Indirect Exoplanet detection

 There are four indirect methods used to detect exoplanets.

The first method indirect is the radial velocity method. It is the most common method used to discover exoplanets. The reflex motion of a star due to the orbiting planet is measures as a change in a stars radial velocity. The radial reflex of the star is compared to the exoplanets orbit, using the measurements of Doppler shifts. These comparisons are used to calculate the mass of the exoplanet, its orbits shape, and its orbital distance. The exoplanets discovered using this method tend to be very low mass planets. 

 The second indirect method is the astrometry method. This method measures a star's position and how it changes over time so is mostly used to discover exoplanets that have very long periods.  After that you can use the acquired information to determine the actual mass of the exoplanet because you can determine the orbital plane of the exoplanet. The best place to use astrometry is in space but you can use this method from the surface of earth. The exoplanets discovered using this method tend to be very far from the solar system. 

 

The third indirect method is the transit method. A transit is an event that occurs when a celestial object moves in-front of another celestial object. When the celestial body moves infront of the other larger celestial body it hides a small portion of it. Observers can see this occurrence at particular orbital points. This method reveals exoplanets when they transit their larger companion stars. Observers see a drop in the visual brightness of the companion star. The exoplanets orbit has to be perfectly aligned with the observers viewing point or the observer could easily miss the exoplanet. Also there is a very high amount of false exoplanet detections when using the transit method because dust, gas, and even planetary debris can easily cause a star to appear dimmer. 

The fourth indirect method is gravitational lensing. Gravitational lensing occurs when the presence of matter effects the path of a light ray. The light ray, from the observers view can appear to be curved or highly unusual. The gravity field of a star can behave like a lens and it can magnify the light of a background star. The star, the background star, and the Earth all move relative to each other. If the lensing star has a companion exoplanet, then the exoplanet's gravitational field can be detected through its contribution to lensing effect. This effect only occurs when the stars are almost perfectly aligned. The lensing events are very short and they can never be repeated so it is very difficult to detect exoplanets using this method.