The Life of Super-Earths

by Dimitar Sasselov

  Table of Contents

  Title Page

  Acknowledgments

  Introduction

  PART I - SUPER-EARTH

  CHAPTER ONE - EXTRASOLAR PLANETS AT LAST

  CHAPTER TWO - THE WORLD OF PLANETS

  CHAPTER THREE - COMPLETING THE COPERNICAN REVOLUTION

  CHAPTER FOUR - CHASING TRANSITS

  CHAPTER FIVE - SUPER-EARTH

  CHAPTER SIX - SUPER-EARTHS

  PART II - ORIGINS OF LIFE

  CHAPTER SEVEN - THE SCALE OF LIFE

  CHAPTER EIGHT - ORIGINS OF LIFE

  CHAPTER NINE - LIFE AS A PLANETARY PHENOMENON

  CHAPTER TEN - PLACES WE COULD CALL HOME

  CHAPTER ELEVEN - THE PASSAGE OF TIME

  CHAPTER TWELVE - THE FUTURE OF LIFE

  NOTES

  INDEX

  Copyright Page

  ACKNOWLEDGMENTS

  This book grew out of the general education lecture course, Life as a Planetary Phenomenon, that my colleague Andrew Knoll and I designed and have taught together at Harvard since 2005; I am much indebted to Andy for encouraging and teaching me along the way.

  My book introduces a general audience to new ideas on old big questions about life and the cosmic perspective on life. These questions formed the foundation for the research agenda of the Harvard Origins of Life Initiative and its core team of scientists—I thank my colleagues for the amazing collaboration and for teaching me what little I know in their fields. In particular, I owe much to Jack Szostak for being a great teacher and partner from the very beginning; to Andy Knoll and my late friend Mike Lecar for getting me into this in the first place; to George Whitesides for his wise advice about chemistry and much more; to Stein Jacobsen, Scot Martin, George Church, Ann Pearson, Rick O’Connell, David Latham, and Sarah Stewart for pushing the boundaries. I was inspired by the books of Erwin Schroedinger and Freeman Dyson and by the brave experiments of Gerald Joyce and Craig Venter, and many others; I apologize to all whose contributions are not mentioned in my short book.

  My goal was to write a popular book because the new science concepts are truly wonderful and exciting, and because they have immediate implications for all of us. My approach was to provide a thorough introduction in order to make the science accessible, followed by my own views on the unanswered questions, and keeping the technical details to the endnotes only. Thanks to John Brockman and Katinka Matson my book took the right path toward that goal; my agent Max Brockman made sure the project was accomplished—I owe them a lot for their guidance and support.

  The road from lab to paper and to readable prose is torturous; I was incredibly lucky to have TJ Kelleher as my editor, from the first meeting years ago to the final touches. He understands the science and he is a talented writer! My friend Peter Abresch helped me with the baby steps and showed me, in my first chapter, how to write well. To both of them I am very grateful, as I am to Andy Knoll for his critical reading of the biology-related chapters of the book. The beautiful and intelligent illustrations are the work of gifted artists Sandra Cundiff and Michael Hardesty, and I am very grateful to them.

  My deepest thanks are due to my family because this book would not have come to be without their encouragement, patience, and support: to my dear parents whom I owe for who I am and what I can do, and to my dear wife, Sheila, for being by my side throughout the entire process.

  INTRODUCTION

  The Mystery of Life

  There are few big questions that rival this one: What is life and how did it come to be? It has always been a big question, though not always for science alone. And there have always been numerous models, scenarios, speculations, and ideas offered in response—most of them not terribly successful. The middle of the nineteenth century was no different. But some samples of slimy mud scooped up from the depths of the North Atlantic along the route of a telegraph cable would change that.

