The rings of Saturn as observed by the Voyager 1 spacecraft. Saturn is just out of the picture in the upper right-hand corner. The minor differences between these many flat rings have been greatly enhanced by computer processing. The large dark gap in the rings, about three-quarters of the apparent distance to the periphery in this picture, is the Cassini Division. For Kant and Laplace, the rings of Saturn served as a model for the structure of the solar nebula that surrounded the Sun at the time the planets were being formed. Courtesy National Aeronautics and Space Administration.
However arbitrary the system of the planets may be, there exists between them some very remarkable relations, which may throw light on their origin; considering them with attention, we are astonished to see all the planets move around the Sun from west to east, and nearly in the same plane, all the satellites moving around their respective planets in the same direction, and nearly in the same plane with the planets. Lastly, the Sun, the planets, and those satellites in which a motion of rotation have been observed, turn on their own axis, in the same direction, and nearly in the same plane as their motion … A phenomenon so extraordinary is not the effect of chance, it indicates an universal cause …
—LAPLACE, “ON THE REGULARITY OF THE SOLAR SYSTEM,”
SYSTEM OF THE WORLD, PART 1, CHAPTER 6, 1799
To this he added two more regularities, the near circularity of the orbits of the planets, and the great eccentricity and random inclinations of the orbits of the (long-period) comets.
Kant had accepted and further developed Thomas Wright’s insight and believed that the Milky Way is a thin plate of stars, one of which is the Sun. Astronomers were discovering strange flattened luminous forms in the night sky—spiral nebulae they were later called. (Nebula, plural nebulae, is a Latin word meaning cloud.) Nature seemed to have a propensity for flattening extended systems whether they are made of dust or stars.
The Kant-Laplace hypothesis for the origin of the solar system involves the interplay of rotation and gravitation. Imagine some irregular cloud of interstellar matter made of gas and dust and destined to form the solar system. All such clouds known today exhibit slow rotation. If the cloud is sufficiently massive, the random molecular motions are overwhelmed by self-gravity—the mutual attraction of the atoms and grains in the cloud for one another. The cloud then begins to contract, the distant provinces falling inward; the density of the cloud increases as a fixed amount of matter squeezes itself into progressively smaller volumes. As it contracts, the cloud spins faster, for the same reason that a pirouetting ice skater does as she brings her arms in. (The experiment can also be done with a small person, seated on a rotating piano stool, holding a brick in each outstretched hand, and then rapidly drawing them in. This demonstration must be performed with caution.) The principle of physics involved is called the conservation of angular momentum, and can be derived from Newton’s laws of motion.
But as the gas and dust and occasional condensations that make up the cloud spin faster around their common axis of rotation, they experience an increasing reluctance to continue falling inward, sometimes called centrifugal force (centrifugal meaning fleeing the center). A pail of water on a rope whirled sufficiently fast around your head does not spill—at least not until you stop whirling. The centrifugal force balances gravity.
There is a category of amusement park ride in which a hollow cylinder is made to spin rapidly, with a crowd of laughing, screaming people—poised somewhere between terror and delight—glued by centrifugal force to the rotating interior surface. When the cylinder stops spinning, the people come tumbling down off the walls.
The contracting cloud also will experience centrifugal force, which will slow it down and eventually stop the contraction—but only in the plane of rotation. If you are standing on a small lump of matter falling toward the center of the cloud but along the axis of rotation, rather than in the equatorial plane, you do not feel any centrifugal force. The result is that matter in the equatorial plane stops collapsing, while matter along the axis continues to fall in. As a result, an initially irregular cloud in time becomes a flattened disk. The further the disk collapses, the more rapidly it rotates, and the denser it becomes at the very center. The collapse stops, or at least slows, when the disk is spinning so fast that matter spews off at the periphery.
The Kant-Laplace hypothesis proposes that, long ago, an irregular, rotating interstellar cloud collapsed in this manner, with the central condensation forming the Sun. There is no doubt today that interstellar matter compressed to the density and temperature of the Sun will initiate thermonuclear reactions and begin to shine like a star. But it was a daring hypothesis for the eighteenth century. Other, smaller nearby condensations, Kant and Laplace proposed, formed the planets, each sweeping out a wide swath of adjacent debris as it grew in size. The result would be a regular spacing of the newly formed planets, something like the layout of the solar system today. Still smaller condensations near the planets would form their moons. The general idea behind the Kant-Laplace hypothesis is more important than the precise details: The solar system, they proposed, evolved—from a very different primordial state and with no outside intervention, natural or supernatural.
The hypothesis of Kant and Laplace is seen to be one of the happiest ideas in science, which at first astounds us, and then connects us in all directions with other discoveries …
—“ON THE ORIGIN OF THE PLANETARY SYSTEM,” LECTURE DELIVERED BY
H. HELMHOLTZ IN HEIDELBERG AND COLOGNE, 1871, PUBLISHED IN POPULAR
LECTURES ON SCIENTIFIC SUBJECTS BY H. HELMHOLTZ, NEW YORK, 1881
Because the word nebula means cloud, and out of analogy with the spiral nebulae (which are, of course, of much larger, galactic dimensions), the contracting cloud that formed the Sun and the planets is traditionally called the solar nebula. Today we know a much larger variety of flat rotating clouds around the nearer stars. They are called accretion disks.
