The Planets. Andrew Cohen

planet. Without some measure of greenhouse effect, Earth would be completely frozen over and life would not be possible on this planet.

      ‘Thirty degrees or so of a greenhouse effect on a planet like the Earth is absolutely wonderful and crucial and what keeps us alive, what makes Earth such a great place for life because it keeps us in that liquid water zone.

      ‘But, of course you can have too much of a good thing, look at Venus for a possible image of Earth’s future, if we’re not careful.’

      Moving even further into the future the outlook becomes bleaker. As the Sun enters old age it will grow into a red giant, engulfing the Earth within its expanding atmosphere. Moonless, lifeless and perhaps reduced to its inner core, our planet and the civilisations it once harboured will be nothing but a distant memory, etched in the atoms that made us all as they are dispersed amongst the cosmos.

      For planet Earth, the clock is ticking and time is slowly running out, but ours is far from the only world to enjoy its moment in the sun. Across the history of our Solar System, stretching deep into its ancient past and reaching far into its future, we see stories of worlds in a constant battle with our ever-changing star. Close in, ancient worlds such as Mercury, which long ago lost their fight with the Sun, are taunted by views of Earth, and what might have been. Further out and even hotter, Venus circles, shrouded in a choking cloak of cloud, and even further beyond the Earth, Mars sits cold and barren. Beyond these planets, frozen worlds await, huddled in perpetual hibernation; anticipating the moment when the warmth of the Sun reaches out far enough, with sufficient heat, to trigger a first spring. On that day, mountains of ice will melt, rivers of water will flow, and where there was once only bleakness, in the distant future on planets once frozen and lifeless we might find a place that looks very much like home.

      The story of our Solar System is not as we once thought it – eternal and unchanging. Instead it is a place of endless transformation. It is a narrative that repeats itself with a predictable rhythm, and as one world passes, another comes into the light. Only one planet has maintained stability for almost the entire life of the Solar System: the Earth has remained habitable for at least 4 billion years while change has played out all around it. What makes the Earth so lucky compared to all of its terrestrial siblings? To answer that question we need to look not just at our planet but at the whole of the Solar System, going right back to the very beginning.

      © NASA, ESA, and K. Noll (STScI)

      The last colourful hurrah of a star. Ultraviolet light from the dying star causes a glow around the white dwarf at the centre, where the star has burned out. This planetary nebula tells the story of the demise of our own Sun.

      IN THE BEGINNING

      © NASA/JPL-Caltech

      This artist’s image represents a dead star known as a pulsar. The disc of rubble that surrounds it resembles the protoplanetary discs of gas and dust that are found around young stars, which collide and coalesce under the gravitational force of attraction to create young planets.

      For the first few million years after its birth, there were no terrestrial worlds to see the Sun rise, and there were no days, no nights, no circular tracks around it. Instead, surrounding our infant star was a vast cloud of dust and gas. A tiny fraction of the material left over from the Sun’s formation, this swirling cloud would one day coalesce to form the various planets of the Solar System, and many other smaller bodies, but at this time, 4.7 billion years ago, there was nothing but tiny specks of dust reflecting back the light of our slowly growing star.

      Only time – vast amounts of empty time – would allow enough of this gas and dust to catch and cluster, randomly forming the smallest of seeds. Most of these seeds would hardly get the chance to grow at all, smashed apart and returned to the immense swirl of dust from whence they arose. Just a few would grow big enough and survive long enough to capture and condense more of the cloud, slowly increasing their mass and density.

      We still do not fully understand the process by which grains of dust no thicker than a human hair can amass to become rocky objects the size of a car, and as yet no model exists to explain this part of the evolution of a planet. But what we do know is that once that disc of gas and dust becomes populated with clumps of rock that make it past the ‘metre-size barrier’, a powerful force comes into play to propel the process forward. These newly formed planetesimals are big enough to allow the great sculpting force of gravity to draw the clumps together, growing to sizes of over a kilometre in length. Swirling around the Sun, thousands upon thousands of these objects live and die, colliding and coalescing under the increasing gravitational forces of attraction until eventually just a few emerge as planetary embryos, moon-sized bodies known as protoplanets. In the last violent steps of the process of planetary birth these protoplanets swirl around in crowded orbits, and many are destroyed, returned to the dust of their origins, but occasionally when a collision brings two or more of these giant objects together, the size of this mass of rock becomes big enough for gravity to pull it in from all sides, creating a sphere of newly formed rock, a new world. In that moment a planet is born.

      Each of the terrestrial planets in our Solar System was born this way. They are the survivors of a process that destroys far more worlds than it ever creates, and which left just four rocky planets remaining – starting closest to the Sun with Mercury, then Venus, Earth and finally, farthest out, the cold and dead world of Mars. Today these four worlds all look vastly different, and yet all were created the same way, made up of the same ingredients and orbiting the same star. So why have they ended up so distinct from each other, and with such starkly different environments? And what makes this place, the Earth, so unique, the only one of the rocks that has blossomed with life? To understand, we have to look deep into the past of our Solar System, to explore the unique history of each of the planets by means of amazing feats of human engineering across billions of miles, and into environments of unknown and unimaginable extremes.

      © NASA/JPL-Caltech

      EXPLORING MERCURY

      Getting to the smallest planet in the Solar System is anything but easy. Skirting past the Sun at a distance of just 46 million kilometres at the closest point in its orbit, Mercury is a planet that is not only held deep in the gravitational grip of our massive star but is also moving at an average orbital speed of 48 kilometres per second (km/s), by far the fastest-orbiting planet in the Solar System and far outpacing the Earth’s more leisurely 30 km/s. It needs to whip around this quickly, otherwise it would have fallen into the Sun’s embrace long ago, but the combination of its speed and position make it a planet that’s immensely difficult to get close to, and even harder to get into orbit around. In order to do so, you have to travel fast enough to catch up with Mercury but not so fast that you cannot somehow slow down to prevent a headlong descent into the Sun, and that challenge has meant that until relatively recently it was the least explored of all the terrestrial planets.

      For many decades our first and only close-up glimpse of the innermost rock orbiting the Sun came from the Mariner 10 spacecraft, when on three separate occasions in 1974 and 1975 it briefly flew past Mercury. This was the first spacecraft to use another planet to slingshot itself into a different flightpath, using a flyby of Venus to bend its trajectory to allow it to enter an orbit that would bring it near enough to Mercury to photograph it close up. Clad in protection to ensure it could survive the intense solar radiation and immense extremes of temperature, Mariner 10 was able to send back the first detailed images of Mercury as it flew past at just over 200 miles above its surface. It passed by the same sunlit side of Mercury each time, so it was only able to map 40 to 45 per cent of Mercury’s surface.

      © Shutterstock