Birth of the Planets

Generations of scientists have tried to explain how the solar system began. None of the explanations is wholly adequate. The trouble is that the solar system is not a haphazard collection of planets buzzing around the sun. It has remarkable "regularities" which indicate that all its major members had a common and systematic origin.

The planets, for instance, revolve in the same direction and in almost the same plane. The four inner ones (Mercury, Venus, Earth, Mars) are smaller and denser than the outer ones (Jupiter, Saturn, Uranus, Neptune). Perhaps the most remarkable peculiarity is that the outer planets possess almost all the "angular momentum" (energy of rotation) in the system. Any satisfactory theory of the solar system's origin must account for these "regularities."

In this week's Science magazine Dr. D. ter Haar of Purdue University sums up the more recent theories. He rules out all theories based on a collision, or near-collision, between the sun and another star. A string of planets drawn out of the sun in this way could not have so large a part of the system's angular momentum. The recent theory of Harvard's Dr. Fred L. Whipple (that the sun and the planets were formed at the same time out of a cloud of mixed gas and smoke particles) is hardly better, says Dr. ter Haar. It accounts for the distribution of the angular momentum of the planets, but not for all of the other "regularities."

The Mother Disc. Dr. ter Haar prefers his own modification of one of the earliest modern theories, originally proposed by Immanuel Kant in 1755. Kant suggested that the planets condensed gradually from a gaseous envelope surrounding the sun. Later cosmogonists discarded this theory mostly because it did not account for the greater angular momentum of the outer planets. But Dr. ter Haar believes that all Kant needs is a little tinkering.

The solar system, says ter Haar, must once have consisted of the sun, surrounded by a vast atmosphere of gas. The system's revolution shaped this atmosphere into a flattish disc. Near the center the gas was dense enough to be somewhat viscous. Its drag gradually slowed the rotation of the sun while the outer parts of the disc revolved faster. This slowing effect, thinks ter Haar, points toward an explanation of the uneven distribution of the solar system's angular momentum.

The Great Grab. The sharp difference; in density between the inner and outer planets he accounts for in another way. The sun's atmosphere was hottest toward the center and cooler farther out. As the gas gradually condensed into solid particles, only substances of high boiling point (metals, mineral compounds) could condense in the hot inner regions. This accounts for the high density of the inner planets. Only in the cool outer parts of the system could light compounds like ammonia and methane get together in masses. The outer "protoplanets" grew faster; they were formed from more of the compounds in the gas.

The gradual growth of "protoplanets" continued, reasons ter Haar, until the largest of them were about the size of Venus. These had enough gravitational force to gather up quickly nearly all the gases left in the sun's atmosphere. Most of it went to the big outer planets, for the inner ones (like the earth) had grown more slowly and were still too small to do much gravitational grabbing. This explains, says ter Haar, why the outer planets are big and mushy, while the inner ones are small and dense.

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