The Planets (3 page)

The young Sun shone but faintly on the planets, growing gradually hotter and more luminous over its first two billion years, as it stored up helium in its core. At present, in middle age, the Sun continues to brighten while converting seven hundred million tons of hydrogen to helium
every second.
Even at this galloping rate of consumption, the Sun’s abundant hydrogen guarantees three to five billion more years of dependable light. But inevitably, as the Sun switches over to helium fusion, it will become so hot as to boil away Earth’s oceans and smite the life it spawned there. The ten-fold
temperature increase required to burn helium will see the hotter Sun turn red, and swell in size until it swallows up the planets Mercury and Venus, and melts the surface of the Earth. One hundred million years later, when the Sun has reduced more helium to carbon ash, it will shrug off its outer layers and dispatch them past Pluto. A larger star could resort to carbon burning at that point, but our Sun, a relatively small star by the standards of the universe, will be unable to do so. Instead, it will smolder as an ember, and shed a fading light on the charred cinder where God once walked among men. This dim future, however, lies so far ahead as to allow the descendants of Adam and Noah ample time to find another home.

The glorious Sun of our time, the planets’ progenitor and chief source of energy, embodies 99.9 percent of the mass in the Solar System. Everything else—all the planets with their moons and rings, plus all the asteroids and comets—account for only 0.1 percent. This gross inequity between the Sun and the sum of its companions defines their balance of power, for the universal law of gravity decrees that the massive shall have dominion over the less massive. The Sun’s gravity keeps
the planets in orbit and also dictates their speeds: The closer they are to the Sun, the faster they go. The Sun, in turn, bends to the will of the concentrated mass of stars at the center of our Milky Way Galaxy, around which it orbits once every 230 million years, carrying the planets along with it.

Just as they feel the Sun’s attraction more or less keenly, according to their distance, so too do the planets partake of the Sun’s light and heat. Solar energy diminishes in intensity as it radiates through interplanetary space. And so, while parts of Mercury bake at five hundred degrees, Uranus, Neptune, and Pluto remain perpetually deep-frozen. Only in the Solar System’s milder middle section, called the habitable zone, have conditions supported the flourishing of “great whales, and every living creature that moveth, which the waters brought forth abundantly, after their kind, and every winged fowl after his kind: and…cattle, and creeping thing, and beast of the earth….”

The planets return the favor of the Sun’s light by reflecting its rays, and in this manner they pretend to shine, though they emit no light of their own. The Sun is the Solar System’s sole light-giving body; all the others glow by reflected glory. Even the full Moon that illumines so many lovely Earthly evenings owes its silvery light to Sunbeams bouncing off the dark lunar soil. The Earth shines just as beautifully when viewed from the Moon, and for the same reason.

The play of mirrored light from Venus, close to the Sun and also closest to Earth, makes that planet appear by far the brightest one to our eyes. Jupiter, though much larger, lies many millions of miles beyond, and therefore pales in comparison in our night sky. The even farther worlds of Uranus and Neptune, immense as they are, catch and toss back so little light that Uranus can only occasionally be discerned (as a mere point of light) by the naked eye, Neptune never.

Although Pluto, too, is impossible for us to see without a telescope, other objects on the outskirts of the Solar System can and sometimes do flare into sudden visibility. When disturbed by a chance encounter, an ice-rock denizen from Pluto’s depths may be pushed Sunward and transformed from a dull lump into a spectacular comet. Basking in the Sun’s warmth, the frozen body heats up and throws out a trailing tail of castoff gas and icy dust that sparkles with the Sun’s reflection. The brilliance fades and disappears, however, after the comet rounds the Sun and returns to the outer Solar System.
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The visits of comets, long interpreted as signs and wonders, have recently sketched the true extent of the Sun’s domain. By tracing the visible parts of comet paths, and extrapolating the rest, astronomers have shown that numerous comets hail from out beyond Pluto’s neighborhood, from a second comet reservoir, hundreds of times farther away. Despite their unimaginable distance, these bodies still belong to the Sun, still heed the Sun’s gravity, still receive some glimmer of the Sun’s light.

