The person who discovered that the Earth is a planet, and not the center of the universe, is the Polish mathematician and astronomer Nicolaus Copernicus. He created the heliocentric view of the solar system, which is the view that the Earth and the other planets rotate around a fixed sun.
Before Copernicus' ideas, people thought that the other planets, stars and the Sun rotated around the Earth. Ancient astronomers found that the planets wouldn't follow a straight path in the sky but would instead continually move forward, stop then move backwards, which caused the Greek astronomer Ptolemy to hypothesize that the planets are spherical in shape and rotate in small circles while moving around the Earth. Copernicus' model went against this belief and took years to be recognized by the scientific community.