The scientist who proposed the heliocentric theory is Nicolaus Copernicus. He published his groundbreaking ideas in the book *De revolutionibus orbium coelestium* (On the Revolutions of the Celestial Spheres) in 1543. This theory posited that the Earth and other planets revolve around the Sun, challenging the long-standing geocentric model that placed the Earth at the center of the universe. Copernicus' work laid the foundation for modern astronomy and paved the way for later scientists, such as Galileo Galilei and Johannes Kepler, to build upon his ideas, ultimately changing our understanding of the cosmos.