One of the discoveries that led to the modern view of the Solar System was that the orbits of the planets were ellipses, not circles. This discovery was made by Johannes Kepler in the early 17th century, and it was a major breakthrough in our understanding of the Solar System. Prior to Kepler's work, it was believed that the planets moved in perfect circles around the Earth. However, Kepler's observations showed that the planets actually moved in elliptical orbits, with the Sun at one of the foci of the ellipse. This discovery was a major challenge to the prevailing Ptolemaic model of the Solar System, which held that the Earth was the center of the universe and that all other objects revolved around it. Kepler's work helped to pave the way for the Copernican model of the Solar System, which placed the Sun at the center of the Solar System and the planets revolving around it.