Nearly all of the ways we detect exoplanets involve indirect observation. Most of them are based on observing the star(s) in the system of the planet. The two main ways we detect exoplanets are:
* Radial Velocity - just as the gravity of a star effects a planet's orbit, the gravity of a planet also has an impact on the star's. It's just this impact is extremely small. By measuring variations in a star's movement due to the planet's gravity we can both detect the planet and estimate its mass.
* Photometry - when a planet moves in front of a star it reduces both the brightness and spectrum of the light we perceive. By carefully measuring this we can determine not only the size of the planet, but also it's chemical make-up - light from the star travels through the atmosphere of the planet, changing its spectrum in response to the elements present.
There are several other ways we can detect and examine planets, but nearly all of them are based not on observing the planet itself but its impact on much larger, detectable things that surround it. There's a comprehensive list here - https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanet...
As an aside, there is an episode of the Cosmos reboot that talked about Photometry at a mile-high level. I had never heard of it before, but it is supremely interesting.
* Radial Velocity - just as the gravity of a star effects a planet's orbit, the gravity of a planet also has an impact on the star's. It's just this impact is extremely small. By measuring variations in a star's movement due to the planet's gravity we can both detect the planet and estimate its mass.
* Photometry - when a planet moves in front of a star it reduces both the brightness and spectrum of the light we perceive. By carefully measuring this we can determine not only the size of the planet, but also it's chemical make-up - light from the star travels through the atmosphere of the planet, changing its spectrum in response to the elements present.
There are several other ways we can detect and examine planets, but nearly all of them are based not on observing the planet itself but its impact on much larger, detectable things that surround it. There's a comprehensive list here - https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanet...