How to Create a Planet Fit for Life
What is it that allows for the formation of a gas giant versus a rocky moon, a comet versus an asteroid, a planet perfectly suited for the origins and evolution of life versus a barren, lifeless world? Life is resilient but also extremely fragile. It needs to find a delicate balance of complementary conditions and some may say serendipitous events for it to originate, persist and thrive. To create a recipe for life, we need to cook up the perfect world for it to live in. The Earth is such a place, but its journey to becoming host to the only intelligent life in the Universe – as far as we know – has not been a smooth one, and several advantageous events have occurred to help it on its way. All life on Earth owes its existence to a single star that burst into light billions of years ago. In fact, it actually owes its gratitude to the star’s death, not its birth. All the elements in existence, apart from hydrogen, helium and tiny amounts of lithium, descend from those cooked inside the fiery hearts of the first long-vanished stars – from the oxygen we breathe, to the carbon in our cells and the silicon in the rocks that built our planet.
A Cosmic Kitchen
Everyone knows the story of the creation of the Universe: there was a Big Bang and from nothing came everything. Okay, there is a little more to it than that, of course, but in terms of life, this multi-billion-year-long event can be summarised into a few key moments. The Universe is roughly 13.8 billion years old, a number hard to wrap the mind around with the sense of the hours we live every day. So instead of billions of years, let’s imagine just one. If the entire creation of the cosmos were squashed into a single year, there would be 438 years per second, 1.58 million years per hour, and 37.8 million years per day – the cosmic calendar. In terms of the Big Bang, we need not bother with an entire year, just the first 15 minutes.
There might not have been an actual physical bang – especially if there was no one in existence to hear it go pop. But this is as good a description as any of a process where one moment there was literally nothing in existence, and then suddenly the entire mass and energy of the Universe was ignited from a single extremely dense and hot spot called a singularity. At this moment, time officially started, the Universe began to expand and grow, the cosmic kitchen was open for business. There is no life without cells, no cells without carbon, no carbon without matter and all of this is created from three basic particles – protons, electrons and neutrons. A millionth of a second after the greatest cosmic event ever not witnessed, a single lowly proton was formed as the newborn Universe began to cool to a sweltering trillion °C (around 1.8 trillion °F). The proton’s partner in crime, the electron, followed a second later, emerging from a broth of particles and antiparticles simmering at just 1 billion °C (a little over 1.8 billion °F).
After this initial birth by searing fire, the cosmos cooled further and grew increasingly menacing; the dark ages of the Universe had begun and lasted for the next 200 million years. During this time the Universe was a smooth soup of energy, its temperature hovering around 10,000°C (just over 18,000°F). The newly crafted protons, neutrons and electrons started to combine and the first atoms began to form. They initially assembled themselves into a single hydrogen atom, the most basic yet most abundant element in the Universe today. From hydrogen came helium and smatterings of lithium and beryllium. The basic ingredients for the recipe of life had been created but in this period the Universe was still completely sterile.
The newly created atoms effectively neutralised the Universe; it was no longer dominated by negatively or positively charged particles, which allowed matter to start to congeal owing to gravity, creating nodules or balls of concentrated matter that threaded across the still growing Universe. This is the birthplace of galaxies. Flurries of proto-dwarf galaxies formed – visually more akin to nebulae than the grand spiral and elliptical galaxies of today. These were the site for the creation of something without which life would not exist. They were the birthplace of the very first stars.
Star Light, Star Bright
Light came to the Universe some 200 million years after the Big Bang. These first illuminating stars were made when a small parcel of gas within one of the newly birthed dwarf galaxies started to feel its own gravity and began a slow but accelerating inward collapse. The earliest stars were simple, formed only of hydrogen and helium as this was the only matter in the Universe to have been created so far. They grew within tremendously hot blobs of gas and were enormous. These first stars illuminated with an extraordinary brilliance but their life was fleeting, and they died in glorious supernova explosions that seeded the surrounding gas with their remains. Through these eruptions, nebulae of dust and gas were created that grew to become the stellar nurseries of the Universe. They provided the starting materials for each new brood of stars, which themselves died and flung even more elements, gas and dust into the Universe to be used to build generation after generation of stars – the stellar circle of life.
Without stars there would be no elements of the periodic table, and without elements there would be no Earth and no life. Stars are like giant nuclear reactors, constantly churning, creating and destroying elements. This deep nuclear fusion is what makes them shine. When their core reaches a high-enough temperature (a few million degrees) atoms are subject to tremendously violent collisions that release an enormous amount of energy; their nuclei begin to fuse together creating an entirely new element. In the early stages of a star’s life, this reaction involves the nuclei of two hydrogen atoms combining to make helium – a process called nucleosynthesis. A star slowly converts its hydrogen into helium but there is not an everlasting supply. As the hydrogen fuelling the star’s very existence is exhausted, nuclear reactions can no longer continue and the core begins to collapse under its own gravity. These higher temperatures cause the star to shine up to 10,000 times more brightly than before. The outer layers of the star then expand outwards, decrease in temperature, and the star becomes a red giant – technically more orange than red, but a giant nonetheless. What happens to the star after this bloating and blushing phase depends upon how large it is.
