The Planets. Andrew Cohen

      Heat-loving extremophiles may flourish for millions of years more, but eventually nuclear physics will have its way and as average temperatures race above 100 degrees Celsius, the last pockets of life will be extinguished from the Earth.

      We can say with confidence this is going to happen because we can plot the future of our Sun far more precisely than the future of the Earth. Our understanding of nuclear physics allows us to predict what happens inside the cores of stars and thus we can see the past, present and future of stars like ours written across the night sky.

      The heavens are filled with shining examples of stars that give us a glimpse into the future of our Sun. Arcturus, for example, in the constellation Boötes, is one of the brightest stars in the Northern Hemisphere. It’s around the mass of the Sun, perhaps a little bit heavier, and so in the distant past would have had remarkably similar characteristics to our own star. Today, though, Arcturus is 6 to 8 billion years old, potentially 3 billion years older than the Sun, and as it is no longer a main-sequence star, it is now in the red giant phase. Its fuel exhausted, it has swollen up to 25 times its original diameter and is around 170 times as luminous, despite the fact that as its core slowly burns out it is cooling.

      To see even further into the future, we need to look towards the brightest star in the northern sky – Sirius. The dog star, as it is commonly known, is twice the mass of the Sun and still fully in the main sequence. But obscured by the glare of Sirius A is a faint companion, Sirius B. This is a star that has already burnt through its fuel, swollen into a red giant and the outer layers have drifted off into space, leaving the fading core of the star about the size of the Earth, known as a white dwarf.

      These stars are just two examples amongst many that point us towards the ultimate fate of our Sun, a fate that we believe will play out over the next 5 billion years or so.

      Just like Arcturus, as the Sun exhausts its hydrogen fuel, its outer edge will inflate and it will enter a red giant phase. Expanding millions of kilometres out into space, it will engulf Mercury first. Venus’s fate will be sealed next as the Sun expands further. Some models predict that Earth may just escape the fiery end of its neighbours – heated to 1,000 degrees Celsius but hanging on beyond the edge of the dying star as its orbit extends out due to the lessening mass of the Sun. Dead but not destroyed, Earth and Mars will orbit as burned-out relics of their former selves. The era of the four rocky inner planets will be over, the billions of lives lived on the surface of one of them nothing but a distant memory, but within our Solar System lies another family of rocky worlds whose moment in the Sun may be to come.

      A NEW HOPE

      Far beyond the asteroid belt, millions of miles away from the sun-drenched planets of the inner Solar System, the gas giants of Jupiter and Saturn are home to another family of rocky worlds. Jupiter alone has 79 known moons orbiting it, a menagerie of satellites of multiple shapes and sizes. We’ve been peering at these moons since Galileo Galilei spotted four of them (Io, Europa, Ganymede and Callisto, known as the Galilean moons) over 400 years ago, with his telescope, transforming our understanding of our place in the Solar System.

      Today we have explored the Galilean moons not just from afar but close up and found them to be dynamic worlds. Io is fiercely volcanic and Europa, the ice moon, shows tantalising evidence on its surface pointing to a sub-surface ocean sitting below its icy crust. Ganymede and Callisto make up the final two Galilean moons, and just like Europa they are rocky worlds with an abundance of water ice on their surfaces and perhaps their own oceans lurking beneath. These three rocky, frozen worlds are all sitting in the cold outreaches of our Solar System, touched by the distant Sun but barely warmed, lying dormant until perhaps one day the ageing Sun will reach out and turn these bodies into ocean worlds for the very first time.

      © NASA/JPL-Caltech/SETI Institute

      Created by images taken by the Galileo spacecraft in the late 1990s, this colour view shows Saturn’s icy moon Enceladus – perhaps our closest candidate for sustaining life as we know it.

      © NASA/JPL/University of Arizona/University of Idaho

      Titan, a frozen moon shrouded in its own atmosphere, as seen from Saturn.

      ‘The world is my country, science is my religion.’

      Christiaan Huygens

      The next planet out, Saturn, also has its ever-growing family of moons. Amongst its collection of over 60 confirmed satellites are Titan, the only known moon with a dense atmosphere and liquid lakes on its surface (though they are primarily methane, not water), and Enceladus, a frozen ice moon just like Europa with a liquid ocean deep beneath its ice. We will come to Enceladus in detail in Chapter 4, but for now it’s intriguing to note that this icy moon may be our best current candidate as a second life-sustaining world in our Solar System. Until we go back and explore further we can’t be certain what lies below its surface, but the possibilities that the Cassini probe has so tantalisingly hinted at make it one of the most exciting places for us to visit within the next generation of interplanetary expeditions.

      All these ice worlds, sitting dormant in the frozen reaches of the Solar System, offer the promise of a very different future, one in which the rocky worlds of the inner Solar System have been reduced to cinders, and a new generation of worlds waits to awaken. Ice worlds will become water worlds, warmed by the expanding Sun, until our dying star ultimately collapses into a white dwarf.

      © NASA/JPL/DLR

      From left to right, the moons of Jupiter – Ganymede, Callisto and Io – are dynamic worlds; the former two lie dormant, waiting to be awakened by the warmth of the Sun.