The history of our species, Homo sapiens, has several big milestones. We know most of them, perhaps we are missing a few. One thing is for sure: they are all milestones important to our history and the history of life on Earth. However, this milestone I am anticipating—synthetic biology—is of a different character and goes beyond the special case of planet Earth and Homo sapiens, because it is significant in the chain of events following the development of ordinary matter in the Universe. Earth life, a single example of the complicated chemistry known as biology, proves feasibility—it is possible for biology to happen, but it proves nothing about how likely it is to happen. Synthetic biology, once its research strategy succeeds, could prove that ordinary matter has an inherent capability to self-organize, to create diversity from a single biochemistry, and ultimately to amplify that diversity by spawning multiple biochemistries. (What we do not know is the magnitude of that amplification.)
That last step is a historic watershed: one tree of life begetting other trees of life (or “roots” of other trees). It is a watershed because it could be a recipe for amplification of diversity on the scale of the Galaxy and on long timescales (billions of years) and because it suggests the existence (now or in the future) of a new generation of life. Let’s call it Generation II life. Its defining feature is that its tree is not rooted in prebiotic chemistry but originates in Generation I life. Life on Earth is Generation I, and the term “generation” is used in the same way as in human society. It implies a cohort of peers, constituting a single step in the line of descent from an ancestor, though not necessarily born strictly at the same time. Generation I may consist of a collection of biochemistries, if such exist.
We do not know how different other biochemistries (or other origins of life) could be from our own. We might discover that the “biochemistry landscape” allows only limited and, perhaps, similar families of biomolecules and biochemistries. If so, the amplification factor afforded by synthetic biology will be considerably diminished, but it will still be an amplification factor, and that implies an increasing role of biochemistry in the redistribution of baryonic (ordinary) matter in the distant future of the Universe. Given what we know about what other planets can be like or will be like—as when the carbide planets come to outnumber the silicate ones, such as our own—there is plenty of room in the chemical landscape for the biochemical landscape to be vast. Generation II life may already exist in the Galaxy.10
The milestone of synthetic biology is, to my mind, one of three ongoing. They appear unrelated and may have happened at once coincidentally, each a product of the highly accelerated rate of our technological development in the past half century. The other two are the completion of the Copernican revolution and the astonishing process of globalization, as humans across the planet have become interrelated not just biologically, but in practical ways and in everyday awareness.
Globalization has already happened and would have been inconsequential to our discussion here, but for its direct relation to the completion of the Copernican revolution (the former having helped make the latter possible) and its not entirely positive effect on Earth’s entire biosphere. I hope that the completion of the Copernican revolution, by showing us that the Earth is just another planet and that other planets may be hospitable to life, will help convince us that we are not special. The humility could do us some good. Looking at life as a planetary phenomenon in which the underlying biochemistry is deeply tied to the planet itself will help reinforce our awareness of being one with our Earth, a product of a unique biochemistry that emerged 4 billion years ago and is distinctly Earthly. We are part of a good thing here, and perhaps learning about it will help motivate us not to screw it up.
The dawn of synthetic biology, then, comes at a fortunate time: it answers the question, What next? that emerges after the completion of the Copernican revolution. It transforms the end of a chapter on humankind’s awareness of the world into the beginning of a chapter about humankind’s place in the world.
In these pages I have painted an optimistic picture in which life is robust and emerges with ease in a Universe full of places where it can grow. We do not really know if life emerges with ease. We only know that it did so here on Earth. One example is not enough to draw conclusions.
I have also suggested that panspermia, whether accidental or purposeful, via rocks, comets, or interplanetary probes, is very possible too. That is one reason why life may be a process that, once it has emerged, can continue indefinitely, never attaining equilibrium with its environment, even on stellar or intergalactic timescales. One piece of evidence for this is us—we know we are life-forms capable of leaving our planet of origin and exploiting other resources. Even if we never leave permanently, the fact that we can do so proves that life is a phenomenon capable of transcending the lifetime of a typical star, such as our Sun.
