At the heart of a trillion worlds.
The ancients imagined the planets to be attachedto invisible machinery--transparent crystal spheres, elegantly coupled and geared. We now know that the ancients were wrong. The planets orbit the Sun guided only by the invisible hand of Newtonian gravitation. Some worlds are rock, some gas, some ice, and nowhere, from Mercury to Pluto, is there anything like a crystal sphere. But imagine ourselves leaving the solar system at some impossible speed, until even the orbits of the outermost planets are too small for us to see, until even the Sun is only a point of light no brighter than the brightest stars seen from Earth. Then we do encounter something like a crystal sphere, but a shattered one--a cloud of a trillion shards and fragments of ice, little worlds each the size of a city, feebly illuminated in the great dark between the stars.
We live at the heart of a trillion worlds, all of theminvisible. It sounds like the teaching of some New Age sect. And we are not talking of metaphorical worlds; rather a trillion places, every one of them as distinct a world as ours is, every one gravitationally bound to the Sun, every one with a surface and an interior and on occasion even an atmosphere.
If there is a ceiling above you, step outside. Castyour eye upward. Concentrate on the smallest piece of sky you can make out. Imagine it extending in a widening wedge far out into space, to the stars. In that little patch of sky are a hundred thousand worlds or more, worlds unseen, unnamed, but in some sense known. These distant cousins of the Earth are the cometary nuclei--cold, silent, inactive, slowly tumbling in the interstellar blackness. But when they are induced to fall into our part of the solar system, they creak and rumble, begin to evaporate and jet, and eventually produce the tails so admired by the inhabitants of Earth. How we know of this invisible multitude of icy worlds is one more scientific detective story, one of the many that begin with Edmond Halley.
After Halley made the first inventory of cometaryorbits, it was clear that many comets return infrequently, once every few centuries or even longer. At any given moment, he knew, there must be unseen long-period comets that have not lately visited the Sun. And if by chance we see comets with periods of years to centuries, perhaps there are others with periods measured in millennia or more. Halley was prepared to believe in a large population of undiscovered comets with very long periods and high eccentricities. But he did not envision truly immense numbers of comets. When an 18th-century astronomer, Thomas Wright of Durham, drew a rosette of orbits surrounding the Sun, he was not tempted to include more than the few known comets, although he did conclude, "the Comets. . . I judge to be by far the most numerous Part of the Creation."
The key to the discovery ofthe comet cloud is the orbits of the comets we see. We must bear in mind that this constitutes only a small sample of all cometary orbits. For all we know, our sample may not even be representative of the larger population. But it is our only starting point.
An elliptical cometary orbithas a certain size. Its near point to the Sun is called its perihelion and its far point, its aphelion. The line from perihelion to aphelion, running through the Sun, is the major axis of the ellipse, and half of the major axis is called the semi-major axis. The semi-major axis of the Earth's orbit is one astronomical unit (A.U.). Comets with small semimajor axes never leave the planetary part of the solar system, and constitute the kingdom of short-period comets. Such comets are tightly bound to the Sun's gravity; it would take a very major influence to perturb their motion significantly. But comets with large semimajor axes spend most of their time far beyond the region of the planets, and less often than once a human lifetime they make a brief foray into the inner solar system. Such long-period comets, much more loosely bound to the Sun, are most easily perturbed. The convention is to call a comet with a period less than 200 years a short-period comet, and one with a period longer than 200 years a long-period comet. There is nothing magic about 200 years; it is chosen only because this is very roughly (now a little less than) the period of modern astronomical study of the comets. So a comet like Halley's (by this definition) is a short-period comet, while one like Comet Kohoutek, which passed by the Earth in 1973 and will not return for another 10 million years, is a long-period comet.
Jan Oort has been for several decadesthe dean of the distinguished modern school of Dutch astronomers. Among his many contributions to science are the first correct estimate of the distance of the Sun from the center of the Milky Way, the first use of radio astronomy to map the spiral structure of the Milky Way, and the discovery of episodic and titanic explosions at the hub of the Milky Way--which may indicate the presence of a massive black hole there. It was also Oort who, shortly after the end of World War II, proposed the existence of a distant cloud of comets, loosely bound to the Sun. Although some aspects of the theory were anticipated by an Estonian-Irish astronomer, Ernst Opik, the full beauty of the idea was first glimpsed and developed by Oort.
