11-15-2025, 10:05 AM
Chapter 11 — Exoplanets & The Search for Life
Exoplanets are planets that orbit stars outside our Solar System.
Over the last 30 years, the discovery of thousands of exoplanets has transformed astrophysics
and opened the door to one of the greatest scientific questions:
Are we alone in the universe?
This chapter explores how exoplanets are found, what types exist, and how scientists search for life.
---
11.1 What Is an Exoplanet?
An exoplanet is any planet that orbits a star other than the Sun.
Exoplanets can be:
• small and rocky like Earth
• gas giants like Jupiter
• super-Earths
• mini-Neptunes
• exotic worlds with extreme temperatures, winds, and compositions
The first confirmed exoplanet was discovered in 1992.
Today we know of more than 5,000.
---
11.2 How Exoplanets Are Detected
Because exoplanets are small and dim, astronomers rarely see them directly.
Instead, they use indirect detection techniques:
1. Transit Method
When a planet passes in front of its star, the star dims slightly.
From the light dip we can determine:
• planet size
• orbital period
• sometimes atmosphere
This is the most common detection method.
2. Radial Velocity Method
A planet’s gravity makes its star wobble.
This wobble changes the star’s light via redshift/blueshift.
This reveals:
• planet mass
• orbital shape
3. Direct Imaging
Rare and difficult — taking actual pictures of exoplanets.
Works best for:
• young planets
• far from their star
• bright in infrared
4. Gravitational Microlensing
The gravity of a star (and its planets) bends light from a background object.
Ideal for discovering:
• very distant planets
• planets in other parts of the galaxy
---
11.3 Types of Exoplanets
Rocky worlds (Earth-like)
Small, solid surfaces. Potentially habitable.
Super-Earths
1–10 times Earth’s mass.
Most common type discovered so far.
Mini-Neptunes
Small gas-rich planets.
May have thick atmospheres or ocean layers.
Gas giants
Large planets like Jupiter and Saturn.
Hot Jupiters
Gas giants extremely close to their star — tidally locked with extreme temperatures.
Rogue planets
Planets drifting in interstellar space with no star.
---
11.4 Habitable Zones & Conditions for Life
The habitable zone (also called the “Goldilocks zone”) is the region around a star where liquid water can exist.
But habitability depends on more than distance:
• atmosphere
• magnetic field
• planet size
• stability of the star
• chemical composition
A planet may lie in the habitable zone but still be uninhabitable, depending on these factors.
---
11.5 Can We Detect Life on Exoplanets?
Scientists look for:
Biosignatures — molecules that may indicate life, such as:
• oxygen
• ozone
• methane
• water vapour
• unusual gas combinations (e.g., O₂ + CH₄ together)
Technosignatures — signs of intelligent life, such as:
• artificial radio signals
• unusual infrared patterns
• atmospheric pollutants
Upcoming telescopes like the James Webb and the ELT will make these searches more powerful.
---
11.6 Famous Exoplanet Systems
TRAPPIST-1
• 7 Earth-sized planets
• Several in the habitable zone
• Ideal for atmosphere studies
Kepler-186f
• First Earth-sized planet in a habitable zone
Proxima Centauri b
• Closest exoplanet to Earth
• In the habitable zone
• Possible atmosphere
Kepler-452b
• “Older cousin of Earth”
• Orbiting a Sun-like star
---
11.7 Future Missions & The Search for Life
Several upcoming missions are designed to detect life or Earth-like planets:
• LUVOIR
• HabEx
• Roman Space Telescope
• Extremely Large Telescope (ELT)
Goals include:
• mapping exoplanet atmospheres
• detecting biosignatures
• finding true Earth analogues
• studying exomoons
Humanity may discover signs of life within the next decades.
---
Chapter Summary
• Exoplanets orbit stars beyond our Solar System.
• Most are detected using transit and radial-velocity methods.
• They come in many types, including super-Earths and hot Jupiters.
• Habitability depends on more than distance — atmosphere is crucial.
• Scientists search for biosignatures and technosignatures.
• Modern telescopes are rapidly advancing the hunt for life.
---
Practice Questions
1. What is the transit method and what information can it reveal?
2. Why are super-Earths important to exoplanet science?
3. What factors besides distance affect planet habitability?
4. What is a biosignature? Give an example.
5. Which exoplanet system is most promising for finding life?
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive
Exoplanets are planets that orbit stars outside our Solar System.
