11-15-2025, 09:54 AM
Chapter 4 — Planets
Planets are the worlds that orbit stars.
Our solar system contains eight planets, but the universe contains trillions of them —
possibly more planets than stars.
Astrophysics studies how planets form, what they are made of, and how they evolve over time.
Understanding planets also helps us explore one of the biggest questions in science:
Could life exist elsewhere in the universe?
---
4.1 What Makes a Planet a Planet?
A planet must meet three conditions:
1. It orbits a star.
2. It is large enough for gravity to shape it into a sphere.
3. It has cleared its orbit of smaller objects.
Planets do NOT produce their own light — they reflect the light of their star.
---
4.2 Types of Planets
Planets generally fall into two major categories:
1. Rocky (terrestrial) planets:
• Small
• Dense
• Solid surface
Examples: Mercury, Venus, Earth, Mars.
2. Gas giants (and ice giants):
• Huge
• Made mostly of gas or ice
• No solid surface
Examples: Jupiter, Saturn, Uranus, Neptune.
These categories also apply to exoplanets found around other stars.
---
4.3 The Structure of a Planet
Planets have layered structures depending on their type.
Rocky planets:
• Crust
• Mantle
• Core (liquid or solid metal)
Gas giants:
• Thick atmosphere
• Liquid metallic hydrogen layers
• Small rocky/icy core
Ice giants:
• Atmospheres of hydrogen and helium
• Deep icy layers of water, ammonia, and methane
• Rocky core
Each type forms under different temperature and distance conditions.
---
4.4 How Planets Form
Planets form in rotating disks of gas and dust around young stars — called protoplanetary disks.
The formation steps are:
1. Tiny grains of dust stick together.
2. Small rocks collide and grow into kilometre-sized planetesimals.
3. Gravity takes over, pulling material together.
4. Rocky planets form close to the star (where it is hot).
5. Gas giants form farther out (where ice can exist).
6. Leftover debris becomes asteroids and comets.
Planet formation takes a few million years — a short time in cosmic terms.
---
4.5 The Solar System’s Planets
Mercury: Small, rocky, extremely hot and cold.
Venus: Thick toxic atmosphere, hotter than Mercury.
Earth: Liquid water, atmosphere, and life.
Mars: Cold, thin air, potential past oceans.
Jupiter: Largest planet, giant storms.
Saturn: Iconic rings, icy moons.
Uranus: Tilted sideways, cold and windy.
Neptune: Strongest winds in the solar system.
Many of their moons may also have oceans and geology, making them targets for future exploration.
---
4.6 Exoplanets
Exoplanets are planets orbiting other stars.
We have discovered over FOUR THOUSAND of them — and more every year.
They come in many types:
• Hot Jupiters
• Super-Earths
• Mini-Neptunes
• Earth-like planets in the habitable zone
The diversity is far greater than in our own solar system.
Some exoplanets may even have oceans, atmospheres, and conditions suitable for life.
---
4.7 Why Planets Matter
Planets are essential because:
• They are the homes of life.
• They teach us how solar systems form.
• They reveal how common (or rare) Earth-like worlds may be.
• They help us understand the chemistry and physics of different environments.
The discovery of Earth-sized planets in the habitable zones of nearby stars has changed
our expectations about life in the universe.
---
Chapter Summary
• Planets orbit stars and come in a huge variety of types.
• Rocky planets form near the star; gas giants form farther away.
• Planets grow from dust and gas in young solar systems.
• Our solar system’s planets show a wide range of temperatures, atmospheres, and geology.
• Exoplanets are common, diverse, and often very different from the planets we know.
• Understanding planets brings us closer to answering whether life exists elsewhere.
---
Practice Questions
1. What three criteria define a planet?
2. What is the main difference between rocky planets and gas giants?
3. Why do rocky planets form closer to the star?
4. What is a protoplanetary disk?
5. Why are exoplanets important for the search for life?
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive
Planets are the worlds that orbit stars.
Our solar system contains eight planets, but the universe contains trillions of them —
possibly more planets than stars.
Astrophysics studies how planets form, what they are made of, and how they evolve over time.
Understanding planets also helps us explore one of the biggest questions in science:
Could life exist elsewhere in the universe?
---
4.1 What Makes a Planet a Planet?
A planet must meet three conditions:
1. It orbits a star.
2. It is large enough for gravity to shape it into a sphere.
3. It has cleared its orbit of smaller objects.
Planets do NOT produce their own light — they reflect the light of their star.
---
4.2 Types of Planets
Planets generally fall into two major categories:
1. Rocky (terrestrial) planets:
• Small
• Dense
• Solid surface
Examples: Mercury, Venus, Earth, Mars.
2. Gas giants (and ice giants):
• Huge
• Made mostly of gas or ice
• No solid surface
Examples: Jupiter, Saturn, Uranus, Neptune.
These categories also apply to exoplanets found around other stars.
---
4.3 The Structure of a Planet
Planets have layered structures depending on their type.
Rocky planets:
• Crust
• Mantle
• Core (liquid or solid metal)
Gas giants:
• Thick atmosphere
• Liquid metallic hydrogen layers
• Small rocky/icy core
Ice giants:
• Atmospheres of hydrogen and helium
• Deep icy layers of water, ammonia, and methane
• Rocky core
Each type forms under different temperature and distance conditions.
---
4.4 How Planets Form
Planets form in rotating disks of gas and dust around young stars — called protoplanetary disks.
The formation steps are:
1. Tiny grains of dust stick together.
2. Small rocks collide and grow into kilometre-sized planetesimals.
3. Gravity takes over, pulling material together.
4. Rocky planets form close to the star (where it is hot).
5. Gas giants form farther out (where ice can exist).
6. Leftover debris becomes asteroids and comets.
Planet formation takes a few million years — a short time in cosmic terms.
---
4.5 The Solar System’s Planets
Mercury: Small, rocky, extremely hot and cold.
Venus: Thick toxic atmosphere, hotter than Mercury.
Earth: Liquid water, atmosphere, and life.
Mars: Cold, thin air, potential past oceans.
Jupiter: Largest planet, giant storms.
Saturn: Iconic rings, icy moons.
Uranus: Tilted sideways, cold and windy.
Neptune: Strongest winds in the solar system.
Many of their moons may also have oceans and geology, making them targets for future exploration.
---
4.6 Exoplanets
Exoplanets are planets orbiting other stars.
We have discovered over FOUR THOUSAND of them — and more every year.
They come in many types:
• Hot Jupiters
• Super-Earths
• Mini-Neptunes
• Earth-like planets in the habitable zone
The diversity is far greater than in our own solar system.
Some exoplanets may even have oceans, atmospheres, and conditions suitable for life.
---
4.7 Why Planets Matter
Planets are essential because:
• They are the homes of life.
• They teach us how solar systems form.
• They reveal how common (or rare) Earth-like worlds may be.
• They help us understand the chemistry and physics of different environments.
The discovery of Earth-sized planets in the habitable zones of nearby stars has changed
our expectations about life in the universe.
---
Chapter Summary
• Planets orbit stars and come in a huge variety of types.
• Rocky planets form near the star; gas giants form farther away.
• Planets grow from dust and gas in young solar systems.
• Our solar system’s planets show a wide range of temperatures, atmospheres, and geology.
• Exoplanets are common, diverse, and often very different from the planets we know.
• Understanding planets brings us closer to answering whether life exists elsewhere.
---
Practice Questions
1. What three criteria define a planet?
2. What is the main difference between rocky planets and gas giants?
3. Why do rocky planets form closer to the star?
4. What is a protoplanetary disk?
5. Why are exoplanets important for the search for life?
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive