So the lovely comets that on occasion rouse us humans to wonder and to awe, that crater the surfaces of inner planets and outer moons, and that now and then endanger life on Earth would be unknown and unthreatening had Uranus and Neptune not grown to be giant worlds four and a half billion years ago.
THIS IS THE PLACE for a brief interlude on planets far beyond Neptune and Pluto, planets of other stars.
Many nearby stars are surrounded by thin disks of orbiting gas and dust, often extending to hundreds of astronomical units (AU) from the local star (the outermost planets, Neptune and Pluto, are about 40 AU from our Sun). Younger Sun-like stars are much more likely to have disks than older ones. In some cases, there's a hole in the center of the disk as in a phonograph record. The hole extends out from the star to perhaps 30 or 40 AU. This is true, for example, for the disks surrounding the stars Vega and Epsilon Eridam. The hole in the disk surrounding Beta Pictoris extends to only 15 AU from the star. There is a real possibility that these inner, dust-free zones have been cleaned up by planets that recently formed there. Indeed, this sweeping-out process is predicted for the early history of our planetary system. As observations improve, perhaps we will see telltale details in the configuration of dust and dust-free zones that will indicate the presence of planets too small and dark to be seen directly. Spectroscopic data suggest that these disks are churning and that matter is falling in on the central stars—perhaps from comets formed in the disk, deflected by the unseen planets, and evaporating as they approach too close to the local sun.
Because planets are small and shine by reflected light, they tend to be washed out in the glare of the local sun. Nevertheless, many efforts are now under way to find fully formed planets around nearby stars—by detecting a faint brief dimming of starlight as a dark planet interposes itself between the star and the observer on Earth; or by sensing a faint wobble in the motion of the star as it's tugged first one way and then another by an otherwise invisible orbiting companion. Spaceborne techniques will be much more sensitive. A Jovian planet going around a nearby star is about a billion times fainter than its sun; nevertheless, a new generation of ground-based telescopes that can compensate for the twinkling in the Earth's atmosphere may soon be able to detect such planets in only a few hours' observing time. A terrestrial planet of a neighboring star is a hundred times fainter still; but it now seems that comparatively inexpensive spacecraft, above the Earth's atmosphere, might be able to detect other Earths. None of these searches has succeeded yet, but we are clearly on the verge of being able to detect at least Jupiter-sized planets around the nearest stars—if there are any to be found.
A most important and serendipitous recent discovery is of a bona fide planetary system around an unlikely star, some 1,300 light-years away, found by a most unexpected technique: The pulsar designated B1257+12 is a rapidly rotating neutron star, an unbelievably dense sun, the remnant of a massive star that suffered a supernova explosion. It spins, at a rate measured to impressive precision, once every 0.0062185319388187 seconds. This pulsar is pushing 10,000 rpm.
Charged particles trapped in its intense magnetic field generate radio waves that are cast across the Earth, about 160 flickers a second. Small but discernible changes in the flash rate were tentatively interpreted by Alexander Wolszczan, now at Pennsylvania State University, in 1991—as a tiny reflex motion of the pulsar in response to the presence of planets. In 1994 the predicted mutual gravitational interactions of these planets were confirmed by Wolszczan from a study of timing residuals at the microsecond level over the intervening years. The evidence that these are truly new planets and not starquakes on the neutron star surface (or something) is now overwhelming—or, as Wolszczan put it, "irrefutable"; a new solar system is "unambiguously identified." Unlike all the other techniques, the pulsar timing method makes close-in terrestrial planets comparatively easy and more distant Jovian planets comparatively difficult to detect.