  The year 1857 could be celebrated as the time humanity took the first practical step to create a global world on this planet—the global-awareness world we live in today. Converted British and American warships laden with rolls of cable were laying the first intercontinental telegraph connection on the bottom of the Atlantic Ocean between Europe and America. The human timescales of news traveling on foot or by horse or by pigeon were giving way, ultimately to be replaced by instantaneous communication at the speed of light. Days and weeks were being replaced by hours and minutes. It seemed as if the oceans that had separated humans for millennia “had suddenly dried up,” as newspapers at the time wrote.

  In preparation for laying the telegraph cable, ships like HMS Cyclops and USS Arctic were sounding the Atlantic Ocean floor and sampling the ocean bottom. In 1868 Thomas Henry Huxley, an English biologist with major achievements in comparative anatomy (although better known today for his role as a popularizer of Darwin’s theory of evolution), discovered among the samples taken from the Atlantic a substance—gelatinous, colorless, and formless—that he thought was a new life-form. Not just any life-form, Huxley thought, but the primordial organic substance, the undifferentiated protoplasm from which life originates.

  It was an audacious idea for a heady time in the quest to understand life and its origins, and Huxley was in the middle of it all. First, in 1859 Charles Darwin published his seminal On the Origin of Species, and the theory of evolution had become a topic of broad and heated debate. Then, between 1860 and 1863, Louis Pasteur completed his famous experiments with sterilization. Between them, long-held concepts about the origin of life were being completely upended.

  Before Darwin and Pasteur, Western science had attempted to explain life’s origins through a combination of spontaneous generation and vitalism. Spontaneous generation was the idea that life emerges from decomposing matter, the latter being imbued by a vital force (common to all organic material, and the air as well). Vitalism was already under attack from chemistry. In its early development, chemistry had separated inorganic compounds from organic ones, the latter being erroneously assumed to derive from living forms only. Once an organic compound was synthesized in a laboratory in 1828, the need for a vital force was on its way out (although organic chemistry still keeps its name).

  The fallacy of spontaneous generation had been exposed in experiments involving extensive boiling of meat broths before Pasteur, but his elegant experiments allowed access to air and thus proved that life emerges only from life. The long, sharply curved swan-neck flasks that he used to boil the broth prevented germs (i.e., bacteria and spores) from entering the sterilized liquid but still let in air. It appears that Pasteur convinced everyone.

  None of that could help scientists understand life’s origin, except that now they could clearly state the problem. Both Pasteur and Darwin described the origin as a single act of abiogenesis: that the first life-form emerged from inanimate matter, which happened just once. For Pasteur it was an act of God’s creation, while Darwin left it to a “warm little pond,” according to a letter he wrote in 1871.

  Against this backdrop, it is no wonder that Huxley thought he had something big on his hands. Indeed, he named the discovery Bathybius haeckelii, for the German biologist Ernst Haeckel, who had recently proposed that all life descended from a primordial ooze that he called Urschleim. Indeed, Huxley was convinced that he had found the Urschleim, and the “discovery” helped prompt the dispatch of the HMS Challenger on a systematic exploration of the depths of the world oceans. No trace of Bathybius haeckelii, or Urschleim, was found; instead, the chemist aboard the ship found that Huxley’s curious substance was simply a chemical precipitate (a hydrated calcium sulfate). In 1875 Huxley acknowledged his error.

  The hunt for beginnings has never ceased, despite Huxley’s e
rror. The twentieth century had its share of milestones and conceptual breakthroughs, though sometimes they felt like a replay of nineteenth-century events but at the molecular level: the germs and microbes were replaced with the molecules of life, but the mystery surrounding life remained.

  In 1953 Stanley Miller, working in Harold Urey’s lab, showed that amino acids—the building blocks of all proteins, and the same protein compounds Darwin mused had formed chemically in the “warm little pond”—can be synthesized in a flask containing ammonia, methane, water, and an electric discharge. Good first step! In the same year Watson and Crick resolved the structure of the DNA molecule. It was the high point for twentieth-century biology as a whole, but much less so for research into the origin of life: how could primitive life come up with such a complex molecule?

