Humans live short lives, but as a species we have always thought and planned for the distant future. In the past, this might have meant simply caring for offspring who would outlive us; increasingly, we plan for the future as a society. This capacity—underlined by our ability for abstract thought that can reach beyond the horizons of space or time—is perhaps our most remarkable trait. Microbial life may be able to survive most of the slings and arrows the Universe can throw at it, but as we’ve seen, the Sun will someday put an end to life on this planet. If anything will enable life to endure past the limited lifetime of the planets, it will have to be our ability to think.
There are even bigger implications to the argument that life is a planetary process. We often imagine our place in the Universe in the same way we experience our lives and the places we inhabit. Just as it is easy to think of the rocks at the Harvard College Observatory as static objects, we imagine a practically static eternal Universe where we, and life in general, are born, grow up, and mature; we are merely one of numerous generations, but the Universe itself is still immeasurably older.
This is so untrue! We now know that the Universe is close to 14 billion years old and that life on Earth is 4 billion years old: life and the Universe are almost peers. To put it in more human terms, if the Universe were a fifty-five-year-old, life would be a sixteen-year-old. What’s more, the Universe is nothing like static or unchanging.
All of this brings us back to the question, What is our place in this young world? This is a profound question, and there are many ways it can be asked. One of them is simply, Are we alone? I am going to touch upon that question in just one aspect. It has something to do with the recent realization that the Universe is young and is still actively undergoing changes.
The answer to the question could be yes, for a number of different reasons. For one, we (life, not just humans) may be alone because life is an exceedingly rare event, and in 13.7 billion years of history of the Universe we are it. On the other hand, we may be alone because we are latecomers to the party. After all, almost 9 billion years passed before our Sun and Earth formed, and so life could have already emerged and died out elsewhere in the Universe, without our knowing it. Or we may be first!
Central to this discussion is the so-called Fermi paradox, named for the renowned physicist Enrico Fermi, who asked the question, “Where are they?” Beneath this question lies the assumption that if there are advanced civilizations out there, astronomers ought to notice them, because surely any advanced civilization would have the power to alter the galaxy sufficiently for us to see. Fermi argued that given the old age of the Universe and the short timescale it took humans to develop technology, other origins of life and civilizations in our galaxy that had a head start should be significantly more advanced than we are. Being significantly more advanced, they would need huge energy resources on the scale of stellar systems and galaxies, which we couldn’t help but notice. If we have not noticed anything yet, then, it follows that we may be alone in our Galaxy and technological civilizations must be a very rare occurrence. (I am reminded of Arthur Clarke’s statement: “Any sufficiently advanced technology is indistinguishable from magic,” which makes me less confident that we know what to look for.)
In the 1990s Paul Horowitz of the Harvard physics department recorded the recollections of Herb York and Phil Morrison about the origin of Fermi’s famous question.2 It was the summer of 1950 at Los Alamos, where a number of American physicists had reassembled, a few years after the Manhattan Project, to develop the hydrogen bomb. Fermi liked to ask rhetorical questions during the group’s lunches and then proceed to answer them. So at one of these occasions, according to York’s recollection, he asked his table mates, “Don’t you ever wonder where everybody is?” Fermi argued that given the large number of stars and planetary systems in the Galaxy and their relatively old age, if life arose and acquired technology elsewhere, the others would be far more advanced and would have colonized the Galaxy by now.
Fermi’s conclusion is very sound statistically, as Michael Hart showed in the 1970s.3 However, the statistical argument is strong only if the timescale of emergence of complex life is much shorter than the age of the Universe, and not so if the two are comparable.
Fermi made his point in 1950, and Hart in the 1970s. In both those eras, the consensus of my fellow astronomers was that the Universe was much older than 10 billion to 15 billion years. The estimate then was more like 20 billion to 25 billion years, and some even argued for a steady state, eternal Universe. At the same time, the geological timescales were already well established at about 4 billion years.
A lot has been learned since then due to an unprecedented revolution in astrophysics at the end of the twentieth century. What scientists have established in the past ten years can help us address Fermi’s paradox and the future of life in the Universe. A lot of the history has been pieced together nicely, and for most events we have direct evidence. The story goes as follows:
Light traveling at its limited speed is a great time machine; astronomers train their telescopes on very distant objects and get to see them as they appeared in the historical past. So, when we look back into the sky’s past, we see a time about 13.7 billion years ago when the entire observable Universe was made of hot hydrogen-helium gas with tiny trace amounts of lithium. None of the familiar objects of our night skies—galaxies, stars, planets—existed. More importantly, neither did any other chemical elements.
Astronomers can observe that era directly, using a sensitive heat-measuring device that allows them to observe the cosmic microwave background radiation, or CMB. The CMB is the relic light released when the Universe’s entire inventory of hydrogen was formed, as previously superenergetic particles combined in atoms. It took merely 20,000 years for this to happen and the light to be released. That light—most of it—has been traveling through our expanding and cooling Universe ever since. Today it is diluted and shifted to longer wavelengths, so what used to be visible light became microwaves and radio waves.4
The CMB carries a treasure trove of information via its temperature, temperature variations, and polarization—a subtle measure of how the CMB waves are twisted. These measurements are very challenging and only in the past decade has technology progressed enough to allow such studies, both from the ground and with space missions like COBE, WMAP, and Planck.5 These direct measurements show clearly that 13.7 billion years ago the Universe had no building materials for life or even for planets—just hot hydrogen and helium gas.
