The four planets closest to the sun, called the rocky planets, were born from the same material in the same era. But they couldn’t be more different: Tiny Mercury is the runt of the litter, almost like a moon. Venus is devilishly hot, and Mars is a frozen desert world. Only on Earth do we find the unique conditions for life as we know it. But why only here? Were Earth's neighbors always so extreme? And is there somewhere else in the solar system life might flourish?