The smallest stars only convert hydrogen into helium, and that is the end of their life. Medium-sized stars (such as our Sun and those up to two times the mass of our Sun) will start to convert the newly formed helium atoms in the core into carbon and oxygen as temperatures reach 100 million degrees. Three helium nuclei fuse together to create carbon, and then an addition of another helium to this carbon molecule creates oxygen. The stars now have a core of carbon-oxygen. The largest or most massive stars (greater than five times the mass of the Sun) will convert hydrogen to helium, then helium to carbon and oxygen, followed by fusion of carbon and oxygen to form neon, sodium, magnesium, sulphur and silicon. Further reactions can take place, transforming the core into calcium, nickel, chromium, copper and finally iron. A needlessly confusing side note: most of the physical matter in the Universe is in the form of hydrogen and helium, so astronomers conveniently use the blanket term ‘metal’ to describe all other elements. So when the phrase metal-rich star pops up, the elements described are non-metals as far as chemistry is concerned, but considered metals in astrophysics. As I said, confusing. Through all these stages of nucleosynthesis, each new metallic element created forms in the fiery soul of the star and is surrounded by a shell of the elements that came before it. If a slice were taken through one of these stars, it would display multiple layers of a giant elemental onion. Finally, we know what makes life possible and where the basic ingredients for life came from: stars. These blazing engines are the creators of carbon, the third most abundant element in the Universe, and by far the most important ingredient in our own creation.
But we have to ask – how do all these elements get from inside a star to our bodies? We mentioned before that the earliest stars that lit up the new Universe 200 million years after the Big Bang exploded in what we call a supernova. But what actually is a supernova? Does every star die in this way? And what does that mean for life? The very low mass stars finish nucleosynthesis with a core of helium and a shell of hydrogen but do not go supernova. They have a quiet, dignified death. They simply start to become less luminous, ending their lives as a cool helium white dwarf. When stars of great mass (8–25 times that of the Sun), however, have reached the stage at which they contain a fiery heart of iron, and no more reactions can take place, their elemental factory shuts down and the iron core collapses under the force of the star’s gravity and implodes. This collapse releases a catastrophic wave of gravitational potential energy, causing an explosion that very briefly can outshine an entire galaxy – the star has gone supernova. This explosion spews nearly all of the star’s matter and energy into space at up to 30,000km/s (around 18,640 miles per second, or 10 per cent the speed of light).
Supernovae are essential for the creation of life as they are a vital source of elements heavier than oxygen. Nuclear fusion within the cores of stars creates elements lighter than iron and, although this is where the story ends for the star, it is not the end of nucleosynthesis. The power of the supernova explosion itself causes further chemical reactions that create even more elements, including plutonium and uranium. The Big Bang produced hydrogen, helium and lithium, while stars and supernovae synthesised the rest, enriching the interstellar medium and molecular clouds with metals.
What is left after a supernova is a compact object and a rapidly expanding shock wave of material heading into, and mixing with, the interstellar medium. This process, as brutal and destructive as it sounds, is actually good for life. These death throes of stars created and then seeded the CHNOPS elements (carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur) throughout the Universe – the elements that are needed to build life (and everything life needs).
The Gauntlet of Galaxies
Around 2.5 billion years after the Big Bang, and 2.3 billion years after the first stars sparked into life, gravity began to pull all the generations of stars thus far created into groups or clusters, commonly known as galaxies. A galaxy at its most basic level is a gravitationally bound system of stars and their remains – an interstellar medium of gas, dust and dark matter, all orbiting around a central point. There may be anything from a few hundred thousand stars to many hundreds of billions within a single galactic neighbourhood. Even within the small patch of Universe observable from the Earth there are hundreds of billions of galaxies and thanks to the work of famous astronomer, Edwin Hubble (1889–1953), we know that they will have one of four different shapes: spiral, elliptical, lenticular or irregular – but each as unique as a fingerprint.
One of the most familiar and intricately beautiful galaxy shapes is the spiral galaxy. In fact, when imagining a galaxy, this is what first comes to mind. This is because the Milky Way, the most famous of all galaxies, its neighbour Andromeda, and 77 per cent of all the galaxies so far seen, are winding, flat, disc-shaped spiral galaxies that loosely resemble an octopus. They basically consist of a central bulge (the head) with a number of different arms (the tentacles) spiralling outwards. Unlike those of an octopus, however, these arms are not restricted to eight in number. These twisted galaxies can be tightly wound coils of dust, gas and stars or loosely splayed tendrils, with all degrees in between. The oldest observed spiral galaxy, BX442, is around 10.7 billion years old yet these coiled galaxies are believed to be much younger than the less glamorous elliptical varieties. Since the Earth is currently orbiting within a spiral galaxy itself, it would be easy to think that spiral galaxies may be better suited for life. Our continued existence in this rotating mass of stars and dust, however, is the result of so many more factors than just its image, but the way its stars are able to move in well-defined orbits does make it a safer and more stable design of galaxy for the long-term prospects of life.
