The heliocentric theory of the solar system was proposed by Nicolaus Copernicus. In the 16th century, Copernicus challenged the long-held geocentric model, which placed the Earth at the center of the universe. He argued that the sun, rather than the Earth, was at the center, with the planets, including Earth, orbiting around it. This revolutionary idea was documented in his seminal work "De revolutionibus orbium coelestium," published in 1543. Copernicus's theory laid the groundwork for modern astronomy and eventually played a significant role in the scientific revolution, influencing thinkers like Galileo and Kepler.