Over the last 30 years, the discovery of thousands of exoplanets has transformed astrophysics
and opened the door to one of the greatest scientific questions:
Are we alone in the universe?
This chapter explores how exoplanets are found, what types exist, and how scientists search for life.
---
11.1 What Is an Exoplanet?
An exoplanet is any planet that orbits a star other than the Sun.
Exoplanets can be:
• small and rocky like Earth
• gas giants like Jupiter
• super-Earths
• mini-Neptunes
• exotic worlds with extreme temperatures, winds, and compositions
The first confirmed exoplanet was discovered in 1992.
Today we know of more than 5,000.
---
11.2 How Exoplanets Are Detected
Because exoplanets are small and dim, astronomers rarely see them directly.
Instead, they use indirect detection techniques:
1. Transit Method
When a planet passes in front of its star, the star dims slightly.
From the light dip we can determine:
• planet size
• orbital period
• sometimes atmosphere
This is the most common detection method.
2. Radial Velocity Method
A planet’s gravity makes its star wobble.
This wobble changes the star’s light via redshift/blueshift.
This reveals:
• planet mass
• orbital shape
3. Direct Imaging
Rare and difficult — taking actual pictures of exoplanets.
Works best for:
• young planets
• far from their star
• bright in infrared
4. Gravitational Microlensing
The gravity of a star (and its planets) bends light from a background object.
Ideal for discovering:
• very distant planets
• planets in other parts of the galaxy
---
11.3 Types of Exoplanets
Rocky worlds (Earth-like)
Small, solid surfaces. Potentially habitable.
Super-Earths
1–10 times Earth’s mass.
Most common type discovered so far.
Mini-Neptunes
Small gas-rich planets.
May have thick atmospheres or ocean layers.
Gas giants
Large planets like Jupiter and Saturn.
Hot Jupiters
Gas giants extremely close to their star — tidally locked with extreme temperatures.
Rogue planets
Planets drifting in interstellar space with no star.
---
11.4 Habitable Zones & Conditions for Life
The habitable zone (also called the “Goldilocks zone”) is the region around a star where liquid water can exist.
But habitability depends on more than distance:
• atmosphere
• magnetic field
• planet size
• stability of the star
• chemical composition
A planet may lie in the habitable zone but still be uninhabitable, depending on these factors.
---
11.5 Can We Detect Life on Exoplanets?
Scientists look for:
Biosignatures — molecules that may indicate life, such as:
• oxygen
• ozone
• methane
• water vapour
• unusual gas combinations (e.g., O₂ + CH₄ together)
Technosignatures — signs of intelligent life, such as:
• artificial radio signals
• unusual infrared patterns
• atmospheric pollutants
Upcoming telescopes like the James Webb and the ELT will make these searches more powerful.
---
11.6 Famous Exoplanet Systems
TRAPPIST-1
• 7 Earth-sized planets
• Several in the habitable zone
• Ideal for atmosphere studies
Kepler-186f
• First Earth-sized planet in a habitable zone
Proxima Centauri b
• Closest exoplanet to Earth
• In the habitable zone
• Possible atmosphere
Kepler-452b
• “Older cousin of Earth”
• Orbiting a Sun-like star
---
11.7 Future Missions & The Search for Life
Several upcoming missions are designed to detect life or Earth-like planets:
• LUVOIR
• HabEx
• Roman Space Telescope
• Extremely Large Telescope (ELT)
Goals include:
• mapping exoplanet atmospheres
• detecting biosignatures
• finding true Earth analogues
• studying exomoons
Humanity may discover signs of life within the next decades.
---
Chapter Summary
• Exoplanets orbit stars beyond our Solar System.
• Most are detected using transit and radial-velocity methods.
• They come in many types, including super-Earths and hot Jupiters.
• Habitability depends on more than distance — atmosphere is crucial.
• Scientists search for biosignatures and technosignatures.
• Modern telescopes are rapidly advancing the hunt for life.
---
Practice Questions
1. What is the transit method and what information can it reveal?
2. Why are super-Earths important to exoplanet science?
3. What factors besides distance affect planet habitability?
4. What is a biosignature? Give an example.
5. Which exoplanet system is most promising for finding life?
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive