In the spatial desert, the first molecules begin an uninterrupted rondo and give rise to, in the suburbs of a modest galaxy, a very special planet.
An infinite desert, with here and there clusters of galaxies fragmented into stars ... A billion years after the Big Bang, the puree of matter has taken shape and is more recognizable. The whole picture appears stable, and the universe might very well have remained at this stage. But once again, evolution is going to intervene. Why?
Because the first stars are going to reverse the course of things. While everywhere else in the universe the cooling process is continually going on, the temperature of the stars is increasing considerably The stars become the crucibles for fashioning matter and are going to be the cause of its taking a new step forward in cosmic evolution. The assemblages of the very first seconds of the universe are going to be replayed again in the stars.
In other words, the stars are in a certain way mini Big Bangs on a local level.
In a certain way, yes. The reheating is produced by the contraction of the star beneath its own weight. When the temperature reaches around ten million degrees, the nuclear force “reawakens.” As was the case for the original Big Bang, the protons combine to form helium.
At its origin, the universe, one recalls, had stopped at this stage.
These nuclear reactions emit an enormous amount of energy into space in the form of light. The star shines. Our Sun has thus been “fueled” by hydrogen for four and a half billion years. The more massive stars shine much more brightly and use up their hydrogen in a few million years, at which point they begin to contract again. Their temperature rises to more than 100 million degrees. Helium, the product of hydrogen fusion, in turn becomes a fuel, at which point a set of nuclear reactions allows for new combinations: three helium nuclei are going to come together as carbon nuclei, and four helium nuclei as oxygen nuclei.
*
But why didn’t these reactions take place at the time of the original Big Bang?
The encounter and fusion of three helium nuclei is an extremely rare phenomenon. It takes a long time for it to occur. In the original Big Bang, the phase of nuclear activity lasted for only a few minutes, which is too short a time to manufacture a meaningful quantity of carbon. This time, in the more massive stars, these agglomerations are going to take place over millions of years.
Each of these more massive stars is therefore going to manufacture carbon and oxygen nuclei?
For the next several million years, the centers of the larger stars will indeed be stockpiled with heavy nuclei, including carbon and oxygen. These elements are going to play a fundamental role in the following phase of our history. Carbon in particular, with its special atomic configuration, lends itself easily to the manufacture of long molecular chains, which will play a key role in the appearance of life. Oxygen will become a component of water, another element that is indispensable to life.
And during this time, these stars continue to contract?
The heart of the star collapses into itself, whereas its atmosphere expands rapidly and becomes red, creating a red giant. When the temperature of such a star exceeds a billion degrees, it engenders heavier atomic nuclei, those of various metals— iron, zinc, copper, lead, gold—until it comes to uranium, made up of 92 protons and 146 neutrons, and even more. The roughly one hundred atomic elements that we know of in nature are thus the products of the stars.
That process could have gone on for a very long time.
No, because now the heart of the star collapses in on itself. The nuclei of the atoms at that point enter into contact and rebound. That provokes a giant shock wave, which results in the explosion of the star. That phenomenon is what we call a supernova, a burst of light that illuminates the sky like a billion Suns. The precious elements that the star has produced within itself throughout its long existence are at that point propelled into space at a speed of tens of thousands of kilometers a second— as if nature had taken the dishes from the oven at the proper moment, just before they would have burned.
But blowing up the oven at the same time!
*
That’s the way massive stars die. Still, they leave on the premises, so to speak, a contracted stellar residue, which will become either a neutron star or a black hole. The small stars, such as our Sun, go out much more quietly. They exhaust their matter nonviolently and turn into white dwarfs. They cool off slowly and are transformed into nonradiant celestial corpses.
What happens to the atoms that escape from the dying stars?
They wander in interstellar space and mingle with the great clouds scattered throughout the galaxies. Now space becomes a chemistry laboratory. As a result of the electromagnetic force, electrons begin to orbit around atomic nuclei to form atoms. These atoms in turn combine to produce heavier and heavier molecules. Some of them contain more than ten atoms. The association of oxygen and hydrogen forms water. Nitrogen and hydrogen form ammonia. We have even found the molecule of ethyl alcohol, the component of our alcoholic beverages, which consists of two atoms of carbon, one atom of oxygen, and six atoms of hydrogen. All these are the same atoms that, later on Earth, will combine to form human beings. We are truly made of the dust of the stars.
