Lesson 9: Exoplanets, Detecting Planets Around Other Stars

Question 1

What is an extrasolar planet? (We shorten this to ‘exoplanet’)

Answer 1

Planets are close to their stars, relative to the distance from us to our Sun

Question 2

Why is it so difficult to detect a planet around another star?

Answer 2

• A sun-like star is about a billion times brighter than the light reflected from its planets
  (Its difficult to see planets because they’re star’s are so bright)

Question 3

What is the difference between a direct planet detection and an indirect planet detection?

Answer 3

Direct: pictures or spectra of the planets themselves
Indirect: measurements of stellar properties revealing the effects of orbiting planets

Question 4

Describe how/why Jupiter makes the Sun move? What is the Sun orbiting?

Answer 4

• The Sun and Jupiter orbit around their common centre of mass
• The Sun therefore wobbles around that centre of mass with the same period as Jupiter

Question 5

Describe the Sun’s motion around the solar system? If some alien civilization was looking at our solar system from 30 light years away, how much motion would the
Sun have on the sky?

Answer 5

• The Sun’s motion around the solar system’s centre of mass depends on tugs from all the planets

0.0005 arcsecond = angular radius of the Sun from 30 lightyears
(VERY TINY)

Question 6

What is the Doppler Technique? Why do we also call this the ‘radial velocity’ method?

Answer 6

A light source is moving towards you, the frequency changes (the waves are closer), and when moving away, the frequency changes (the waves are stretched)

Why call this the ‘radial velocity’?
- the velocity component along the radius between observer and target

Question 7

Why would a massive planet be easier to detect than a less-massive planet using the Doppler Technique?

Answer 7

the parent star will wiggle more with a large planet nearby, thereby creating a larger and more easily detectable spectral shift

Question 8

What is the name of the first ever exoplanet discovered? What technique was used?
What was the period of the planet around its star?

Answer 8

51 Pegasi - radial velocity method (doppler)
4-day orbital period

Question 9

Why is 51Peg b referred to as a ‘hot Jupiter’

Answer 9

7 million km from its star A ‘hot Jupiter’
very close to its host star

Question 10

What is the Transit Method? What exactly is a ‘transit’?

Answer 10

• Measuring the apparent brightness of the star
  ○ The apparent brightness dips when a planet travels/transits in front of it
  ○ The planet is blocking some of the light

Watching for the trends of dips to know when the repeated cycle

Question 11

Why would a large (ie, in diameter) planet be easier to detect than a small planet using the Transit Method?

Answer 11

larger dip in apparent brightness, which means there could be planet(s) surrounding star

Question 12

Compare the Kepler Space Telescope with the TESS telescope, what were their scientific objectives?

Answer 12

Kepler Space Telescope
mission searched for transits from 2009-2013

Transiting Exoplanet Survey Satellite (TESS)
mission is focused on G, K, and M-type stars

Question 13

What is the difference between a transit and an eclipse in an exoplanet system?

Answer 13

• A transit is when a planet crosses in front of a star
  ○ This reduces the star’s apparent brightness and tells us a planet’s radius
• Sometimes an eclipse — the planet passing behind the star, can also be detected
  Star actually blocks some light when it goes behind the star (not as much as transit)

Question 14

What is the Direct Imaging Method? How successful has it been at finding
exoplanets? Why or why not has it been successful?

Answer 14

Direct Imaging - Taking Pictures

• Seeing light bouncing off the planet coming from the star
• Covering the star with a mask, so we can bring the contrast down and find the dimmer things around it
  
  • Works at most angles
    ○ Can take pictures at all angles especially ones at birds eye view at system

Question 15

What is the Gravitational Microlensing Method? How successful has it been at finding exoplanets?

Answer 15

Gravitational Microlensing - Light in a Gravity Len
When light curves - called lensing
Mass bends light - gravitational lensing

As a planet passes in front of the star relative to the observer (i.e. makes a transit), the light rays of the background source star become bent due to the gravitational “attraction”

• The star bending light towards us
  ○ Therefore, more light is sent toward us
  Angle matters (event has to be aligned with us)

Question 16

What is the Astrometry Method? How successful has it been at finding exoplanets?

Answer 16

• Astrometry - The precise cataloguing of objects in the sky
  The more precise you are at measuring the location of an object, the more able you are to see it move

WORST METHOD

Question 17

Overall, what has been the most successful method so far at finding exoplanets?

Answer 17

Transit Method- Searching for Shadows

Question 18

What types of properties are we able to measure from an exoplanet?

Answer 18

Measurable Properties:

• Orbital period, distance, and shape
• Planet mass, size, and density
• Atmospheric properties

With doppler method and transit method we can find planet density

• Radial method gives mass
• Transit gives volume

Denisty = Mass/Volume

Question 19

What is a ‘super-Earth-size’ planet? What is a ‘sub-Neptune-size’ planet?

Answer 19

• Super Earths (if they’re rocky, bigger than Earth)
• Sub-Neptune (if they’re gaseous, smaller than Neptune)

Question 20

How do types of planets in the Milky Way galaxy compare to the types of planets in
our Solar System?

Answer 20

• Most planets that have been found are a size not relevant in our solar system
• We’re missing a common size planet in our universe - In between Neptune and Earth

Question 21

What is the Habitable Zone?

Answer 21

• Habitable Zone — Region around a star, where we believe there is liquid water on their surface
  Found near inner system

Question 22

What is the Habitable Zone?

Answer 22

• Habitable Zone — Region around a star, where we believe there is liquid water on their surface
  Found near inner system

Question 23

What does ‘selection effect’ mean, and how does that apply to the discovery of exoplanets?

Answer 23

Selection effect—where doppler technique of discovery selects certain kinds of objects as “easy finds.”
When the technique was first used, it selected massive planets close to their stars as the most likely discoveries

Question 24

What does Doppler and Transit allow you to calculate?

Answer 24

The **Doppler method **allows us to estimate the mass of a planet.
If we can determine the size of the star, the transit method tells us the volume of the planet

Both Calculations: used to derive the **average density (mass/volume)
**