Planet C, some 2.8 times more massive than the Earth, orbits the pulsar every 98 days at a distance of 0.47 astronomical units1 (AU); Planet B, with about 3.4 Earth masses, has a 67-Earth-day year at 0.36 AU. A smaller world, Planet A, still closer to the star, with about 0.015 Earth masses, is at 0.19 AU. Crudely speaking, Planet B is roughly at the distance of Mercury from our Sun; Planet C is midway between the distances of Mercury and Venus; and interior to both of them is Planet A, roughly the mass of the Moon at about half Mercury's distance from our Sun. Whether these planets are the remnants of an earlier planetary system that somehow survived the supernova explosion that produced the pulsar, or whether they formed from the resulting circumstellar accretion disk subsequent to the supernova explosion, we do not know. But in either case, we have now learned that there are other Earths.
The energy put out by B1257+12 is about 4.7 times that of gun. But, unlike the Sun, most of this is not in visible light, but in a fierce hurricane of electrically charged particles. Suppose that these particles impinge on the planets and heat them. Then, even a planet at 1 AU would have a surface around 280 Celsius degrees above the normal boiling point of water, greater than the temperature of Venus.
These dark and broiling planets do not seem hospitable for life. But there may be others, farther from B1257+12, that are. (Hints of at least one cooler, outer world in the B1257+12 system exist.) Of course, we don't even know that such worlds would retain their atmospheres; perhaps any atmospheres were stripped away in the supernova explosion, if they date back that far. But we do seem to be detecting a recognizable planetary system. Many more are likely to become known in coming decades, around ordinary Sun-like stars as well as white dwarfs, pulsars, and other end states of stellar evolution.
Eventually, we will have a list of planetary systems—each perhaps with terrestrials and Jovians and maybe new classes of planets. We will examine these worlds, spectroscopically and in other ways. We will be searching for new Earths and other life.
ON NONE OF THE WORLDS In the outer Solar System did Voyager find signs of life, much less intelligence. There was organic Matter galore—the stuff of life, the premonitions of life, perhaps but as far as we could see, no life. There was no oxygen in their atmospheres, and no gases profoundly out of chemical equilibrium, as methane is in the Earth's oxygen. Many of the worlds were painted with subtle colors, but none with such distinctive, sharp absorption features as chlorophyll provides over much of the Earth's surface. On very few worlds was Voyager able to resolve details as small as a kilometer across. By this standard, it would not have detected even our own technical civilization had it been transplanted to the outer Solar System. But for what it's worth, we found no regular patterning, no geometrization, no passion for small circles, triangles, squares, or rectangles. There were no constellations of steady points of light on the night hemispheres. There were no signs of a technical civilization reworking the surface of any of these worlds.
The Jovian planets are prolific broadcasters of radio waves—generated in part by the abundant trapped and beamed charged particles in their magnetic fields, in part by lightning, and in part by their hot interiors. But none of this emission has the character of intelligent life—or so it seems to the experts in the field.
Of course our thinking may be too narrow. We may be missing something. For example, there is a little carbon dioxide in the atmosphere of Titan, which puts its nitrogen/methane atmosphere out of chemical equilibrium. I think the CO2 is provided by the steady pitter-patter of comets falling into Titan's atmosphere—but maybe not. Maybe there's something on the surface unaccountably generating CO2 in the face of all that methane.
The surfaces of Miranda and Triton are unlike anything else we know. There are vast chevron-shaped landforms and crisscrossing straight lines that even sober planetary geologists once mischievously described as "highways." We think we (barely) understand these landforms in terms of faults and collisions, but of course we might be wrong.
The surface stains of organic matter—sometimes, as on Triton, delicately hued?
??are attributed to charged particles producing chemical reactions in simple hydrocarbon ices, generating more complex organic materials, and all this having nothing to do with the intermediation of life. But of course we might be wrong.
The complex pattern of radio static, bursts, and whistles that we receive from all four Jovian planets seems, in a general way, explicable by plasma physics and thermal emission. (Much of the detail is not yet well understood.) But of course we might be wrong.
