Origin Story, page 7
For a long time, people knew about just one solar system. But in 1995, astronomers identified exoplanets, planets orbiting other stars in our galaxy. They did this by detecting tiny wobbles in the motions of stars or tiny variations in their brightness as planets crossed in front of them. Since then, we have learned that most stars have planets, so there could be tens of billions of planetary systems of many different types just in our galaxy. By the middle of 2016, astronomers had identified more than three thousand exoplanets. In the next decade or two, the study of other planetary systems should give us a better sense of the most common configurations. Soon, we should be able to study their atmospheres, which may give us a sense of how many could be life-friendly. We already know that many are about the same size as Earth, and many orbit at the right distance from their stars to have liquid water—a crucial ingredient for life.
The discovery of exoplanets tells us that, like threshold 3, threshold 4 has been crossed many times, and it may have been crossed for the first time quite early in the universe’s history around a star we will probably never detect. But we now know quite a lot about what the crossing of this threshold looks like.
The formation of planetary systems is a messy and chaotic process, a by-product of star formation in chemically enriched regions of space. Billions of years after the big bang, interstellar space was filled with clouds of matter containing many different chemical elements. Hydrogen and helium still made up almost 98 percent of those clouds, but it was the remaining 2 percent that made the difference. As in the early universe, gravity liked to make these clouds more clumpy. In our region, gravity may have been helped by a nearby supernova explosion that shook things up and started the contraction of a huge cloud of gas and dust about 4.567 billion years ago. The supernova left its calling card in distinctive radioactive materials that show up in meteorites within our solar system.
As it contracted, the dust cloud broke up into multiple solar nebulae, one of which formed our sun. The sun gobbled up 99.9 percent of all the matter in its dust cloud. But what interests us now is the leftovers, the rings of debris orbiting the young sun. As gravity shrank the solar nebula, its swirling mass of gas, dust, and ice particles spun faster and faster until centrifugal forces flattened it like pizza dough to create the thin plane of today’s solar system. We can now observe such protoplanetary disks in nearby regions of star formation, so we know they are very common.
Two processes turned a spinning disk of matter into planets, moons, and asteroids. The first was a type of chemical sorting. Violent bursts of charged particles from the young sun, known as the solar wind, blasted lighter elements, such as hydrogen and helium, away from the inner orbits to create two distinct regions. The outer regions of the young solar system, like most of the universe, consisted mainly of the primordial elements, hydrogen and helium. But the inner regions, where the rocky planets—Mercury, Venus, Earth, and Mars—would form, lost so much hydrogen and helium that they had a rare chemical diversity. Oxygen, silicon, aluminum, and iron make up over 80 percent of Earth’s crust, with elements such as calcium, carbon, and phosphorus playing lesser roles. On Earth, hydrogen plays only a medium-size role, and helium is hardly ever sighted.
The second process that formed our solar system was accretion. Within different orbits around the young sun, bits of matter slowly gathered together. In the gassier outlying regions, this was probably a fairly gentle process. Gravity collected matter into huge gassy planets, such as Jupiter and Saturn, that consisted mostly of hydrogen and helium with a thin sprinkling of dust and ice. In the inner regions, though, accretion was a more violent and chaotic process, because here, a lot more matter was solid. Particles of dust and ice stuck together to form small blobs of rock and ice, which careened around, sometimes smashing each other into pieces, sometimes sticking together to form larger objects. Eventually, even larger objects appeared, like meteors and asteroids, and within each orbit, these smashed into each other or merged to form objects so large that their gravity could sweep up most of the remaining debris. Eventually, these processes generated the planets we see today, spaced out in distinct orbits around the sun.
Such an account gives little sense of the chaos and violence of accretion. Some objects crossed orbits, knocking young planets and moons out of alignment or smashing them to pieces. The vast protoplanet of Jupiter may have migrated inward, its gravitational pull breaking up any would-be planet that was forming in what is now the asteroid belt. Uranus’s odd tilt and rotation are probably the result of a violent collision with another large body. And the jagged forms of many asteroids are the scars of brutal collisions early in the history of our solar system.
Collisions continued for a long time, even as the solar system stabilized. Indeed, our own moon was probably formed from a collision between the young Earth and a Mars-size protoplanet (Theia) about one hundred million years after the birth of the solar system. That collision sent huge clouds of matter into orbit around Earth, where they probably circled like the rings of Saturn (which might also be the debris from a smashed-up moon) until they accreted to form our moon.
Within fifty million years, our solar system had acquired the basic shape it has today, and since then it has proved quite stable. The billions of planetary systems in our universe probably formed in similar ways, though they exist in a great variety of different configurations. But all planetary bodies are cooler than stars, and chemically richer and more diverse, and that’s why they provided Goldilocks conditions that allowed the building of new forms of complexity. Eventually, at least one of these objects, and probably many more, generated life.
