Planet Earth: In the beginning…
So where did the Earth and all its fellow planets come from? In the beginning, the universe and all the matter in it essentially sprang from absolutely nothing, a minute “singularity” which erupted in a “big bang” to produce the expanding universe we see today. All this happened 14 billion years ago, and this was truly the very beginning: there is no point asking what there was before, because time itself, as we know it, was also generated at the Big Bang. This is because time slows down in gravitational fields, and that “singularity” contained all the matter in the universe.
The idea of a “big bang” seems counterintuitive. Indeed, the term was coined by the British cosmologist Sir Fred Hoyle (1915-2001) in an attempt to ridicule it. But in 1965 Arno Penzias and Robert Wilson found the leftover heat from the Bang – more formally known as the cosmic background radiation – which for most people settles the argument.
Since the Big Bang, the universe has been expanding steadily. We know this because of extremely elegant observations that show that further away objects such as very distant galaxies are receding from us faster than nearby ones. This only matters on a cosmic scale and does not measurably affect the solar system or even a whole galaxy. If it seems further than it used to when you walk to the shops, you should take more exercise.
One thing that is important about the cosmic background radiation is that it is not absolutely even. In the jargon, it is anisotropic. This is another word of Greek origin and simply means that it looks different from different directions. The background radiation itself is incredibly faint, let alone the anisotropy, and it takes all our ingenuity to detect the very slight ripples that it contains. But these inhomogeneities reflect basic irregularities in the early universe, of which the background radiation is a sort of souvenir. If they had not been there, it is even possible that no matter would ever have clumped together to make solid objects. Instead, the atoms that make it up would just have got steadily further apart as the universe expanded.
As you are reading this, and sitting on a planet as you do so, we know that things did not work out this way. Instead, the slight irregularities in the universe allowed atom to meet atom and eventually entire galaxies to form. The structure that these irregularities produced is seen to this day when astronomers examine the distribution of galaxies in the sky. They are not randomly distributed but form clusters and other patterns. Some are very close, and collisions between galaxies are common. By contrast, stars within galaxies are comparatively far apart and close encounters between unrelated stars are rare.
A huge amount is now known about galaxies, including the fact that we live in one ourselves. The Milky Way, a band of light that crosses the night sky, had long been thought to consist of closely packed stars, and Galileo confirmed this by looking at it with his crude telescope. Now we know that when we look at the Milky Way we are looking at the disc of our own galaxy and that the centre of the galaxy is in the
direction of the constellation Sagittarius. Here a huge black hole – matter collapsed into a miniature version of the singularity from which the universe itself sprang – forms the central mass around which the rest of the galaxy rotates. It takes the Sun about 225 million years to complete a single lap.
Even within a galaxy, however, there is far too little material for everything to aggregate into stars or planets. Instead, matter has to be concentrated for the process to get going. You can see it in action if you get someone to show you the constellation Orion in the sky. As well as being big and distinctive, it lies across the Equator in the sky so it can be seen from New Zealand or Norway with equal ease in the southern hemisphere summer or the northern hemisphere winter. Just to the south of the three stars called Orion's Belt is a shiny, patchy bit. It consists of dense, glowing gas and dust. More precisely, it is dense by astronomy standards but is nowhere near as dense as the Earths atmosphere which you are breathing now. It is opaque because it is as deep as a solar system. How it got that way is just being elucidated. The smart money backs the idea that a shock wave from an exploding star (or supernova) compresses material together until it is dense enough for stars to form. Similar pressure waves could also be produced when galaxies collide, and there is some evidence that this has happened in our corner of space. Within this mass of material, stars are being formed, so rapidly that it is almost possible to watch them turning on.
Many other such “stellar nurseries” have also been charted. In these areas, stars are forming in such dense and close-packed zones of space that most new stars exist as mutually orbiting double or multiple stars. However, since most older stars are so far apart, there must be some force that removes them from their siblings shortly afterbirth. Quite likely, gravitational disturbances in the gas cloud fling them out to make their way in the world.
The next part of our story demands a change of focus, from the vast dimensions of outer space to the atomic scale. The early universe produced by the Big Bang consisted mainly of one element, hydrogen. A hydrogen atom is the simplest atom possible, consisting of a proton with a positive charge being orbited by a negatively charged electron. These made up 74 percent of the universe while almost all the rest was helium, the next simplest atom, whose nucleus contains two protons and two neutrons, which as the name implies have no charge. A tiny amount of lithium, the third-lightest element, also formed in the Big Bang. The amount of helium in the universe is in agreement with theory and is powerful evidence that the Big Bang happened.
