The Planets (2 page)

After the science fair, my class staged a planets play. I got the part of “Lonely Star” because the script called for that character to wear a red cape, and I had one, left over from a Halloween costume. As Lonely Star, I soliloquized the Sun’s wish for companionship, which the planet-actors granted by joining up with me, each in a speech admitting his own peculiarities. The play’s most memorable performances were delivered by “Saturn,” who twirled two Hula-hoops while reciting her lines, and “The Earth,” plump and self-conscious, yet forced to announce matter-of-factly, “I am twenty-four thousand miles around my middle.” Thus was the statistic of our Earth’s circumference indelibly impressed upon me. (Note that we always said “the earth,” in those days. “The earth” did not become “Earth” until after I came of age and the Moon changed from a nightlight to a destination.)

My role as Lonely Star helped me appreciate the Sun’s relationship to the planets as parent and guide. Not for nothing is our part of the universe called the “Solar System,” in which each planet’s individual makeup and traits are shaped in large measure by proximity to the Sun.

I had omitted the Sun from my diorama because I hadn’t understood its power, and besides, it would have posed an impossible problem of scale.
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Another reason for leaving out the Sun, and likewise the Moon, was the bright familiarity of both objects, which seemed to render them regular components of the Earth’s atmosphere, whereas the planets were glimpsed only occasionally (either before bedtime or in a still-dark, early-morning sky), and therefore more highly prized.

On our class trip to the Hayden Planetarium, we city kids saw an idealized night sky, liberated from the glare of traffic signals and neon lights. We watched the planets chase each other around the heavens of the dome. We tested the relative strength of gravity with trick scales that told how much we’d weigh on Jupiter (four hundred pounds and more for a normal-sized teacher) or Mars (featherweights all). And we gawked at the sight of the fifteen-ton meteorite that had fallen from out of the blue over Oregon’s Willamette Valley, posing a threat to human safety that few of us had thought to fear.

The Willamette meteorite (still on permanent display at what is now the Rose Center for Earth and Space) was said to be, incredibly, the iron-nickel core of an ancient planet once in orbit around the Sun. That world had shattered somehow, several billion years back, setting its fragments adrift in space. Chance had nudged this particular piece toward the earth, where it hurtled down to the Oregon ground at tremendous speed, burning up from the heat of friction, and hitting the valley floor with the impact of an atom bomb. Later, as the meteorite lay still over eons, the acidic rains of the Pacific northwest chewed large holes in its charred and rusted hulk.

Here was a primal scene to upset my innocent planet ideas. This dark, evil invader had no doubt consorted in space with hordes of other stray rocks and metal chunks that might strike the earth at any moment. My Solar System home, till that moment a paragon of clockwork regularity, had turned into a disorderly, dangerous place.

The launch of
Sputnik
in 1957, when I was ten, scared me to death. As a demonstration of foreign military strength, it gave new meaning to the
school-wide air raid drills in which we crouched under our desks for pretended safety, our backs to the windows. Clearly we still had more to dread from angry fellow humans than from wayward space rocks.

All through my teens and twenties, as the country realized the young president’s dream of a rocket to the Moon, clandestine rockets in missile silos kept collective nightmares alive. But by the time the Apollo astronauts brought back their last batch of Moon rocks in December 1972, peaceful, hopeful spaceships had landed also on Venus and Mars, and another, the U.S.
Pioneer 10,
was en route to a Jupiter flyby. Throughout the 1970s and ’80s, hardly a year passed without an unmanned excursion to another planet. Images radioed home to Earth by robot explorers painted detail upon detail on the planets’ long-blank faces. Whole new entities came to light, too, as spacecraft encountered literally dozens of new moons at Jupiter, Saturn, Uranus, and Neptune, as well as multiple rings around all four of those planets.

Even though Pluto remained unexplored, deemed too distant and too difficult to visit, its own unexpected moon was discovered accidentally in
1978, through careful analysis of photographs taken by ground-based telescopes. Had my daughter, born in 1981, attempted her own diorama of the revised and expanded Solar System when she turned eight, she would have needed handfuls of jellybeans and jawbreakers to model the many recent additions. My son, three years her junior, might have opted to model his on our home computer.