Planet Earth
Our solar system lies in the galaxy we call the Milky Way in a stellar suburb on one of the Milky Way’s spiral arms, the Orion spur. The Milky Way is one member of a group of about fifty galaxies, known, unromantically, as the Local Group. The Local Group lies in the outer regions of the Virgo Cluster, which has about a thousand galaxies. This is part of the Local Supercluster, which includes hundreds of groups of galaxies. It would take you one hundred million years traveling at the speed of light to cross it. In 2014, it was found that the Local Supercluster is part of a vast cosmic empire with perhaps a hundred thousand galaxies, and to cross that would take you four hundred million years traveling at the speed of light. This empire is the Laniakea (Hawaiian for “immeasurable heaven”) Supercluster. At present, this is the largest structured entity we know of in the universe. We assume that Laniakea is built around a scaffolding of dark matter whose gravitational pull holds all these galaxies together as the universe expands.
Now we must travel back to the suburbs of Laniakea, to our own local group, our own galaxy, and out to the Orion spur, where we find our own sun and planet Earth. After Earth formed by accretion, one final display of chain-saw sculpture gave it its distinctive inner structure. Geologists call this process differentiation.
The young Earth heated up and melted. It was heated by the violent collisions of accretion, by the presence of radioactive elements (created in the supernova that provided much of the material for our solar system), and by increasing pressure as it grew in size. Eventually, the young Earth was so hot that much of it melted into a gooey sludge, and as it liquefied, its different layers sorted themselves by density, giving it the structure it has today.
The heavier elements, mainly iron and nickel and some silicon, sank through the hot sludge to the center to form Earth’s metallic core. As Earth spun, the core generated a magnetic field that shielded the surface from the damaging charged particles of the solar wind. Lighter rocks, such as basalts, gathered above the core to form a second layer, a three-thousand-kilometer-deep region of semimolten rock mixed with gas and water that’s known as the mantle. This is where the lava belched up by volcanoes comes from. The lightest rocks, many of them granites, floated to the surface, where they cooled and solidified to form a third layer: the eggshell-thin stratum known as the crust, which is covered today by oceans and continents. Beneath the oceans, the crust is sometimes just five kilometers thick, but under the continents, it can be up to fifty kilometers thick. The crust is particularly interesting chemically. In it, you can find solids, liquids, and gases, and it was repeatedly heated and cooled by volcanoes, asteroid impacts, the harsh glare of the young sun, and the eventual condensation of Earth’s first oceans. Here and in the mantle, heat and the circulation of elements generated perhaps two hundred and fifty new minerals.2 Gases, including carbon dioxide and water vapor, bubbled up from the mantle through volcanoes and cracks in the surface to form a fourth layer: Earth’s first atmosphere. The crust and atmosphere were also enriched by gases, water, complex molecules, and other materials brought in by asteroids and comets.
The hot, molten core kept the young Earth dynamic, as energy from the center worked its way through the planet, heating and churning up its upper layers to create circulating currents of soft rock in the mantle and a surface dotted with volcanoes. Heat from the core still drives change in the upper levels of planet Earth. Today, we can track the movement of the surface using GPS systems, and we know that crustal plates on the surface move around at about the speed that your fingernails grow; the fastest of them cruise at about twenty-five centimeters a year.
Geologists divide Earth’s history into subdivisions, the largest of which is the eon. The first is the Hadean (“hell-like”) eon. This lasted from when Earth formed to about four billion years ago, when the Archean eon began. If you’d visited during the Hadean eon, you’d have found a planet still affected by the demolition derby of accretion. Gouges and tears on the surface of the moon and other planets show that between 4.0 and 3.8 billion years ago, the inner solar system was subjected to a massive pummeling by asteroids and other stray objects. This is known as the Late Heavy Bombardment, and it was probably caused by shifts in the orbits of Jupiter and Saturn, which sprayed objects at random around the young solar system. Today, most of the asteroids live between Jupiter and Mars, so they may be the bricks and struts of a planet that was never built because of Jupiter’s disruptive gravitational tug. At present, we know of some three hundred thousand asteroids. Though most are small, that’s a lot of stray matter with which to bombard the inner planets.3
Studying Earth: Seismographs and Radiometric Dating
Despite what Hollywood might have us believe, we cannot dig deep into Earth. The deepest dig so far is about twelve kilometers, which is about 0.2 percent of the distance to Earth’s core. That hole was drilled in the Kola Peninsula in the far northwest of Russia as part of a geological investigation. We know about the interior because of another neat scientific trick, the geologist’s equivalent of an X-ray. Earthquakes generate tremors that travel through Earth’s interior. Seismographs measure those tremors at different places on the surface. By comparing results from different regions, you can figure out how fast and how far tremors have traveled through the interior. We also know that different types of tremors travel at different speeds through different materials, and some travel only through solids, while others can travel through liquids as well. So tracking these tremors with multiple seismographs can tell us a lot about Earth’s interior.
Determining Earth’s age and the many other dates scattered throughout the modern origin story became possible only in the second half of the twentieth century, and it depended on some very clever science.