Elliptical galaxies are the most massive of galaxies with few or no dust lanes, being all central bulge and no disc. They are largely composed of older mature stars and have little to no rotation, so the stars display a variety of orbits, haphazardly moving around like a swarm of flies. These galaxy types seldom have stellar nurseries or new star–forming regions. Owing to stars orbiting in any manner of directions they commonly find themselves heading on a collision course with each other and the centre of the galaxy. As they draw closer, the increasing proximity and density of other stars produces an environment of high radiation and gravitational instability. If that were not chaotic enough, the statistical chance of these stars coming into close contact with more than one ancient star about to go supernova is much higher. These galaxies are therefore believed to be incredibly unfavourable for the emergence of worlds suitable for and capable of sustaining life.
A Galactic Goldilocks Zone
Planets qualified to support life are much more likely to exist around stars that reside in certain parts of a galaxy. The galactic habitable zone (GHZ) is a galaxy-wide Goldilocks zone, a theoretical ring threaded through a galaxy, where conditions exist that are favourable for supporting life should it happen to arise in any orbiting solar system. It covers a region lying in the plane of the Galactic disc that possesses enough of the heavy metallic elements that would be needed to build terrestrial planets like the Earth or Mars. As mentioned before, supernova explosions were responsible for the creation of interstellar dust clouds. These became increasingly more metal-rich with every additional stellar deposit and formed a nest filled with new baby stars. With more metal-rich material available, these younger stars were more likely to be able to grow planets to orbit around them. Where there are planets, there may be life. Galaxies, therefore, effectively have a Goldilocks zone of metallicity, a belt stretching across their waist whereby the amount of metals is just right to go into the formation of planets and where a planet fit for life can exist.
Within the GHZ, the cosmic environment needs to be sufficiently accommodating over several billion years to allow for the biological evolution of complex multicellular life, i.e. us. A major threat to this is the, up until now, very helpful supernova explosion. The blast waves created during the detonation, despite sending biologically useful elements into space, also release deadly cosmic rays, gamma rays and X-rays that can be fatal to any life form watching wide-eyed on a nearby planet or moon. This supernova fear factor is greatest in areas with the most stars and the largest amount of star formation. Keeping out of the way of the Galaxy’s spiral arms is another requirement of a GHZ. The sheer density of gases and interstellar matter in the spiral arms leads to the birth of new stars. Although this is a good thing and can lead to the creation of planets and ultimately life, it would be dangerous for an already inhabited solar system to cross paths with one of them. The intense radiation and gravitational chaos of entering a spiral arm would cause catastrophic and life-threatening disruptions in our Solar System.
So to build an ideal planet suitable for life, we would start with a spiral galaxy and a metal-rich young star. This star would be orbiting around the core of the galaxy within a ring-shaped Goldilocks region at just the right distance from the galactic centre so that it has the minimum metallicity needed to form some rocky life-friendly planets, but is far enough away from the centre so that its solar system would not be continually plagued by swarms of exploding stars.
More Than Just Chocolate
After the chaos of the Big Bang, stellar births and deaths, and the formation of the first galaxies, there is really only one spiralling neighbourhood we are personally invested in: the Milky Way. It is home to the only example of life that we know of, and therefore rather an interesting place. It measures between 100,000 and 120,000 light years in diameter – a light year actually being a measure of distance, not time; it represents the distance that light can travel in a year, a cosmic speed limit, if you like. To put this expanse in perspective, light travels 9,460,528,398,225km (5,878,499,810,000 miles) over the course of one year, so multiply that by 100,000 years and the size of the galaxy is almost unfathomable.
The Milky Way is almost as old as the Universe itself. Recent estimates put the age of the Universe at 13.7 billion years, and our Milky Way has been around for up to 13.6 billion of those, give or take 800 million years. This is measured based on the age of the oldest stars in the Galaxy, so the Galaxy must be older than they are. It is part of a larger family of at least 100 galaxy groups, each made up of 50 individual Local Group galaxies that include our neighbour Andromeda, forming a team known as the Virgo Supercluster. Within this family of galaxies, the Milky Way is moving through the Universe at a speed of 600km/s (over 370 miles per second). It contains between 200–400 billion stars but when you look up at the night sky with the naked eye, the most you can see from any one point on the globe is about 2,500 stars. The number of stars in the Milky Way changes yearly owing to deaths and births; about seven new stars are born every year. Although this stellar headcount sounds impressive, the Galaxy is only a middleweight. The largest galaxy we know of is IC 1101 which has more than 100 trillion stars. Despite all this stellar illumination you cannot actually see 90 per cent of the Milky Way as most of its mass consists of dark matter that creates an invisible veil. In fact, every picture ever published of the Milky Way in its entirety is in fact not the Milky Way at all but another galaxy or an artist’s interpretation. Currently, we cannot actually take a picture of the Milky Way from above because we are buried inside the galactic disc, about 28,000 light years from the galactic centre. It would be like trying to photograph your own house from the inside. But we do know that although incredibly beautiful and perfect for us, it is actually imperfect and warped. Neighbouring dwarf galaxies of the Large and Small Magellanic Clouds which are made up of a mere 10 billion stars are playing a game of tug-of-war with the Milky Way, pulling on its halo of dark matter and distorting the vast quantities of hydrogen gas. The result is a disc that resembles the profile of a sombrero.