At this point in the evolution of the universe, there is only gas, balls of stellar fire, but not yet any solid matter.
It’s about to happen. As they cool down, certain atoms emanating from the stars, such as silicon, oxygen, and iron, are going to come together to form the first solid elements. These are tiny grains, smaller than a micron (one thousandth of a millimeter) in diameter, that contain hundreds of thousands of atoms. The force of gravity acts on the interstellar clouds and forces them to collapse into themselves, producing a generation of new stars. Some of these stars will have a suite of planets revolving around them, as is the case in our own solar system. And these planets will contain within them the atoms engendered by the dead stars.
In other words, for new stars to be born, old stars have to die. In space, too, the appearance of the new requires the death of the old.
The atoms of our biosphere have necessarily been created in the crucibles of stars and are sent forth into space when the stars die. These intertwined generations of stars and atoms begin to take place several hundred million years after the Big Bang and will go on for several billion years thereafter. Space becomes a kind of forest of stars: stars big and small, young and old, die, disintegrate, and enrich the terrain to nourish new growth. In our galaxy alone, an average of three stars come into being every year. Thus it is that, relatively late in the game, roughly four and a half billion years ago, one star of particular interest to us, our Sun, is born on the fringe of a spiral galaxy, the Milky Way.
Why “spiral”?
It’s the rapid rotation of the stars around its center that gives our galaxy its shape, which is that of a flattened disk. The origin of the spiral arms is a result of complex gravitational phenomena. The Milky Way, that great luminous arc that crosses the sky at night, is the image of all the stars strewn the length of the disk of the galaxy and revolving around its center: our solar system makes a complete revolution around that center roughly once every two hundred million years.
What distinguishes our Sun from the other stars?
In our galaxy, our Sun is an ordinary, run-of-the-mill star. Of the hundred billion stars, there are at least a billion that are so similar to our Sun you couldn’t tell them apart. When our Sun came into existence four and a half billion years ago, it was much larger than it is today, and it was red. By slow degrees, it contracted, it became yellow, and its internal temperature increased. After a dozen million years or so, it began to transform its hydrogen into helium, like a giant H-bomb, with the difference that its output is controlled. This phenomenon of nuclear fusion is what assures our Sun’s stability and its luminosity.
This run-of-the-mill star nonetheless managed to attract a number of planets and create a solar system around itself.
This phenomenon—stars with planetary systems around them—is probably rather common in our galaxy, although we’Ve been able to detect relatively few because of the still-limited technology at our command. The formation of planets like Earth must be relatively recent. The solid bodies of our planetary parade are made up for the most part of oxygen, silicon, magnesium, and iron; the atoms were formed progressively by the activity of generations of successive stars. It took several billion years for them to come together in sufficient quantities in the interstellar clouds. we’ve been able to measure the age of the Moon, as well as that of many meteorites. The numbers are identical: 4.56 billion years, to be precise. The Sun and its planets appeared at the same time, at a point in time when our galaxy was already more than eight billion years old.
How did the planets form?
Interstellar dust gathers around embryos of stars and forms disks analogous to the rings of Saturn. Then, over long periods of time, these small objects come together to form rocky structures that become larger and larger. Some of the larger bodies attract other smaller bodies to them and eventually become planets. The countless craters of the Moon, and of other bodies in our solar system, attest to the violent shocks of the impact with interstellar matter, which added to their mass. These shocks emit a great deal of heat, to which is added the nuclear energy from the decay of radioactive nuclei.
So all these planets are in a state of molten incandescence?
