by Ben Bova
The distances between the stars are truly vast, which means that such a close encounter must be extremely unusual, which in turn means that planetary systems must be equally rare. Where the nebular hypothesis was evolutionary, the stellar encounter hypothesis was catastrophic: Stars only form planets when they have close encounters with other stars, a type of catastrophe that is as abnormal as a tidal wave in Kansas.
If the stellar encounter hypothesis is correct, the Sun must be one of a very, very few stars that possess a planetary system; perhaps the only one in our entire Milky Way galaxy.
Is our solar system unique? If there is one lesson that astronomers have learned from Copernicus, it is to be suspicious of anything that says our Earth is special in any way. We are not the center of the universe, and the idea that our solar system is unique bothered many astronomers down to their bones. Yet the stellar encounter theory, for all of its statistical improbability, was accepted for several decades. Thanks to Jeans’ popular books on astronomy and astrophysics, the general public believed that our solar system was probably the only one of its kind—a belief that comforted many but disgruntled those who wanted to believe in a universe teeming with other worlds and life.
As the decades rolled by, though, physical evidence against the stellar encounter hypothesis began to accumulate. For one thing, the filament of gas pulled out of the Sun would be no more likely to condense into planets than the Kant/Laplace nebula. Even though the filament might be much more massive than the nebula, it would also be much hotter, since it came from the Sun’s seething interior. The higher the temperature, the more quickly it would dissipate into space.
Then, too, studies of the Sun showed it consisted almost entirely of the two lightest elements: hydrogen and helium. The planets have very different chemical compositions. If the material that formed the planets was pulled from the Sun’s interior, how do you account for the very different chemical compositions?
THE BIRTH OF A SOLAR SYSTEM
By the middle of the twentieth century, new observations and new ideas came together to generate a new and more detailed understanding of how the solar system originated. Superficially, it resembles the old nebular hypothesis. But only superficially. However, like the Kant/Laplace premise, the current explanation for the birth of the solar system is evolutionary: It could happen to other stars. As we will see in Chapter 17, there is strong evidence that it has. Planetary systems are common among the stars; there is an abundance of worlds on which life might exist.
To understand the origins of the Earth, we must expand our mental horizons to consider the long, majestic sweep of time in terms of billions of years. And we must realize that our planet did not come into existence by itself; our Earth is part of the solar system that includes the Sun, the eight other major planets, their various moons, the rocky debris of the asteroids, and the icy fragments of untold myriads of comets.
Our Earth was born together with the Sun and other planets, moons, comets, and asteroids that comprise our solar system. Our world has been shaped, influenced, and molded by its interactions with the Sun and these other bodies of the solar system for nearly 5 billion years. We are not separate and apart, and we never have been.
The Solar System was born roughly 4.6 billion years ago. The evidence of this enormous age comes from studies of the rocks of Earth and the Moon,8 from meteorites that fall to Earth from outer space, and from the composition and behavior of the Sun (see Appendix 6). They all tell a consistent tale: The Earth was created at the same time that the Sun and other bodies of our solar system came into existence, some 4.6 billion years ago.
Picture the scene as it must have been then. There were other stars shining in the eternal darkness of space. The Milky Way, that titanic pinwheel colony of billions of stars, glowed steadfastly against the black void even then. But where our solar system was to be born there was only a dark swirling cloud of dust-laden gas, about a light-year wide.
The cloud consisted of cool gases laced with dark flecks of microscopic dust grains. Most of the gas was hydrogen. The dust grains were minuscule motes of metals and silicates (silicon and oxygen) covered with thin coatings of ice: frozen water, methane, or other light molecules. Astronomers have seen such cold dark clumps of protostar clouds in interstellar space and have even watched some of them begin to glow. The birth of new stars has been observed and photographed.
Where did this protoSun cloud come from? From the death throes of earlier stars. The stars, which seem so unchanging and eternal to us, have life cycles of their own. Though their lives are measured in billions of years, stars are born and they die. Stars are being created now; astronomical telescopes have captured scenes of breathtaking beauty where new stars are being born. Stars die, too, often violently. Supernova explosions that mark the death of massive stars can put out as much energy in a day as our Sun does in billions of years. The supernovas of 1054 and 1572 were so bright that their light could be seen at high noon.
When stars explode, their gases are flung into interstellar space and eventually form the building material for new stars. Also, less spectacularly, all through their life spans stars emit thin winds of gases, mostly hydrogen. Our own Sun’s solar wind pervades interplanetary space and eventually wafts out beyond the edges of our solar system to join the ultrarare gases of interstellar space. That cold, tenuous interstellar gas becomes the building material for new stars.
The cloud that was the progenitor of our solar system contained mostly hydrogen, with a smattering of helium and heavier gases, plus a sprinkling of dust grains. All this material originally came from older stars. The atoms that compose our Sun, our planet Earth, and our own bodies originated in other stars. The carbon, oxygen, nitrogen, and other elements in our bodies were created inside stars that died billions of years ago. We are stardust, quite literally.
