Copyright: BBC Studios The first episode traces the development of the four rocky worlds closest to the Sun: Mercury, Venus, Earth and Mars. Born together, they battled the violence of the early Solar System to become stable planets. For a while each had a moment when they experienced almost Earth-like conditions. In contrast, today Mercury is a scorched world bearing the scars of its primordial richness in volatile elements. Venus is a runaway greenhouse world with a hothouse atmosphere and Mars is a frozen desert. Only on Earth do oceans – and life – persist, although billions of years from now its fate will replicate that of its sister planets.

Copyright: BBC Studios Early in the story of The Planets, there was a beautiful watery world, but this was not Earth - this was the young Mars. Professor Brian Cox continues his tour of the Solar System revealing that it was once home to not one, but two blue planets.

Sadly, with the decay of its magnetic field Mars also lost most at its atmosphere and with that almost all its water. Today, Mars is an arid desert world that long ago had similar potential to the Earth.  Which begs a big question: did life get a start on both worlds? In search of the answer, plans are afoot to take humans to the red planet; so perhaps one-day soon we will become the Martians.

Copyright: BBC Studios Saturn is the jewel of the Solar System, the most seductive of all the planets, but as Professor Brian Cox reveals - it wasn’t born that way. Raised in the freezing distant reaches of the solar system Saturn began as a strange planet of rock and ice. Born beyond the snow line, with an abundance of building materials, it soon grew to dwarf the Earth, drawing in colossal amounts of the hydrogen and helium that permeated the early Solar System. In time Saturn was transformed into a gas giant, ring-less and similar looking to its great rival, Jupiter.

As NASA’s Cassini probe has discovered, Saturn probably remained ring-less most of its life - until a fateful encounter changed everything. Less than a hundred million years ago, one of Saturn’s ice moons was drawn too close to the planet. In a truly cataclysmic event the entire moon was destroyed and the rings where born.

But Saturn’s true beauty may have remained hidden if it wasn’t for the decision to send Cassini on a risky close encounter with another moon, Enceladus. There we discovered an internal ocean sandwiched between ice and rock, with similar conditions to places on Earth where life thrives, fed by chemical energy where hot springs emerge onto the ocean floor.

Copyright: BBC studios In the final episode of The Planets, Professor Brian Cox journeys to the remote outskirts of the Solar System., Here, beyond Saturn we have so far only visited each world once.

Uranus – barely visible to the naked eye - was once thought to be the furthest planet from the Sun. But with the telescope and some careful viewing the more distant planet Neptune was discovered. Thanks to a rare alignment of the planets in 1976, Voyager 2 was sent for our only flyby of these two ice worlds. There we discovered far more vibrant planets that we ever imagined. Even at such cold temperatures, great storms whip around these frozen worlds that are home to spectacular moons and intricate ring systems. After a few hours of observation at each planet, Voyager 2 left them behind. We have never returned.

For decades that’s as far as we got, until 2015 when NASA’s New Horizons probe pushed the frontier even further into space with its extraordinary passage to the dwarf planet Pluto. Once again, all our assumptions about this distant world were wrong. Pluto’s surface is marked by ice volcanoes, dunes, and geysers – and there is even evidence for a subsurface ocean. What’s more, we discovered that Pluto isn’t alone out there - there’s a plethora of other icy worlds in what we now call the Kuiper belt.