The Life of Super-Earths

by Dimitar Sasselov

  Smaller things—our bodies, for example—are held likewise together in a pressure balance, but it is not the balance between gravity and electromagnetic forces. (Our bodies have too little mass for our own gravity to help hold us together.) Instead, we are pulled down by the gravity of planet Earth and simultaneously we are under pressure by the air above our heads—about fifteen pounds of it on each square inch. The air, of course, is also pulled down by gravity. As we go to even smaller scales, electromagnetism gets more and more dominant and gravity all but drops out of the competition. Electromagnetism, as manifested by the chemical bond, makes our tissues structurally sound, keeping cells and larger multicell bodies together.

  FIGURE 7.1. Comparative sizes: the molecules of life are huge when compared to the common molecule of water (H2O); they define the large-molecule scale.

  To everything, then, it seems there is the right size, as J. B. S. Haldane—an early geneticist and famous science popularizer—mused in a 1928 essay. He was answering the question we always ask as children: How are insects built to be able to remain unscathed after a fall from a height that is multiple times their size? Why do some insects walk on water while others drown in it?

  Haldane gave the right answers, but only in a general sense. A former colleague, Bill Press, was more mathematical about it and got the correct answer for the size of the human body. To get the correct answer, he had to make three assumptions: that a human body (1) is made of complicated molecules that are strings (polymers) rather than amorphous, (2) requires an atmosphere that is not of hydrogen and helium, and (3) is as large as possible to carry its large brain, but liable to stumble and fall, and in so doing should not break.5 Thus he defined the right size of an active animal living on the solid surface of planet Earth. That includes the ambient temperature at which humans (and most animals) live, which—not surprisingly—is very close to the temperature of their chemical bond energy. As Bill points out, that fact makes cooking practicable.

  The big Universe, however, is definitely not built to our human scale, and the problem is not only the vastness of space, but also the unimaginable scale of time. Time and space are inseparably linked, as we learned from twentieth-century physics, so it should not be surprising that in a big place things take a long time to change. It is essential to my story that you get a feel for this. Nevertheless, while the Universe as a whole is not built to a human scale, it is obviously possible to find places that are, for example, our home planet. This, as I will argue in the next chapter, is not an accident. Life needs planets. Let’s see why.

  CHAPTER EIGHT

  ORIGINS OF LIFE

  Why Planets?

  During the Middle Ages, when every continent was a separate world, people seriously considered multiple origins.1 During the Renaissance, with Earth now just another planet, people seriously considered multiple origins of life on the other planets, as well as the Moon. Thus I am bringing up an old question and revisiting it with fresh evidence about newly discovered worlds and about what it takes for life to emerge and survive on them. Some of this evidence I have touched on earlier, but I am going to bring together the various threads here to argue that for life to emerge, it must happen on planets.

  So, let’s take a step back and think of the cosmic perspective. Why planets? Aren’t there other places in the Universe that could be equally good cradles for life?

  Here is a recipe to answer these questions: get what we know about other places in the Universe and mix it with what we know about life, and see if anything useful comes out. The first ingredient is easy to procure. During the past fifty years astronomers have revolutionized our knowledge of the observable Universe and have a pretty complete census of what is in it—lots of galaxies, many more stars, and a list of planets that grows practically by the day.

  What we know about life is the difficult part—we know a lot and very little, at the same time. Scientists have revolutionized our understanding of the building blocks and the amazing interconnectedness of living forms, yet the parts are in a system,e they form networks,f and there is something essential but elusive in all that.

  I’ll begin with the difficult part and list what we know about life—not a definition of life, but just some essential attributes:2

  1. Life is chemical in essence.3

  2. Life is a system that is not in equilibrium.4

  3. Life is adaptive and self-optimizing.5

  4. Life is compartmentalized—it needs cells, enclosures, vesicles.

  5. Life uses molecules that are suited to water.

  The list is easy enough to compile; the difficult part is discerning what may be missing from it, since all we know about life comes from a single example—life on Earth. Curiously, each one of the five items taken separately can describe a nonliving system; life seems to need all five together.

  The first attribute is obvious but very useful to the big picture because it is an absolute for any origin of life in our Universe. There is no other form of matter or system capable of ordered networks; chemical bonds can do wonders under natural conditions. We cannot exclude the possibility that extant life-forms might develop a system (or develop into a system) that is not based on molecules, but that is not the question I am asking. I am interested in the process by which life emerges and the environments that allow it; that is, I am interested in the cosmic transition from chemistry to life. People have described nonmolecular (nonbiological) life-forms (in, e.g., the writings of Hans Moravec, Ray Kurzweil, and Steven Dick), but always as derivative or evolving from biological predecessors and based, at least partially, on molecules and molecular bonds.6 We humans may be capable of creating ordered networks in silicon, but this is the result of a long history of the development of technology, not of a lifeless planet’s original condition.

