Why do planets farther from the Sun take longer to complete an orbit?

Planets farther from the Sun take longer to complete an orbit due to the relationship between their distance from the Sun and their orbital velocity, as described by Kepler’s laws of planetary motion.1

According to Kepler’s third law, the square of a planet's orbital period (${\tau }^2$) is proportional to the cube of its mean distance from the Sun ($a^3$). Mathematically, this is expressed as ${\tau }^2\propto a^3$, meaning that as the distance from the Sun increases, the time required to complete one full revolution increases significantly.12

Physically, this occurs because planets farther from the Sun must travel along a much larger orbital path at a slower orbital speed. Because the gravitational force from the Sun weakens with distance, planets further out require less orbital velocity to maintain their path compared to those closer to the Sun. Consequently, the combination of a longer orbital circumference and a lower required velocity results in a longer orbital period for more distant planets.32