Whence the Planets?

Ever since man perceived that the solar system is a cohort of planets revolving around a central sun, at different distances but in the same direction and in almost the same plane, he has wondered how it all started. Pierre Laplace, French mathematician (1749-1827), devised the celebrated "nebular hypothesis": that the solar system was originally a diffuse, whirling, gaseous mass. As this nebular mass became smaller and denser, it whirled faster, until centrifugal force threw off a ring of gas. The process was repeated, each gas ring coalescing into a planet and the sun finally settling down to its present calm estate.

Later astronomers abandoned the nebular hypothesis. They found two things wrong with it: 1) it was unlikely that the rings would gather into planets; 2) the "angular momentum" was wrong. Angular momentum can be roughly defined as the amount of rotation in a system. In the solar system the planets have about 98% of the angular momentum, but only one-seventh of 1% of the solar system's mass, the rest being in the sun. The Laplace theory could not account for this discrepancy. It seemed more likely that there had been interference from outside.

The need for some outside disturbance gave rise to the "encounter theory"--that the planets were born when another star happened to pass close by the sun. The disturbance caused the sun to throw off jets of matter, "planetesimals," which later collected in large masses to make the planets. Sir James Jeans pictured the passing star as raising a huge tide on the sun, eventually pulling out a long filament of sun-stuff which broke up into major pieces. British Harold Jeffreys favored an actual collision of sun and star, the two bodies drawing out a ribbon of their commingled substance as they veered away from each other. Other theorists supposed that the sun had originally been a double star, that an intruding third star had carried away the sun's companion but had left behind enough debris for planet-building.

In spite of this lack of agreement on forms of the encounter theory, modern astronomers have favored the theory in general. But last week it looked as though the encounter theory was about ready for the scrap heap, along with the nebular hypothesis of Laplace.

The surface temperature of the sun is about 5,400DEG C. Matter gouged out of the sun by a passing star would be much hotter than that--1,000,000DEG C. or more.

At such temperatures hydrogen atoms are whipped up to speeds around 100 miles per second. A hot gob of sun-matter would thus tend to expand and dissipate into a formless cloud. Hopeful theory was that this tendency would be counteracted by rapid cooling in open space.

Lately Dr. Lyman Spitzer Jr., a National Research Fellow at Harvard, mathematically analyzed the conditions which would exist in the gob of hot sun-stuff. He showed that, even under the most favorable conditions, expansion would take place more than 100 times as fast as cooling--therefore that a planet would never have a chance to form. In the Scientific American last week his work was explained by Princeton's grey, gentle Henry Norris Russell, a great authority on the solar system, under the gloomy title, "A Famous Theory Weakens."

A few astronomers now favor the notion that the planets were born of explosion--that the sun once flared up as a nova or "new star," scores of which have been spotted in the far sky. Novae throw off shells of gas, potential planetary material. But Dr. Russell declared that even this idea offered no escape from the Spitzer mathematics. The shells of gas would still be hot, they would still expand too fast to permit planetary formation.

Last week, in short, science might have been glad to write off the solar system as an optical illusion.

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