Sunlight, which darts through space at the dazzling speed of 186,000 miles per second, takes ages to emerge from the dense interior of the Sun. Light advances only a few miles per year near the Sun’s core, where the crush of matter repeatedly absorbs it and impedes its escape. Radiating this way, light may journey for a million years before reaching the Sun’s convective zone, there to catch a quick ride up and out on roiling eddies of rising gas. As soon
as these eddies release their cargoes of light, they sink back down, to soar again later with more.

The light-emitting, visible surface of the Sun—the photosphere—seethes as though boiling from the constant tumult of energy release. Gas bubbles bursting with light give the photosphere a grainy complexion, marred here and there by pairs of dark, irregularly shaped sunspots, with black centers and gray, graded shading around them like the penumbras of shadows. Sunspots designate areas of intense magnetic activity on the Sun, and their darkness bespeaks their relative coolness of about 4000 K, compared to neighboring areas at nearly 6000 K.
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The level of solar activity rises and falls in cycles averaging eleven years, and sunspots mingle, morph, and multiply according to this same schedule. Their number and distribution vary like famine and plenty, from no spots at “solar minimum,” or just a few spots dotting the Sun’s high latitudes, to “solar maximum” five to six years later, when hundreds of them crowd closer to the
equator. Although sunspots seem to gather and scud like clouds across the photosphere, really it is the Sun’s rotation that carries them around.

The Sun rotates on its axis approximately once a month, in a continuation of the spinning motion it was born in. Being an enormous ball of gas, the Sun spins complexly, in layers of various speeds. The core and its immediate surroundings turn at one rate, as a solid body. The overlying zone spins faster, and, above that, the visible photosphere whirls around at several different rates, more quickly at the Sun’s equator than near its poles. These combined, contrary motions whip the Sun into a fury, with consequences felt clear across the Solar System.

The “solar wind,” a hot exhalation of charged particles (reminiscent of the “wind from God”), blows out from the turbulent Sun and keeps up a constant barrage on the planets. Were it not for the protective envelope of Earth’s magnetic field, which deflects most of the solar wind, we could not withstand the onslaught. From time to time, especially during solar maximum, the steady solar wind is augmented by sudden blasts of higher-energy particles from solar flares on the Sun’s surface, or by gargantuan blobs of ejected solar
gas. Such outbursts can disable our communications satellites and disrupt power grids, causing blackouts. In milder doses, particles of solar wind trickle into the upper atmosphere near the North and South Poles, initiating cascades of electrical charge that draw curtains of colored lights across the sky—the so-called Northern and Southern Lights. Other planets also sprout colorful auroras in response to the solar wind, which billows on past Pluto all the way to the heliopause—the undiscovered boundary where the Sun’s influence ends.

From Earth, we see the Sun as a blazing circle in the sky, brighter but no bigger than the circumference of the full Moon. The “two great lights,” as the Sun and Moon are described in Genesis, make a matched pair. For although the Moon measures only one four-hundredth the Sun’s million-mile diameter, it nevertheless lies four hundred times closer to Earth. This uncanny coincidence of size and distance enables the puny Moon to block out the Sun whenever the two bodies converge on their shared path across Earth’s sky.

Approximately once every two years, some narrow swath of Earth—as often as not a godforsaken, all but inaccessible place—is blessed
with a total solar eclipse. There, dusk falls and dawn breaks twice on the same day, and the stars come out with the Sun still overhead. Temperatures may drop ten or fifteen degrees at a stroke, allowing even the most jaded observer to sense the bizarre disorientation that birds and animals share as they hasten to their nests or burrows through the sudden midday darkness.

No total eclipse can last much longer than seven minutes, because of Earth’s persistent turning on its axis and the Moon’s unwavering march along its orbit. But totality of the briefest duration affords sufficient reason for scientific expeditions and curious individuals to travel halfway around the world, even if they have seen one or more eclipses before.

