An Exoplanet is a planet that is outside of our own Solar System. 

The first Exoplanets to be discovered were in orbit around a Pulsar in 1992. Since then, hundreds more Exoplanets have been discovered. (4284 Exoplanets as of 20th of September 2020)

This number changes regularly as new Exoplanets are being discovered all the time, this is a field  that is at the forefront of modern scientific research. 

Why search for Exoplanets?

The search for exoplanets is tied up with one of the biggest questions for Humanity - 


                                                     Are we alone in the Universe? 

In the effort to answer this question, scientists have searched our Solar System for signs of habitable places, and as technology improves, this search has been widened to include other star systems. 

The video below gives a brief introduction into the methods of detecting Exoplanets.

There are several methods that can be used to detect Exoplanets, some are much more successful than others. The graph below shows the number of Exoplanets detected by each method (up to September 2014):-

            Direct Imaging        Micro-lensing        Transit          Radial Velocity        Timing

As can be seen from the above graph, almost all known Exoplanets have been discovered using one of two Methods:-

Transit Method

The Transit method currently has the highest success rate for finding Exoplanets.

The Transit method is a type of indirect detection, the planet itself is not seen, but its effect on the brightness of the parent star can be detected, and information about the planet can be deduced.

The Transit method works by observing small changes in the brightness of a star as a planet passes between the star and the Earth. 

This method can be thought of as similar to the change in brightness of the Sun during an eclipse, with the Moon blocking almost all of the sunlight reaching Earth. The only difference is that the amount of the starlight being blocked is very small, so the drop in brightness can be hard to spot. 

By observing a star over a long period of time and measuring its brightness, the following diagram can be generated:- 

As can be seen above, the drop in brightness will show the presence on an Exoplanet, without being able to see the Exoplanet directly. 

The above shows the effect of one Exoplanet transiting a star; when more than one Exoplanet is involved, the effects become much more complex:-

The video below gives a summary of the Transit method of detection.

Radial Velocity Method ( Intro to Higher Physics ) 

The main issue with the Transit method is that the Exoplanet must pass between the star and the Earth. This only occurs in a small number of cases, meaning to detect other Exoplanets, another method must be used. 

The radial Velocity method is another example of Indirect detection, as again the Exoplanet itself if not seen.

In this method, the Exoplanet is detected by its gravitational effect on the parent star. 

As the planet orbits the star, the stars position is changed slightly by the gravitational effect between them. This causes the star to orbit a point where the centre of Mass of the system occurs. 

By following this small orbit, the star will be at certain points be moving towards the Earth, and at other times the star will be moving away. 

Just like the sound of an ambulance changing pitch as it passes by you (Doppler Effect), the colour of the observable light will be shifted by the motion:-

1. If the star is moving towards the Earth - the light is blue shifted.

2. If the Star is moving away from the Earth - the light is red shifted.

The below diagram shows the effect of an Exoplanet on the Parent Star, and how the Light from the Star is Red or Blue Shifted because of this motion. 

The video below gives a summary of the Radial Velocity Method of Detection.