Why did Kepler conclude that planets move in elliptical orbits?

Johannes Kepler concluded that planets move in elliptical orbits primarily through the rigorous analysis of the highly accurate observational data of Mars inherited from the Danish astronomer Tycho Brahe.12

Initially, Kepler attempted to model planetary motion using circular paths, consistent with the Copernican heliocentric theory and traditional geometric models. However, he found that these circular models could not reconcile with Tycho’s precise observations, leaving a discrepancy of about 8 minutes of arc—a margin of error Kepler deemed unacceptable given the accuracy of the data.12

Because Mars possesses a significantly eccentric orbit, its deviations from a perfect circle were large enough to be detected. Forced to reexamine the fundamentals of planetary motion, Kepler abandoned the assumption of circularity and determined that the paths of planets are ellipses with the Sun at one focus. This discovery, published in his 1609 work Astronomia Nova, provided a far more accurate description of planetary movement than previous circular models.12