  Planets help make this possible by providing a physical screen against space radiation, through their bulk and their atmosphere. Planets provide stability over an average timescale longer than the development of chemical networks—years. For example, interstellar molecular clouds (like any cloud) cannot provide a macro- or microenvironment that is stable or protected from destructive cosmic radiation on such timescales, while planets can.

  The second attribute is also an absolute: a system at equilibrium is a dead system in which nothing happens; you need energy to accomplish anything. To assemble an ordered network—chemical or not—you need energy. You need energy just to keep it ordered, to keep the inside separate from the outside.

  The third attribute is the most fascinating thing about Earth life, and probably of any life—it can adapt itself to both fast and slow changes. Charles Darwin’s genius was to see the essence of that property, which is based in part on the process by which new and different organisms develop as a result of changes in genetic material. We call it evolution, or Darwinian evolution in his honor. Most scientists consider it so important that they would define life through it. Gerald Joyce of the Scripps Institute summarized a NASA committee proposal in 1994 in a famous short definition of life as “a self-sustaining chemical system capable of Darwinian evolution,” sometimes referred to as the “NASA definition.” 7 Even though Darwinian evolution is an obvious way8 to achieve diversity in response to changing environmental conditions and interaction with other living forms,9 strictly speaking, it is not the only one.

  The alternative is the existence of environmental conditions that allow continuous creation of life-forms with no inheritable molecules, but still with random variations.10 The burden to sustain such a biochemistry then rests on the existence of environmental conditions that allow the continuous synthesis of life-forms. Evolution offers a simpler and more straightforward mechanism, and is very likely a universal attribute to different origins of life and different biochemistries.

  In the cosmic transition from chemistry to life there is no substitute for molecules and what they can do; only one of the essential attributes—the second, describing energy dissipation—can be worked out without m
olecules. Any of the others will be impossible to do without some sort of complex molecules—usually large ones, called polymers. Polymers are very long molecules made of smaller units called monomers (see Figure 7.1 ). The basic polymers of life, called biopolymers, are proteins—made by linking together amino acids and nucleic acids—made from nucleotides.

  We might not know which chemical networks and attributes are essential to life, but as long as we are certain that some of them are, then molecules become building blocks and we can’t do without them. Moreover, even though there are millions of molecules, the toolbox for building them is very limited, so we can identify some fairly narrow constraints on what must be around for life to happen. This is a good thing, as it gives us a fighting chance to understand them.

  As we’ve seen, life must exist on a scale at which both gravity and electromagnetism are noticeable forces, and life needs protection from the environment of space. Consider temperature. In the Universe there is a wide range—from a few degrees above absolute zero to several hundred million degrees.11 The coldest places are clouds of gas away from stars in the outskirts of small galaxies. Many planets are very cold too. In our Solar System the dwarf planets Sedna and Eris are more than three times farther from the Sun than Neptune, and their surface temperature never rises above a bone-chilling 30 K (–400°F). The gas giant planets that were photographed recently around stars HR 8799 and Fomalhaut are at similarly great distances from their parent stars. It is expected that many more planets are equally cold.

  At the hot extreme of temperature are the envelopes of massive stars and gas near the central regions of galaxies. We know the destruction tolerances of molecules, and most of the high-temperature range is off-limits. While a few very hardy ones, such as carbon monoxide, survive at high stellar temperature (in low densities, they can survive temperatures as high as 6,000 K), any molecule with more than three atoms needs temperatures lower than those found in stars. Gas near the central regions of galaxies won’t work either. The X-ray and UV light emitted by stars, as well as the energy released during the explosions of supernovae, keep lots of the gas in galaxies hot too.

  The very low temperatures have their problems too—polymers survive but lose chemical functionality. Just as we know from personal experience (and make use of, through refrigeration), chemistry happens very slowly if at all at very low temperature. So, there is a narrow range of temperatures in which large molecules thrive and complex networks of chemical reactions can take place. That temperature range is from about 100 K to 600 K if we feel generous, but much narrower if we consider Earth biochemistry as we know it today.

  That is a remarkable conclusion. In a Universe where temperatures of millions of degrees are common from a cosmic perspective, life is a low-temperature phenomenon. At these low temperatures there are only a couple of large long-lasting types of objects in our Universe—clouds of gas between the stars, called molecular clouds, and planets (as well as their satellites and other small planetary bodies).

  Access to energy is another important environmental condition. In the above temperature range, there are two benign, steady, and long-lasting sources of energy in moderate amounts: starlight and internal heat of planets. Being a certain distance away from a star, such as Earth is from the Sun, is an excellent place. The energy flow is steady and moderate, meaning that it matches the energy needs of large molecules without destroying them readily. To access this stellar energy, you do not have to be on a planet. On the other hand, planetary hydrothermal vents, such as those in Yellowstone National Park or Iceland, or on the ocean floor, are a good example of how internal heat can provide a similar source of energy, both locally and globally on a planet, no matter how far from the star. Planets cool slowly, especially larger super-Earths, and this source can be steady for a very long time. In addition to heat, vents provide a rich source of chemicals that can be used as a source of energy. Hydrogen sulfide, for example, a chemical poisonous to you and me, is harnessed by bacteria at vents through a process known as chemosynthesis to power their cellular machinery.