Martin Rees, Astronomer Royal of Great Britain, points this out:
Our cosmic habitat is like an ecosystem. Gas is recycled through successive generations of stars. Fast-burning heavy stars transmute pristine hydrogen into carbon, oxygen, iron, and the rest of the elements in the periodic table; they then throw their spent fuel back into space, either via stellar winds or in the final supernova outburst. An oxygen atom expelled from a massive star may have wandered for hundreds of millions of years in interstellar space. It may then have found itself in a dense cloud, contracting under its own gravity to make a new star, surrounded by a dusty disk. That star might have been our Sun, and that particular atom could have ended up on Earth, perhaps someday to be cycled through a human cell. To understand our origins, we must understand stars that formed long ago, in the remote parts of our Milky Way.
A STAR IS BORN
The light-year-wide solar cloud was collapsing, sinking into an ever-tighter mass under the pull of its own gravity. And it was rotating, slowly at first, but its rotation became faster and faster as it shrank. This is due to the conservation of angular momentum, the same effect that makes an ice skater spin faster and faster as she pulls her arms in close to her body.
As the cloud shrank, its density increased. As the density increased in the core of the cloud, so did the pressure and the temperature. The cold, dark cloud started to heat up; soon it began to glow a sullen deep red as it grew hotter, heated by the continuing infall of more than 2 billion billion billion (2 ¥ 1027) tons of gas and dust. The temperature at the core of the cloud climbed to thousands of degrees, then millions, and still the cloud spun, shrank, and became denser. The temperature and pressure became so high in the cloud’s core that the atoms were stripped of their orbital electrons. The hydrogen atoms, which consist of a single proton as the nucleus and a single electron orbiting around it, became a seething sea of bare protons and free electrons.
In only 100,000 years, an eyeblink in the eons-long lifetime of a star, the temperature at the cloud’s core reached approximately 10 million degrees. The naked protons began to fuse together. Even though all protons carry a positive electrical
charge, and under normal conditions would repel one another, under those unbelievable temperatures and pressures they were forced together. The process is called thermonuclear fusion, or simply hydrogen fusion. Four hydrogen nuclei—bare protons—join together to form a nucleus of the element helium.
The helium nucleus is 0.7 percent lighter than the combined mass of the four protons. That tiny fraction of mass is converted into energy in the fusion process.9 We call that energy sunlight. The cloud began to glow with the energy of hydrogen fusion. A star was born: our Sun.
The tremendous energy generated by the fusion process put the brakes on the Sun’s shrinking. With the heat and radiation pressure now welling up from its core, the Sun maintains itself at a diameter of 1.39 million kilometers, 109 times larger than the Earth.
Every second, the Sun converts 4 million tons of its hydrogen into energy. The Sun has been losing 4 million tons of its matter every second for nearly 5 billion years! Yet this rather average star contains enough hydrogen to keep on producing energy in this manner for another 5 billion years. And at the end of that time, even if all its hydrogen is consumed, it will only have lost 0.7 percent of its total mass.
The heat pouring from the Sun quickly began to effect the chunks of rock and metal that were condensing into planets around it. Ices were melted, vaporized, and blown to the farthest reaches of the solar system. Only dense, rocky bodies could remain close to the Sun. Farther out, larger planets could form by drawing in some of the hydrogen and helium gases that still composed the major share of the solar cloud. Far out in the dark, cold fringes of the solar system, icy bodies could form, their distance protecting them from the newborn Sun’s heat.
Closer, though, there was a zone where the temperature was warm enough for liquid water to exist. Several planets were being built in that region of the solar system. One of them became our Earth.
7
The Birth of Our World
I am the daughter of Earth and Water,
And the nursling of the sky.
I pass through the pores of the ocean and shores;
I change, but I cannot die . . .
—Percy Bysshe Shelley
”The Cloud”
THE NEWBORN SUN provided the energy for life. Its heat created a region in the solar system where water could exist in its liquid form. Astrobiologists call that region the thermally habitable zone. Earth orbits squarely inside the Sun’s TH zone.
The newly forming Earth was blessed with energy from the Sun and a temperature environment that would allow water to remain liquid on its surface and even deep underground. There was plenty of carbon available: It is the fourth most common element in the universe. Energy. Liquid water. Carbon. The three requirements for life existed from the very beginning when the Earth formed, some 4.6 billion years ago.
But that is not the entire story. The early solar system was a violent, dangerous place. It was by no means certain that life could arise. And once it came into existence there was an excellent chance that it would be quickly extinguished.
A MATTER OF MOMENTUM
The time is roughly 4.5 billion years ago. The Sun is shining. But instead of a placid, orderly system of planets and their moons, the solar system is a furious mass of tiny chunks of rock, metals, and ices careening wildly, smashing into one another in violent collisions, looking more like a celestial shooting gallery than a likely abode for living creatures.
And the Sun was spinning fast, far faster than it does today. Astronomers trying to understand how the solar system was created were puzzled by the question of angular momentum, the energy that a spinning object possesses. Today the Sun rotates rather slowly on its axis: once in about thirty days. The nine planets, with their wide-ranging orbits, contain almost all the angular momentum (spin energy) of the solar system.