We have found nothing on dozens of worlds so clear and striking as the signs of life found by the Galileo spacecraft in its passages by the Earth. Life is a hypothesis of last resort. You invoke it only when there's no other way to explain what you see. If I had to judge, I would say that there's no life on any of the worlds we've studied, except of course our own. But I might be wrong, and, right or wrong, my judgment is necessarily confined to this Solar System. Perhaps on some new mission we'll find something different, something striking, something wholly inexplicable with the ordinary tools of planetary science—and tremulously, cautiously, we will inch toward a biological explanation. However, for now nothing requires that we go down such a path. So far, the only life in the Solar System is that which comes from Earth. In the Uranus and Neptune systems, the only sign of life has been Voyager itself.
As we identify the planets of other stars, as we find other worlds of roughly the size and mass of the Earth, we will scrutinize them for life. A dense oxygen atmosphere may be detectable even on a world we've never imaged. As for the Earth, that may by itself be a sign of life. An oxygen atmosphere with appreciable quantities of methane would almost certainly be a sign of life, as would modulated radio emission. Someday, from observations of our planetary system or another, the news of life elsewhere may be announced over the morning coffee.
THE VOYAGER SPACECRAFT are bound for the stars. They are on escape trajectories from the Solar System, barreling along at almost a million miles a day. The gravitational fields of Jupiter, Saturn, Uranus, and Neptune have flung them at such high speeds that they have broken the bonds that once tied them to the Sun.
Have they left the Solar System yet? The answer depends very much on how you define the boundary of the Sun's realm. If it's the orbit of the outermost good-sized planet, then the Voyager spacecraft are already long gone; there are probably no undiscovered Neptunes. If you mean the outermost planet, it may be that there are other—perhaps Triton-like—planets far beyond Neptune and Pluto; if so, Voyager 1 and Voyager 2 are still within the Solar System. If you define the outer limits of the Solar System as the heliopause—where the interplanetary particles and magnetic fields are replaced by their interstellar counterparts—then neither Voyager has yet left the Solar System, although they may do so in the next few decades.1 But if your definition of the edge of the Solar System is the distance at which our star can no longer hold worlds in orbit about it, then the Voyagers will not leave the Solar System for hundreds of centuries.
Weakly grasped by the Sun's gravity, in every direction in the sky, is that immense horde of a trillion comets or more, the port Cloud. The two spacecraft will finish their passage through
the Oort cloud in another 20,000 years or so. Then, at last, completing their long good-bye to the Solar System, broken free of the gravitational shackles that once bound them to the Sun, the Voyagers will make for the open sea of interstellar space. only then will Phase Two of their mission begin.
Their radio transmitters long dead, the spacecraft will wander for ages in the calm, cold interstellar blackness—where there is almost nothing to erode them. Once out of the Solar System, they will remain intact for a billion years or more, as they circumnavigate the center of the Milky Way galaxy.
We do not know whether there are other space-faring civilizations in the Milky Way. If they do exist, we do not know how abundant they are, much less where they are. But there is at least a chance that sometime in the remote future one of the Voyagers will be intercepted and examined by an alien craft.
Accordingly, as each Voyager left Earth for the planets and the stars, it carried with it a golden phonograph record encased in a golden, mirrored jacket containing, among other things; greetings in 59 human languages and one whale language; a 12-minute sound essay including a kiss, a baby's cry, and an EEG record of the meditations of a young woman in love; 116 encoded pictures, on our science, our civilization, and ourselves; and 90 minutes of the Earth's greatest hits—Eastern and Western, classical and folk, including a Navajo night chant, a Japanese shakuhachi piece, a Pygmy girl's initiation song, a Peruvian wedding song, a 3,000-year-old composition for the ch'in called "Flowing Streams," Bach, Beethoven, Mozart, Stravinsky, Louis Armstrong, Blind Willie Johnson, and Chuck Berry's "Johnny B. Goode."
Space is nearly empty. There is virtually no chance that one of the Voyagers will ever enter another solar system—and this is true even if every star in the sky is accompanied by planets. The instructions on the record jackets, written in what we believe to be readily comprehensible scientific hieroglyphics, can be read, and the contents of the records understood, only if alien beings, somewhere in the remote future, find Voyager in the depths of interstellar space. Since both Voyagers will circle the center of the Milky Way Galaxy essentially forever, there is plenty of time for the records to be found—if there's anyone out there to do the finding.