At their birth, the large planets are incandescent balls of fire. The more massive the planet, the hotter it is, and the longer it takes to cool down. In the smaller bodies like the asteroids, the cooling process takes place very quickly. The planet Mercury and our Moon dissipated their initial heat into space over a few hundred million years. For a long time now, neither the Moon nor Mercury has had any internal fire and therefore no further geological activity As for the Earth, this cooling process has taken longer. Today, the core of the Earth is still hot, and this causes the convective motion of still-fluid stone. These phenomena are what make continents shift, volcanoes erupt, earthquakes occur. This geological instability is actually a boon: it brings about variations of climate, which play a major role in the evolution of living things.
What makes our planet different from the others?
Our planet has plenty of water in liquid form. There’s water elsewhere in the solar system, plenty of it. The satellites of Jupiter and of Saturn contain water in the form of ice, because of their very low temperatures. Recent measurements by the Galileo satellite suggest that liquid water may be present on the surface of Europa. Venus also has water in the form of vapor, because, being second closest to the Sun, Venus has extremely high temperatures. The Earth’s orbit keeps us just far enough from the Sun to allow water to remain liquid.
Mars also used to have liquid water, as the so-called canals and dried-up wadis that spacecrafts have shown seem to indicate.
As recently confirmed by the Mars mission Pathfinder, torrents of water did flow on the surface of Mars some billion years ago. But for a long time now, there has been none. Why? We really don’t know. Given its relatively small mass, its tectonic activity is now very weak.
But where does Earth’s water come from?
Let’s go back to those torrents of matter projected into space at the death of the stars. Dust was formed—literally Stardust—on which ice and frozen carbon dioxide came to rest. When agglomerations of dust grew large enough to give birth to planets, the ice volatilized and escaped outside in the form of geysers. What’s more, comets, which are made up largely of frozen water, fell on the planets, bringing water with them.
And the Earth retained that water?
Its field of gravity is strong enough to retain the water molecules on its surface, and its distance from the Sun allows it to retain water in liquid form, at least in part. In these early days of the Earth’s formation, it was constantly bombarded by ultraviolet rays emitted by the young Sun,
Why didn’t the same evolution take place on Venus?
We don’t quite know. The two planets are so much alike they have virtually the same mass and contain the same amount of carbon. On Venus, however, this carbon is in the atmosphere, whereas on Earth, it is to a large extent in the ocean. Yet the atmospheric compositions of the two planets were very much alike in the early stages of their formation.
*
What does the difference stem from?
We think that water in the liquid state on the surface of our planet played a crucial role. Thanks to this blanket of water, the carbon dioxide in Earth’s early atmosphere was dissolved and wound up at the bottom of the oceans in the form of carbonates. Venus is slightly closer to the Sun than we are. The difference in temperature was in all likelihood responsible for the absence of liquid water in that planet’s early stages. Its atmospheric envelope of carbon dioxide created an enormous greenhouse effect, which kept its surface temperature in the vicinity of five hundred degrees. So it was that two planets, alike in many respects, evolved in two very different ways.
Without water—liquid water—it’s safe to say that it would have been the end of our story.
I think so. Water played a primordial role in the appearance of cosmic complexity. Within the ocean blanket, sheltered from the ionizing rays from outer space, intense chemical reactions would occur. By means of various encounters and associations, those chemical reactions would produce molecular structures that were increasingly large. In the early stages of prebiotic evolution, carbon, born of the red giants, would play a major role.
Why is carbon so successful?
It’s the ideal atom for molecular constructions. It has what we call a valence of four, meaning that it has four electron “holes” that can act as harnesses for numerous other atoms. The links it creates are sufficiently supple to allow easy and quick association or disassociation, which is indispensable to life. Silicon also has a valence of four, but the links it makes are much more rigid. It creates stable structures, such as sand, but it has no capability to yield to the constraints of metabolism.
It’s therefore absurd to imagine that somewhere out there in the universe there is life based on silicon.
It’s highly unlikely. In our galaxy, as in the neighboring galaxies, the various molecules of more than four atoms that we’ve been able to identify by radio telescope always contain carbon, never silicon. This observation strongly suggests that if life does exist elsewhere in the universe, it is also made out of carbon.
Once Earth’s atmosphere was formed, life soon followed, isn’t that so?