How can this be? If the solar system began as a spinning cloud, and its spin grew faster and faster as the cloud collapsed and coalesced into the Sun, the Sun must have gained virtually all that angular momentum for itself. How did the Sun get rid of all that energy of motion and transfer it to the planets? Four and a half billion years ago there weren’t any planets, only the gases and dust grain–sized particles that formed the raw materials from which the planets would be built.
While the early Sun was spinning, shrinking, and turning itself into a star, its spin rate became so rapid that the Sun began to cast off a flattened disk of gas and dust from its middle. That dust-laden gas spread around the Sun’s middle in a wide, relatively thin disk. Actually, since the gas was ionized, it should be called a plasma.
Thanks to plasma physicists such as Sweden’s Hannes Alfvén (1908–1995), astrophysicists were able to puzzle out the mystery of the solar system’s angular momentum.
Within the seething sphere of the protoSun and the thin disk of plasma that was spinning out of its middle, powerful electrical currents flowed. Electrical currents generate magnetic fields. The early Sun and its disk were linked by electromagnetic forces. Much of the energy of the Sun’s rapid rotation was transferred to the disk through these electromagnetic forces. The Sun began to slow its spin rate, while the disk absorbed most of that angular momentum.
PLASMA: THE FOURTH STATE OF MATTER
Astronomers have noted that some stars have rapid spin rates, while others spin more sedately, like our Sun. The Russian-American astronomer Otto Struve (1897–1963) pointed out that slowly spinning stars are probably accompanied by planetary systems that have absorbed their stars’ angular momentum, just as the planets of our solar system soaked up the angular momentum of the Sun.
Moreover, other stars have been found to be accompanied by dusty clouds extending from their middles. Starting in the 1990s astronomers have found dozens of young stars that have spun off dust-laden disks of gas, just as our Sun did in its early days. These disks are the building materials for planets.
FROM DISK TO PLANETS
Astronomers speak of the Sun’s accretion disk, because within that disk whirling around the Sun’s equator the gas and dust motes began to clump together in a process called accretion. This is the process that built the Earth and all the other planets, moons, asteroids, and comets. The dust particles consisted of metals, silicates, and ices: all made of atoms created in the hearts of long-dead stars. Much of the ice was frozen water, the most common triatomic molecule in the universe.
Studies of the fluid dynamics of such a disk, greatly aided by computer simulations, have shown that the disk would not be uniform throughout, but would actually break up into a series of rings spiraling around the Sun. Some of them would fall back into the Sun; others would spiral out farther and farther away and even escape the solar system entirely.
Within the rings, the gas would tend to aggregate into clumps, and the microscopic grains of metallic and silicate dust would drift toward these clumps, attracted by their gravitational pull.
All around the newborn Sun bits and pieces of the original accretion disk began coalescing into larger bodies, pulled together by gravity, smashing into one another, often breaking apart only to come together again. Simple atoms and molecules began to combine, as well, in a primeval bit of chemistry that led to the formation of more complex molecules. Tarry, sticky polycyclic aromatic hydrocarbons (PAHs) probably began to form, and they may have served as a primitive glue to hold colliding particles together and allow them to grow.
Some collisions within the accretion disk were gentle enough for the colliding particles to remain together. Others were so violent that they split the pieces apart. Slowly, though, microscopic grains grew to the size of pebbles, then boulders, mountains, and larger bodies that astronomers call planetesimals.
VIOLENT CHILDHOOD
Our solar system today seems placid and well-ordered, the planets swinging smoothly in stable, widely spaced orbits around a calm and constant Sun.
It was not always so.
Chaotic violence churned through the solar system in its earliest days, as dozens, perhaps hu
ndreds, of embryonic planets swung on wildly eccentric orbits through the clutter of rocks and ice chunks of the newborn Sun’s accretion disk. Asteroids and comets bombarded the growing worldlets, pounding the planetary embryos into molten spheres of slag. One planetesimal the size of Mars ploughed into the young Earth in a grazing collision that blasted more than a hundredth of our planet’s material into space, eventually to form the Moon. Another smashed into the early Mercury so hard it ripped away most of that world’s crust, to be siphoned into the voracious gravitational trap of the nearby Sun.
Out beyond the orbit of Mars, other Earth-sized worlds were building, only to collide with one another; they were shattered into fragments and ejected from the solar system entirely by the gravitational kick of massive Jupiter.
After hundreds of millions of years of such cosmic violence, the solar system was cleared of most of the dangerously unstable planetesimals, leaving the sedate group of planets and moons we have today, gliding smoothly through mostly empty interplanetary space, with only an occasional meteor or comet to remind us of the mayhem that once racked the Sun’s newborn family.
In the thermally habitable region where our Earth now orbits, the temperature was warm enough for liquid water to exist. Comets crashed into the early Earth, bringing megatonnages of water. More water was literally sweated out of the Earth’s hot interior. In time, a “pale blue dot” of a world came into being—planet Earth, its sunlit surface glittering with vast oceans of liquid water.