Despite the increased population of the Solar System, its planets stayed stable at nine, at least through 1992. That year, a small, dark body, independent of Pluto, was detected on the Solar System’s periphery. Similar discoveries soon followed, until the total number of diminutive outliers grew to seven hundred over the ensuing decade. The abundance of mini-worlds made some astronomers wonder whether Pluto should continue to be regarded as a planet, or reclassified as the largest of the “trans-Neptunian objects.” (The Rose Center has already excluded Pluto from the planetary roll call.)

In 1995, only three years after the first of Pluto’s numerous neighbors was found, something even more remarkable came to light. It was a bona fide
new planet—of another star. Astronomers had long suspected that stars other than the Sun might have their own planetary systems, and now the first one had surfaced at 51 Pegasi, in the constellation of the flying horse. Within months, other “exoplanets”—as the newly discovered extra-solar planets were quickly dubbed—turned up at stars such as Upsilon Andromedae, 70 Virginis b, and PSR 1257+12. Nearly 200 additional exoplanets have since been identified, and refinements in discovery techniques promise to uncover many more in the near future. Indeed the number of planets in our Milky Way Galaxy alone may far exceed its complement of one hundred billion stars.

My old familiar Solar System, once considered unique, now stands as merely the first known example of a popular genre.

As yet, no exoplanets have been imaged directly through a telescope, so their discoverers are left to imagine what they look like. Only their sizes and orbital dynamics are known. Most of them rival giant Jupiter in heft, because large planets are easier to find than small ones. Indeed, the existence of exoplanets is deduced from their effect on their parent star: Either the star wobbles as it yields to
the gravitational attraction of unseen companions, or it periodically dims as its planets pass in front and impede its light. Small exoplanets, the size of Mars or Mercury, must also orbit distant suns, but, being too tiny to perturb a star, they elude detection from afar.

Already planetary scientists have appropriated the name “Jupiter” as a generic term, so that “a jupiter” means “a large exoplanet,” and the mass of an extremely large exoplanet may be quantified as “three jupiters” or more. In the same fashion, “an earth” has come to represent the most difficult, most desirable goal of today’s planet hunters, who are devising ways to probe the Galaxy for petite, fragile spheres in the favored shades of blue and green that hint at water and life.

Whatever daily concerns dominate our minds at the dawn of the present century, the ongoing discovery of extra-solar planetary systems defines our moment in history. And our own Solar System, rather than shrink in importance as one among many, proves the template for comprehending a plethora of other worlds.

Even as the planets reveal themselves to scientific investigation, and repeat themselves across
the universe, they retain the emotional weight of their long influence on our lives, and all that they have ever signified in Earth’s skies. Gods of old, and demons, too, they were once—they still are—the sources of an inspiring light, the wanderers of night, the far horizon of the landscape of home.

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In his ingenious pamphlet, “The Thousand-Yard Model, or, The Earth as a Peppercorn,” Guy Ottewell guides the construction of a scale model Solar System using a bowling ball for the Sun. The eight-thousand-mile-wide Earth, here reduced to a peppercorn, takes its rightful place seventy-eight feet (!) from the bowling ball.

“I
n the beginning, God created the heaven and the earth,” the first book of the Bible recounts. “And the earth was a formless void and darkness covered the face of the deep, while a wind from God swept over the face of the waters. Then God said, ‘Let there be light’; and there was light.”

The energy of God’s intent flooded the new heaven and earth with light on the very first day of Genesis. Light’s potent good thus pervaded the evenings and the mornings when the seas separated from the dry lands, and the earth brought forth grass and fruit trees—even before God set the sun, moon, and stars in the firmament on the fourth day.

The scientific Creation scenario likewise unleashes the universe in a burst of energy from a void of timeless darkness. About thirteen billion years ago, scientists say, the hot light of the “Big Bang” erupted, and separated itself instantly into matter and energy. The next three minutes of cooling precipitated all the atomic particles in the universe, in the unequal proportions of 75 percent hydrogen to 25 percent helium, plus minuscule traces of a few other elements. As the universe expanded exponentially in all directions and continued to cool, it shed no new light for at least a billion years—until it begat the stars, and the stars began to shine.