At totality, when the Moon is a pool of soot hiding the bright solar sphere, and the sky deepens to a crepuscular blue, the Sun’s magnificent corona, normally invisible, flashes into view. Pearl and platinum-colored streamers of coronal gas surround the vanished Sun like a jagged halo. Long red ribbons of electrified hydrogen leap from behind the black Moon and dance in the shimmering corona. All these rare, incredible sights offer themselves to the naked eye, as totality provides the only safe
time to gaze at the omnipotent Sun without fear of requital in blindness.

Moments later, the shadow of the Moon passes and the natural world order is restored by the ordinary grace of the Sun’s familiar light. But visions of the eclipse persist among viewers, as though a miracle had been witnessed. Is it an accident that the Solar System’s lone inhabited planet possesses the only satellite precisely sized to create the spectacle of a total solar eclipse? Or is this startling manifestation of the Sun’s hidden splendor part of a divine design?

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Discarded comet dust litters interplanetary space, and when the Earth trundles into a patch of it, the particles that fall through the atmosphere are incinerated, appearing as isolated “shooting stars” or whole showers of meteors.

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Degrees K, for Kelvin, are the same size as degrees Celsius (or centigrade)—almost double the value of Fahrenheit degrees. However, the Kelvin scale starts lower, at –273º C, or “absolute zero,” the point at which all motion ceases, and has no upper limit, which makes it useful for describing the temperatures of stars.

T
he planets speak an ancient dialect of myth. Their names recall all that happened before history, before science, when Prometheus hung shackled to that cliff in the Caucasus for stealing fire from the sky, and Europe was not yet a continent but still a girl, beloved by a god, who beguiled her disguised as a bull.

In those days Hermes—or Mercury, as the Romans renamed the Greek messenger god—flew fleet as thought on divine errands that earned him more mentions in the annals of mythology than any other Olympian: After the goddess of the harvest lost her only daughter to the god of the
underworld, Mercury was sent to negotiate the victim’s rescue, and drove her home in a golden cart pulled by black horses. When Cupid got his wish, making Psyche immortal and therefore fit to marry him, it was Mercury who led the bride into the palace of the gods.

The planet Mercury appeared to the ancients, as it appears to the naked eye today, only on the horizon, where it coursed the twilight limbo between day and night. Swift Mercury either heralded the Sun at dawn, or chased after it through dusk. Other planets—Mars, Jupiter, Saturn—could be seen shining high in the sky all night for months on end. But Mercury always fled the darkness for the light, or vice versa, and hastened from view within an hour’s time. Likewise the god Mercury served as a go-between, traversing the realms of the living and the dead, conducting the souls of the deceased down to their final abode in Hades.

Myth may have conferred the god’s name on the planet, because it mirrored his attributes, or perhaps the observed behavior of the planet gave rise to legends of the god. Either way, the union of planet Mercury with divine Mercury—and with Hermes, and the Babylonian deity Nabû the Wise before him—was sealed by the fifth century
B.C.

The persistent image of Mercury, lean and hell-bent as a marathon runner, personifies dispatch. Wings on his sandals urge him on, spurred faster by the wings on his cap, and the magic powers of his winged wand. Although speed tops the panoply of his powers, Mercury also gained fame as a giant-killer (after he slew thousand-eyed Argus) and as the god of music (because he invented the lyre, and his son, Pan, fashioned the shepherd’s pipe of reeds), god of commerce and protector of traders (for which he is remembered in words like “merchant” and “mercantile”), of cheats and thieves (since he stole herds from his half-brother Apollo on the very first day of his life), of eloquence (having given Pandora the gift of language), as well as of cunning, knowledge, luck, roads, travelers, young men in general, and herdsmen in particular. His snake-entwined wand, the caduceus, has invoked fertility or healing or wisdom over the ages.