And a good thing too! Imagine our Solar System 5 billion years from now. The Sun—the parent star, source of light, provider of warmth and energy to living things on our life-transformed planet—is taking a well deserved retirement and is about to begin spending its retirement account faster than a savings-free baby boomer. And the Earth? Well, the Earth will have to go. Venus and Mercury will have to go too—engulfed, molten, and vaporized in the slowly expanding hot sphere of the red giant star that is now our Sun emeritus. Planet Earth and its 9 billion-year-old biosphere are gone for good! The microbes do not have an evacuation plan.
No need to panic, though, since 5 billion years is a bit beyond your retirement age. Nevertheless, the Sun’s retirement (like our own) is something we ought to plan for well in advance. And perhaps instinctively, humankind is already on a path to do just that. Understanding the essence of life here on planet Earth will help us understand the origins of life in other places in the Universe. With this knowledge we will seek and find friendly harbors. And one day we will throw anchor there. This is the day humankind, and with it Earthly life, will free itself from the cosmic fate of planet Earth and our Sun.
We humans have made this kind of trip numerous times in our brief history. Here is just one example.11 About 4,000 years ago tribes in south central Europe had domesticated horses and invented carriages. They could move entire villages over vast distances—a thousand miles, maybe more. After several generations, living conditions in the European steppes had grown unbearable, so one day they packed up and left for the east, where sparsely populated lowlands opened before them. For another century or more—nobody knows exactly how long—these people moved east until they reached the towering mountains of central Asia. Through local tribes they heard of a fertile valley just southeast of the Pamir and Tienshan mountains—hard to reach but uncontested land. These people of the steppes had the knowledge and the technology to cross the high mountain passes, some exceeding 10,000 feet. The other side must have appeared to the worn-out travelers like a place out of this world. Today this is the land-locked Tarim basin in the heart of Asia, mostly a salty sand desert—the Ta-klimakan. But geological evidence shows that as late as 2,000 years ago the Tarim appears to have been rich in water and vegetation. 12 The tribes from Europe not only survived but prospered. Today we marvel at their exquisite clothing, beautiful artifacts, and rich culture in the amazing mummified burials discovered in the dessicating sands of Tarim in the past decade.13
This is just one such story. The time for such migrations on planet Earth has ended. Today the planet is densely populated and globalized. Our planet is a beautiful place and we could be happy living here for thousands of years to come. But we already know that one day our kind will face the same decision the Tarim migrants faced all those years ago. Will our future relatives have the knowledge and technology to make it across?
NOTES
CHAPTER ONE
1 For a detailed account of this history, see Charles A. Whitney, The Discovery of Our Galaxy (New York: Knopf, 1971).
2 Temperature is measured by different scales—Celsius, Fahrenheit, Kelvin, each with a dif
ferent zero point. The Kelvin scale begins at “absolute zero,” while the Celsius scale has its zero point at the temperature distilled water freezes under sea level pressure. Therefore, 0 degrees C corresponds to 273 K, while 170 K is a very cold minus 103 degrees Celsius. D. Sasselov and M. Lecar, “On the Snow Line in Dusty Protoplanetary Disks,” Astrophysical Journal 528 (2000): 995.
3 Planets orbiting other stars are named after the star followed by a lowercase letter “b,” “c,” and so on, in order of discovery. The shortened constellation name (e.g., 51 Peg for 51 Pegasi) is commonly used. Whenever the star has no previous common name, the name of the project responsible for the discovery is used, followed by a consecutive number and by a lowercase letter “b,” “c,” and so on.
