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.