FIGURE 11.1. There are as many grains of sand in all the beaches on Earth as there are stars and planets in the Universe. The bright dots in the vicinity of our Sun denote some planetary systems we have already discovered.
To begin, the number of known planets to date, summer of 2011, is in the mid-hundreds (about 600), and most of them are in our neighborhood of the Galaxy (see Figure 11.2). They will be a useful reference.
First, I need to know the number of stars in the Galaxy. This number is being constantly updated but has not changed much in the past decade, and is based on many different surveys. Many millions of stars, of different types and in different parts of our Galaxy, have now been counted. With these counts and a measure of the extent of the Galaxy, I multiply to obtain the total number of stars: about 200 billion stars in total. Of these only 90 percent are small enough and long-lived enough to develop and have planets. In addition, only 10 percent of these smaller stars were formed with enough heavy elements to have Earth-like planets. So far, our estimates are very secure and robust. But now I need to know how many of that 10 percent of stars actually harbor Earth-like planets.
I turn to planet counts, just as with the stars. I count how often Earths and super-Earths pop out in the planet surveys done so far. This is a difficult task because few super-Earths have been discovered (no Earths to date); as we’ve seen, they are much more difficult to find, compared to the large and heavy giant planets, so any census of them needs to take this difficulty into account. One way to do that is to compare two different methods of planet discovery and see if they lead to the discovery of different ratios of super-Earths to giants. Comparing the Doppler shift method with the gravitational lensing method suggests that about half of the stars with heavy elements should have at least one Earth or super-Earth planet. If we assume that there are no privileged orbits—that all else being equal, planets can form and remain in any orbit around a star—then only about 2 percent of these Earths and super-Earths will happen to be inside their star’s habitable zone. The remaining 98 percent will orbit too close or, more often, too far from their star. The ultimate answer will come from the Kepler mission, near its nominal end in a few years, but the preliminary data already is consistent with this estimate.14
FIGURE 11.2. Most extrasolar planets discovered so far are close to our Solar System; about 80 percent of all known planets are within 500 light-years and are around stars from the Orion arm of the Milky Way.
Now I am ready to sum up the numbers. With all these fractional reductions to the population of 200 billion stars in our Galaxy, I end up with 100 million planets with habitable potential today.
This number is not precise, but there is no escaping how big it is. On the other hand, the majority of these planets are of an age similar to our Earth’s. Some are younger, but only a few, given the recent advent of heavy elements, should be much older. With such time constraints, we need to be cautious about drawing conclusions of inevitability. All we can be certain of is that life is not an impossibly rare phenomenon—it definitely has odds of 1 in 100 million. These odds aren’t so bad; events of such rarity do happen. As of May 2011, the US Megamillions lottery jackpot had been won four times that year, with odds of 1 in 176 million. In fact, using the binomial distribution, we can see that there is an 18 percent chance of two successes in 100 million trials.
As I explained in the previous chapter, we live in a very young and changing Universe, so the estimate of 100 million planets with habitable potential is just a snapshot of our Galaxy now. A real estate developer once asked me about the trend in the number of habitable planets—are they growing or diminishing in number? Good question!
The stars tell a story. About ten years ago, thanks to the Hubble space telescope, it became possible to measure many very distant galaxies, looking back through time in the process. Astronomers could look at the colors of those galaxies. The color of a galaxy is an aggregate of the colors of its myriads of stars. When this was done carefully, with astronomers making sure to note “redness” due to dust, and so on, a fascinating pattern emerged. 15 The colors of galaxies got “bluer” early on in their existence, then peaked about 7 billion years ago, when they began getting redder very quickly.
What do these changing colors mean? Galaxy colors reflect the colors of their constituent stars, so we have to talk about star colors. For about 90 percent or more of its life, a star’s color tells us how heavy that star is. As we saw in the last chapter, the more massive a star is, the faster it fuses hydrogen. That makes such stars hotter and more luminous. The extra energy they possess causes them to shine with bluer light, just as the filaments of an incandescent bulb do relative to an electric stove heater. Burning off faster also means that bigger stars have short lives.
