AST101 Lecture 21: Extrasolar Planets

Properties of Exoplanets

• AKA Extrasolra planet

• Name the star (David)

  • Planets follow by letter David 38a, David38b

• Exoplanets are faint and harder to see than the star

• Through a standard telescope, this makes them essentially invisible

• Been able to block out the bright light from the parentstars revealing the planets

• However the vast majority of exoplants have been found by indirect methods

What are the methods of finding exoplanets?

• Indirect way

  • Leaves traces 

• Transit Method (2969 found)

• Doppler (radial velocity) (wobble) method (863 exoplanets)

What is the Transit Method?

• When one celestial object passes in front of another 

• Planet blocks out a bit of the sun’s light

• Brightness of the sun dips and that can be measured

• Only occur if the planet’s orbit happens t be on edge as viewed from Earth (most exoplanets never produce a transit)

Light Curve

Amount of light in image

• Not wavelength, just all types of language

What can we learn from the transit method?

• Can find multiple planets by finding gaps in perioding spacing

• Orbit must be edge on or nearly edge on

• Using Kepler’s THird law we can use the period to calculate the distance of the exoplanet from its parent star

• Easily revels multiple planets around the same star (a solar system)

  • Transit changes depending on the the amount of light that is blocked out

  • Bigger planet blocks out more light (deeper)

  • Planets that take the longest also have a longer curve

How can we learn about size through the transit method?

• We can calculate the size of the exoplanet, because deeper the transit, bigger the planet

• Compared to spectrum which is brightness as a function of wavelength

  • Need enough light to make wavelength

• Kepler has shown that three are many more small planets than large ones

• Number of small large planets might be underestimating

  • Giant planets have orbital periods greater than 12 years

  • Maybe we haven’t watched long enough

How can we learn about temperature through the transit method?

• During a transit, an exoplanet show different phases and therefore different depths of transit

  • Difference between night and day sizes gives temperatures

  • How many hot and cold slides as seen through infrared can tell us the temperature

How can we learn about atmosphere through the transit method?

• 1 and 4 are combined light from star plus exoplanet

• 6 (eclipse) is the light from the star only

• Subtracting step 6 from 1 and 4 can tell us the light of the planet only

• Repeating at different wavelengths gives the spectrum of the planet

Steps of the Transit Method

• First 3 are when Transit starts

  • Level of light curve we see is planet + sun

  • 1 is the Sun and the exoplanet

  • 2 This is just starting

    • Period between 2 and 3 when it edges the star

    • Darkening of the star between the edges

    • Moves in the transit view

    • Maximum as close to the Transit

  • 3 is the maxiumum block

  • 5 and 6 it’s behind the star

  • Position 4 are kinda the same because we can see the star and the exoplanet

  • 5 and 6 there is change in brightness but that’s because all we see is the star

What are the strengths of the transit method?

Does not need a big telescope

- Can detect very small exoplanets

- Only possible way of 

 measuring exoplanet sizes

- Can study many stars at once

- Gives atmospheric composition 

 if we can see eclipses

What are the weaknesses of the transit method?

Can only see exoplanets in the

 rare case their orbits are edge-on

- Needs continuous monitoring to 

 ensure transits are not missed

 and to measure orbital periods

- Finding small exoplanets is only 

 possible from space

- Biased toward close-in exoplanet