Overview
This lecture explains the nebular theory of solar system formation, the processes of accretion and differentiation, and provides perspective on the scale and structure of our solar system.
The Nebular Theory and Solar System Formation
- The solar system consists of the sun, eight major planets, and smaller objects.
- The nebular theory explains that the solar system formed from a large cloud of gas and dust called a nebula.
- Star and planet formation in nebulae takes millions of years.
- The process is understood by observing objects at various stages, like looking at snapshots in a timeline.
Stages of Solar System Formation
- Stage 1: Nebula (cloud of gas and dust).
- Stage 2: Gravity causes the nebula to spin and flatten into a protoplanetary disk.
- Stage 3: Material in the disk forms a dense center (the sun) and planetesimals (baby planets) start to clump.
- Stage 4: Planetesimals grow by accretion and clear gaps in the disk as they sweep up material.
- Stage 5: The sun ignites, planets are mostly formed, and little gas/dust remains.
- Stage 6: The completed solar system has all material collected into planets or the sun.
Accretion and Differentiation
- Accretion is when small particles stick together to form larger bodies, eventually forming planets.
- Collisions and radioactive decay cause young planets to melt partially or completely.
- Differentiation occurs as denser materials sink to the core while lighter materials form mantles and crusts.
Structure and Scale of the Solar System
- The sun and planets formed in the same plane (plane of the ecliptic).
- The inner planets are rocky; the outer planets are gas giants due to cooling temperatures farther from the sun.
- Size and distance in the solar system vary dramatically, with vast spaces between the outer planets.
- The planets differ greatly in size, with Jupiter much larger than Earth, and the sun far larger than any planet.
Regions Beyond the Planets
- The Kuiper Belt is a donut-shaped region beyond Neptune containing many icy objects, including Pluto.
- The Oort Cloud is a distant spherical shell of icy bodies and is the source of long-period comets.
- Some objects, like Kuiper Belt objects, have tilted (inclined) and elongated (eccentric) orbits.
Key Terms & Definitions
- Nebula — A giant cloud of gas and dust in space.
- Protoplanetary disk — A spinning, flattened disk of material around a young star where planets form.
- Planetesimal — A small, early-stage planetary body formed from dust and gas.
- Accretion — The process where small particles combine to create larger bodies.
- Differentiation — Separation of planetary material by density, forming core, mantle, and crust.
- Ecliptic — The plane of Earth's orbit, the reference plane for the solar system.
- Kuiper Belt — A doughnut-shaped region beyond Neptune with icy bodies.
- Oort Cloud — A distant, spherical shell of icy objects surrounding the solar system.
- Eccentricity — The measure of how oval-shaped an orbit is.
- Inclination — The tilt of an orbit relative to the ecliptic plane.
Action Items / Next Steps
- Complete assignments on size scale and distance scale models of the solar system.