The Planets

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DAYS YET TO COME

E arth sits a mere 150 million kilometres from the Sun – not too hot, not too cold, with surface temperatures ranging from minus 88 to plus 58 degrees Celsius. This ‘Goldilocks’ location has created a stability of climate that, despite the best efforts of ice ages and impacts, has allowed life to maintain an unbroken chain for nearly 4 billion years, and yet we know for certain that it cannot last.

Our Sun, like every star in the universe, is far from static. Stars have life cycles of their own and, eventually, the hydrogen fuel that powers the nuclear reactions within a star will begin to run out and the star will enter the final phases of its lifetime. It will expand, cool and change colour to become a red giant. Small stars, like our sun, will undergo a relatively peaceful and beautiful death, which will see it pass through a planetary nebula phase to become a white dwarf, which will cool down over time to leave a brown dwarf. Life on Earth has prospered through our sun’s middle years, but these optimum conditions are waning. At first the changes will be invisible, but a billion years from now they will be obvious to any life forms left on the planet – an immense sun, filling the sky, will warm and transform itself and the Earth that it shines upon. The Sun is both the giver and the taker of life on our planet.

‘What has been is what will be, and what has been done is what will be done, and there is nothing new under the sun.’

Ecclesiastes 1:9

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It is one of the great paradoxes of the Universe that as the life of a star like ours begins to wane, its size and luminosity will increase. A rise in luminosity of just 10 per cent will see the average surface temperature on Earth rise to 47 degrees Celsius instead of the 15 degrees Celsius that it is today. The effect of this rise in temperature manifests in a lifting of vast amounts of water vapour from the oceans into the atmosphere, creating a greenhouse effect that could quickly and rapidly run out of control, evaporating the oceans and sending the surface temperature skyrocketing. Astrobiologist David Grinspoon explains,

The greenhouse effect is the name we give to the physical process by which planets heat up through the interaction of their atmospheres and solar radiation. Solar radiation comes in what we call the visible wave lengths, primarily wave lengths that we can see, and most atmospheric gases are very transparent to visible radiation. So light from the Sun comes through pretty much unimpeded by an atmosphere and reaches the surface of a planet. Then the surface of the planet reradiates that radiation in infrared, because planets are much cooler than the Sun. And that means they radiate at much longer wave lengths – what we call infrared. That infrared radiation doesn’t make it through an atmosphere so easily. Some of the atmospheric gases, the ones we call greenhouse gases, block infrared radiation and so therefore the more of those greenhouse gases that are in a planet’s atmosphere the harder it is for that surface radiation to make it back out into space and the more that planet will heat up.

Estimates of the timescale that will see our oceans disappear vary massively, and are heavily influenced by a multitude of factors, but few are in doubt that by the time our planet reaches its 8-billionth birthday (in 3.5 billion years’ time) the end will be in sight. With temperatures heading above a thousand degrees, life will have long disappeared from a surface that is beginning to melt under the burning Sun.

© DETLEV VAN RAVENSWAAY / SCIENCE PHOTO LIBRARY

These computer artworks show how the Earth might appear in 5–7 billion years’ time. As the Sun swells and becomes a red giant, temperatures on our planet’s surface will soar, making life untenable.

© NASA/GSFC/SDO

Our Sun is far from static, and NASA’s Solar Dynamics Observatory regularly and consistently tracks its rise to solar maximum. This composite image shows 25 shots taken between 16 April 2012 and 15 April 2013, which reveal an increase in solar activity.

© MSFC

This series of photos, captured by the Hubble Space Telescope in 2002, demonstrates the reverberation of light through space. A burst of light from an unusual star in the constellation spreads through space and reflects off surrounding dust. During this activity, the red star at the centre brightens to more than 600,000 times the Sun’s luminosity. It will continue to expand before eventually disappearing.

Astrobiologist David Grinspoon on the greenhouse effect

‘The greenhouse effect gets a bad rap these days and that’s understandable because we are tweaking it in ways we don’t fully understand, in changing the climate balance that we depend upon on this planet.

‘And yet it’s important to understand that the greenhouse effect is an essential part of what makes the Earth a habitable 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

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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.

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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.

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© NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The first glimpse of Mercury from Messsenger, nearly 40 years after Mariner 10’s historic mission.

The spacecraft took over 2,800 photos, which gave us never-before-seen views of the planet’s cratered, Moon-like surface, a surface that we had never previously been able to fully resolve through Earth-based observation. Despite the beauty of the pictures taken, it wasn’t the images from Mariner 10 that really surprised us, it was the data the probe collected relating to Mercury’s geology, which pointed to a much more surprising history of the planet than had previously been imagined. Mercury, it seemed, was far from being just a scorched husk.

Mariner was able to sense the remains of an atmosphere consisting primarily of helium, as well as a magnetic field and a large iron-rich core, opening a mystery that would remain unexplored for another 30 years. As it flew past Mercury for the last time on 16 March 1975, the transmitters were switched off and its contact with Earth silenced. Mission completed, Mariner 10 began a lonely orbit of the Sun that, as far as we know, continues to this day.

‘We have Mercury in our sights.’

MDIS Instrument team, 10.30 am EST, 9 January 2008.

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Mariner 10, launched on 4 November 1973 from Cape Canaveral, was the first unmanned spacecraft to fly past Mercury, managing to map half of the planet’s surface in the process through over 2,800 photos, and giving us a unique insight into its history and makeup.

© NASA/JPL

© NASA, COLOURED BY MEHAU KULYK / SCIENCE PHOTO LIBRARY

These images, both in their original glory and false-coloured, clearly show some of the thousands upon thousands of impact craters that make up the surface of Mercury.

© NASA Wikimedia Commons