Origins: Fourteen Billion Years of Cosmic Evolution (15 page)

Simply for the cosmic hell of it, let’s have a quick look at some far more obscure entries on the periodic table. You will almost certainly never own any significant quantities of these elements, but scientists find them not only intriguing riffs on nature’s bounty but also highly useful in special circumstances. Consider, for example, the soft metal
gallium
(thirty-one protons per nucleus). Gallium has such a low melting point that the heat from your hand will make it liquefy. Apart from this parlor demo opportunity, gallium provides astrophysicists with the active ingredient in gallium chloride, a variant on table salt (sodium chloride) that proves valuable in experiments that detect neutrinos from the Sun’s core. To capture these elusive neutrinos, astrophysicists create a 100-ton vat of liquid gallium chloride and set it deep underground (to screen out effects from less penetrating particles), then watch it carefully for the results of any collisions between the neutrinos and the gallium nuclei, which turn the nuclei into germanium nuclei, each of which has thirty-two protons. Every transformation of gallium into germanium produces X-ray photons, which can be detected and measured every time that a nucleus gets slammed. By using these gallium-chloride “neutrino telescopes,” astrophysicists resolved what they had called the “solar neutrino problem,” the fact that earlier types of neutrino detectors found neutrinos in smaller numbers than the theory of thermonuclear fusion in the Sun’s core had predicted.

Every nucleus of the element
technetium
(atomic number 43) is radioactive, decaying after a few moments or a few million years into other types of nuclei. Not surprisingly, we find technetium nowhere on Earth except in particle accelerators, where we make it on demand. For reasons not yet fully understood, technetium lives in the atmospheres of a select subset of red giant stars. As we noted in the previous chapter, this would cause astrophysicists no alarm—except that technetium has a half-life of a mere 2 million years, far, far shorter than the ages and life expectancies of the stars in which we find it. This proves that the stars cannot have been born with the stuff, for if they had been, none would be left by now. Astrophysicists also lack any known mechanism to create technetium in a star’s core
and
to have it dredge itself up to the surface where they observe it, a matter of uneasy fact that has spawned exotic explanations, still shy of consensus within the astrophysics community.

Along with osmium and platinum,
iridium
gives us one of the three densest elements on the periodic table—two cubic feet of iridium (atomic number 77) weighs as much as a Buick, which makes it one of the world’s best paperweights, able to defy all known office fans and window breezes. Iridium also gives scientists the world’s most famous smoking gun. All over the world, a thin layer of iridium-rich material appears at the geological layer that marks the famous K-T boundary, laid down 65 million years ago. Not coincidentally, most biologists believe, that boundary also marks the time when every land species larger than a breadbox, including the legendary dinosaurs, went extinct. Iridium is rare on Earth’s surface, but ten times more common in metallic asteroids. Whatever might have been your favorite theory for destroying the dinosaurs, a ten-mile-wide killer asteroid from outer space, capable of raising a worldwide blanket of light-blocking debris before slowly raining downward several months later, now seems quite compelling.

It’s not clear how Albert would have felt about this, but physicists discovered a previously unknown element in the debris from the first hydrogen bomb test in the Pacific (November 1952) and named it
einsteinium
in his honor. Armageddium might have been more suitable.

While helium derives its name from the Sun itself, ten other elements in the periodic table draw their names from objects that orbit the Sun:

Phosphorus
, which means “light-bearing” in Greek, was the ancient name for the planet Venus when it appeared before sunrise in the dawn sky.

Selenium
comes from
selene
, the Greek word for the Moon, so named because this element was always found in association with the element tellurium, which had already been named for Earth, from the Latin
tellus
.

