The Life of Super-Earths (18 page)

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 (SO
₂
) 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 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?

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 different 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).

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.

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.