Laplace suggested that during the formation of the solar system, the Sun’s atmosphere once extended far out into space, perhaps in consequence of an enormous explosion in the Sun, like Tycho’s supernova of 1572, produced by a star in the constellation Cassiopeia. Or perhaps it was the residuum of the original solar nebula. Laplace’s interstellar comets were, he imagined, falling in toward the Sun. The material in the solar nebula slowed comets down in the inner solar system, altered their orbits and induced them to impact the Sun. The drag of the solar nebula cleaned the inner solar system of comets with nearly circular orbits, but left the comets at much greater distances unaffected. Through gravitational perturbations by the jovian planets, an occasional comet is induced to visit the inner solar system. The idea is remarkable in several respects. It indicates a kind of natural selection in the physical world well before Darwin; it proposes that there were once many more objects in the solar system than there are now; and it hypothesizes a large repository of comets beyond the most distant planet known.
Why then were the planets not similarly disturbed, and induced to collide with the Sun? Laplace proposed that the planets had formed by successive condensations in the early solar nebula. A tube of nearly empty space, centered around the orbit of each new planet, was formed as the planet, growing at the expense of adjacent material, swept its surroundings clean of nebular debris. Perhaps he toyed with the idea that many dark breaks should exist in the rings of Saturn if there are moons among the rings. However, he urged caution in accepting his hypothesis which he offered “with that distrust which every thing ought to inspire that is not the result of observation or calculation.” Probably because of his flirtation with a possible interstellar origin of comets, it seems not to have occurred to him that comets as well as planets might condense out of the solar nebula.
That the rotation and revolution of the satellites are in the same direction as the rotation of their planets; that the planets rotate in the same sense that they revolve; and that the orbits of the planets are close to circular, while the com
ets have highly eccentric orbits, all followed naturally if everything (including or excluding the comets) had condensed out of the same rotating and collapsing cloud.
For both Kant and Laplace, the nebular hypothesis explained the regularities of the solar system as the end result of an evolution of worlds. Both believed that other stars were surrounded by planetary systems evolved from their own accretion disks. In the last few years, groundbased and spaceborne observations have confirmed that many nearby stars are surrounded by accretion disks. The initial discovery was made by a space observatory called IRAS, the Infrared Astronomy Satellite, a joint Anglo-Dutch-American endeavor. Vega is one of the brightest stars in the night sky, only twenty-six light-years distant, and it was a real surprise to discover that this well-studied star is surrounded by a previously unsuspected disk of debris. It showed up as an extended source of infrared radiation centered on Vega. Now, Vega is a star considerably younger than the Sun. To find an accretion disk around Vega strongly suggests that most, perhaps even all, ordinary stats are surrounded by such a disk during and immediately after their time of formation. Something eventually tidies up the disk—perhaps a combination of radiation pressure, the stellar wind, and planetary formation. But it takes time. And in that time, additional bodies may be condensing out of the nebula.
What may be another solar system in the late stages of formation is shown in this picture of the star Beta Pictoris, masked at the telescope (and at the center of the picture) in order to bring out the fainter disk of debris (the diagonal feature). This faint disk, seen nearly edge-on, may be no more than a few hundred million years old. The picture was taken by Bradford A. Smith (University of Arizona) and Richard J. Terrile (jet Propulsion Laboratory) at the 2.5-meter telescope of the Las Campanas Observatory, Chile. Courtesy Bradford A. Smith and Richard J. Terrile.
IRAS also provided infrared evidence of an accretion disk around a star called Beta Pictoris, among many others. Soon after, Bradford Smith of the University of Arizona and Richard Terrile of the Jet Propulsion Laboratory attached a special highly sensitive camera, developed for a forthcoming space observatory, to a groundbased telescope, and were able to photograph the Beta Pictoris accretion disk in ordinary visible light. The disk extends at least 400 Astronomical Units from the central star (here blocked out, so its radiation will not overwhelm and wash out the much more feeble light reflected off the disk). If this were a picture of the Sun in its early history, the accretion disk would extend much farther from the Sun than does the orbit of the farthest known planet (some 30 to 40 A.U. out). Smith and Terrile deduce a relative absence of debris in the interior of the disk, and suggest that this region has already been swept up by the condensation of planets—that are, however, much too small to be seen directly. Many other accretion disks around adolescent stars have been sighted recently. Accretion disks have also been found around infant stars formed only a million years ago.
Thus, it now seems that the Kant-Laplace hypothesis is in its fundamentals verified, and by a technology that would have delighted both of them. The Sun, the planets, and their moons all condensed out of the same rotating and collapsing disk of gas and dust. This is why all the planets revolve in the same plane in which the Sun rotates. Newton’s view that the regularity of the planetary motions is direct evidence of divine intervention has been superseded by another more evolutionary view—still determined, to be sure, by the laws of nature, which, if we wish, we may still attribute to a god or gods. But when queried—by Napoleon, of all people—about why his account of the origin and history of the solar system made no mention of God whatever, Laplace replied, “Sire, I have no need of that hypothesis.”