Before continuing with the rest of the story, I need to address the timing of different events in the early history of the Universe. The age of the Universe is known as approximately 13.7 ± 0.1 billion years, meaning that our measurements can’t tell if the age is 13.6 billion or 13.8 billion years, or anywhere in between. At the same time, some events can be timed more precisely in relative terms. Therefore it has been easier to refer to the times for different events, as times since time-zero (called the big bang). For example, the CMB was released 379,000 years after the big bang. This timing of the CMB is a measurement and the preceding statement remains true regardless of whether the age of the Universe (i.e., the time of the big bang) is 13.6 billion, 13.7 billion, or 13.8 billion years ago. Alternatively, if we fix the big bang at 13.7 billion years ago, this same event (the creation of what has become the CMB) occurred 13.6997 billion years ago.
Now, back to our story. The obvious question we have to answer is, Where did the building materials—all the chemical elements like carbon, oxygen, silicon, and iron—come from? The answer is well-known—they all came later, from stars. That brings us to the next notable event in our Universe—the formation of the first stars.6 This is an event that we do not yet see directly, although the successor to the Hubble space telescope is being built to do that. Nevertheless, scientists already have plenty of indirect evidence that this happened about 13.1 billion years ago.
Stars, including the first stars, are very unusual objects w
hen you look at the big picture. They are stable and long-lived concentrations of ordinary, or baryonic, matter. There is nothing unusual about that. Ordinary matter is found all over the Universe (billions of galaxies’ worth) in big and small clumps that just sit there and do nothing—except when some of these clumps get compressed under their own weight and form stars. It just happens that the balance between gravity pull and matter repulsion is achieved at temperatures and densities inside the star that allow the atomic nuclei of hydrogen and helium to fuse. When you fuse atomic nuclei, two important consequences follow: lots of energy is released and new, heavier nuclei are formed. That is how our Sun shines.
Stars are the queens of fusion—they do it admirably well! They literally light up the place and proceed to transform it from a boring simple gas to the richness of the entire table of the elements.7 The process is orderly: first, hydrogen fuses into helium until the central regions of the stars are chock-full of helium, which, being heavier than hydrogen, shrinks and heats up. Helium heats up until its threshold for fusion is reached, and then a new stage in the life of the star begins, at least inwardly.
While fusing hydrogen produces mostly helium (fusion would be a clean, powerful source of energy for humankind, if we ever learn to do it in a controlled fashion), the fusion of helium produces a number of heavy elements, most notably carbon and oxygen. Stars can fuse elements all the way up to iron, at which point they stop, lacking sufficient energy to go any farther—unless the star is big enough to explode. In such a supernova, more fusion can happen that produces many more elements and frees the rest to capture electrons and become the atoms of heavy elements with all the rich chemistry they can cook up.
Astronomers can observe how the stars enriched the Universe in heavy elements. The large telescopes of the past ten to twenty years have allowed them to peer back to about 12 billion years ago. They see some heavy elements, such as iron; they see patterns in which elements are relatively enriched and that reveal how stars produced them. The picture that emerges is one of generations of stars steadily transforming the hydrogen and helium of the young Universe into all the heavy elements.
Stars form out of low-density gas, which must cool while being compressed (under its own weight) for a star to be able to condense. Because hydrogen and helium are terribly bad at such cooling, the first stars must have been super-size only—hundreds of times larger than our Sun. The first stars were massive, had short lives, and produced some heavy elements that were dispersed inside the nascent galaxies and made it possible to form smaller, less massive stars. The addition of even a sprinkling of elements to the hydrogen-helium gas helps it cool, so the next generation of stars can be formed from a wider range of gas clumps. With each generation, progressively smaller stars can form—and they do. Today our Galaxy has many stars smaller than our Sun. Partly this is due to the fact that smaller stars live longer by burning their nuclear fuel slowly, but mostly because small stars have formed in increasingly larger numbers as the Universe has evolved.
Small stars disperse a portfolio of heavy elements when they die, so the enrichment of the Universe with heavy elements continues at a slow, steady pace. In fact, after 13 billion years only about 2 percent of the original mixture has been transformed to heavy elements; the enrichment, as astronomers like to call it, is very slow indeed.
The brief story of the Universe, then, looks like this: from just hydrogen and helium about 13 billion years ago, generations of stars made enough iron and oxygen, silicon and carbon, and all other elements, to be able to form Earths and super-Earth planets. There are at least two important morals to this story regarding life.
First, it took a long time before stars anywhere in the Universe could have planets. Stable environments in normal galaxies that were enriched enough to have planets became available about 9 billion years ago.8 If you ask about large terrestrial planets, such as rocky super-Earths and Earths, then it is more like 7 billion to 8 billion years ago. We can imagine that the emergence of life had to wait until that time in the history of the Universe, if not later.
