By about 3 million years, Jupiter and Saturn had formed and were cooling down. But the protoplanetary disc was still very active. Closer to the Sun, the rocky planetesimals were continuing to gather. And much further from the Sun – twice as far out as Saturn is, and beyond – so too were the last of the icy planetesimals. Despite the abundance of ice there, it took longer for icy protoplanets to accrete to the dimensions where, like Jupiter and Saturn, they could pull in gas directly from the disc, because the orbital speeds there were slower. Eventually, though, two more dominant protoplanets of ice and rock did develop. These would become the outermost giants, Uranus and Neptune.

In time these kernels of rock and ice, each about as massive as the modern Earth, began to stockpile hydrogen and helium, just as the larger cores of the gas giants had done a couple of million years earlier. But they had arrived on the scene too late. The Sun was by now past its T-Tauri phase, and very little gas remained in the protoplanetary disc. For a few more million years Uranus and Neptune seized what little gas they could from the ever-diminishing supply, but their growth ceased after about 10 million years – the exact time remains uncertain. The end result was a pair of planets a little over one-third the diameter of Jupiter and only 5 per cent of its mass. And yet, despite their diminutive statures compared with Jupiter, Uranus and Neptune are each still heavier than 15 Earths. They were more than capable of joining in the game of cosmic billiards demonstrated earlier by Jupiter and Saturn.

The terrestrial planets were latecomers. Because ices could not condense near the Sun, the materials (rock and metal) from which these planets coalesced were a lot less abundant than those that formed the giants further out. So, while the gas planets had formed within a million years – or at most a few million years – and the ice giants took maybe ten million years, for the terrestrials the formation process was even longer.

At least the initial growth of the terrestrial planets, within a few astronomical units of the Sun, had been very fast. Once the first rocky planetesimals had appeared, they had begun gravitationally to attract smaller bits of nearby debris. As we have seen, these first planetesimals grew to dimensions of hundreds or thousands of kilometres in less than 100 000 years. After about one million years the innermost regions of the Solar Nebula were populated by several large rocky and metallic protoplanets approaching the size of Mercury. And by 10 million years these protoplanets had grouped together through gravitation so that only four dominant spheres remained. These, at last, were the primitive terrestrial planets: from the Sun outwards, Mercury, Venus, Earth and Mars. But even after all four of the giants and their satellites had emerged, the terrestrial planets had grown to only half their eventual masses. And they had a very long way to go to make up that missing half – because the supply of available fragments in the disc was now much lower. Moreover, the terrestrial protoplanets had become large enough for the addition of more planetesimals to have a smaller and smaller effect on their size as they continued to accrete.

While the four giant planets were forming, they were not doing it alone. As each of the giant protoplanets stole gas from the Solar Nebula, the material had swirled around the icy kernels to form gas discs like the Solar Nebula on a much smaller scale. Exactly as in the Solar Nebula itself, the particles in these discs had begun to lump together into larger building blocks – and new, independent worlds had started to appear in orbit around the planets. These would become the giant planets' satellite systems – their moons. Because these moons formed from discs, like the planets, they now tend to orbit their planetary hosts in a thin plane, each in the same direction as the others and in fairly circular paths. Moons with these orbital characteristics also tend to be large. They are known as regular satellites.

It is probable that the regular satellites grew to maturity very quickly, even before their planets did. Why? Simply a question of scale. The discs that surrounded the newly emerging giant planets were much smaller than the Solar Nebula, so they had correspondingly shorter orbital timescales. Their rich cargoes of icy volatiles grew to protoplanet dimensions much more quickly than the planets did. But not all of the moons formed at the same time. The Jovian disc, right on the snow line, would have been the richest. So Jupiter's regular satellites – Io, Europa, Ganymede and Callisto – no doubt formed first, alongside their planet, at T-plus 2–3 million years. These are known today as the Galilean moons, after their discoverer.

Thirty million to 50 million years. That's all the time it took to form the star we call the Sun. This may sound like a long time, but let's put it in perspective. Since the last dinosaurs walked the planet, enough time has passed for at least one and possibly two stars like the Sun to have formed, one after the other – utterly from scratch. The details of this miraculous creation are not exceptionally well understood, but astronomers at least have a good grounding in the basics. Perhaps ironically, one star's birth starts at the other end of the line – when other stars die.