Let us now follow a modern rendition of the Kant-Laplace hypothesis, in which we pay special attention to the origin and evolution of the comets. From direct spectroscopic evidence, we know the interstellar gas to be composed mainly of hydrogen and helium, although it is rich in many other materials, including complex organic molecules. Besides the gas, the other chief constituent of interstellar space is an enormous number of motes of dust. One of them, placed on a table before you, would be entirely invisible. They are, typically, a thousandth of a millimeter across. But concentrate enormous numbers of them over hundreds or thousands of light-years, and you can have enough dust to blot out the stars behind them. The chemistry of the grains can also be inferred. Most seem to be made of ices, silicates, and organics—very roughly in equal proportions. Since this mix of gas and grains makes up the interstellar clouds everywhere in the Milky Way, it must also have constituted the early collapsing solar nebula. Since interstellar space ordinarily holds much more gas than grains, this should have been true for the solar nebula as well.
As the nebula contracts and its density increases, collisions of grains with one another become more frequent. In part because of the organic and icy content of these grains, when they collide they tend to stick. Big grains annex smaller ones. But all this does not go on in the dark. The primitive Sun has begun brightly shining. In the outer parts of the disk, it is still sufficiently cold that exotic ices such as methane or carbon monoxide are perfectly stable in the growing condensations of matter. But in the very inner solar system, it is too hot even for water ice. There the ices on the grains evaporate and dissipate, and what survives is made mainly of silicates. You have to carry a rock very close to the Sun, only a few million kilometers away, for it to boil. As a result of all this, the chemistry in the inner solar system must have been very different from the chemistry in the outer solar system—silicates predominating inside and ices and organics outside.
According to several calculations, a vast number of kilometer-sized objects should have accumulated throughout the nebula—silicate-rich ones on the inside, ice-rich ones on the outside. These objects may have been generated, not primarily through grain-by-grain collisions, but by a fundamental gravitational instability in the solar nebula, in which objects a few kilometers in size were quickly and preferentially formed.
Both dust and gas gravitationally collapsed to form the disk. But it takes a great deal of gravity to hold on to so lightweight and thus fast-moving a molecule as hydrogen. In the middle part of the nebula, the kilometer-sized lumps collided and grew into still larger objects, until a few aggregations of matter were able to retain the cold gas around them. This was the evolutionary line to the jovian planets. The original accretion core is smothered in a vast sphere of gas. In the warmer inner solar system the grains, divested of their ice, grew more slowly, and the temperatures were higher—both effects making it more difficult for the gas to be captured by the growing rocky spheres. This was the evolutionary line to the terrestrial planets.
Schematic illustration of condensation in the early solar nebula. We are looking down on the nebula, its bright hot interior shown at left. The farther from the forming Sun, the cooler it is. Three different materials are shown: methane (CH4); water (H2O); and silicates (SiO2). The cubes indicate solids, the clouds vapors. In the outermost portions of the solar nebula, methane condensed as a solid; farther in, it was present only as a gas. Water is solid as ice well into the interior of the solar system, and silicates survived as solids almost to the surface of the Sun. Thus rocky planets were formed in the interior of the solar system and icy bodies farther out. Diagram by Jon Lomberg/BPS.
Big objects would sweep up smaller ones on adjacent orbits. Because the relative velocities were low, the two bodies would tend to collide softly and merge. Eventually, a few large objects were produced, in orbits that never intersect. These became the planets. There is a kind of collisional natural selection at work here. You start out with a large number of growing objects in chaotic orbits, but through a process of collision and growth and only occasionally the shattering of worlds, the solar system becomes regularized, simplified. The number of worlds steadily declines, from trillions to thousands to dozens. If you look at the planets today you find them decorously spaced, their orbits by and large almost perfectly circular; except for
the case of Pluto,* planets give each other wide berth. Those early bodies on highly eccentric orbits were in danger; very soon they would collide with a world or be ejected from the solar system. Eventually, the only planets left were those that had by chance developed on orbits that quarantined them from their neighbors. It is just as well for us that they did; frequent world-shattering collisions are probably not good for the development of life.
The planets so formed would be orbiting the Sun in the sorts of orbits we recognize for the planets today. While no one has been able to prove that exactly nine planets should form—and not, say, six or forty-three, the entire question of the ultimate number of planets being a matter of collision statistics—the general picture is very successful, and explains not only the orbits, but also the overall chemical differences between the terrestrial and the jovian planets that we observe today.
If you picture the collapsing disk of gas and dust flattening and spinning faster, the grain collisions generating larger and larger objects, the eventual formation of kilometer-sized objects that collide and grow still further, a question may occur to you: What happened to all those kilometer-sized objects? Are there any left? Were they all swept up as the growing planets ran into them, or might some of them still exist somewhere, unchanged from the epoch in which the solar system was formed?