The Life of Super-Earths (15 page)

The hierarchy of timescales that involve life is very interesting, and they intertwine with the spatial scale of life. The large molecule scale, as we saw, is on average 10
^—7
meters, and the chemical reaction of replicating one unit of DNA lasts about 10
^—3
seconds.
¹
This is slow compared to what can be accomplished by atoms in the tiny volume defined by 10
^—7
meters. However, it is extremely fast compared to any planetary process. This means that life as a process (or sum of processes) will have time to adapt to, coopt, or simply survive whatever it is that happens on the longer, geological timescales, such as the ups and downs of global temperature or the rearrangement of continents. But a chemical reaction is a chemical reaction, and a planetary geochemical process is most often simply the sum of the myriad chemical reactions underlying it. So all those individual geochemical reactions would appear to have similar (short) timescales to the individual biochemical ones that make up the processes of life.

How, then, could processes of life rise above the destructive chemistry of the planetary environment? If it takes the same time to do them, then ordinary chemistry wins out. For life and its biochemistry to prevail, their timescales should be
shorter. Life on Earth is competitive that way by using two tricks. One is getting help from special molecules (catalysts, typically called enzymes) to speed up reactions; the other is by keeping tabs on how it does that. In other words, biochemistry has neatly ordered sequences of reactions that do something very well and fast (like storing and releasing energy, forming an enclosure, etc.). In addition, a special molecule keeps tabs on the ordered sequences so they don't have to be reinvented each time. We know such molecules—we all have them and we call them DNA and RNA.

The genetic molecules of Earth life, DNA and RNA, are unique and common to all of Earth biochemistry. Their complexity is the result of a long process of evolution. Interestingly, a molecule can have a structure that encodes a sequence, and that code can be copied and inherited. Equally interesting is that RNA—as has been shown on multiple occasions in laboratories—is capable of catalyzing its own replication. The result is a dramatic shortening of the biochemical timescales—fast rates that can rise above the destructive chemistry of the planetary environment.

Most objects in the Universe that retain their identity over long periods of time are either very big (such as galaxies) or very stable (such as stars and planets). Our Sun will be a star to a venerable age of 10 billion years by being very thrifty in how it spends its energy; it is very stable indeed. But life, thanks to these two tricks it has, presents a third way. It endures thanks to its individual, short-lived, and localized units—organisms that have the flexibility to adapt by doing
chemistry faster than the changing environment—that are nonetheless balanced by longer-lived and global entities such as entire populations or species. These larger groups are flexible and allow various members to try different things to survive. That is an extraordinarily smart invention! For all we know, thanks to this, life may be a cosmic phenomenon that, once it has emerged, can continue for an indefinite time.

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 when 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.