You might want to recollect some of the topics you have studied before going into this, if you haven’t don’t worry I will explain it all in this summary. Let us begin the journey………..
You might have studied about Doppler Effect in physics. Of course it deals with the sound from a moving object while listening from a stationary point. The same applies in the light spectrum too. If a light source is drawn away from an observer the most of the blue light in that spectrum will get scattered and the red light is all that remains in that spectrum. This is because the low wavelength of the blue light makes it to scatter when travelling through air molecules and the red light has a larger wavelength which mays it to travel longer distance without getting scattered.
We all know that stars and planets are bound together by a weak and a long range force called gravitational force of attraction. Each and every mass in space possess gravity. Depending upon its mass and density the gravitational magnitude will vary. If an object is lot denser it’s force of gravity will be huge, if its density is low the force of gravity will be low. That’s why extremely dense black holes possess inescapable gravitational force and it can even rip the space and time. We’ll talk about the black holes later.
So, you might ask what this has to do with the spotting of an exo-planet, here’s the reason why.
When a planet revolves around its host star its gravitational force affects the star’s axis of rotation, i.e. the star wobbles due to the gravitational force of the planet.
When the planet comes between the earth and the host star the host star is pulled towards earth for a fractional amount, this makes the light spectrum from the star to possess some blue light wavelength too and when the planet goes behind the star, the star gets pulled away from the earth and the starlight spectrum goes red. By detecting these variations in a starlight spectrum we can easily confirm that a planet definitely revolves around it. By using this method we can also find the mass and density of the planet revolving around it. Although this method can only be used for detecting large planets because only large planets have enough gravity to make the host star wobble along its rotational axis. Remaining ways to find an exo-planet will follow later.