How exoplanets are detected

Planets that orbit a star other than the sun are referred to as exoplanets or extrasolar planets. The latest set of exoplanets discovered are those that orbit the star TRAPPIST - 1 about 40 light year away. The first discovery of an exoplanet came on January 9th, 1992 when radio astronomers Alexander Wolszczan and Dale Frail revealed the discovery of 2 exoplanets orbiting a pulsar (a rotating neutron star.

Artist's rendition of how prevalent exoplanets are in the Milky Way galaxy

Detecting exoplanets is quite a challenging task : stars such as our sun are many-fold brighter than the light that is reflected off of the planets orbiting it. Due to this as well as additional reasons, astronomers generally tend to use indirect methods to detect exoplanets rather than directly observing them.

Detection methods

There are numerous ways through which astronomers can detect these planets. The most common ones are mentioned below : 

Radial velocity -   Stars with planetary systems such as the solar system experience variations in their speed due to the gravitational force exerted on the parent star by the planets. The speed here is relative to earth; either the planets cause the speed of the star moving away from earth to increase or they cause the star's speed towards the earth.

The variations of speed are in the radial velocity of said star relative to earth. The radial velocity can be calculated from the displacement in the star's spectral lines due to the Doppler effect. The Doppler effect can be best understood using the following analogy. Consider an ambulance or a police car with the siren on. As the vehicle moves towards you, the sound becomes louder and as it moves away the opposite happens. This is an example of the Doppler effect.

Regarding how spectral lines are used, look at the graph below : 

The graph shows the normalised flux (apparent brightness) versus the wavelength in nm. The graph shows a zoomed in region of the spectrum of each star and the absorption lines are shown. Using the data, the radial velocity of Arcturus can be derived. 

The sun has a wavelength of approximately 882.4 nm (nanometre) at it's lowest apparent brightness and Arcturus has a wavelength of about 882.55 nm at it's lowest apparent brightness. The difference between these values is about 0.15 nm. Dividing this wavelength by the sun's wavelength and multiplying by the speed of light in vacuum gives a quantity of 50 km/s. It is thus deduced that Arcturus is moving away from us when the spectrum was recorded.

Bar chart of exoplanet discoveries by year. Radial velocity is dark blue, transit is dark green, timing is dark yellow, direct imaging is dark red and microlensing is dark orange

Transit photometry -  This method can be used to calculate the exoplanet's radius. If it exists, then the planet crossing its parent star leads to a dimming in the apparent brightness of the star by a relatively small amount. A light curve can be used to record the dimming as well.

The star Kepler 6's light curve showing a dimming caused by the planet Kepler 6b

Transit-timing variation -  This method is similar to the transit photometry, it detects planets by observing variations in the time it takes for the planet to transit (move across) it's star. The variations refer to whether the transit occurs with periodicity or it has changes. This is quite a sensitive method, in that, it's very precise. 

It is capable of detecting additional planets around the star, some as small as earth! The simulation below shows the difference between planet transit timings for uni-planetary systems and multiple planetary systems.