the model to accommodate discrepancies

• In other words, carry out the scientific process

• Let’s look at the solar system characteristics comparative planetology has to work with… Solar System Formation

1. Large bodies have orderly motions and are isolated from each other
   • All planets and most moons have nearly circular orbits going in the same direction in nearly the same plane
   • The Sun and most of the planets rotate in this same direction as well Solar System Formation -- Characteristics of Our Solar System

1. Large bodies have orderly motions and are isolated from each other
   • All planets and most moons have nearly circular orbits going in the same direction in nearly the same plane
   • The Sun and most of the planets rotate in this same direction as well
   • And most moons orbit their planet in the direction it rotates Solar System Formation -- Characteristics of Our Solar System

2. Planets fall into two main categories Solar System Formation -- Characteristics of Our Solar System

3. Swarms of asteroids and comets populate the solar system
   • Asteroids are concentrated in the asteroid belt Solar System Formation -- Characteristics of Our Solar System

4. Several notable exceptions to these general trends stand out
   • Planets with unusual axis tilts
   • Surprisingly large moons
   • Moons with unusual orbits Solar System Formation -- Characteristics of Our Solar System

1. Large bodies in the solar system have orderly motions and are isolated from each other - All planets and most moons have nearly circular orbits going in the same direction in nearly the same plane - The Sun and most of the planets rotate in this same direction as well - Most moons orbit their planet in the direction it rotates
2. Planets fall into two main categories
   • Small, rocky terrestrial planets near the Sun
   • Large, hydrogen-rich jovian planets farther out
     • The jovian planets have many moons and rings of rock and ice
3. Swarms of asteroids and comets populate the solar system
   • Asteroids are concentrated in the asteroid belt
   • Comets populate the regions known as the Kuiper belt and the Oort cloud
4. Several notable exceptions to these general trends stand out
   • Planets with unusual axis tilts
   • Surprisingly large moons
   • Moons with unusual orbits any successful theory of solar system formation must account for these Summary of Characteristics of Our Solar System

• It starts with cold interstellar clouds of gas and dust
• These clouds are mostly hydrogen and helium from the Big Bang
• But they contain heavier elements that were not formed in the Big Bang
• Astronomers call these “metals” (even though they’re not necessarily metallic elements)
• Where did these heavier elements come from?
• They came from stars! Solar System Formation – The Nebular Theory

• Stars make heavier elements from lighter ones through nuclear fusion Solar System Formation – The Nebular Theory

• Stars make heavier elements from lighter ones through nuclear fusion
• The heavy elements (the “metals”) mix into the interstellar medium when the stars die
• And then new stars form from the enriched gas and dust
• And the cycle continues Solar System Formation – The Nebular Theory

• And at the same time stars are forming… …planetary systems can form
• Here’s how it works… Solar System Formation – The Nebular Theory

• A large cloud -- a nebula perhaps 1 light year across -- floats in space
• The cloud begins to collapse Solar System Formation – The Nebular Theory …WHY would this happen?... Local density increase

• A large cloud -- a nebula perhaps 1 light year across -- floats in space
• The cloud begins to collapse -- local density increase
• As it collapses it begins to spin faster Solar System Formation – The Nebular Theory …WHY would this happen?... Conservation of angular momentum