Professor Brian Cox examines the remotest part of the solar system, a place that the most mysterious planets - Uranus, Neptune and dwarf planet Pluto - call home. Barely visible to the naked eye, Uranus was once thought to be the furthest planet from the Sun, before Neptune was discovered. Thanks to a rare alignment of the planets in 1976, Voyager 2 was sent for mankind's only flyby of these ice worlds. Even at such cold temperatures, great storms whip around these frozen worlds, which are home to spectacular moons and intricate ring systems. Then in 2015, Nasa's New Horizons probe pushed the frontier even further into space with its extraordinary passage to the dwarf planet Pluto.