Chapter 7

Section 7.1: Overview of Our Planetary System

The Structure of the Solar System

• The Solar System consists of:

  • The Sun – a star at the center providing energy.

  • 8 Planets – divided into terrestrial (rocky) and Jovian (gas/ice giants).

  • Dwarf planets – smaller planetary bodies that haven’t cleared their orbits.

  • Moons (natural satellites) – orbit planets (e.g., Earth's Moon, Jupiter’s Io).

  • Asteroids and comets – rocky or icy remnants from solar system formation.

  • Interplanetary dust and gas – leftover particles spread throughout space.

Classification of Planets

Special Notes on Dwarf Planets

• Pluto, Eris, and Ceres are dwarf planets because they haven’t cleared their orbit.

• Kuiper Belt: Region beyond Neptune containing icy bodies, dwarf planets, and comets.

• Oort Cloud: Hypothetical, distant spherical shell of icy objects surrounding the solar system.

Section 7.2: Composition and Structure of Planets

What Are Planets Made Of?

• Terrestrial Planets: Composed of silicate rock & metal cores.

• Jovian Planets: Composed mostly of hydrogen, helium, and ice.

• Dwarf Planets: Made of rock and ice mixtures.

• Asteroids: Primarily rocky with some metal.

• Comets: Icy bodies mixed with dust that vaporize when close to the Sun.

Atmospheres & Surface Conditions

• Terrestrial planets: Thin atmospheres (except Venus), high-density interiors.

• Jovian planets: Thick hydrogen/helium atmospheres, lack solid surfaces.

• Planetary Atmospheres Depend On:

  • Mass: Strong gravity holds gases.

  • Temperature: Hot planets lose light gases (Mercury, Moon).

  • Magnetic Field: Protects atmosphere from solar wind (Earth’s magnetic field prevents stripping).

Section 7.3: Dating Planetary Surfaces

How Do Scientists Determine Planetary Age?

• Radiometric dating: Measures the decay of radioactive isotopes (e.g., Uranium-238 → Lead-206).

• Crater counting: More craters = older surface.

• Geologic Activity: Planets with volcanoes/plate tectonics have younger surfaces.

Crater Counting & Surface Age

• Old surfaces: More craters (e.g., Mercury, Moon’s highlands).

• Young surfaces: Fewer craters due to geological processes erasing impact scars (e.g., Earth, Io).

Differentiation: How Planets Develop Layers

• Heavier materials sink (core), lighter materials rise (crust).

• Earth’s layers: Core (iron/nickel), Mantle (semi-molten rock), Crust (solid rock).

Section 7.4: Origin of the Solar System

Nebular Hypothesis: How Did the Solar System Form?

• Step 1: Giant molecular cloud collapses due to gravity.

• Step 2: A spinning disk forms, with the Sun at the center.

• Step 3: Planetesimals (small bodies) collide and merge to form planets.

The Role of Angular Momentum

• As the cloud collapsed, it spun faster (like an ice skater pulling arms in).

• This caused the protoplanetary disk to form.

The Frost Line (Snow Line)

• Defines where planets form:

  • Inside frost line: Too hot for ice, only rocky planets form (Mercury, Venus, Earth, Mars).

  • Outside frost line: Cold enough for gas & ice giants to form (Jupiter, Saturn, Uranus, Neptune).

How Did Planets Form? (Accretion & Differentiation)

1. Dust & gas collide → forming small grains.

2. Grains stick together → forming planetesimals.

3. Planetesimals collide → forming protoplanets.

4. Protoplanets clear their orbits → forming true planets.

Why Are Asteroids & Comets Leftovers?

• They are remnants of planetesimals that never became full planets.

• Asteroids are rocky, mostly found in the Asteroid Belt (between Mars & Jupiter).

• Comets are icy, found in the Kuiper Belt & Oort Cloud.