Exoplanets (AQA A Level Physics): Revision Note

Exoplanets

• An exoplanet is:

A planet found outside our Solar System, in orbit around another star

• Exoplanets are difficult to detect directly because:

  • light from the host star is much brighter than the reflected light from the planet

  • they subtend extremely small angles compared to the resolution of telescopes

• Astronomers must use indirect detection techniques to observe exoplanets, such as:

  • the transit method

  • the radial velocity method

The transit method

• When a planet passes in front of a star (as seen from Earth), some of the light from the star is obscured

• Therefore, the total amount of light reaching the Earth is reduced

• By measuring the apparent magnitude, or intensity of light received from the star over time, a light curve can be obtained

Light curve using the transit method

• The dip in brightness can be used to determine the size of the planet

• The duration of the dip can be used to determine the orbital period of the planet

• Some of the limitations of this technique are:

  • the accuracy can be reduced if the Earth, planet and star are not aligned in the same plane

  • only planets with a short orbital period can be detected

The radial velocity method

• As a planet orbits its host star, they both orbit around a common centre of mass

• During the orbit, the star will move slightly towards, or away from the Earth as the planet moves to different positions in the orbit

  • The line spectrum of the star will show blueshift when it moves towards the Earth, then redshift when it moves away

Doppler shift of an exoplanet

• This causes very small, but measurable, periodic shifts in the wavelength of the light received from the star

  • The time period of the planet’s orbit is equal to the time period of the Doppler shift

• The main limitation of this technique is that:

  • low-mass, or Earth-like, planets do not cause as much 'wobble' as high-mass planets since they have a greater gravitational pull on the star