4 This is the first valid detection of a planet outside our Solar System (D. Latham et al., “The Unseen Companion of HD 114762: A Probable Brown Dwarf,” Nature, May 4, 1989), but it was not announced as such because the authors of the work were cautious not to overinterpret their evidence. Discovered with the same technique used to find 51 Peg b, the planet’s mass is derived only in its minimum limit, meaning that if we happen to be observing the planet’s orbit face-on (i.e., from its pole), its mass must be larger. The probability is not negligible, particularly when compounding the case with two unusual properties of the HD 114762 companion: (1) its mass exceeds that of Jupiter, yet its orbit is smaller than that of Mercury, and (2) it has a substantial orbital eccentricity. For comparison, 51 Peg b at least has a noneccentric orbit, though what an orbit it is!
5 G. Walker et al., “A Search for Jupiter-Mass Companions to Nearby Stars,” Icarus 116 (1995): 359.
6 S. Ida and D. Lin, “Toward a Deterministic Model of Planetary Formation,” Astrophysical Journal 626 (2005): 1045.
7 The question of the Other has fascinated writers, philosophers, and anthropologists; a nice analysis of Western thought, albeit confined to mostly French sources, is contained in the seminal monograph by Tzvetan Todorov, Nous et les autres (Paris: Editions du Seuil, 1989; On Human Diversity Eng. trans., Cambridge: Harvard University Press, 1993).
CHAPTER TWO
1 Dava Sobel, The Planets (New York: Penguin, 2005), 145.
2 Helium was discovered remotely in the Sun through spectral analysis of the signatures of gases in solar light, not in a mineral or laboratory on Earth. Hence its name from Helios, the Sun.
CHAPTER THREE
1 D. C. Black, “Completing the Copernican Revolution: The Search for Other Planetary Systems,” Annual Reviews of Astronomy and Astrophysics 33 (1995): 359. This fascinating and insightful review was written at the dawn of the age of extrasolar planet discovery. It shows the awesome technical challenges, the frustrations, and the gnawing doubts after the many empty-handed searches. M. Mayor and P.-Y. Frei, in New Worlds in the Cosmos (Cambridge: Cambridge University Press, 2003), give an account of the beginnings and provide full quotes from the writings of C. Huygens and B. de Fontenelle.
2 The Hubble space telescope observed the area in the sky known as the Hubble Deep Field for ten consecutive days, taking multiple images in four different filter passbands: near-ultraviolet (300nm), blue-yellow (450nm), red (606nm), and near-infrared (814nm), for a total of 342 individual exposures.
3 There is an extensive literature on direct imaging for planet detection for both ground-based and space-based telescopes. There are two general types of solutions. One tries to minimize the light of the star by directly blocking it inside the telescope, while the other tries to minimize the light of the star by combining it in at least two telescopes and eliminating it through interference. The latter device is known as an interferometer, the former as a coronograph. The most ambitious interferometer proposed is a flotilla of telescopes orbiting around the Sun and maintaining a precise formation. The design is often associated with NASA’s Terrestrial Planet Finder project and the European Space Agency’s (ESA) Darwin project. Webster Cash, in “Detection of Earth-like Planets Around Nearby Stars Using a Petal-shaped Occulter,” Nature, July 6, 2006, has put forward a similarly ambitious proposal for an enormous coronograph telescope in space.
4 In special cases, when the planets are young, large, and orbit far from their stars, it is possible to discover them directly, as in the spectacular infrared images of star HR 8799 with its coterie of four planets found by Christian Marois et al., “Direct Imaging of Multiple Planets Orbiting the Star HR 8799,” Science 322 (2008): 1348; and Marois et al., “Images of a Fourth Planet Orbiting HR 8799,” Nature, December 23, 2010.
5 SIM PlanetQuest was a NASA mission that was in a detailed design phase a few years ago. S. Unwin et al., “Taking the Measure of the Universe: Precision Astrometry with SIM PlanetQuest” (Astronomical Society of the Pacific, January 2008).