On January 1, 1801, the first day of the nineteenth century, the Italian astronomer Giuseppe Piazzi discovered a new planet orbiting the Sun within the suspiciously large gap between Mars and Jupiter. Maintaining the tradition of naming planets after Roman gods, Piazzi called the object Ceres after the goddess of harvest, which also provides the root for our word “cereal.” The excitement in the scientific community over Piazzi’s find caused the next element to be discovered to be named
cerium
in its honor. Two years later, another planet was found, orbiting the Sun within the same gap as Ceres. This object received the name Pallas, from the Roman goddess of wisdom; like cerium before it, the next element discovered thereafter was named
palladium
in its honor. The naming party ended a few decades later, after dozens more of these planets were discovered in much the same location, and after closer analysis revealed that these objects are much, much smaller than the smallest known planets. A new swath of real estate had come into view within the solar system, consisting of small, craggy chunks of rock and metal. Ceres and Pallas turned out to be not planets but asteroids, objects only a few hundred miles across. They live in the asteroid belt, now known to contain millions of objects, of which astronomers have catalogued and named upward of fifteen thousand—somewhat more than the number of elements in the periodic table.

The metal
mercury
, which assumes a viscous liquid form at room temperature, owes its name to the speedy Roman messenger god. So too does the planet Mercury, the fastest-moving of all the planets in the solar system.

Thorium
’s name comes from Thor, the hammer-and-thunder-wielding Scandinavian god, who corresponds to the lightning-bolt-wielding Jupiter in Roman mythology. By jove, recent Hubble Space Telescope images of Jupiter’s polar regions reveal extensive electrical discharges deep within its turbulent cloud layers.

Saturn, most people’s favorite planet, has no element named for it, but Uranus, Neptune, and Pluto are famously represented. The element
uranium
, discovered in 1789, received its name in honor of William Herschel’s planet, discovered by him just eight years earlier. All isotopes of uranium are unstable, spontaneously but slowly decaying to lighter elements, a process accompanied by the release of energy. If you can arrange to speed up the rate of decay with a “chain reaction” among uranium nuclei, you have the explosive energy release required for a bomb. In 1945, the United States exploded the first uranium bomb (familiarly called an atomic bomb or A-bomb) to be used in warfare, incinerating the Japanese city of Hiroshima. With ninety-two protons packed in each nucleus, uranium wins the prize as the largest and heaviest element to occur naturally, although trace amounts of still larger and heavier elements appear in places where uranium ore is mined.

If Uranus merited an element, so did Neptune. Unlike uranium, however, which was identified soon after its planet, neptunium was discovered in 1940 in the particle accelerator called the Berkeley Cyclotron, ninety-seven years after the German astron-omer John Galle found Neptune in a spot on the sky predicted as the most likely spot by the French mathematician Joseph Le Verrier, who studied Uranus’ unexplained orbital behavior and deduced the existence of a farther planet. Just as Neptune comes immediately after Uranus in the solar system, neptunium comes right after uranium in the periodic table of the elements.

Particle physicists working at the Berkeley cyclotron discovered more than half a dozen elements not found in nature, including
plutonium
, which immediately follows neptunium in the periodic table and bears the name of Pluto, which the young astronomer Clyde Tombaugh found in 1930 on photographs taken at Arizona’s Lowell Observatory. As with the discovery of Ceres 129 years earlier, excitement ran high. Pluto was the first planet discovered by an American and, in the absence of accurate observational data, was widely believed to be a planet of size and mass commensurate with those of Uranus and Neptune. As our measurements of Pluto’s size improved, Pluto kept getting smaller. Our knowledge of Pluto’s dimensions did not stabilize until the late 1970s, during the
Voyager
missions to the outer solar system. We now know that cold, icy Pluto is by far the Sun’s smallest planet, with the embarrassing distinction of being smaller than the solar system’s six largest moons. As with the asteroids, astronomers later found hundreds of other objects in similar locations, in this case in the outer solar system with orbits similar to Pluto’s. These objects signaled the existence of a heretofore undocumented reservoir of small icy objects, now called the Kuiper Belt of comets. A purist could argue that like Ceres and Pallas, Pluto slipped into the periodic table under false pretenses.

Like uranium nuclei, plutonium nuclei are radioactive. These nuclei formed the active ingredient in the atomic bomb dropped on the Japanese city of Nagasaki, just three days after the uranium bombing of Hiroshima, bringing a swift end to World War II. Scientists can use small quantities of plutonium, which produces energy at a modest, steady rate, to power radioisotope thermoelectric generators (abbreviated as RTGs) for spacecraft that travel to the outer solar system, where the intensity of sunlight falls below the level usable by solar panels. One pound of this plutonium will generate 10 million kilowatt-hours of heat energy, sufficient to power a household light bulb for eleven thousand years, or a human being for just about as long. Still drawing on their plutonium power to send messages to Earth, the two
Voyager
spacecraft launched in 1977 have now traveled far beyond Pluto’s orbit. One of them, at nearly one hundred times Earth’s distance from the Sun, has begun to enter true interstellar space by leaving the bubble that the Sun’s outflow of electrically charged particles creates.

