The time it takes for a particle to complete an orbit, T, is independent of the radius of the orbit because the speed of the particle changes as it moves around the orbit. Specifically, the velocity of the particle is proportional to 1/r, so as the radius of the orbit increases, the speed of the particle decreases in such a way that the time to complete one orbit remains constant. This is known as Kepler's second law of planetary motion, which states that a planet (or any other object) moves faster when it is closer to the sun (or any other central object), and slower when it is farther away, such that the area swept out by the planet in a given time is always the same.