Oort proposed that a vast cloud ofunseen comets surrounds the Sun at immense distances and that all the comes we see are the deserters and refugees from that distance assemblage. Most of these comets are on fairly circular orbits, with modest eccentricity. They never enter the planetary part of the solar system, and we never see them. But occasionally a cometary nucleus leaves its fellows and plummets into the inner solar system, where it may come close enough to the Sun for us to designate it as a long-period comet; or else it might make a close pass by one or more of the major planets, and have its orbit progressively altered, so that eventually we describe it as a short-period comet.
But what induces this occasionalcomet, weakly held by the Sun's gravity, to enter the inner solar system? Oort calculated that the Sun, in its motion about the center of the Milky Way galaxy, would sometimes come close enough to other stars to make a kind of gravitational flurry in the comet cloud--spilling numbers of them in all directions, including to the vicinity of the Sun. A typical comet in the Oort Cloud is circuiting the Sun at the leisurely pace of about a hundred meters per second, around 220 miles an hour. The change in speed administered by the passing star is only a few tens of centimeters a second, near the top speed your fingers can manage to walk across a tabletop. It represents a very small change in the overall speed of the comet, but it's enough to send a few of them careening down among the planets. No single gravitational impulse from a passing star causes the comets to flutter about. Rather, the accumulation of a few dozen close stellar passages has produced a jittery population of faster-moving comets, and the latest stellar encounter provides the small increment needed to drive some of them down toward the Sun or out into the interstellar medium. It's the straw that breaks the camel's back.
Even if a star were to plow straightthrough the comet cloud, the consequences would not be spectacular. Opik likened it to a bullet traversing a swarm of gnats: comparatively few of the gnats are scattered or destroyed; the swarm continues almost undisturbed. (The star would drill a hole through the Oort Cloud about 1,000 Astronomical Units (AUs) across that would gradually be repaired). And comets deep within the Oort Cloud are not ejected by stellar perturbations at all; residing closer to the Sun, they are more tightly bound to it by gravity, a d cannot readily be shaken loose by a passing star, unless it comes very near the Sun.
In addition to nearby stars, todaywe also know that large clouds of interstellar molecules exist in our part of the galaxy, and the solar system should plow through a few of them every billion years. Each time this happens, there will be an additional gravitational stirring within the circumsolar cometary halo, and a further flurry of comets will be sent into the inner solar system. Oort deduced a vast reservoir of comets in deep freeze and mint condition. How vast? An 18th-century German astronomer, Johann Heinrich Lambert, argued that the space around the Sun was probably filled with as many comets as could be squeezed in without frequent collision, and from this deduced that there were "at least 500 millions of comets" in the solar system. With the present size of the Oort Cloud and with the present rate of gravitational perturbation from passing stars, astronomers deduce at least a trillion cometary nuclei. The number of comets in the Oort Cloud is thus larger than the number of stars in the Milky Way galaxy. However, this estimate is almost certainly too modest. A still larger number of comets seems likely. Recent evidence supports the idea that the Oort Cloud stretches from its periphery nearly 100,000 AUs from the Sun continuously inward, reaching almost to the orbit of Pluto.
Such comets are too far out for encounterswith the planets to change their orbits, and too close in for the usual passing stars or interstellar clouds to perturb them. But once in a geological age, a star will come much closer to the Sun, actually passing within the boundaries of the Oort Cloud. If there is an inner Oort Cloud, a very closer stellar encounter can produced a major disruption, scattering a billion comets at once, and showering the inner solar system with comets at a rate of about one an hour for a million years. If we allow for the inner Oort Cloud, we find that the total number of comets orbiting the Sun may be 100 trillion. This is roughly the same as the number of stars in a hundred thousand galaxies like ours. The comets constitute a small universe.
These numbers, even the mere trillioncomets in the outer cloud, are so staggering that they invite disbelief. "The chief difficulty about this hypothesis," wrote a highly respected American astronomer known for his openness to new ideas, "is that it demands the existence of an enormous number of comets of large perihelion distance." Well, exactly.