So, then, if we see “bluish” galaxies, we can conclude that they have plenty of big, heavy stars and that such big stars are being formed at a high rate. If not for the latter, “bluish” galaxies would be very rare. Therefore, the colors of distant galaxies can tell us the rate of star formation.
In the Hubble images, the changes of galaxy colors with time show us the changing rate of star formation. Stars and galaxies formed early on, and then stars kept forming at a steadily increasing rate during the following 5 billion years. In the past few billion years that rate has plummeted, worse than the 2008 world markets downturn and with no prospect for recovery. The heady days of big stars are over! Our Galaxy and other galaxies in today’s Universe form ten to fifty times fewer stars, and most of the stars are small.
This history is good for chemistry and for life. The bumper crop of early, short-lived stars enriched the Universe and the next generation of stars (and planets) with heavy elements. A galaxy with fewer big blue stars is better for chemistry and life too. X-rays and UV light, which large, “blue” stars create in large quantities, are health hazards. In the past 7–8 billion years our Universe has become more and more hospitable, and will continue to evolve in this direction for a long time in the future.
In the meantime, there are at least two reasons why we should be optimistic about the plurality of worlds with separate origins of life in our Galaxy. First is the apparently short time it took life to emerge and take hold on the surface of the young Earth. Second, we now know that the family of hospitable planets is even larger than we thought, with super-Earths being excellent cradles for life.
CHAPTER TWELVE
THE FUTURE OF LIFE
Earth is our home planet. We call it the cradle of life. These notions are deeply ingrained—but wrong. A home is a place we live in, grow up in, and leave; we buy and sell them. A planet is not a home to life. Rather, the planet and the life on it are the same thing. If I were to leave this discussion here, an astute reader might accuse me of making a trivial point, akin to the politician who refers to the same thing by a different name in order to tax it more. After all, life is a planetary phenomenon, albeit not as transient as a hurricane or as inconsequential as a continental tectonic plate. Yet even the single example of life we know has the property of being transplantable (to another “lifeless” planet). Or at least we think so.
How can this be so? As I suggested in my parable of the three planets, we already know that the chemical process known as life is sufficiently powerful to transport itself from Earth to the Moon, and there’s a good chance that we’ve already sent bacteria as passengers on our spacecraft to live permanently on Mars—and maybe far beyond, whether on Titan thanks to Huygens or via Voyager to who knows where. But this kind of transplantation of microbial spores could have happened long before we began exploring space, and over greater distances. When Earth was being bombarded by asteroids early in its history, the outflow of material would have been nearly as great as the inflow, and it’s not hard to imagine some of that material being life, ultimately to travel interstellar distances, embedded in dust particles or comets, until it found a new home on some other habitable planet. In that way, we can imagine planet Earth as a home or a crad
le for life in the conventional sense. The idea is old and is called panspermia.
Panspermia, which means “seeds everywhere,” is an old idea that can be traced back to the Greece of Socrates. Svante Arrhenius, a Swedish chemist, revived the idea in the early twentieth century. Additional arguments were put forward by Fred Hoyle and Chandra Wickramasinghe in the 1970s, with comets as the delivery vehicle for spores, in order to protect them from radiation damage. (Hoyle and Wickramasinghe were also proponents of an eternal steady state Universe, with plenty of time for any spores to make their journey across our Galaxy and mix well among its stars. That kind of thinking made the Fermi paradox seem very appealing.) There is also a local version that postulates exchange within a planetary system, such as from Mars to Earth. The idea goes as follows: a large asteroid impacts young, wet Mars and ejects debris into space and some into independent orbits around the Sun. Though such impacts resemble dramatic explosions (they melt and vaporize huge amounts of rock), many of the ejected rocks should remain entirely unheated and undisturbed. If Mars was alive, with microbes deep inside rocks, many microbial colonies should have survived inside large chunks of rock.