We cannot know how much of the records they would understand. Surely the greetings will be incomprehensible, but their intent may not be. (We thought it would be impolite not to say hello.) The hypothetical aliens are bound to be very different from us—independently evolved on another world. Are we really sure they could understand anything at all of our message? Every time I feel these concerns stirring, though, I reassure myself. Whatever the incomprehensibilities of the Voyager record, any alien ship that finds it will have another standard by which to judge us. Each Voyager is itself a message. In their exploratory intent, in the lofty ambition of their objectives, in their utter lack of intent to do harm, and in the brilliance of their design and performance, these robots speak eloquently for us.
But being much more advanced scientists and engineers than we—otherwise they would never be able to find and retrieve the small, silent spacecraft in interstellar space—perhaps the aliens would have no difficulty understanding what is encoded on these golden records. Perhaps they would recognize the tentativeness of our society, the mismatch between our technology and our wisdom. Have we destroyed ourselves since launching Voyager, they might wonder, or have we gone on to greater things?
Or perhaps the records will never be intercepted. Perhaps no one in five billion years will ever come upon them. Five billion years is a long time. In five billion years, all humans will have become extinct or evolved into other beings, none of our artifacts will have survived on Earth, the continents will have become unrecognizably altered or destroyed, and the evolution of the Sun will have burned the Earth to a crisp or reduced it to a whirl of atoms.
Far from home, untouched by these remote events, the Voyagers, bearing the memories of a world that is no more, will fly on.
CHAPTER 10: SACRED BLACK
Deep sky is, of all visual impressions, the nearest akin to a feeling.
—SAMUEL TAYLOR COLERIDGE, NOTEBOOKS (1805)
The blue of a cloudless May morning, or the reds and oranges of a sunset at sea, have roused humans to wonder, to poetry, and to science. No matter where on Earth we live, no matter what our language, customs, or politics, we share a sky in common. Most of us expect that azure blue and would, for good reason, be stunned to wake up one sunrise to find a cloudless sky that was black or yellow or green. (Inhabitants of Los Angeles and Mexico City have grown accustomed to brown skies, and those of London and Seattle to gray ones—but even they still consider blue the planetary norm.)
And yet there are worlds with black or yellow skies, and maybe even green. The color of the sky characterizes the world. Plop me down on any planet in the Solar System; without sensing
the gravity, without glimpsing the ground, let me take a quick look at the Sun and sky, and I can, I think, pretty well tell you where I am. That familiar shade of blue, interrupted here and there by fleecy white clouds, is a signature of our world. The French have an expression, sacre-bleu!, which translates roughly as "Good heavens!"1 Literally, it means "sacred blue!" Indeed. If there ever is a true flag of Earth, this should be its color.
Birds fly through it, clouds are suspended in it, humans admire and routinely traverse it, light from the Sun and stars flutters through it. But what is it? What is it made of? Where does it end? How much of it is there? Where does all that blue come from? If it's a commonplace for all humans, if it typifies our world, surely we should know something about it. What is the sky?
In August 1957, for the first time, a human being rose above the blue and looked around—when David Simons, a retired Air Force officer and a physician, became the highest human in history. Alone, he piloted a balloon to an altitude of over 100,000 feet (30 kilometers) and through his thick windows glimpsed a different sky. Now a professor at the University of California Medical School in Irvine, Dr. Simons recalls it was a dark, deep purple overhead. He had reached the transition region where the blue of ground level is being overtaken by the perfect black of space.
Since Simons' almost forgotten flight, people of many nations have flown above the atmosphere. It is now clear from repeated and direct human (and robotic) experience that in space the daytime sky is black. The Sun shines brightly on your ship. The Earth below you is brilliantly illuminated. But the sky above is black as night.