The first steps toward a modern history of planet Earth were taken in the seventeenth century. That’s when some of the pioneers of modern geology realized that it might be possible to determine the order of events in Earth’s history, even if no one had any idea of exactly when things happened. In the seventeenth century, a Danish priest who lived in Italy, Nicholas Steno, showed that by carefully studying sedimentary rocks, you could determine the order in which different rock strata had been laid down. All sedimentary rocks are built up layer by layer, so we know that the oldest layers are normally the lowest ones. Anything cutting through them had to have been younger.
Early in the nineteenth century, an English surveyor, William Smith, showed that identical suites of fossils appeared in rock formations in different places. On the reasonable assumption that similar fossils must have come from about the same time, you could identify strata around the world that had been laid down at the same time. Taken together, these principles allowed nineteenth-century geologists to create a relative timeline for Earth’s history. That timeline still lies behind modern geological dating systems, and it begins with the Cambrian period, the first period whose strata contained fossils visible to the naked eye.
But no one knew when the Cambrian period had occurred, and many geologists despaired of ever finding absolute dates for different strata. In 1788, James Hutton wrote: “We find no vestige of a beginning, no prospect of an end.”4 Even early in the twentieth century, the only way to give an absolute date to an event was to find a written record that mentioned it. And that meant, as H. G. Wells pointed out when he tried to write a modern origin story just after World War I, that absolute timelines could reach back no farther than a few thousand years.
Though H. G. Wells didn’t know it, some of the discoveries that would eventually provide better dates had already been made. The key was radioactivity, a form of energy discovered by Henri Becquerel in 1896. In atoms with large nuclei, such as uranium, the repulsive power of lots of positively charged protons can destabilize the nucleus until, eventually, it breaks down spontaneously, emitting high-energy electrons or photons or even whole helium nuclei. As chunks of the nucleus are ejected, the element is transformed into different elements with fewer protons. For example, uranium eventually breaks down to lead. In the first decade of the twentieth century, Ernest Rutherford realized that, even if you could not tell when a particular nucleus was about to break apart, radioactive breakdown was a very regular process when averaged over billions of particles. Every isotope of the same element (isotopes have the same number of protons but different numbers of neutrons) breaks down at different but regular rates, so it is possible to determine precisely how long it will take for half of the atoms in a given isotope to decay. For example, the half-life of uranium-238 (with 92 protons and 146 neutrons) is 4.5 billion years, while uranium-235 (with 92 protons and 143 neutrons) has a half-life of 700 million years.
Rutherford realized that radioactive breakdown could provide a sort of geological clock if you could measure how much a given sample had decayed. In 1904, he tried to measure the breakdown of a sample of uranium and came up with a figure of about five hundred million years for the age of Earth. The basic idea was right, but his estimate of Earth’s age was controversial because it was much older than the accepted age of less than one hundred million years.
Over time, an increasing number of geologists began to agree that Earth might be much older than they had once thought. But the technical problems of measuring radioactive breakdown were formidable. They were solved only in the late 1940s, using methods developed as part of the Manhattan Project, which had manufactured the first atomic bomb. To make the bomb, it was necessary to separate different isotopes of uranium in order to produce purified samples of uranium-235. An American physicist, Willard Libby, helped develop the techniques for separating and measuring different isotopes of uranium, and these would prove crucial in the task of measuring radioactive breakdown.
In 1948, Libby’s team managed to give accurate dates for material from the tomb of the pharaoh Zoser, which had been provided by the Metropolitan Museum.5 They used carbon-14, a radioactive isotope of carbon that has a half-life of 5,730 years, which makes it extremely useful when studying organic materials such as wood. Different radioactive materials worked at different scales and with different materials. For geologists, the decay of uranium to lead was particularly valuable, and the fact that different isotopes of uranium decay at different rates allowed cross-checking.6 In 1953, Clair Patterson dated the age of an iron meteorite using the decay of uranium to lead. He made the correct assumption that meteorites were made up of primordial material from the young solar system and could therefore provide an age for the entire solar system. His measurements suggested Earth was about 4.5 billion years old, much older than Rutherford had estimated. Patterson’s date still stands today.
Along with radiometric dating techniques, there have emerged other dating techniques that can be used to check each other. Dates within recent millennia can sometimes be determined by counting the annular rings of ancient trees such as bristlecone pines, which can live for several thousand years. Astronomers use their own techniques for dating the history of the universe, and biologists have found that DNA evolves at a reasonably regular pace, so you can roughly date when two species diverged from a common ancestor by measuring differences in their genomes. Such techniques, based on careful study of processes such as radioactive decay, as well as the development of new instruments for measuring them precisely, have given us the timelines around which the modern origin story is built.
So far, we have watched complexity increasing in entities that are interesting but not alive. Now we reach one of the most fundamental of all our thresholds: the appearance of life. With life, we encounter an entirely new type and level of complexity and a whole series of new concepts, including information, purpose, and even, eventually, consciousness.
PART II
