No one really knows for sure how the Solar System began. It would be like ask­ing a child to give an account of his birth or a descrip­tion of his conception. Religious scriptures explain the creation of the Earth in compelling ways, but no two accounts agree exactly. Some of them, however, do come quite close to the scientist's idea of creation-or, at least, to the readings of the evidence lodged in the Earth's ancient rocks.

In exploring the origin of the Earth we must at the same time try to explain the beginning of the Solar Sys­tem, for the Earth's past is intimately tied to the history of our nearest neighbors in space.

In 1755 the German philosopher Immanuel Kant published his theory of the heavens, postulating that in the beginning there was an immense, cold whirling cloud of dust and gas. This suggestion is accepted readily by astronomers today. Their extremely powerful modern telescopes show re­mote, dark clouds of dust floating between distant stars -clouds that must even now be similar to the local, swirling cloud that Kant had in mind.

In 1796 Kant's contemporary, the French mathemati­cian Pierre Simon Laplace, took his idea a step further by suggesting how the Solar System might have formed from such a cloud.

The immense mass was set spinning by cosmic forces, Laplace hypothesized. At the same time it began to shrink in size under the gravitational pull of its own matter. At intervals, the contracting cloud shed veils of particles into space, which eventualy condensed into the planets. Shrinking under the force of its own gravity, meanwhile, the central mass became the Sun.

As potent as Laplace's concept was, it fell victim to fundamental physical laws of more recent discovery. Calculations based on these laws show that a shrinking Sun would spin faster and faster as it grew smaller and smaller, until today it would be rotating at a far greater speed than it actually is.

After Laplace's brilliantly imaginative picture was shown to contain flaws, several other seemingly plausi­ble suggestions were put forward by astronomers. One theory assumed the formation of the Sun first, with no planets. Then, a second star passing close by in space tore out a long stream of material. The planets, it was suggested, might then have condensed around the Sun, with the passing star continuing on its way. Unfortu­nately, calculations show that such hot material from the Sun would disperse, rather than form planets. Even if by some unknown process planets were to condense, their orbits would be much more irregular than those found in the Solar System today.

Another theory held that in the distant past of the cosmos, or universe, the Sun had a twin companion, and a passing star collided with its twin. Out of the debris resulting from such a collision, planets might possibly form in orbits around the single remaining sun. But the great distances at which the stars are scattered in space make collisions of this type most unlikely. If such a catastrophe did occur, it seems impossible that planets could form directly from the intensely hot and volatile material of the exploding stars. Both the "close encoun­ter" theory and the "collision" theory fail on one fur­ther count; neither explains how most of the planets have obtained moons.

More recently, cosmologists went back to the sugges­tion of Kant, careful to avoid the pitfall of Laplace. A theory took shape from the combined efforts of astronomers, mathematicians, chemists and geolo­gists. This hypothesis is called the "nebular" or "proto-planet" theory. It gives unity to so many seemingly disparate details of material reality that a majority of cosmologists have become convinced that it cor­rectly accounts for at least the broad features of cosmic evolution.

Protoplanetary diskHarking back to Kant and Laplace, the proto-planet theory assumes that a large cloud of gas once filled the region of space where the Solar System now exists. This gas consisted of the "cosmic mix"-a mixture of gaseous molecules found everywhere in the universe. In every 1000 atoms, 900 are' hydrogen, 97 helium, with the re­maining three atoms being heavier elements, such as carbon, oxygen and iron. Slowly the primordial cloud began to turn. Its rotation probably did not develop smoothly; from radio-telescopic observation of similar gaseous clouds in distant space, astronomers be­lieve that turbulence must have developed. Indeed, the swirling cloud must have looked something like a whirl­pool-with small local eddies forming and re-forming as the entire volume turned in space. A large eddy at the center, contracting more rapidly than the rest of the cloud, formed a dark, denser object, the "proto-Sun."

In the cold depths of the cloud surrounding the proto-Sun, certain atoms of gas combined to form com­pounds, such as water and ammonia. Slowly, solid dust crystals began to grow as did metallic crystals, includ­ing iron and stony silicates. And, gradually, gravita­tional and centrifugal forces at work in the spinning cloud flattened it into the shape of an enormous protoplanetary disc.

If we could have viewed the events at a great distance, our eyes would have beheld something like a gigantic, re­volving vinyl record, with the proto-Sun in the hole at the center.

Within the huge whirling disk, local eddies continued to appear. Some of the swirls were doubtless torn apart in collisions, while others were broken up by the in­creasingly strong gravitational pull of the proto-Sun. In a sense, each small eddy was carrying on a fight for survival. To hold itself together in the face of such dis­ruptive forces, an eddy had somehow to collect a cer­tain critical amount of substance to provide its own center of gravity.

In a kind of cosmic battle within the wheeling system, some local swirls gained material as others lost it. Ultimately a series of large whirling disks developed in the region around the Sun. Each was a proto-planet.