With time—millions of years—the orbits of small rocks are perturbed by the changing gravitational pull of the planets. Often those orbits change dramatically and cross the orbits of large planets. Some of them end up on the surface of a neighbor planet as meteorites. There are many Martian meteorites on Earth; they are relatively easy to identify by their minerals and inclusions of small bubbles containing Martian atmosphere. This suggests that if life ever existed on Mars, it could have reached this planet too. Interestingly, it is far less likely to have gone the other way, as the predominant traffic flow is from the outer Solar System in. When orbits change, they tend to preferentially move toward the Sun.
Whether this has happened is simply an empirical question because the mechanism itself is quite plausible. We know that there are microbes durable enough to make the trip into space and endure the low temperatures and high radiation, even over the million years required for a typical “meteoritic” trip from Mars to Earth. Similar trips between stars, though possible, would take much longer.1 Even if spores might survive billion-year journeys (no such spores are known today), the question remains if there has been enough time in the history of our Galaxy for such journeys to be completed yet. As noted in previous chapters, our Universe is young and the components needed for life are even younger. Just as there has been barely enough time for life to have evolved at all, there is even less for spores to have dispersed through the volume of our big Galaxy. Even if panspermia is not the state of life today, it surely could be in its future.
Panspermia is a long-term phenomenon, however, and so the future of life, at least on our planet, may seem doomed to go on much as it has in the sense that evolution will continue operating on the same basic rules, with the same basic tools, such as DNA and RNA. But that scenario, I think, is far from the truth. In fact, we are living through, in the first years of the twenty-first century, a remarkable turning point in the history of life on Earth. For the first time in about 4 billion years a new species is not going to emerge from the set of processes that led to the diversity of life on this planet. Instead, one species is going to synthesize another—a life-form that is unique, but not in the way that a new dog breed or a genetically modified corn plant is made unique by some cosmetic differences with its progenitor. It will be new in terms of its unique biochemistry, a new life-form that has no place on Earth’s tree of life, a new life-form at the root of a new tree of life.
I am describing the dawn of a new field—synthetic biology. All Earth life has a striking unity of shared biochemistry, which is why we can use E. coli, fruit flies, and lab mice as proxies for our own biology in the laboratory. Much of the cellular machinery is the same. Through synthetic genomics (or, more generally, bioengineering) scientists use that unity to create diversity in form and function. Think of age-old breeding, or the current promise of fully designed microbial genomes.2 Synthetic biology, the way I refer to the field here, goes beyond synthetic genomics, both conceptually and in practice. It is no longer a matter of modifying a genome or even writing a new one, but of synthesizing biological systems that do not exist as such in nature and using this approach to understand life processes better.3 This amounts to changing the basic biochemistry and that is why this new life-form cannot belong on Earth’s tree of life.
Defined thus, synthetic biology research is closer to chemistry than to biology, while the opposite is true for genetic engineering.4 (This led Pier Luigi Luisi to coin the term “chemical synthetic biology” in 2007.)5 I see as one of its central concepts and objects of study the minimal artificial cell—a hypothetical chemical system enclosed in a vesicle and capable of life’s main functions.6 It is a cell because of the vesicle; it is “minimal” because it is stripped down to a bare minimum of function needed for self-sustained existence and evolution; it ought to be “artificial” because no one expects to discover such a primitive (and vulnerable) chemical system on Earth, where even the simplest microbes are highly sophisticated, complex cells and have occupied every conceivable space and niche. At the opposite end, a genetic engineer works with these same highly sophisticated forms, including plants and animals, and strives to modify them to perform in ways that go beyond their naturally endowed skills.
Synthetic biology brings the prospect that an alternative biochemistry is possible and may have developed independently on other planets (or on Earth). This prospect is remarkable in that it involves ordinary chemistry, as opposed to discovering a new form of matter or a new force. Instead, the anticipated discovery lies in uncovering new basic rules (or “laws”) of nature. It is also remarkable because it allows us to ask the origins of life question in a way that may lead science to a breakthrough: What is the role of initial conditions in shaping the unity of Earth biochemistry? Does the diversity of planetary environments map onto a diversity of alternative biochemistries?