The Milky Way is structured like billions of other spiral galaxies; it is not particularly special in that regard. Strong emissions of infrared radiation and X-rays leaking from its galactic centre have strongly hinted that clouds of ionised gas are rapidly moving around some sort of dark object – a black hole, and a supermassive one at that, called Sagittarius A*, believed to measure about 22,530,816km across (14 million miles), or about the extent of Mercury’s orbit around the Sun.
Around this black hole we find the highest density of stars in the galaxy and they are some of the oldest. Surrounding this ‘nuclear bulge’ is the galactic disc containing a lot of interstellar matter (dust and gas), as well as young and intermediate or middle-aged stars. Extending beyond this disc is a swollen ‘galactic halo’ where very old star populations are clustered. The high volume of stars in the central bulge influences the metallicity gradient spanning the Milky Way from highest in the galactic centre and decreasing outwards. More stars also means more supernova explosions and more metals returned to the interstellar medium to go into making yet more stars. Larger galaxies with a greater number of stars therefore tend to have a higher metallicity than their smaller counterparts. The Milky Way is by no means the largest galaxy out there, but it isn’t the smallest either. Some 80 per cent of galaxies are less luminous than the Milky Way, but this is not vanity talking. A galaxy’s brilliance is positively affected by its metallicity; the more metals present, the brighter a galaxy shines, and the more metals it has, the greater the likelihood of life arising. This statement depressingly puts 80 per cent of the Universe in a category in which life is far less probable.
The vast Milky Way halo and also the thick inner disc region are dominated by the older stars with low metallicity, so any planets that may arise around such stars are not predicted to have the materials needed for life. Some of the inner regions of the Galaxy, however, have the high metallicity required for the formation of terrestrial planets, but they are also unlikely to be suitable for life as they would be much more prone to suffering from extremes of radiation, being violently thrown around by gravitational fluctuations and hit by supernova shockwaves. The main region of the Milky Way that would be amenable for life is the thin disc where, coincidently, our Sun is found. The Sun’s metallicity is used as a baseline (as we know it is good for life). Our Sun is exceptional in being both long-lived – currently 4.5 billion years young – and having a 40 per cent greater metallicity than most stars of the same age. Stars having between 60 per cent and 200 per cent of this level of metals are found in a region encircling the galactic centre at a distance between 15,000–38,000 light years – the Milky Way’s GHZ. Unfortunately, for the possibility of other life-bearing worlds in the Milky Way, however, this area contains only about 20 per cent of the total stars in the Galaxy. Also, just because a star falls within this 20 per cent, does not automatically mean it has the potential to sustain life.
The Milky Way is not only moving but growing; it is a cannibalistic galaxy currently gobbling up hydrogen from those same Magellanic Clouds that are causing it to warp. They are currently only 80,000 light years away from the centre of our Galaxy, technically already within the embrace of the Milky Way. They will probably be entirely absorbed into our Galaxy in about one billion years. The Andromeda Galaxy also seems set on a collision course with the Milky Way, a convergence that may begin about three billion years from now. The two galaxies will collide head-on and fly through one another, leaving gassy, starry trails, and scrambling stars to create new constellations. This may prove disastrous for any and all life forms, should they have remained in the area.
A Solar Family
The Solar System lies on the edge of the Orion–Cygnus spiral arm of the Milky Way, a nice secure distance away from the galactic centre, around halfway out, in a relatively uncrowded part of the Galaxy. Its orbit is remarkably circular around the galactic centre, which as a consequence keeps the Solar System safely away from the supermassive and super-destructive black hole, Sagittarius A*, at its core. A location in this suburban region also protects our solar family from the huge gravitational tug of stars clustered near the centre, keeping other planetary bodies such as planets, moons, asteroids and comets in their orbits and out of our way. The threat of any nearby stars going supernova and wiping us out is also reduced.
The Milky Way revolves around Sagittarius A* once every 250 million years but not uniformly, with the inner regions moving faster than the outer, and stars constantly overtaking each other. The four spiral arms do not rotate as a rigid structure either, so the orbit of the Solar System will inevitably pass through one of these spiral arms one day; every 100 million years to be exact, and it will take a further 10 million years to make it through, hopefully unscathed! As it takes this daring manoeuvre it will have to avoid increased supernova risks as the concentration of stars grows greater, and dodge the large dusty stellar nurseries, whose dust could infiltrate the Solar System and block out sunlight to any planets and moons. This has the potential to cause life reliant on sunshine to shut down and cool a planetary surface enough to cause an ice age. Luckily, however, the Solar System moves at nearly the same rate as the Galaxy’s spiral-arm rotation. This synchronisation or ‘co-rotation cycle’ prevents our Solar System from crossing a spiral arm too often.