These proto-planets were sufficiently large to hold together under the strength of their own gravitational fields. As each moved through space around the Sun, it acted as a sort of scavenger, sweeping up leftover mate­rial from the original cloud.

At this stage thermonuclear fusion began in the core of the proto-Sun releasing large amounts of energy, and the proto-Sun began to shine. It "burned" fitfully at first, a dull red. In time it was to become the golden yellow star that we see today. Remember that the proto-Sun was about one hundred times larger in diam­eter than the proto-planets. It was this immense differ­ence in size, of course, that caused it to become a star rather than a planet. Its strong gravitational pull was sufficient to trap light hydrogen atoms in its interior, triggering thermonuclear fusion. Such was not the case with the smaller proto-planets.

Somewhere in the region of the proto-Sun, then, proto-Earth was born as a whirling cloud of icy parti­cles and solid fragments-a cosmic dust storm. Only later did this material collect into a ball, sticking to­gether because of the cohesive attraction of water and ice molecules. As proto-Earth orbited around the Sun, it swept up more material by gravitational attraction. Thus the Earth and the other planets formed by the process of accumulation of cold dusts from the region of space near the Sun.

Gradually radioactive elements within the cold ball of dust that was Earth began to give off heat. After mil­lions of years the Earth's temperature became high enough to melt the material at its center. At that time, the heavy metals-iron and nickel-that were spread throughout the ball began to sink to form the molten core of the planet. Afterward, molten rock frequently broke through fissures to the surface. And slowly, molecules of hydrogen, water vapor and other gases escaped from within to create an atmosphere above the planet's surface. But these light gases did not stay with the Earth for long. A second major source of heat was already in action-the rays of the Sun.

The Sun's radiation was now striking the Earth with full intensity, breaking up the molecular compounds in its primitive atmosphere and scattering them into space. Thus most of the atmospheric hydrogen and other light elements escaped from the Earth. This process eventu­ally left behind a high concentration of the heavier, rarer elements of the universe-elements essential for the formation of rocks, plants and our own bodies.

Be­cause of the escape into space over billions of years of such light atoms as hydrogen, the Earth now contains about one thousand times less mass than was present in proto-Earth when it condensed from the dust cloud. The origin of the Moon remains an enigma to scien­tists. Did it form at the edge of proto-Earth? Or did it form elsewhere in space as a separate planet that was later captured by the Earth's gravitational field? Or another theory is that the Moon was the result of a massive asteroid impact with the Earth. Cosmologists favor these last two possibilities rather than the older theory that the Moon was ripped out of that part of the Earth that is now the Pacific Ocean basin. And with the advent of manned exploration of the Moon like to be restarted very soon, it seems likely that the scientific enigma of the Moon will one day be solved.

The story of the Earth has almost reached the point where it can be taken up by a geologist. After the Earth stopped collecting debris from its path in space, its sur­face gradually cooled and became solid. A crust of rock formed; land masses appeared. But the Earth was not yet ready to support life as we know it today; its sur­face was still too hot for living organisms and the atmosphere was heavy with poisonous methane and ammonia. Molten lava flowed from fissures in the crust, allowing the escape of steam that had been trapped in the Earth's molten interior. In fact, many geologists think that this early volcanic activity brought to the surface most of the water that forms the present-day oceans-water originally trapped in icy dust.

As volcanic activity decreased on the Earth, intense ultraviolet radiation from the Sun broke up a portion of the atmospheric water molecules into separate atoms of hydrogen and oxygen. The Earth's gravitational pull wasn't strong enough to retain the lighter hydrogen atoms, and most of them drifted off into space. The heavier oxygen atoms would have remained. Although some free oxygen was thus liberated in the Earth's evolv­ing atmosphere, the gases methane and ammonia must have remained preponderant for a long time, since most of the free oxygen in today's atmosphere is known to exist as the byproduct of photosynthesis in plants, in­cluding the algae of lakes and oceans.

Year by year the Earth became cooler as it radiated heat and proto-Sun faded to the intensity of brightness we know now.

Soon the Earth's atmosphere had cooled enough to cause water vapor in the air to condense and fall back to the surface as rain. At first, the raindrops spattering on the hot surface boiled back in a hiss of steam. Eventually, though, the Earth cooled sufficiently to permit pools of water to collect over the surface. Soon the cooling atmosphere must have begun to yield tremendous amounts of rain.

All the water in the seven seas may have descended in one long continuous deluge. Gradually the shallow areas in the wrinkled crust filled, and oceans appeared on the face of the Earth.

Although scientists are generally convinced that the Earth on which we live has passed through the stages of development outlined in the previous paragraphs, no one, of course, can vouch for the exact chronology. Probably, proto-Earth reached its present size and shape some four and a half billion years ago.

After this, one and a half billion years may have passed before conditions on the Earth became hospitable to early forms of life.