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Mercury, May/June 2005 Table of Contents

Courtesy NASA/JPL/Space Science Institute

by Richard H. Durisen

In 1755, Immanuel Kant had the remarkable insight that stars and planetary systems form by the gravitational collapse and rotational flattening of interstellar clouds. Later in the 18th century, Marquis Pierre Simon de Laplace refined Kant’s idea by suggesting that planets condensed from rings produced at the edge of a contracting disk while the Sun formed at the center—a picture thereafter known as the Nebular Hypothesis.

Kant’s physical intuition has stood the test of time and is the basis of all contemporary theories of planet formation. Disks of dust and gas are now routinely observed around young stars in our own galaxy. Formed by gravitational collapse and flattened by orbital motion around their central star, the disks typically have sizes comparable to that of the Solar System or larger.

These flat swirling “nebulae” are ripe for planet formation and are now called “protoplanetary disks.” The Solar System itself probably formed from such a disk. In the Sun’s case, we call that disk the Solar Nebula. Although details of Laplace’s ring-formation dynamics did not survive closer scrutiny in the 19th century by James Clerk Maxwell, Laplace’s ideas may still have had a “ring” of truth about them, as we will see.

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