This prospect is significant because it pulls together the various threads woven throughout this book, in a synthesis that was hardly possible until very recently. The work in the lab has direct implications for the astronomical search for life on distant planets, and vice versa: these remote findings help hone the lab search for pathways to alternative biochemistries. To find if we are not alone in the Universe, we must understand life, and to understand life, we must learn how to build its chemistry.
When we try to grasp the challenge, the unity of Earth biochemistry can provide a helpful clue. Think of all chemistry—its millions of molecules and all the reactions that can take place among them under different conditions—as a large space. Mathematically speaking, that is a multidimensional space that human brains can’t visualize, but if we choose a few relevant parameters and reduce the many dimensions to two or three, we can see patterns that often resemble a geographical map with valleys and mountains. Scientists often call such spaces “landscapes.” The chemical landscape is shown in Figure 12.1.
FIGURE 12.1. The chemical landscape shown here is a metaphor illustrating the space of all molecules and their chemical reactions, in terms of, for example, molecular reactivity. Surrounding the space are the entry points for assembling molecules (from atoms) under different environments. We call these “initial conditions.” Often an initial condition can occur in nature, as on early Earth; often it can only be achieved in a lab. One of the highest peaks in the landscape represents Earth’s life biochemistry; a pathway from early Earth initial conditions leads to it. We don’t know if other biochemistries exist, nor do we know if multiple pathways reach them or reach Earth’s biochemistry (or its mirror handedness version: the second peak nearby).
The most extraordinary feature of the chemical landscape is a high peak—narrow and well-defined—which encompasses all of Earth life biochemistry. The peak is high because life is a highly efficient chemical system today; evolution has raised its height through time.
Its narrow base, on the other hand, illustrates how constrained in its chemical choices life is. It uses only twenty amino acids, only left-handed ones, only some proteins, only RNA and DNA, and so on, out of many millions of available choices in the vast chemical landscape. Are there any other tall peaks in the landscape? If so, which of them are alternative biochemistries (“other life”)? Are any of them accessible from plausible planetary environments?
We can answer the question about alternative biochemistries in the affirmative because of the twin peak corresponding to “mirror life”—same choice of biomolecules but opposite symmetry (handedness) (see Figure 12.1). There is no reason to suspect that a mirror biochemistry would behave any differently in a lab or on a planet. While scientists understand why a mix-and-match biochemistry fails, the choice of left versus right symmetry might have simply been a matter of chance.7 In that sense, the mirror biochemistry is trivial—it provides no direct insight whether truly alternative biochemistries are possible. However, if it can be built in the lab, such a biochemical system will deliver powerful tools for understanding life’s fundamental principles. For one, it might be the shortcut to the successful assembly and maintenance of a minimal cell that will open numerous opportunities.8 At least, having a functioning mirror system will allow testing the importance of initial conditions (e.g., different planetary environments).
Chemists today have analytical tools at their disposal that allow them to explore large swaths of the unknown interior territory of the chemical landscape.9 Similarly, for the first time in history, astronomers have tools to explore planetary environments unknown on Earth or in the Solar System. Soon they will explore and understand alternative global cycles to Earth’s carbonate-silicate cycle (see Chapter 10) on Earth-like exoplanets orbiting other stars, and map the hitherto unknown regions of the periphery of the chemical landscape, where the initial conditions reside (see Figure 12.1). Working together, chemists armed with information provided by the planetary astronomers could venture into the interior in search of high peaks (alternative biochemistries). Would a super-Earth planet with a dominant sulfur (SO2) cycle lead them to a new “sulfur-inspired” biochemistry? Or would they find an alternative pathway to our own Earthly biochemistry? Whatever they found, it would be a trip to remember! Perhaps the most valuable thing we’d learn is how to look intelligently for signs of life on those distant planets, as opposed to simply, and naively, looking for carbon copies of Earth’s biosphere.
The Life of Super-Earths Page 12