The Sun is the source of all life on Earth and only its continued presence allows life to exist. A ball of gas held together by its own gravity, it is composed of hydrogen (92.1 per cent), helium (7.8 per cent), and less than 0.1 per cent metallic elements. The diameter of the Sun is about 109 times that of Earth (we could fit some 1.3 million Earths inside it) and is an almost perfect sphere, with a difference of just 10km (6.2 miles) in diameter between the poles and the equator. It is actually the closest thing to a perfect sphere that has ever been observed in nature. The Sun is a G-type main-sequence star (G2V), informally designated as a yellow dwarf, with a surface temperature of 5,500°C (9,932°F). The temperature in its core however is about 15 million °C (27 million °F) – it is a nuclear reactor after all.
But where did our Sun and its planet suitable for life come from? Well, about 4.6 billion years ago in a nebulous cloud of gas dust and stars far, far away, a grandparent star exploded in an incredible supernova event. It sent a powerful shockwave through the nebula, compressing regions sufficiently so that they began to collapse under their own gravity. The contracting gas started to move and rotate, whirling faster and faster, until it became a violently swirling storm that eventually broke apart into smaller vortices – each with the potential to form a solar system. The death throes of this single star gave birth to a whole brood of new stars and solar systems. One of these galactic tornadoes began to collapse and flatten into a rotating accretion disc swirling around a region that would become the point of origin for a single central star, the Sun. Of the entire disc, 99.86 per cent of its mass, which itself is 99 per cent gas, made its way into the Sun, but even the seemingly tiny amount of mass left was more than enough to make a family of planets, moons, asteroids and comets. The disc was enormous, stretching ten times the distance Pluto currently orbits from the Sun, and apart from gas was formed of tiny grains of dust – the seeds used to grow our planet.
The Rocky Worlds
As the Sun began to shine out across this foetal landscape, dust grains were drawn to each other, sticking and clumping together, growing into larger and larger lumps of rocky material. Once they became large enough, gravity caused them to be pulled towards each other. These rocky embryos became rounded because gravity pulls equally towards the centre of large masses, so anything jutting out was pulled back to form a ball. Within about one million years the accretion disc had become a rock garden, with several hundred rounded planetary pebbles orbiting around the infant Sun. This was not a calm period. The Solar System resembled a cosmic pinball machine with planetary embryos thrown in all directions by gravity, crashing into each other, breaking apart and recombining into larger bodies, or sent hurtling towards the Sun or even flung out of the Solar System entirely. These colliding embryos became the building blocks for the rocky inner planets of the Solar System, including the Earth. The heat of these impacts and the decay of radioactive elements within the rocks themselves melted the interior of the growing planets causing the heavier elements such as iron to sink to the centre, forming a core. These embryos took 100 million years to become fully grown planets but at this stage remained barren rocky worlds, lacking water, carbon and life.
The Gassy Worlds
A stream of charged particles heading straight out from the Sun, called the solar wind, swept away the lighter elements such as hydrogen and helium, from the Solar System’s inner regions, leaving only heavy rocky materials to create the smaller terrestrial worlds such as Earth. Further away, the solar winds had much less strength, allowing hydrogen and helium to remain and be included into the building of the outermost worlds.
Just as when climbing a mountain an elevation is reached at which the ground begins to be covered in snow, at a certain point within the solar nebula an imaginary snow line forms, where falling temperatures freeze water and other volatile chemical compounds such as methane, carbon dioxide and carbon monoxide, and cluster them together. During the formation of our Solar System the water snow line was found around five times the distance from the Earth to the Sun, more or less where the asteroid belt is found today. Inside this boundary was a zone too warm for volatile compounds to be incorporated into the growing dust grains, so they became a gas and were lost. Beyond it, they condensed into solid ice grains and were built into the growing worlds. Not all chemical compounds have the same freezing point as water, however, so different molecules will freeze at different distances from a central star and may be the reason of why there are specific types of planet. For example, the carbon monoxide line in our system corresponds to the orbit of Neptune, and could also mark the starting point from where smaller icy bodies such as comets and dwarf planets like Pluto began to form.
The temperatures in the outer Solar nebula during this phase were well below –120°C (–184°F), creating many more solid grains available for incorporation into planets – and so the large gas giants were born. Of the mass not taken up by the growth of the Sun, the remaining 90 per cent went into Jupiter and Saturn. The cores of these larger planets are rocky and icy but still more than 30 times bigger than the Earth. This means that their gravitational attraction was strong enough to draw in large amounts of hydrogen and helium-rich nebula gas, making the planets even more massive, which in turn pulled in even more gas and dust – the ultimate snowball effect. At the furthest reaches of the Solar System there was little material left to make whole planets, so smaller icy bodies formed, like Pluto and the comets.