And so we end our cosmic journey through the periodic table of the chemical elements, right at the edge of the solar system. For reasons we have yet to determine, many people don’t like chemicals, which may explain the perennial movement to rid foods of them. Perhaps sesquipedalian chemical names just sound dangerous. But in that case we should blame the chemists, and not the chemicals. Personally, we are quite comfortable with chemicals. Our favorite stars, as well as our best friends, are made of them.

Part IV

The Origin
of Planets

CHAPTER 11

When Worlds Were Young

I
n our attempts to uncover the history of the cosmos, we have continually discovered that the segments most deeply shrouded in mystery are those that deal with
origins
—of the universe itself, of its most massive structures (galaxies and galaxy clusters), and of the stars that provide most of the light in the cosmos. Each of these origin stories fills a vital role, not only in explaining how an apparently formless cosmos produced complex assemblages of different types of objects but also in determining how and why, 14 billion years after the big bang, we now find ourselves alive on Earth to ask, How did this all happen?

These mysteries arise in large part because during the cosmic “dark ages,” when matter was just beginning to organize itself into self-contained units such as stars and galaxies, most of this matter generated little or no detectable radiation. The dark ages have left us with only the barest possibilities, still imperfectly explored, for observing matter during its early stages of organization. This in turn implies that we must rely, to an uneasily large extent, on our theories of how matter ought to behave, with relatively few points at which we can check these theories against observational data.

When we turn to the origin of planets, the mysteries deepen. We lack not only
observations
of the crucial, initial stages of planetary formation but also successful
theories
of how the planets began to form. To celebrate the positive, we note that the question, What made the planets?, has grown considerably broader in recent years. Throughout most of the twentieth century, this question centered on the Sun’s family of planets. During the past decade, having discovered more than a hundred “exosolar” planets around relatively nearby stars, astrophysicists have acquired significantly more data from which to deduce the early history of planets, and in particular to determine how these astronomically small, dark, and dense objects formed along with the stars that give them light and life.

Astrnophysicists may now
have more data, but they have no better answers than before. Indeed, the discovery of exosolar planets, many of which move in orbits far different from those of the Sun’s planets, has in many ways confused the issue, leaving the story of planet formation no closer to closure. In simple summary, we can state that no good explanation exists of how the planets
began
to build themselves from gas and dust, though we can easily perceive how the formation process, once well underway, made larger objects from smaller ones, and did so within a rather brief span of time.

The beginnings of planet building pose a remarkably intract-able problem, to the point that one of the world’s experts on the subject, Scott Tremaine of Princeton University, has elucidated (partly in jest) Tremaine’s laws of planet formation. The first of these laws states that “all theoretical predictions about the properties of exosolar planets are wrong,” and the second that “the most secure prediction about planet formation is that it can’t happen.” Tremaine’s humor underscores the ineluctable fact that planets do exist, despite our inability to explain this astronomical enigma.

More than two centuries ago, attempting to explain the formation of the Sun and its planets, Immanuel Kant proposed a “nebular hypothesis,” according to which a swirling mass of gas and dust that surrounded our star-in-formation condensed into clumps that became the planets. In its broad outlines, Kant’s hypothesis remains the basis for modern astronomical approaches to planet formation, having triumphed over the concept, much in vogue during the first half of the twentieth century, that the Sun’s planets arose from a close passage of another star by the Sun. In that scenario, the gravitational forces between the stars would have drawn masses of gas from each of them, and some of this gas could then have cooled and condensed to form the planets. This hypothesis, promoted by the famed British astrophysicist James Jeans, had the defect (or the appeal, for those inclined in that direction) of making planetary systems extremely rare, because sufficiently close encounters between stars probably occur only a few times during the lifetime of an entire galaxy. Once astronomers calculated that almost all the gas pulled from the stars would evaporate rather than condense, they abandoned Jeans’s hypothesis and returned to Kant’s, which implies that many, if not most, stars should have planets in orbit around them.

