Exoplanets are planets beyond our own solar system. Thousands have been discovered in the past two decades, mostly with NASA's Kepler Space Telescope.
These exoplanets come in a huge variety of sizes and orbits. Some are gigantic planets hugging close to their parent stars; others are icy, some rocky. NASA and other agencies are looking for a special kind of planet: one that’s the same size as Earth, orbiting a sun-like star in the habitable zone.
The habitable zone is the area around a star where it is not too hot and not too cold for liquid water to exist on the surface of surrounding planets.
Scientists discovered a very efficient way to study these occurrences; planets themselves do not emit light, but the stars around which they orbit do. Considering this fact into account scientists at NASA developed a method which they called Transit method in which a digital-camera-like technology is used to detect and measure tiny dips in a star’s brightness as a planet crosses in front of the star. With observations of transiting planets, astronomers can calculate the ratio of a planet’s radius to that of its star — essentially the size of the planet’s shadow — and with that ratio, they can calculate the planet’s size. Kepler Space Telescope’s primary method of searching for planets was the “Transit” method.
Source: https://exoplanets.nasa.gov/