Exoplanets are planets that orbit stars other than our Sun. A common way to detect exoplanets involves measuring the change in the amount of light we see as the planet passes in front of the star. This method uses photometry to determine how bright the star is over time. The AAVSO has a good guide on doing photometry, in multiple languages.
To learn about NASA’s TRAnsiting Planets and Planetesmals Small Telescope (TRAPPIST) new discovery of the TRAPPIST-1 System, click link below. The TRAPPIST System is the first known system of seven planets that orbit around a single star.