Astrophysicists now have good evidence that stars form, not one by one but by the thousands and tens of thousands, within giant clouds of gas and dust that may eventually give birth to about a million individual stars. One of these giant stellar nurseries has produced the Orion nebula, the closest large star-forming region to the solar system. Within a few million years, this region will have produced hundreds of thousands of new stars, which will blow most of the nebula’s remaining gas and dust into space, so that astronomers a hundred thousand generations from now will observe the young stars unencumbered by the remnants of their starbirthing cocoons.

Astrophysicists now use radio telescopes to map the distribution of cool gas and dust in the immediate vicinities of young stars. Their maps typically show that young stars do not sail through space devoid of all surrounding matter; instead, the stars usually have orbiting disks of matter, similar in size to the solar system, but made of hydrogen gas (and of other gases in lesser abundances) sprinkled throughout with dust particles. The term “dust” describes groups of particles that each contain several million atoms and have sizes much smaller than that of the period that ends this sentence. Many of these dust grains consist primarily of carbon atoms, linked together to form graphite (the chief constituent of the “lead” in a pencil). Others are mixtures of silicon and oxygen atoms—in essence tiny rocks, with mantles of ice surrounding their stony cores.

The formation of these dust particles in interstellar space has its own mysteries and detailed theories, which we may skip past with the happy thought that the cosmos
is
dusty. To make this dust, atoms have come together by the millions; in view of the extremely low densities between the stars, the likeliest sites for this process seem to be the extended outer atmospheres of cool stars, which gently blow material into space.

The production of
interstellar dust particles provides an essential first step on the road to planets. This holds true not only for solid planets like our own but also for gas-giant planets, typified in the Sun’s family by Jupiter and Saturn. Even though these planets consist primarily of hydrogen and helium, astrophysicists have concluded from their calculations of the planets’ internal structure, along with their measurements of the planets’ masses, that the gas giants must have solid cores. Of Jupiter’s total mass, 318 times Earth’s, several dozen Earth masses reside in a solid core. Saturn, with ninety-five times Earth’s mass, also has a solid core with one or two dozen times the mass of Earth. The Sun’s two smaller gas-giant planets, Uranus and Neptune, have proportionately larger solid cores. In these planets, with fifteen and seventeen times Earth’s mass, respectively, the core may contain more than half of the planet’s mass.

For all four of these planets, and presumably for all of the giant planets recently discovered around other stars, the planetary cores played an essential role in the formation process: First came the core, and then came the gas, attracted by the solid core. Thus all planet formation requires that a large lump of solid matter must form first. Of the Sun’s planets, Jupiter has the largest of these cores, Saturn the next largest, Neptune the next, Uranus after that, and Earth ranks fifth, just as it does in total size. The formation histories of all the planets pose a fundamental question: How does nature make dust coagulate to form clumps of matter many thousand miles across?

The answer has two parts, one known and one unknown, with the unknown part, not surprisingly, closer to the origin. Once you form objects half a mile across, which astronomers call planetesimals, each of them will have sufficiently strong gravity to attract other such objects successfully. The mutual gravitational forces among planetesimals will build first planetary cores and then planets at a brisk pace, so that a few million years will take you from a host of clumps, each the size of a small town, to entire new worlds, ripe to acquire either a thin coat of atmospheric gases (in the case of Venus, Earth, and Mars) or an immensely thick one of hydrogen and helium (for the four gas-giant planets, which orbit the Sun at distances large enough for them to accumulate huge quantities of these two lightest gases). To astrophysicists, the transition from half-mile-wide planetesimals to planets reduces to a series of well-understood computer models that produce a wide variety of planetary details, but almost always yield inner planets that are small, rocky, and dense, as well as outer planets that are large and (except for their cores) gaseous and rarefied. During this process, many of the planetesimals, as well as some of the larger objects that they make, find themselves flung entirely out of the solar system by gravitational interactions with still larger objects.