Second, the enrichment continues to this day, and we have a fairly clear idea of how our Universe will be transformed in the eons to come. For example, we see that massive stars have been forming less frequently for the past 5 billion years, so the small stars will dominate element production and enrichment in the future. Generally, that means more carbon than oxygen. Today there are three times more oxygen atoms than carbon atoms in most of our Galaxy, but eventually a point will be reached when carbon and oxygen exist in equal abundance. When this happens, the mineralogy of rocky planets changes. Carbides dominate silicates, and there will be important implications for the origins of life on such planets, as the carbon planets described earlier in the book go from being rare to being common.
In general, though, the future of life looks excellent. Unless life is an exceedingly rare phenomenon, there should be more of it, and more diverse forms of it, in the future. Planets may be just a tiny fraction of the Universe because they are so small, yet there are so many of them that there are plenty of places for life. We now know that our Universe is passing through its peak of forming stars (known as the stelliferous era), but it appears that it is still peaking in terms of forming planets.9
This implies that the Fermi paradox, which is about the past, is the wrong way to look at the question of whether there is life elsewhere. The paradox assumes that there was enough time before us for others to emerge and develop. The new evidence does not support such an assumption easily. Of course, when it comes to technology, not microbial life, we can only speculate–our own technological capabilities have grown exponentially recently, and if such growth were used as a basis, then the Fermi paradox remains strong statistically. But for life, the logical sequence I follow is: (1) complex chemistry is necessary for life to emerge—enough heavy elements are needed; (2) stable environments that allow chemical concentration are also necessary—terrestrial planets (Earths, super-Earths) are needed. When in its past did our Galaxy (and our Universe) fulfill these requirements?
The answer is, Between 7 and 9 billion years ago. I arrive at this answer via two independent paths. The first path, much of which relies on what we’ve been considering, is to observe the stars and gas in distant galaxies, measure their abundance in heavy elements (the ones needed for life and planets), and thus see how their abundance grows with time. When we begin seeing stars with just enough heavy elements to allow forming Earths and super-Earths, we have pinpointed the time in the past we are looking for. The only problem is that we need to know how much heavy elements are enough to form big terrestrial planets. That’s a tough question. If our computer models for planet formation are accurate, then a solar system requires at least 1/1,000 of the proportion of heavy elements that our Sun has. Our Galaxy reached this state about 9 billion years ago.10
The second path to answering the above question goes directly to the planets. Do we observe a decline in the number of planets around stars that are poor in heavy elements? Yes. This evidence surfaced early on in the planet-hunting game. It was so pronounced that most teams were tempted to select stars rich in heavy elements in order to discover more planets. Nobody was surprised that such a trend—more metals, more planets—existed, but the strength of the trend was surprising. The trend drops off to practically no planets so fast that even the proportion I mentioned above—1/1,000 of the heavy elements of the Sun—seems too generous. It comes out to something like 1/100 of heavy elements compared to the Sun.11 This would put the time in the past when planets that were capable of cradling life could form at just about 7 billion to 8 billion years ago.
A word of caution is due here. The “more metals, more planets” trend is currently only observed for Jupiter-like and Saturn-like planets, and for hot Jupiters in particular. I have to assume for now that it holds for terrestrial planets, but the Kepler mission is working to answer that question accurately.
Today astronomers know with cert
ainty that less than 13 billion years have passed since our Universe was capable of having stars and planets. This makes the stellar, planetary Universe very young. (Because we see that our Galaxy and the rest of the observable Universe, and its 200 billion galaxies, show a clear potential to continue on as we see them today for hundreds of billions of years, if not much longer, I feel that the words “very young” describe the Universe adequately.) The anthropomorphic analogy to parent-daughter, when we talk about a Universe with planets and Earth life, is then pretty good, as well. Life on Earth could really be among the first older siblings in the family.
So far I have been talking about microbial life. But what about the bigger question: Are we humans alone? That is a far more difficult question to answer. However, if planets and life are so young in our Universe, perhaps we are not latecomers to the party. We may be among the early ones. That could explain why we see no evidence of “them.” This does not necessarily mean, however, that no one is there.
By all accounts, today the Fermi paradox remains unresolved and allows for a fascinating range of possible solutions—from the very deep to the very entertaining, all of them worth more attention than I plan to give them here, but for recommending the rich literature that does. 12
With this answer to the Fermi paradox in hand, we can now estimate just how big the family of life—the census of habitable planets—is. The answer is, Pretty big. Consider this: there are more stars in the Universe than there are grains of sand in all the beaches on Earth.13 And there are equally as many planets (see Figure 11.1). Of course, as I noted at the beginning of this book, those astronomical numbers do not imply inevitability, no matter how good we feel about our models. We have to go and find out for ourselves. The survey by the NASA Kepler mission will accomplish that. In the meantime, we can use the current discoveries of extrasolar planets to make a preliminary estimate.
The Life of Super-Earths Page 11