So there we have it – from a swirling nebula of gas and dust we have a solar system family of inner rocky terrestrial planets and outer gas giants orbiting around a middle-aged yellow dwarf star. In actual fact, the Sun is encircled by eight planets, at least five dwarf planets, tens of thousands of asteroids, and up to three trillion comets and icy bodies. Yet, within this extended family there is only a single known planet that supports one of the most precious commodities in existence: life.
Building a Home in the Goldilocks Zone
Planets, unlike stars, do not run a nuclear reactor in their cores and so are relatively cool, grateful for the warmth provided by their suns. In a star system such as ours, consisting of multiple planetary bodies, there is a special region that is uniquely suitable for planets to grow and nurture the conditions amenable to life; we call it the Goldilocks Zone, or more officially the circumstellar habitable zone or CHZ. In this imaginary region that encircles the Sun, surface temperatures on planets and moons have the potential to be able to sustain liquid water – they are wet. If worlds were to exist inside this zone and closer to the Sun, such as where Venus and Mercury reside, they would be too hot on their surface and any liquid water would boil away. They would be dry and desiccated. Planets and moons further out past the CHZ would be colder and any water present would freeze, as it has beneath the surface of Mars or on the surface of Europa. They would be frozen worlds (and incidentally dry worlds, too). Estimates of the CHZ boundaries in our Solar System are 0.75–0.95 astronomical units (AU) for the inner boundary and 1.37–1.90AU for the outermost, with the Earth positioned comfortably in the middle at 1AU (an AU being a unit of length roughly the distance between the Earth and the Sun). This positioning of the CHZ, however, is quite a simplistic view as for each individual solar system there are many variables to consider that might move or stretch its boundaries, such as the size and radiation of the central star, and the dimensions, mass and atmosphere-holding abilities of its individual planets and moons. Present CHZ models also do not include secondary regions where life not reliant on the Sun as a source of heat and energy could exist.
A Family Affair
Is it solely the distance a planet or moon sits from its sun that determines whether life arises or not? Although a critical dimension, there are many other factors, particularly in our Solar System, that have allowed Earth to hold this unique and privileged position. The first is, perhaps, the protection Jupiter offers the Earth. The comet Shoemaker-Levy 9, discovered in 1993 by astronomers Carolyn and Eugene Shoemaker and David Levy, was observed in July 1994 from hundreds of observatories around the world as more than 20 fragments of it crashed into Jupiter’s southern hemisphere. In July 2009, a second comet or asteroid this time ripped a Pacific Ocean-sized hole in its surface. This gives the impression that Jupiter is acting as a protective big brother for planet Earth – a celestial shield if you will, sacrificing itself and deflecting asteroids and comets away from the inner Solar System. The planet’s enormous mass – more than 300 times that of the Earth – is enough to catapult comets that might hit Earth out of the Solar System. It is also thought that Jupiter’s gravitational pull could thin the crowd of dangerous asteroids and other objects, making Earth less impact-prone. Models of the formation of the Solar System suggest that the presence of a planet as massive as Jupiter also helped to conserve the Sun’s angular momentum and stabilised the entire planetary system, especially the motions of the inner terrestrial planets. Three cheers for Jupiter …
Next, it is perhaps the presence of the Earth’s unusually large Moon that may have helped the Earth become stable enough to be a home for life. There is some dispute as to the origins of the Moon, but today the most commonly accepted theory is that about 30–50 million years after the initial formation of the Earth, a huge object with a similar mass and size to Mars smashed into it. As a result of this collision a significant part of the proto-Earth was ejected into space. This debris and that of the impactor coalesced into what became the Moon, one of the largest planetary satellites in the Solar System, with a diameter 25 per cent that of the Earth.
Earth’s only satellite, it settled into orbit at a relatively close distance of about 30,000km (some 18,600 miles) but is gradually receding from us by about 3.8cm (1.5in) a year. Our Moon is also slowly braking the rotation of the Earth to the tune of about 1 second roughly every 67,000 years. These are both effects of tidal forces occurring between the Moon and the Earth. The Earth’s rotation is slowing down due to rotational energy transfer to the Moon through the tides and because of this the Moon is very slowly increasing its orbital radius – and moving away from us.
The Moon has played a number of roles in the evolution and continued presence of life on Earth, although how necessary these have been is not entirely clear. The most familiar effect the Moon has is on the liquid envelope of the Earth, driving the tides, but should we lose it for some terrible (and unimaginable) reason, the Sun could in theory take over. Life wouldn’t suddenly be extinguished because of this. The chief role the Moon plays pertains to the stabilisation of the Earth’s axis over time. The tilt of the Earth is the main driver of the seasons, and this varies from 22.1 degrees to 24.5 degrees and back (known as the change in obliquity) over a span of 41,000 years, currently at a value of 23.4 degrees and decreasing. Without the large Moon to dampen this change in tilt, much wider and life-threateningly unpredictable swings would occur. This stability of Earth’s seasons and climate has allowed for even the most complex multicellular organisms to evolve and thrive.