All this works rather well on a computer, but building the half-mile-wide planetesimals in the first place still lies beyond astrophysicists’ present abilities to integrate their knowledge of physics with their computer programs. Gravity can’t make planetesimals, because the modest gravitational forces between small objects won’t hold them together effectively. Two theoretical possibilities exist for making planetesimals from dust, neither of them highly satisfactory. One model proposes the formation of planetesimals through accretion, which occurs when dust particles collide and stick together. Accretion works well in principle, because most dust particles
do
stick together when they meet. This explains the origin of dust bunnies under your couch, and if you imagine superdust bunnies growing around the Sun, you can, with only minimal mental effort, let them grow to become chair-sized, house-sized, block-sized, and before long the size of planetesimals, ready for serious gravitational action.

Unfortunately, unlike the production of actual bunnies, the dust-bunny growth of planetesimals seems to require far too much time. Radioactive dating of unstable nuclei detected in the oldest meteorites implies that the formation of the solar system required no more than a few tens of millions of years, and quite possibly a good deal less time than that. In comparison with the current age of the planets, approximately 4.55 billion years, this amounts to a dram in the bucket, only 1 percent (or less) of the total span of the solar system’s existence. The accretion process requires significantly longer than a few tens of millions of years to make planetesimals from dust; so unless astrophysicists have missed something important in understanding how dust accumulates to build large structures, we need another mechanism to surmount the time barriers to planetesimal formation.

That other mechanism may consist of giant vortices that sweep up dust particles by the trillions, whirling them quickly toward their happy agglomeration into significantly larger objects. Because the contracting cloud of gas and dust that became the Sun and its planets apparently acquired some rotation, it soon changed its overall shape from spherical to platelike, leaving the Sun-in-formation as a relatively dense contracting sphere at the center, surrounded by a highly flattened disk of material in orbit around that sphere. To this day, the orbits of the Sun’s planets, which all follow the same direction and lie in nearly the same plane, testify to a disklike distribution of the matter that built the planetesimals and planets. Within such a rotating disk, astrophysicists envision the appearance of rippling “instabilities,” alternating regions of greater and lesser density. The denser parts of these instabilities collect both gaseous material and dust that floats within the gas. Within a few thousand years, these instabilities become swirling vortices that can sweep large amounts of dust into relatively small volumes.

This vortex model for the formation of planetesimals shows promise, though it has not yet won the hearts of those who seek explanations of how the solar system produced what young planets need. Upon detailed examination, the model provides better explanations for the cores of Jupiter and Saturn than for those of Uranus and Neptune. Because astronomers have no way to prove that the instabilities needed for the model to work actually did occur, we must refrain from passing judgment ourselves. The existence of numerous small asteroids and comets, which resemble planetesimals in their sizes and compositions, support the concept that billions of years ago, planetesimals by the millions built the planets. Let us therefore regard the formation of planetesimals as an established, if poorly understood, phenomenon that somehow bridges a key gap in our knowledge, leaving us ready to admire what happens when planetesimals collide.

In this scenario,
we can easily imagine that once the gas and dust surrounding the Sun had formed a few trillion planetesimals, this armada of objects collided, built larger objects, and eventually created the Sun’s four inner planets and the cores of its four giant planets. We should not overlook the planets’ moons, smaller objects that orbit all of the Sun’s planets except the innermost, Mercury and Venus. The largest of these moons, with diameters of a few hundred to a few thousand miles, appear to fit nicely into the model that we have created, because they presumably also arose from planetesimal collisions. Moon building ceased once collisions had built the satellite worlds to their present sizes, no doubt (we may assume) because by that time the nearby planets, with their stronger gravity, had taken possession of most of the nearby planetesimals. We should include in this picture the hundreds of thousands of asteroids that orbit between Mars and Jupiter. The largest of these, a few hundred miles in diameter, should likewise have grown through planetesimal collisions, and then have found themselves stymied from further growth by gravitational interference from the nearby giant planet Jupiter. The smallest asteroids, less than a mile across, may represent naked planetesimals, objects that grew from dust but never collided with one another, once again thanks to Jupiter’s influence, after attaining sizes ripe for gravitational interaction.