  Next came a gift from the heavens, literally, with the Murchison meteorite that fell in Australia in September 1969. Quick analysis of this piece of primitive unprocessed material from the early history of the Solar System revealed a rich set of organic molecules and many amino acids among them—not that different from the ones synthesized in the Miller-Urey experiments. Here we had rocky material that had never been incorporated into a big planet or asteroid, though from a big enough chunk that had warmed up just enough to briefly have liquid water inside it, and the primitive material had produced the building blocks for proteins by pure chemistry. Studies in 2008 and 2010 have revealed about 14,000 different organic compounds, including two nucleobases.

  As exciting as these discoveries are, they still don’t answer our big question. The actual origin of life on Earth remains as elusive as ever and may well stay that way. After all, it is a historical question that requires knowing environments that are not preserved in the Earth’s geological record. The more general question—about possible pathways from chemistry to life—appears more within reach of today’s science.

  Astronomy and the hunt for exoplanets—planets orbiting other stars—offer an approach to the problem. Exploring other Earth-like planets gives us the opportunity to investigate analogs of our own planet under conditions that held before life emerged. This approach has been wildly successful in astronomy. We study stars by proxy, getting to “know” our Sun through time by examining similar stars at other stages of their lifecycle. So, in a sense, we can answer the general questions about the origins of life, about what life is, and how environments determine its appearance, by simply asking, Is there life on other planets? There are more stars in the Universe than there are grains of sand in all the beaches on the Earth. And there are at least as many planets as there are stars. If only 1 percent of them are like Earth, does this make life on them inevitable?

  Astronomy has always been about big numbers—astronomical numbers—and experience with big numbers has taught us that they do not guarantee inevitability. We have to go and find out for ourselves. Still, it seems likely that on some of those Earth-like planets, we will find signs of life. When we discover New Earth—a planet we could call home—the question of the “plurality of worlds” will come front and center, reminding us yet again that we are not the center of the Universe. The Copernican revolution, which placed the Sun, not Earth, at the center of our system of planets, did it first. That shift launched modern science and technology. Today, two efforts have placed us on the verge of completing the Copernican revolution. One is the discovery of a new Earth. The other is the era of synthetic biology. These two milestones are going to teach us about our place in the universe in ways we could never have imagined.

  Want a front-row seat for these unfolding events? Climb aboard and we’ll get under way.

  PART I

  SUPER-EARTH

  CHAPTER ONE

  EXTRASOLAR PLANETS AT LAST

  In October 1995, I was attending a conference in Florence, Italy, that beautiful old city where the Medicis were the patrons of astronomy during the seventeenth century. I was there to exchange new ideas and thinking with my colleagues. Then during an unguarded moment of casual conversation, as often happens, a bold new concept exploded amid my deeply held presumptions.

  At the day’s end, a couple of us were talking to Swiss astronomer Michel Mayor about his discovery of a small companion—a planet about the size of Jupiter—around a star named 51 Pegasi. The claim itself was not a “wow” moment; such claims had come and gone in decades past. What really caught my attention was that Michel and his graduate student, Didier Queloz, had measured the orbital period in days, not in years, as one would expect. This new planet circled its sun in just four hundred days!

  I was incredulous.

  Okay, stars are my specialty, not planets, but I know the basics, and this did not fit. As far back as my last year of high school I had known about the Kant-Laplace model of the formation of our Solar System. Although you may know Immanuel Kant as a philosopher, as a young man he was an astronomer and an Isaac Newton groupie. He was at the University of Koenigsberg, today’s Kaliningrad on the Baltic Sea, and he used Newton’s new calculus and theoretical mechanics to solve an obvious but unexplained feature of the Solar System.