What is the scale of the classicalOort Cloud? One hundred thousand AUs is a little less than two light-years, roughly halfway between the Sun and the nearest star. If we were standing on the comet, we would be so far from the Sun that we would see it as it was almost two years ago. The typical period for a comet in the Oort Cloud to circle the Sun is a few million years. Since the age of the solar system is about 4.6 billion years, a typical such comet has made a thousand revolutions of the Sun. A year on the comet is a million times longer than a year on the Earth, and there the words of the Psalmist are literally correct: "For a thousand years in Thy sight are but as yesterday when it is past, and as a watch in the night" (365 days/year x 1,000 years = 365,000:1, crudely 1,000,000:1).
With a trillion or more comets inthe Ort Cloud, you might think that comets are closer together there than anywhere else in the solar system--perhaps huddling together, far from the Sun, like one of Dore's illustrations of the souls of the dead. But the number of comets is more than balanced by the immensity of the space they occupy, and the typical distance from one comet to another out there is about 20 AUs, roughly the same as the distance from the Earth to the planet Uranus. The greatest concentration of comets in the solar system happens, it seems, to be in the very innermost parts, where by lucky chance we happen to live. The three or four long-period comets that each year are passing through together with all the short-period comets constitute, so far as we know, the greatest density of comets anywhere between here and the nearest star system, Alpha Centauri.
The Voyager 2 spacecraft waslaunched in 1977 on an unusual high-energy trajectory so that it would reach the planet Uranus in 1986 and Neptune in 1989. If our present spacecraft could get out to the Oort Cloud, it would take us a decade or more to fly from comet to comet. But we cannot soon get to the Oort Cloud. The Voyager spacecraft--the fastest ever launched by the human species--took nine years to go from Earth to Uranus; it will take them 10,000 years to reach the main comet repository. The comets themselves take a few million years to fall from the solar-system frontiers to the vicinity of the Earth. The Oort Cloud is very far away.
Altogether, how much do a trillioncomets weigh? If every one of them is about a kilometer across, then the total mass of all the comets in the outer Oort Cloud is about the same as the present mass of the Earth. Put another way, if you took the Earth and divided it up into the little kilometer-sized chunks, you would have in numbers and size (but not in composition) some approximation of the current population of the outer Oort Cloud. If, as seems likely, the typical comets is a little bigger, or if you include the inner Oort Cloud, then the total mass of the cloud will be considerably more.
A typical comet in the Oort Cloudhas existed for billions of years at a temperature only a few degrees above absolute zero. There are no collisions, no heating of the comet, no outgassing. It is very quiet in the Oort Cloud. There is a flux of galactic cosmic rays that slowly intrudes into the top meter or two of the comet. Each cosmic ray leaves a trail of broken chemical bonds behind it. As the fragments slowly reassemble themselves in the frigid and solid surface, new molecules are generated. If there is initial methane ot carbon-monoxide ice, the net result over the age of the solar system will be the generation of a considerable abundance of complex organic molecules, but only in the outer shell of the comet. If you took a very slow stop-motion film of the cometary surface (one frame every million years, say), the ices would gradually become darker and redder due to the complex organic molecules being synthesized. Suppose the comet is then nudged into the inner solar system. During a single perihelion passage, all of the processed ices would be vaporized to space--the work of a billion years undone in a month. The underlying ices revealed after the blowoff of the top meter will be close to their pristine form--pure white ice, if that is how the comet formed--or with a reddish primordial taint from the organic molecules of the interstellar medium that went into the comet in the first place. In any case, under the thin surface layer worked by cosmic rays is material virtually untouched since the beginning of the solar system.
Why is the outer boundary of theOort Cloud a hundred thousand astronomical units away? Does the population of comets in the Oort Cloud fall off slowly thereafter, perhaps bumping into the Oort Cloud of some other star? We might imagine the two cometary halos interdigitating, interpenetrating--the individual comets mingling, but widely separated--while remaining loyal to the stars around which they were born. Eventually, they pass on. But we now know that the Oort Cloud cannot extend very far beyond a hundred thousand AUs. A Soviet astrophysicist, G.A. Chebotarev, has shown that the massive center of the Milky Way galaxy, 30,000 light-years from the Earth and Sun, is adequate to loosen the feeble gravitational grip of the Sun on any comet more distant than about 200,000 AUs. A small part of the mass of the center of the galaxy is probably provided by a black hole that resides there. Without the black hole, the Oort Cloud would be slightly bigger.