From the Inside Out
We know with some certainty the internal structure of the Earth – it looks a bit like the inside of a gigantic plum or peach. The Earth’s innermost part, its inner core, is extremely dense and mostly made up of iron and nickel. It is unbelievably hot with a temperature of around 7,000°C (12,632°F) but instead of melting to form a liquid, it is completely solid. This is because of the tremendous pressures weighing on it, measured in gigapascals (GPa), as the mass of the entire planet pushes down exerting in the order of 360GPa. The source of these soaring temperatures this far underground cannot be due to the Sun. They are partly caused by left over heat produced during formation of the Earth and partly due to the decay of radioactive isotopes of potassium, uranium and thorium, whose half-lives are in excess of a billion years. This heat diffuses outwards, and makes a small contribution to the temperature balance of the Earth’s surface. Interestingly, it also provides the heat and energy used by microbes that inhabit the outermost 3km- (1.86-mile-) thick layer of the planet.
The outer core is liquid and this 2,253km- (1,400-mile-) thick layer of iron and nickel moves or convects. This flow of metallic liquid is believed to create a geodynamo that influences the Earth’s magnetic field, a shield that extends from the Earth’s interior through the planet and several tens of thousands of kilometres out into space where it does battle with the solar wind. The magnetosphere deflects the Sun’s charged particles and cosmic rays away from the Earth, protecting the atmosphere, which would otherwise get stripped away, thus protecting life on the Earth from exposure to deadly ultraviolet radiation.
Above the outer core lies the mantle, composed of hot but not quite solid rock. Its topmost layer, the asthenosphere, flows like a liquid but moves extremely slowly. This upper mantle layer and the outer crust together make up the lithosphere, the rigid outermost shell of the Earth, which is broken up into plates. The movements of these lithospheric plates over the mantle are known as the process of plate tectonics, and are the source of numerous phenomena that strongly affect life.
Earth’s Recycling Plant
The seven major and many minor plates of the Earth’s lithosphere ride atop the asthenosphere, travelling from only a few millimetres to up to 15cm (6in) a year. Each lithospheric plate can be topped by up to two types of crust – oceanic crust and continental crust. As their names suggest, one is created under the seas of the Earth and the other builds the land. The plates topped with continental crust can be up to 200km (124 miles) thick when they are carrying mountains, whereas those shifting oceanic crust are only 80–100km (50–62 miles) thick. There are no gaps between the plates; they all touch, forming a fractured but continuous rocky skin around the Earth. Where each plate meets another, however, their relative movements create different types of boundary: divergent, where the plates are slowly pulling away from each other; convergent, where the plates are colliding with each other; and transform, where the plates are slowly rumbling past one another in opposite directions.
As the lithospheric plates ride over the convecting mantle, new oceanic crust is formed along mid-ocean ridges, pushing the plates along and forcing the older oceanic crust back inside the Earth – a process called subduction. As such, these undersea rocks are relatively young (less than 180 million years old). Continental rocks, however, can be as old as 4 billion years – almost as old as the Earth itself. When two continental plates converge neither sinks and the plates buckle and crumple together, rising up to form massive mountain ranges. The Himalayas formed as a result of the Indian and Eurasian plates running into each other. Many of the characteristic features of the Earth, such as earthquakes, tsunamis, volcanoes, black smokers and mountain ranges, can be explained by these plate tectonics and the types of boundary between them – as such the consequences of plate movements are of great importance to the evolution and continuation of life.
The surface environment of the Earth shows long-term relative stability because by using plate tectonics it can regulate its own temperature through a process called the carbon cycle. It works like this: carbon dioxide from deep within the Earth is constantly pumped into the atmosphere via volcanoes and deep-sea hydrothermal vents, which in turn are made possible by plate tectonics. But the carbon dioxide from volcanoes does not stay in the atmosphere indefinitely. It is actively removed by a process called chemical weathering whereby it reacts and combines with the rocks on the Earth’s surface, and is then returned to the inside of the planet through plate subduction, preventing any life-threatening accumulations of carbon dioxide from occurring in the atmosphere. This process is, of course, as everything else, temperature sensitive and works faster at higher temperatures. By this mechanism, the Earth regulates its temperature and keeps it habitable for life: if the planet’s atmosphere starts to get too hot, the weathering rate will increase and more carbon dioxide will be drawn down and chemically captured within the rocks of the Earth. The concentration in the atmosphere will then decrease, reducing the greenhouse warming caused by too much carbon dioxide, and the temperature of the planet will drop. It is a delicate balance though. If the temperature of the planet drops too much and it starts to freeze, tectonic processes will work to ensure that carbon dioxide is pumped back into the atmosphere to help it warm up again. But without liquid water (as it would all be frozen) there would be no carbon dioxide removal by weathering, so the carbon dioxide concentration will build up in the atmosphere until the temperature rises to the point at which the ice melts, and weathering commences again. During weathering, carbon dioxide is converted to a soluble ion known as bicarbonate (HCO3–), which precipitates in the oceans as minerals such as calcite and dolomite that go into making seashells, coral reefs and the white cliffs of Dover. These minerals are decomposed when subducted and drawn back into the Earth, releasing their load of carbon dioxide into the mantle, ready to be erupted again by volcanoes. The cycle is complete and the Earth, through plate tectonics, maintains a warm, water-rich environment suitable for both simple and advanced life.