New stars lit up by pressuring the hydrogen atoms deep within themselves to fuse with one another, yielding helium and releasing energy. Energy fled the stars as light and heat, but helium accumulated inside them, until eventually it, too, became a fuel for nuclear fusion, and the stars melded atoms of helium into atoms of carbon. At later stages of their lives, stars also forged nitrogen, oxygen, and even iron. Then, literally exhausted, they expired and exploded, spewing their bounty of new elements into space. The largest and brightest stars bequeathed to the universe the
heaviest of elements, including gold and uranium. Thus the stars carried on the work of Creation, hammering out a wide range of raw materials for future use.

As the stars enriched the heavens that had borne them, the heavens gave rise to new generations of stars, and these descendants possessed enough material wealth to build attendant worlds, with salt seas and slime pits, with mountains and deserts and rivers of gold.

In its own beginning, some five billion years ago, the star that is our Sun arose from a vast cloud of cold hydrogen and old stardust in a sparsely populated region of the Milky Way. Some disturbance, such as the shock wave from a nearby stellar explosion, must have reverberated through that cloud and precipitated its collapse: Widely dispersed atoms gravitated into small clumps, which in turn lumped together, and kept on aggregating in an ever-quickening rush. The cloud’s sudden contraction raised its temperature and set it spinning. What had once been a diffuse, cool expanse of indeterminate shape was now a dense, hot, spherical “proto-solar nebula” on the verge of starbirth.

The nebula flattened into a disk with a central bulge, and there in the heart of the disk the Sun
came to light. At the moment the Sun commenced the self-consumptive fusion of hydrogen in the multi-million-degree inferno of its core, the outward push of energy halted the inward gravitational collapse. Over the ensuing few million years, the rest of the Solar System formed from the leftover gas and dust surrounding the infant Sun.

The Book of Genesis tells how the dust of the ground, molded and exalted by the breath of life, became the first man. The ubiquitous dust of the early Solar System—flecks of carbon, specks of silicon, molecules of ammonia, crystals of ice—united bit by bit into “planetesimals,” which were the seeds, or first stages, of planets.

Even as they assembled themselves, the planets asserted their individuality, for each one amassed the substances peculiar to its location in the nebula. At the hottest part, flanking the Sun, Mercury materialized from mostly metallic dust, while Venus and Earth matured where rocky dust as well as metal proliferated. Just past Mars, tens of thousands of rocky planetesimals availed themselves of plentiful carbon supplies, but failed to amalgamate into a major planet. These herds of unfinished worlds, called “asteroids,” still roam the broad zone between Mars and Jupiter, and their territory,
the “Asteroid Belt,” marks the Solar System’s great divide: On its near-Sun side lie the terrestrial planets. On the far side, the frigid gaseous giants grew.

The planetesimals at greater distances from the Sun, at lower temperatures, assimilated stores of frozen water and other hydrogen-containing compounds. The first one to reach appreciable proportions then attracted and held onto great quantities of hydrogen gas, and thus grew into Jupiter, the mammoth planet whose mass doubles that of all the others combined. Saturn, too, aggrandized itself with gas. Farther out from the Sun, where dust proved even colder and scarcer, planetesimals took longer to develop. By the time Uranus and Neptune had achieved sufficient mass to pad themselves with hydrogen, the bulk of that gas had already dissipated. At Pluto’s remove, only rock shards and ices could be had.

All the while the planets were forming, projectiles flew through the young Solar System like avenging angels. Worlds collided. Icy bodies struck the Earth and disgorged a few oceans’ worth of water. Rocky bodies rained fire and destruction. In one such cataclysm 4.5 billion years ago, a hurtling Mars-sized object (roughly half as big as Earth) thrust itself into the Earth. The impact and upheaval
blasted molten debris into near-Earth space, there to orbit as a disk before cooling and coalescing into the Moon.

The violence of the Solar System’s formative period ended shortly thereafter, about four billion years ago, in a final paroxysm descriptively termed “the late heavy bombardment.” In those ancient days, many still-wandering planetesimals crashed into existing planets, which incorporated them at once. Multitudes of other small bodies were forcibly ejected, by gravitational interactions with the giant planets, to a distant land of Nod in the outer Solar System.