  Speaking of chemicals, there is a third important environmental condition—life seems to need environments allowing chemical concentration. Most environments in the Universe are dilute. Relatively complex molecules are observed to form in molecular clouds, but their concentrations are always extremely dilute. This severely limits the complexity in molecules and their reaction networks. Gas giant planets like Jupiter or Neptune are another example of chemically dilute places. Their cloudy atmospheres and bright display of colors are all there is—no solid or liquid surface. As you plunge deeper below the clouds, the gas gets denser and hotter, and all but the smallest molecules are destroyed. Life on Earth uses enclosures such as cells and vesicles within them to further concentrate chemicals in useful places; maintaining this kind of disequilibrium, of course, requires energy.

  The combination of relatively low temperatures, steady sources of energy, and access to chemical concentration leaves us with planets as the best, if not the only, places where the molecules of life and their interrelated reactions can emerge and sustain themselves. No other type of object has the complete set of conditions. And then only some planets—gas giants won’t do. Terrestrial planets are unique in providing a range of rich chemical concentrations, energy sources, and sheltered environments. This is a profound realization!

  CHAPTER NINE

  LIFE AS A PLANETARY PHENOMENON

  Half a day’s sail northwest of the Cape of Good Hope lies Cape Town, South Africa. Even today, the city feels like an outpost, a cozy yet uneasy harbor at the edge of the world. After all, there is, to human eyes, nothing south of there but cold and ice, a frigid southern ocean encircling the least hospitable landmass on Earth.

  There, in the shadow of exotic Table Mountain, in November 1873 an unusually outfitted British ship, HMS Challenger, was being readied for a grueling journey to the icy shores of Antarctica, then on to Australia and around the world. Forty years after the famous trip of the HMS Beagle with Charles Darwin onboard, HMS Challenger was on a four-year voyage to explore a new frontier—the depths of the world oceans.1 HMS Challenger had a complement of six scientists in its crew and a deck loaded with curious instruments: for measuring oceanic temperatures, depths, and currents; for taking deep-ocean samples; and for capturing living specimens of whatever creatures might call the strange place home.2

  The results astounded laypeople and scientists alike. From the total darkness and tremendous pressure found eight or more kilometers below the surface, HMS Challenger collected 600 cases of specimens. The rich harvest proved that the vast underwater landscape of the oceans is not a desert. Life was everywhere on the surface of our planet.

  More recently, research in the last twenty years has led to two more astonishing realizations. First, rather than simply providing a home for life, planet Earth has been thoroughly transformed by life, which has accompanied it almost since its formation 4.5 billion years ago. What’s more, as fragile as life may appear to be, clinging to the surface of a small planet that is subject to violent cosmic events, what we have learned has led us to conclude that life on Earth is virtually indestructible. The evidence suggests that it has been that way for a very long time, perhaps 3 billion years.3 Destroying all living things on Earth, including spores and complex biomolecules, would take nothing short of melting and vaporizing the planet in the sterile interior of the Sun. Destroying life would also require annihilating all the spacecraft in Earth orbit, as well as the other places we’ve sent space probes. Though unlikely, the possibility that we’ve colonized Mars with some microbes, for example, is not that far-fetched; many extremely resilient microbes have been discovered in the past thirty years, and some—such as Deinococcus radiodurans—are hardy enough to survive the trip to Mars.4 Real space travelers!

  The secret behind the indestructibility of the Earth biosphere lies in the sheer diversity and inventiveness of the organisms that have always ruled “our” plane
t—the microbes. The hardiest among them are called extremophiles, meaning that they inhabit extreme environments. Some are able to withstand 250°F (122°C) in hot springs and in ocean floor “black smokers”—hot volcanic vents. Others survive high pressure levels, for example, in the sterilizing high pressure vats for orange juice5 or in the natural habitat of the Marianas trench in the Pacific Ocean at 800 atmospheres.6 Yet others have made their home in the tiny cracks of rocks four kilometers underground, discovered as history has gone through another cycle, and a flotilla of twenty-first-century ships, mimicking the Challenger before them, has drilled into the ocean floor.7

  Scientists have probably not yet reached the bottom of the Earth’s biosphere. Microbes that live deep within the Earth’s crust are often dubbed SLiME communities, for subsurface lithoauthotrophic microbial ecosystem. They depend on nothing from the surface. The heat comes from the depths of the planet, the chemicals and water are already there, and sunlight is not needed. Though extreme, their environment is very protected.