What would happen to life if the tectonic plates stopped drifting? Well, the Earth would be a very different place. The volcanoes of the Pacific Ring of Fire, in South and North America, Japan, the Philippines and New Zealand, for example, would shut down, and there would be far fewer earthquakes. Erosion would continue to wear down the mountains, but with no tectonic activity to refresh them, over a few million years the whole planet would be a great deal flatter. The level of the seas would rise as the polar caps melted, and most of today’s dry land would be submerged. Only a small number of isolated dry islands would survive. Would intelligent life have arisen on Earth if it were a mostly aquatic planet? What would happen to all the land-based life? We don’t know. If the plates stopped moving, the planet would need to find a new way to regulate its temperature if life were to survive. It is not clear what that mechanism might be, or even if one exists. Perhaps the Earth’s crust would appear like the single plate crust of Venus and fall victim to catastrophic volcanic episodes? What is clear to us is that for a world to be habitable for life it needs a way to regulate its temperature so as to keep it suitable for its indigenous life forms. As far as we know, plate tectonics is the perfect mechanism to achieve this.
Part of the handicap we face when designing the perfect world for life to thrive is that there is only one planet in the Solar System where we can currently observe processes such as plate tectonics – any evidence for it on Venus and Mars is at best very tenuous. Life on Earth is adapted to the effects of plate tectonics, but until we find another example somewhere nobody can say if tectonics are crucial for life to exist. There are, however, three factors that we know with complete certainty are essential for life: carbon to build cells, water as described in Chapter 2, and energy; and the availability of each of these is linked to special properties of planet Earth.
The where and why of how we think the Earth obtained its water has also been discussed in Chapter 2, but the most important aspect for life is the fact that we have it and it is wet. Most of the planet is at the perfect temperature, fluctuating within the boiling and freezing points of water, so for the most part water stays liquid. Apart from the role plate tectonics plays in this, where the Earth sits in our Solar System, in the Goldilocks Zone, is the main cause. The amount of solar warmth that envelops the Earth is dependent upon the Suns’ brightness as dictated by its dimensions and chemical composition. All planets and moons in the Solar System receive some degree of warmth from the Sun, but their distance from it is key to whether they receive too little, too much, or just enough. As luck would have it, the Earth sits at the perfect distance, where the warmth it receives is just right to allow water to be a liquid on its surface.
Distance from the Sun is only one lucky factor that makes our planet nice and cosy for life. The Earth also has an atmosphere full of greenhouse gases, in particular carbon dioxide, which help to warm the surface of the planet below. Without any greenhouse gases and their warming effects, and with the Earth’s surface reflecting sunlight away back into space (an effect called its albedo), Earth would be frozen and hover around –15°C (5°F). It is not as simple as Earth sitting at the right distance from the Sun therefore: the Earth itself promotes life. It can be easily imagined that higher or lower carbon dioxide levels would be necessary to maintain a habitat on a planet whose distance from its Sun is lesser or greater, in response to the volumes of solar radiation it receives. The outer reaches of a habitable Goldilocks zone are achieved when the levels of carbon dioxide in an atmosphere become so high that it forms clouds, blocking incoming solar radiation from reaching the surface and causing an increase in the planetary albedo … the end result is a frozen world.
The radiation-absorbing talent of Earth’s atmosphere also supports this ideal surface temperature for water and consequently life. Our atmosphere has a window; it allows some infrared radiation from the cloud tops and surface to pass through it directly to space without intermediate absorption and re-emission, and thus without heating the atmosphere. Without this infrared atmospheric window, the Earth would become much too warm to support life, and possibly so warm that it would lose its water and come to resemble the planetary greenhouse that is Venus.
A third atmospheric quirk is that it is thick enough to exert a pressure on the surface of the Earth, which suppresses the rapid evaporation of liquid water. Atmospheric pressure is the force per unit area exerted on a surface by the weight of air above. On Earth, the atmosphere around us is filled with air molecules that collectively weigh on our bodies. Although you cannot feel it, Earth’s atmosphere presses down with the force of 1kg/cm2 (or some 14.7lb/in2) and terrestrial biology has evolved to operate quite easily under it.
The final special property of the Earth is its gravity. Gravity allows the planet to hold on to its all important atmosphere, allowing only a little to escape into space, which is quickly replaced by outgassing volcanoes. At the same time, the Earth’s gravity is not so strong as to attract a denser atmosphere, which would over-insulate the surface and produce increased surface temperatures unacceptable to life.
The Earth has proven itself physically fit and highly adaptable which enables it to support and nurture life. Luckily it arose within the Goldilocks Zone of the Solar System, which itself formed and sits nicely within the Goldilocks Zone of the Milky Way. The next step is for life to arise and take advantage of this perfectly situated and uniquely designed world.