But look around and you do not see a world dominated by these three elements. Indeed, over 47 percent of the Earths crust is oxygen, and silicon makes up another 28 percent. There is quite a bit of hydrogen about – they don't call water H>0 for nothing – but helium is so rare that it was identified on the Sun (hence the name) before it was found on Earth. Now it is extracted from natural gas (mainly in Amarillo, Texas, for some reason) to make balloons fly and for deep-sea divers to inhale. Lithium has some high-tech uses, such as the handy ability of lithium hydroxide to absorb carbon dioxide and prevent astronauts from dying of asphyxiation, but it is hardly central to life for the rest of us.
Instead, there is something else going on, and the big word for it is nucleosynthesis. The term simply means the synthesis (forming) of nuclei, the central cores of atoms. Everyone just assumes that once the nuclei are there, the electrons needed to orbit them will come, and this part of the theory seems to be sound.
The theory of nucleosynthesis is one of the great achievements of twentieth-century science, and the story it tells is a very satisfying and complete one.
Nucleosynthesis happens in two places. The creation of all that hydrogen, helium and lithium mentioned above is called Big Bang nucleosynthesis (BBN is the acronym if you need to impress astrophysicists). But the creation of all the other elements occurs in stars, in what is known as “stellar nucleosynthesis”.
Normal stars like the Sun are powered by the energy released when four protons, or hydrogen nuclei, fuse to form one helium nucleus. This process, called “hydrogen burning”, is a kind of nuclear fusion, and is the same force that powers a hydrogen bomb. But when two helium nuclei collide, there is no stable nucleus that can form. Only in stars older and hotter than the Sun (red giants and red supergiants) is there enough energy for “helium burning” (the fusion of three helium nuclei to produce carbon) to take place, and for even heavier elements such as oxygen to be formed, via these carbon nuclei accumulating other particles.
But even these stars cannot form the heaviest elements. Stars cannot gain energy by making elements heavier than iron, because it takes more energy to power the process than it produces. So these elements – such as uranium, plutonium, gold and silver – are only formed when the most massive stars explode as supernovae. At these extremely high temperatures neutrons are captured by existing nuclei to build up yet heavier atoms before being blasted into space by the exploding star. So any atom of gold you touch started life in the heart of a supernova. This super-stellar origin explains why elements that are important to us, such as gold, are vanishingly rare in the universe overall.
So how do planets themselves form? A range of theories flourished in the nineteenth and twentieth centuries. In one, a passing star would have sucked a cigar-shaped mass of material from the Sun to cool in space and yield planets. This was a neat idea because it allowed the arrangement of the solar systems planets to be explained. Near the Sun, we find small, rocky planets (the Earth, plus Mercury, Venus and Mars). Go a little further out and you find bigger planets in the shape of Jupiter (biggest of the lot), Saturn, Uranus and Neptune. Pluto and other small objects lie beyond.
But the current view is a little simpler. As we have seen, as well as emitting shock waves which compress particles together to form stellar nurseries, exploding supernovae seed the surrounding area with heavy atoms. Through chemical reactions, these atoms build up into more complex molecules, which gradually stick together to form lumps called planetesimals. Over time, planets form from the collision and aggregation of these pieces.
A German-led group of scientists has run an experiment called CODAG, the Cosmic Dust Aggregation Experiment, on NASA's Space Shuttle to prove that dust particles in space can stick together in this way. To see the process in action on a larger scale, we can take a look at the star Beta Pictoris – astronomy code for the second brightest star in the constellation Pictor, the painter. Look at this star with a good enough telescope and you will see that it lies at the centre of a flat disc of muddy – looking dust. Similar dust discs have been observed around other stars and they confirm that when a star forms it does not absorb everything around it. Instead, at least some stars form at the centre of a spinning disc of material. Most ends up in the star but enough is left over to produce planets. Some never does get captured by the star or a planet and remains to yield comets and other small objects.
In a solar system like this one, the star at the centre contains most of the mass, but consists almost entirely of helium and hydrogen. As the star forms and begins to shine, pressure from the light it emits stops more dust and gas falling in. This means that heavier elements are concentrated further out.
Near a stars fierce heat, planets struggle to hold on to water and gases and tend to be almost entirely rock. The nearest planet to the Sun, Mercury, orbits at about 58 million km out, compared to 150 million km for the Earth, and its only atmosphere is a feeble array of passing atoms emitted from the Sun itself. Further out, Venus, Earth and Mars (known as the terrestrial planets) all have reasonably useful atmospheres, but they do not add up to much compared with those of the big planets of the outer solar system, which are often – and with reason – termed the gas giants. Their cores are not unlike those of the inner rocky planets, but they are overlain by many thousands of kilometres of gas which has its own chemistry and weather.
How long does all this planet-making take? Not long by geological standards. In the outer solar system, Neptune and Uranus, despite being far bigger than the Earth, formed in about 10 million years. Nearer the Sun, it takes a little longer. The Earth took about 100 million years to form. Its core would have taken 29 million years to accumulate, as against 13 million for Mars's much smaller core. But the biggest object in the solar system apart from the Sun itself, Jupiter, will have formed very rapidly, with its core building up in only 100,000 years.
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