  Astronomers before Kant had noted that all planets orbit the Sun in the same plane and in the same direction, which is also the direction in which the Sun spins. Most planets spin that way as well. Kant offered an elegant solution for this by analogy with Saturn’s rings. Planets form from particles circling the sun in a rotating flat disk, and the conservation of angular momentum explains its flattened shape.a (Because his publisher went bankrupt, Kant didn’t get the credit due him at the time, as recounted in The Discovery of Our Galaxy by my old mentor, Charles Whitney.)1 Pierre-Simon Laplace added mathematical rigor to Kant’s ideas in 1796, and the Kant-Laplace model has survived 250 years of critiques, changes, and improvements while retaining its basic foundations.

  FIGURE1.1. The newly formed star is surrounded by an orbiting disk of gas and dust, the material from which planets form. The disk is heated by the star and there is a curve at a distance where its temperature drops below freezing, known as the snow line. It is outside this line that snowflakes add to the dust in the formation of planets and help create gas giants like Jupiter.

  There was something else that made me find Michel’s discovery a bit hard to believe. According to the modern version of the Kant-Laplace model, there is a curve, roughly two to three times the distance of the Earth from the Sun, at which the temperature of the gaseous disk surrounding a star falls to just 170 Kelvin, or 150 degrees below zero Fahrenheit, at which point water and ammonia molecules in that rarefied atmosphere form ice grains and snowflakes.2 These two light materials, as well as, ultimately, hydrogen, combine with dust particles and grow into giant gas planets orbiting the sun. Within the so-called snow line, dust particles, with no ice grains and snowflakes to aid their growth, combine to form small, dense planets (see Figure 1.1 on the preceding page). This is the beautifully simple explanation for the makeup of our solar system, gas giant planets orbiting the sun farther out and taking years to complete their journeys, and small, rocky planets orbiting closer in. So you can see why I was surprised by Michel’s claim—there was no way a Jupiter-like gas giant planet could have ever formed inside the snow line. And orbiting 51 Pegasi, a star like our Sun, in just four hundred days—that just seemed impossible.

  At the press conference the next morning, I found out I had been mistaken about the four hundred days.

  It was four days!

  Somehow my brain had locked onto the incredible figure and multiplied it by a factor of one hundred. Yet there was Michel, with the evidence to back his claims, showing that the orbital period of the new planet was 4.2 days!

  My deeply held preconceptions fell apart like ice grains and snowflakes meeting the Sun. It was a powerful—and humbling—lesson.

  News of many more planets has followed the discovery of 51 Peg b.3 Geoffrey Marcy and Paul Butler in California, already pursuing a similar project and technique, discovered several interesting planetary sys
tems within months of Michel’s announcement, allaying any lingering doubts that what Michel interpreted as planet 51 Peg b might have been an unusual property of its star. It was also easier to go back to an early find and accept it as a possible planet—the companion of star HD 114762, discovered in 1989 by my colleague and pioneer planet hunter, David Latham, and his collaborators.4 It was also possible to see why the pioneers of the technique, led by Gordon Walker of the University of Victoria in Canada, had failed to discover a single extrasolar planet: they had done a systematic search from 1986 to 1995 but looked for planets with periods of ten years or longer, which limited the number of stars they could monitor. With some bad luck, the search ended empty-handed.5

  Planets orbiting other stars, dubbed extrasolar planets or exoplanets, now number in the hundreds—about 600 at the time of this writing. All of them lie in our Milky Way Galaxy, relatively close to home, most within a circle of 500 light-years, although a handful are as far away as 5,000 light-years. More than sixty of these planets are similar to 51 Peg b and are referred to as “hot Jupiters” (Figure 1.2). This number, which is fairly high, reflects the fact that the planets are easy to find, not that they are numerous. These planets, which at first seemed so anomalous (how could they have formed so close to the heat of their stars?), ended up having an explanation that didn’t require throwing out the Kant-Laplace model. The hot Jupiters opened our eyes to the phenomenon of planet migration, the result of slow changes to a newly formed planet’s orbit due to its interaction with the disk of gas and dust. As the orbiting planet raises density waves in the disk, its orbit can spiral inward or outward. In most cases, the shift is inward; the result is hot Jupiters.6