Generally speaking, stars make their exit in one of two ways. A low-mass star like the Sun eventually expands its outermost layers until the star becomes a gross, bloated caricature of itself: a red giant. Gradually, the star's envelope expands outwards, all the time becoming thinner, until the dense core of the star is revealed. Such an object is known as a white dwarf. It is a tiny and, at first, white-hot object with a stellar mass – yet confined to live out the rest of its existence within the limits of a planet's radius. The rest of the star meanwhile, the cast-off atmosphere, grows larger and larger. Eventually it becomes nothing but a thin fog of gas spread over more than a light-year. This is the fate that awaits our Sun, as we shall see in detail in Part 4. By contrast, a heavier star dies much more spectacularly. It blows itself to smithereens in a star-shattering explosion called a supernova.

After about 10 billion years, the surface of the white dwarf Sun cools to around 3000–4000 Celsius. At these temperatures the object looks distinctly red (even though it is still called a white dwarf), and is tens of thousands of times dimmer than the main-sequence star it used to be. By now the cooling rate, which slows drastically with age, is incredibly slow. But though it takes an unfathomable amount of time – longer even than the current age of the Universe – the white dwarf Sun one day vanishes totally from the optical window in the electromagnetic spectrum through which we humans today admire the Universe. Too cold to emit any signs of optical radiation at all, the dead Sun ceases to shine. It becomes a black dwarf.

At long last, perhaps 100 billion years from now, maybe even longer, the light will go out in the Solar System. The battered planets still remain, their orbits being stable, huddled around a star that they can no longer ‘see’. But overall, the scale of the planetary realm is almost twice as large as it is today. At only half a solar mass, the dark star's gravity clings quite feebly to its retinue of charred worlds, and they each orbit about 1.85 times further out than they do today. Meanwhile, those planets look little, if anything, like the worlds we know today. Over the tens of billions of years, facing a steadily declining heat source, the terrestrial planets have cooled down to just a few degrees above the coldest temperature possible, absolute zero.

We have seen how the Solar System came to be, and how it has changed in the billions of years since it was born. Now it is time to take a different journey – a journey into the future of the Solar System. This is the subject of Part 4.

We think of the Sun as all-powerful and everlasting. Indeed, on a human timescale it is. Deep within its fiery interior, though, the numbers speak for themselves. On the main sequence the Sun converts a phenomenal 600 million tonnes – the mass of a small mountain – of hydrogen into helium every second, just to keep itself balanced against gravity. At the moment, there is no need for us to worry about this alarming appetite. For the Sun has enough hydrogen to keep its nuclear fires stoked for a good few billion years into the future – long after mankind has vanished. But the day will come when the Sun's fuel heap will run dry, and its useable hydrogen has been totally consumed. When that happens, the Sun will start to die – and with it, the rest of the Solar System.

With the emergence and subsequent evolution of the planetary atmospheres, the Solar System was almost complete. Only two things remained to be added: the rings of the giant planets, and some of the smaller, irregular satellites. The irregular satellites were probably acquired early in the history of the Solar System, when the giant planets captured icy planetesimals from the thinning Solar Nebula. Some are no doubt of more recent origin. The origins of the rings, however, are more difficult to pin down.

The most famous ring system is Saturn's. Consisting of countless boulder-sized, and smaller, icy chunks in individual orbits about the planet, the rings are exceedingly thin – with relative dimensions like those of a sheet of paper the size of a football pitch. But Saturn is not alone, because each of the other giant planets has similar accoutrements, albeit with different characteristics. Indeed, research has shown that no two systems are alike: they differ from each other in terms of diameter, brightness, and in the sizes and compositions of the particles that constitute them. This is a clue to their formation. But the biggest hint is that most of the rings surround their planetary hosts inside their respective ‘Roche limits’. This is the distance from a given planet at which gravitational forces tear apart any body held together mostly by gravity. These clues could mean that the rings are the unassembled ruins of moons that strayed within this danger zone and got ripped to shreds, or the remains of comets that got too close and suffered a similar fate.