In the Beginning ...

Most modern astronomers agree that the sun and its planets condensed from a single great cloud of gas. But they disagree, sometimes sharply, about the details of how it happened--and none of the theories have ever quite fitted together to give a satisfying solution to one of science's most baffling puzzles. Now, from Professor Hannes Alfven of Stockholm's Royal Institute of Technology, comes a promising explanation. To explain the solar system, Alfven says, other scientists have used plain old hydrodynamics (the behavior of fluids, including gases); if magnetohydrodynamics (the behavior of ionized gases in a magnetic field) is used instead, many things become clear.

Alfven points out in Britain's New Scientist that ordinary hydrodynamics rules only in tiny crannies of the universe, such as the earth's ocean and the lower levels of the earth's atmosphere. The great bulk of the universe, including the stars and most of the matter between them, is made of ionized gases whose atoms have electric charges caused by the effects of heat or radiation. Unlike the earth's familiar water and air, most of whose atoms are electrically neutral, ionized gases are influenced by the magnetic fields that pervade space.

Three Parts. When the sun formed in the heart of a hot and ionized cosmic cloud, Alfven believes, the sun's powerful magnetic field fended off the distant, electrically charged parts of the cloud. Gradually the cloud cooled, and some of its ionized atoms combined with electrons, making the atoms electrically neutral and permitting them to fall toward the sun. After they had fallen a few hundred million miles, they acquired tremendous speed, collided with the thin gas that surrounds the sun, were ionized again by the energy of collision, and then were stopped in their tracks by the sun's magnetic field. Easily ionized chemical elements were stopped well away from the sun; some that were harder to ionize got close to the sun before they were halted.

One fraction of cloud, mostly made up of hard-to-ionize elements, stopped near the orbit of Venus (67 million miles from the sun). As it cooled off, some of its material condensed into dust. The dust grains grew bigger and bigger by attracting each other, and they finally coalesced to form three planets: Mercury, Venus and Earth. Another fraction of the cosmic cloud stopped farther away from the sun, forming Mars and the moon. Since these two zones of planet formation overlapped, the earth was able to capture the moon as its satellite. The big outer planets, Jupiter, Saturn, Uranus and Neptune, were formed from a third fraction of the cosmic cloud, whose chemical composition allowed it to be ionized and stopped at a very great distance from the sun.

Third Planet. Alfven believes that his theory gives nearly every star a retinue of planets. "In fact," he says, "if we consider a star with the same mass as the sun, we should expect that the third planet from it moves at about the same distance as the earth from the sun, and has a constitution which is similar."

A reasonable extension of Alfven's theory: the third planet away from stars like the sun has the conditions most likely to sustain life.

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