Thus the Oort Cloud connects familiarevents in this backwater of the inner solar system not only with the nearby stars but also with the center of the galaxy, so far away that, through the telescope, we see it as it was 30,000 years ago. The comets that rush into our part of the solar system are from a skittish population made excitable by the passage of stars and nebulae. If the solar system were isolated from the rest of the galaxy, we would never know that comets exist--because then the passing stars and interstellar clouds would be unavailable to shake the Oort Cloud occasionally and send some comets fluttering into the inner solar system.
And the number of comets that arrivedown here (as well as the external boundaries of the Oort Cloud from which they come) may be determined in small measure by a black hole at the galactic center--an object undreamed of only a few decades ago. The comets are unexpectedly and profoundly connected with the Milky Way, a conclusion appropriate enough for Jan Oort, who perhaps more than any other person in the 20th century has revolutionized our knowledge of the galaxy.
The scope of Oort's idea isremarkable. To account for the handful of new comets that appear in our skies each year, a vast mind-numbing multitude of invisible comets living far beyond the orbit of Pluto is postulated. The idea explains what we know about comets in an elegant way that no other theory even approaches. The trillions of comets are now widely accepted by astronomers all over the world and called, properly, the Oort Cloud. Many scientific papers are written each year about the Oort Cloud, its properties, its origin, its evolution. Yet there is not yet a shred of direct observational evidence for its existence. We are not yet able to poke around in the Oort Cloud. No spacecraft has voyaged to count the comets there. It will be awhile before any do. There is one recent measurement that might conceivably be relevant, the discovery of distant wispy aggregations of matter by the Infrared Astronomy Satellite; the contention that this is the structure of the Oort Cloud is, however, wildly controversial.
But with the refinement ofour scientific instruments, and the development of space missions to go far beyond Pluto, our chances of observing Oort Cloud comets will improve. There will be some day in the future of our species--provided we are not so foolish as to destroy ourselves first--when we will directly measure the population of the Oort Cloud, designate and characterize each of the large comets there, plot their orbits into the future, and perhaps make plans for their utilization. We do not knopw how long it will be until some presentation of the Oort Cloud will be accumulated from real data. We wish the astronomers of that distant epoch well, and in our mind's eye share with them the joy that they will surely take in those great discoveries to come.
Editor's Note: in last month's issue,we published the chapter "A Mote of Dust" from Comet, by Carl Sagan and Ann Druyan. In the editing, several paragraphs were omitted and are printed here at the request of the authors.
Comets may act as the creators, thepreservers, and the destroyers of life on Earth. A surviving dinosaur might have reason to mistrust them, but humans might more appropriately consider the comets in a favorable light--as bringers of the stuff of life to Earth, as ocean--builders, as the agency that removed the competition and made possible the success of our mammalian ancestors, as possible future outposts of our species, and as providers of a timely reminder about large explosions and the climate of the Earth.
A comet is also a visitor from thefrigid interstellar night that constitutes by far the greatest part of the known universe. And a comet is, further, a great clock, ticking out decades or geological ages once each perihelion passage, reminding us of the beauty and harmony of the Newtonian universe, and of the daunting insignificance of our place in space and time. If, by chance, the period of a bright comet happens to be the same as a human lifetime, we invest it with a more personal significance. It reminds us of our mortality.
And why would anyone goto such lengths, when there are perfectly adequate means of communication at hand? Because, after the imminent arms race in anti-satellite weaponry gets going, communications satellites would be among the early casualties in a nuclear war. Meteor Burst Commnications has been developed so that a nuclear war can be fought. The comets have been enlisted. For the first time since they were thought to be warnings sent by angry God, comets have practical value. But they have not been singled out. The entire world-wide enterprise of human knowledge is being drawn upon for similar services, and something approaching half the scientists on Earth now work for the various national military establishments.
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|Author:||Sagan, Carl; Druyan, Ann|
|Publication:||Saturday Evening Post|
|Date:||Apr 1, 1986|
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