6 A. Wolszczan and D. Frail, “A Planetary System Around the Millisecond Pulsar PSR1257 + 12,” Nature 355 (1992): 145.
7 Pulsars are the remnants of supernova explosions—the end product of the development of a star about ten times more massive than our Sun. Even if the original star had planets, the planets around the remnant pulsar today are not those. We do not have a good idea how the pulsar planets formed after the explosion of the star, and what these planets are made of is not clear.
8 The technique was proposed by M. Holman and N. Murray, “The Use of Transit Timing to Detect Terrestrial-Mass Extrasolar Planets,” Science 307 (2005): 1288, and by E. Agol et al., “On Detecting Terrestrial Planets with Timing of Giant Planet Transits,” Monthly Notices of the Royal Astronomical Society 359 (2005): 567, with the practical use of the transit method in mind—transit timing variations. However, discovering an unseen planet by watching its effect on the orbit of a known planet has a venerable history. This is how the planet Neptune was discovered.
9 J. Lissauer et al., “A Closely Packed System of Low-Mass, Low-Density Planets Transiting Kepler-11,” Nature 470 (2011): 53. In the case of Kepler-11, all planets were discovered by the transiting method, but transit timing variations allowed for the candidate planets to be confirmed and their masses measured.
10 When applied to stars, the effect is technically referred to as gravitational microlensing, in order to distinguish it from lensing between galaxies.
11 A. Einstein, “Lens-like Action of a Star by the Deviation of Light in the Gravitational Field,” Science 84 (1936): 506; S. Mao and B. Paczynski, “Gravitational Microlensing by Double Stars and Planetary Systems,” Astrophysical Journal 374 (1991): L37.
12 J. P. Beaulieu et al., “Discovery of a Cool Planet of 5.5 Earth Mass via Microlensing,” Nature, January 26, 2006; D. Overbye, “Astronomers Briefly Glimpse an Earth-like Planet,” New York Times, January 25, 2006.
13 OGLE stands for the Optical Gravitational Lensing Experiment, a US-Polish project that uses a telescope in Chile to detect stellar gravitational lensing events.
14 As stars orbit the center of the Milky Way Galaxy and we observe them from our own orbit in the Galaxy, they all appear to shift, albeit very slowly, with respect to each other. Occasionally they will literally pass in front of each other from our point of view. This is the moment when for a brief period of time we can see the gravitational bending of light—the effect of gravitational lensing. The smaller the mass of the lens, the briefer the event. With typical orbital speeds of stars in our Galaxy (200–300 km/ sec) and our own motion in the same general direction, the typical duration of a stellar lensing event is several weeks. The lensed star appears to brighten, peak, and then fade back to its original brightness; the peaks typically last just a few days. The signature of a planet is a separate peak—a blip that is superposed on the brightening of the star and lasts for less than a day. Under rare favorable circumstances the orbital motion of the planet may be discernable.
15 A. Gould et al., “Microlens OGLE-2005-BLG-169 Implies That Cool Neptune-like Planets Are Common,” Astrophysical Journal 644 (2006): L37.
CHAPTER FOUR
1 The march, called “The Transit of Venus March,” w
as written by John Philip Sousa in 1882 for the nineteenth-century transit of Venus and to honor the first secretary of the Smithsonian Institution.
2 The transits of Venus occur either in pairs separated by an eight-year interval or as a single transit every 121 years; three transits in a short sequence never occur. We live in an era when the transits of Venus come in pairs. The present era started with the transit in 1631 and will end with the transit in 2984, followed by a cycle of single transits. The mechanics of this are nicely described by Eli Maor in Venus in Transit (Princeton: Princeton University Press, 2004).
3 W. Sheehan and J. Westfall, The Transits of Venus (New York: Prometheus, 2004).
4 See the amazingly successful community of amateur astronomers observing extrasolar planet transits on www.transitsearch.org andthe American Association of Variable Star Observers. In 2007 the former succeeded in discovering the transits of an extrasolar planet that had been discovered by the Doppler shift method—HD 17156b.
The Life of Super-Earths Page 13