When Earth was born roughly four and a half billion years ago, the conditions were scarcely favorable. The temperature on the surface was too high. In addition, at that time, space was rife with countless small celestial bodies that would later be absorbed by more massive planets (the solar system was cleaning up its own house). The constant bombardment of meteorites and comets was extremely violent. Studies of various comets revealed the presence of a considerable quantity of hydrocarbons. The collisions of the first billion years in all likelihood brought, in addition to water, an important quantity of complex molecules to the surface of the Earth. These comets, which in ages past were generally thought to be harbingers of death and destruction, probably played a beneficial role in the appearance of life. Less than a billion years after the birth of Earth, its oceans were swarming with living organisms, including the first blue-green bacteria. This view was strongly confirmed by the rich harvest of organic molecules left behind in the tail of the comet Hale-Bopp in 1997, including formaldehyde, various cyanides, and methanol.
End of Act 1, the longest and slowest. We arrive on Earth after several billion years of the history of the universe. From this point on, things are going to speed up considerably.
This time, the molecular agglomerations are going to take place with hundreds, thousands, millions of atoms. Ever since the Big Bang, matter has been ascending the steps of the pyramid of complexity. Only a tiny fraction of the elements that have reached one step manages to ascend to the next. Only an infinitesimal portion of the protons in the first phase of our story has succeeded in forming heavy atoms. Only a very small number of simple molecules have organized into complex molecules, and only a tiny portion of these complex molecules will participate in the structures of life.
At the same time, it seems that there was a great degree of uniformity during the first part of evolution.
True, the universe has wrought the same structures everywhere throughout space. The fact is, we have never observed in the stars and in the most distant galaxies a single atom that does not exist in our own laboratories.
All of which suggests that the same story of earthly evolution could have occurred elsewhere, and that life could well exist on other planets.
We note that everywhere in the universe, quarks associate themselves into protons and neutrons, these protons and neutrons with added electrons come together to form atoms, and the atoms in turn form molecules. We also know that clouds of interstellar matter collapse to give birth to stars. We can well imagine that some of these stars do indeed have a suite of planets circling them, and we can also conceive that some of these planets may well contain liquid water, which is conducive to the appearance of life. All that is plausible. But as yet, we have no concrete proof.
Time has also contracted: the further we go along in our story, the faster the evolution.
Absolutely If we take the four and a half billion years of our planet and assume that it’s but a single day—point zero of that day being the Earth’s birth—then life begins about 5:00 A.M. and grows in complexity throughout the rest of the day. About 8:00 P.M., the first mollusks appear. Then the dinosaurs appear at 11:00 P.M. and disappear at 11:40 P.M., leaving the field open for the rapid evolution of mammals. Our ancestors put in their appearance at about five minutes to midnight; the capacity of the human brain doubled in the minute from 11:59 P.M. to midnight. The Industrial Revolution began in the last hundredth of a second.
And the world is filled with people who are firmly convinced that what they’ve been doing since this fraction of a second will last indefinitely. One can’t help but see a logic in the unfolding of this first act, a kind of thrust toward complexity that propels the universe toward successive organizations, one inside the other, like so many Russian dolls, from chaos to intelligence. One might go so far as to say a sense, a meaning . . .
All evidence indicates that our universe has transformed its initial amorphous state into a variety of structures that are increasingly organized. This metamorphosis might be explained by the action of the forces of physics on matter that is cooling off. Without the expansion of the universe, without the great interstellar void, this story would have had no second act. But that only moves the interrogation back ever so slightly and brings us back to our reflections about the laws of nature. The question “Why are there laws rather than no laws?” strikes me as being a logical sequel to Leibniz’s “Why is there something rather than nothing?”
Was the appearance of life programmed into the original scenario?
In the past, people used to say that the probability of life appearing was as unlikely as putting a monkey in front of a typewriter and expecting it to produce the complete works of Shakespeare. Today, there are a number of reasons to believe that the appearance of life on an appropriate planet is far from improbable. However, whether probable or improbable, we can say with certainty that from the very first moments of the cosmos, the possibility (but not the necessity) of the appearance of life was inscribed in the very form of the laws of physics—a fascinating question that Joel de Rosnay will explore in the next act of the cosmic drama.