L15 : Exoplanets and their Environments

Question 1

Why is detecting exoplanets challenging?

Answer 1

Small and faint compared to stars they orbit
Very far so angular separation very small due to distance

Question 2

Define exoplanet?

Answer 2

(extrasolar planet)

Planet that orbits star outside of our solar system
Can vary widely in size, composition, orbit

Question 3

What was the first exoplanet discovered around a sun-like star?

Answer 3

51 Pegasi b
Discovered in 1995 by Queloz and Mayor

Question 4

Give an overview of radial velocity and its use?

Answer 4

One of first and most successful methods of exoplanet detection
Over 1060 extrasolar planets since 1995

Question 5

Briefly what is the radial velocity method?

Answer 5

Detects the periodic motion (wobble) of a star caused by a orbiting planet’s gravitational pull

Question 6

How does the radial velocity method detect exoplanets?

Answer 6

Using spectroscopy
By observing Doppler shifts in the star’s light spectrum
Red when moving away
Blue when moving toward

Question 7

What is a radial belocity curve?

Answer 7

A graph showing how a star’s belocity changes over time due to a planet’s orbit

Question 8

What information does the period (T) and amplitude of the radial velocity curve provide?

Answer 8

Period (T)
- Orbital period of the planet

Amplitude
- Planet mass

Question 9

How is the radial velocity method assist in assessing habitability?

Answer 9

Orbital distance helps estimate planetary temperature
Combined with planet mass, helps evaluate in planet might be habitable

Question 10

What happens to starlight during a transit?

Answer 10

Small portion of light is blocked, causing a temporary and repeatable dimming

Question 11

Briefly what is the transit photometry method?

Answer 11

Detects planets by observing periodic dips in the star brightness when a planet transits in front

Question 12

What is required for the transit method to work?

Answer 12

System must be aligned so planet passes between its star and Earth
Must view in plane of orbit

Question 13

What was the first confirmed transit detection?

Answer 13

1999
HD209458
Initially found via radial velocity

Question 14

What can be achieved combining transit and radial velocity data?

Answer 14

Planet density
First order estimate of composition

Question 15

How many exoplanets confirmed as of 27 Feb 2025

Answer 15

5839

Question 16

What is NASA TESS mission?

Answer 16

Transiting Exoplanet Survey Satellite

Survey 85% if the sky for planets transiting nearby bright stars

Question 17

How many exoplanet candidates and confirmations has TESS contributed?

Answer 17

As of 2023
~7000 candidates
403 confirmed

Question 18

What is JWSTs role in exoplanet science?

Answer 18

Observes in infrared
Located at Sun-Earth L2 point
Capable of characterising exoplanet atmospheres and climates

Question 19

What is ARIEL?

Answer 19

Future mission led by ESA and UCL
Atmospheric remote sensing of exoplanets using infrared

Question 20

What is Kepler?

Answer 20

Space Telescope
- Space based observatory

Question 21

What did the Kepler mission prove about planet occurrence?

Answer 21

Very common
- On average, every main-sequence star has at least one planet
- Many have multi-planet systems

Question 22

What is notable about many exoplanet orbits?

Answer 22

Many have highly eccentric orbits compared to Solar System
Some over 0.9 (eg. 0.96 HD29782 b, 0.02 Earth)

Question 23

How does high orbital eccentricity affect habitability?

Answer 23

Causes sever seasonal variations and variable habitability

Question 24

Can planets form in binary systems?

Answer 24

Yes, either around one star (circumstellar) or in circumbinary orbits (eg. Kepler 16)
Gravitational stability zones must be considered

Question 25

What does stellar metallicity indicate in planet formation?

Answer 25

Gas giants are more common around higher mass metal-rich stars Trend is weaker for lower mass planets

Question 26

How does stellar mass affect giant planet occurence?

Answer 26

Giant planets are more common around higher mass stars

Question 27

Why might small planets be more common around low mass stars?

Answer 27

Unclear But suggests planet formation in a wide variety of environments?

Question 28

What is the average age of small rocky planets in the galaxy?

Answer 28

About 7 Gyr

Question 29

Why is direct imaging of exoplanets difficult?

Answer 29

Planet star contrast and small angular separations

Question 30

What kind of planets are most easily imaged directly?

Answer 30

Young, hot, massive gas giants in the infrared

Question 31

Which system is a key example of direct imaging?

Answer 31

HR8799 Four probable plaets ~30 Myr old Bright in IR

Question 32

What is a sub-Neptune or super-Earth? Importance?

Answer 32

Planets 1.8-4 Earth radii Most common exoplanet size Not found in Solar system

Question 33

What does the mass-radius relationship for sub-Neptunes show?

Answer 33

Flat trend Implying structure dominated by hydrogen helium envelopes

Question 34

What characterises gas giants?

Answer 34

Composed mainly of H/He Radius is relatively insensitive to mass due to internal pressure

Question 35

What are super-Puffs?

Answer 35

Planets with low density and unusually large radii for their mass (<1 g/cm3)

Question 36

Compare hot Jupiters vs super-Puffs

Answer 36

Hot Jupiters - Massive - Close to star Super puffs - Less massive - Farther out - Very low density

Question 37

What is the sub-Neptune desert?

Answer 37

Radius gap between 1.5 and 2 Earth radii Likely due to photoevaporation

Question 38

What is synchronous rotation (tidal locking)?

Answer 38

Planet rotates once per orbit One side is always facing the star (substellar point) One in permanent darkness (antistellar point)

Question 39

What are the three tidal rotation regimes?

Answer 39

Rapid rotators (<5 days) Rhines rotators (5-20 days) Slow rotators (>20 days)

Question 40

Expand on rapid rotators?

Answer 40

Multiple climate cells, strong winds

Question 41

Expand on Rhines rotators?

Answer 41

Intermediate circulation with partial asymmetry

Question 42

What affects circulation patterns in tidally locked planets?

Answer 42

Orbital period Atmospheric dynamics Temperature gradients

Question 42

Expand on slow rotators?

Answer 42

Dominated by single global cell, strong day-night contrast

Question 43

Why might older planets be better candidates for life?

Answer 43

Offer more time for biological evolution and system stabilisation

Question 44

Why are habitable zone models limited?

Answer 44

Don't account for all environmental, geological and biological feedback processes