For the planets in a solar system, where T is the orbital period of a planet and r is the radius of circular orbit of planet about its sun:
(1) Show that T∝r23.
(2) Outline what is meant by one astronomical unit (1 AU).
(3) Pluto is a dwarf planet of the Sun that orbits at a distance of 5.9×109 km from the Sun. Determine, in years, the orbital period of Pluto.