The Planets

© Igor Shpilenok / naturepl.com (http://naturepl.com)

Russia’s Kamchatka Peninsula is one of Earth’s most inhospitable areas; the volcanic landscape gives us an insight as to how planet Earth might appear when it becomes too hot for life.

© DSS2 / MAST / STScI / NASA

Arcturus, one of the brightest stars in the Northern Hemisphere, which in its early history would have had similar characteristics to Earth.

Perhaps there are even tougher life forms that we are yet to discover, but the thermophilic microorganisms that we have so far identified and investigated in places like Kamchatka all point to the fact that life has its limits. Evolution by natural selection can only adapt so much, and even though it’s impossible to imagine what life on Earth will look like in a few hundred million or even a few billion years’ time, we know that biology is constrained by thermodynamics, and so we can say with some certainty that there will come a time when the Earth is too hot for any living things to exist. Natural selection will eventually run out of options as the laws of physics outplay it, and all life will come to an end.

© NASA, ESA and G. Bacon (STScI)

The blue white-dwarf star Sirius B (pictured to the right of Sirius A) has burned out to a core the size of Earth, giving us an insight into the future of our planet.

When this will happen no one can be certain, but as the Sun ages and grows hotter, temperatures on Earth will rapidly rise. Today the average surface temperature on the planet is 14.9 degrees Celsius, but with just a 10 per cent rise in the Sun’s luminosity, the average temperature will rise to 47 degrees Celsius and climbing. The increased temperatures will raise great storms across the planet. The rains will remove carbon dioxide from the atmosphere and it will be locked away as newly formed sedimentary rock. Trees and plants will struggle as they are robbed of the gas that sustains them, until eventually photosynthesis will cease. The lungs of our planet will fail and the precious oxygen that green plants and algae produce will dwindle. With the primary food source gone, the food chain will collapse and the age of complex life on Earth will draw to a close.

© Science History Images / Alamy Stock Photo

The diamond-shaped constellation, Boötes, has been known to scientists for centuries, described by Ptolemy in the second century.

Astrobiologist David Grinspoon on Venus as a window on Earth’s future

‘Left to its own devices, Earth will go the way of Venus. Now, this is nothing to lose sleep over right now because we’re talking at least a billion years, probably more like a couple of billion years in the future. We have more immediate concerns, but as we do compare the planetology and look at the exoplanets around other stars and consider the variety of planets in the universe and consider not just the past, but the future of our climate in our solar system, it is something to think about, that the current state of Venus is probably some kind of a window into the distant future of Earth under the warming sun.’

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 (#litres_trial_promo), 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.

No 2

EARTH

+

MARS

THE TWO SISTERS

PROFESSOR BRIAN COX

© Shutterstock

WAR OF THE WORLDS

Mars is a mirror for our dreams and nightmares. To the naked eye, the planet exhibits a reddish hue, blood red in the imagination; God of War, Star of Judgement. Through a small telescope, it is the most Earth-like of planets, with cinnabar deserts and white polar ice caps. A world we could imagine visiting, perhaps even settling in. Nineteenth-century astronomers convinced themselves they saw plains and mountain ranges and canals delivering meltwater from high latitudes to arid equatorial cities. Some thought the Martians a peaceful civilisation, far in advance of our own. Others saw threat. ‘Across the gulf of space, minds that are to our minds as ours are to those of the beasts that perish, intellects vast and cool and unsympathetic, regarded this earth with envious eyes,’ wrote H.G. Wells in his classic science-fiction novel The War of the Worlds, in 1897.

The nature of Mars remained a mystery until well into the twentieth century because the planet is small and far away and therefore difficult to view with ground-based telescopes. Even the Hubble Space Telescope, high above the distorting effects of Earth’s atmosphere, produces images which would not at first sight have prevented Wells from publishing. With a little imagination, the ice caps, high clouds and dark regions circling the deserts could be mistaken for evidence of a water cycle feeding the seasonal advance and retreat of vegetation.

© NASA and the Hubble Heritage Team (STScI/AURA)

The topography of Mars, as captured by NASA’s Hubble Space Telescope. The white ice clouds and orange dust storms characterise the planet’s hostile weather systems.

Photographs from the first flyby of Mars by NASA’s Mariner 4 spacecraft on 15 July 1965 abruptly laid to rest the romantic notion of Mars as Earth’s habitable twin or potential foe. These images revealed an arid surface reminiscent not of our blue planet but of our desiccated Moon. Overnight, we discovered for certain that Earth is the only planet in the Solar System capable of supporting complex life, and contemporary accounts of the impact of the Mariner 4 flyby suggest that this was a powerful realisation. In November 1965, the Bulletin of the Atomic Scientists carried an article entitled ‘The Message From Mariner 4’ – and the message was bleak. ‘The shock of Mariner’s photographic and radiometric reports is caused not only by their denial of the terrestrial image of Mars, but by the revelation that there is no second chance, at least not in the solar system.’ President Lyndon B. Johnson was reported as commenting, ‘It may be – it may just be that life as we know it, with its humanity, is more unique than many have thought.’ The hesitation in the first few words is revealing. Here is Mars as a symbol of our cosmic isolation. It is as though deep, or perhaps not so deep, in the subconscious, the 1960s’ power brokers all the way up to the President suddenly understood that the Earth is far more fragile and precious than a dispassionate analysis of their Cold War brinkmanship might suggest. Or perhaps the perspective delivered by exploration is always shocking. Apollo 8’s Earthrise, the photograph that delivered such a positive end to a troubled 1968 by setting the blue Earth against the grey Moon, was three years away, but red Mars provided a foretaste.

‘The flight of Mariner 4 will long stand as one of the really great advances in man’s unending quest to extend the horizons of human knowledge.’

Lyndon B. Johnson

© NASA Image Collection / Alamy Stock Photo

The Mariner 4 spacecraft began its historic journey to Mars on 28 November 1964. It sent back its first pictures to NASA’s Jet Propulsion Laboratory on 15 July 1965.

© NASA/JPL

One site on Mars seen three ways. First imaged in 1965 by Mariner 4.