planet formation

Introduction The origin of the planets in our Solar System has been debated since 1755, when Immanuel Kant proposed a theory for planet formation. Whether the origin of our Solar System's planetary system was an unusual event or a common consequence of star formation has been at the heart of the debate. During the 1900s, the theory that our planetary system is not unique became commonly accepted among scientists, and a recent discovery of a planetary system around a star other than the Sun supports this theory. The following text discusses our knowledge of planet formation in the past, our present understanding of the subject, and the discoveries we may make in the future.

The Nebular Hypothesis Planet formation was first discussed by Immanuel Kant in 1755. Kant proposed that a nebulae, which is a huge cloud of dust and gas, was pulled together by gravity so that it collapsed into a flat, rotating disk. The disk eventually coalesced into the Sun and planets. Kant also stated that because a similar process occurs around other stars, our Solar System is not alone in the universe.  Pierre Laplace expanded on Kant's theory in 1796 when it became known as the nebular hypothesis. Laplace proposed that the planets were formed by rings of matter split off a rotating nebulae by centrifugal force. After the matter split off, it coalesced into a planet. The process repeated itself, resulting in a planet each time. The matter left over was the Sun.

Problems With the Nebular Hypothesis The nebular hypothesis used a flat, rotating nebulae as the Solar System's origin to explain why all the planets orbit in nearly the same plane and in the same direction. The theory, however, contained a few problems. It contradicted the observation that the Sun contains most of the Solar System's mass but only a small fraction of its angular momentum. If the theory were correct, the Sun must have most of the Solar System's angular momentum. This is because the Sun's angular momentum would increase as it contracted, much like a spinning ice skater who rotates faster as he brings in his arms.  Another problem with the nebular hypothesis was that if rings of matter were split off, as Laplace stated, they wouldn't be pulled together to form planets but would disperse into space.

The Protoplanet Hypothesis After faults were found with the nebular hypothesis, other explanations of planet formation were sought. After many failures, such as the encounter theory, astronomers returned to the nebular hypothesis to find improvements during the mid 1900s.  A modern version of the nebular hypothesis, called the protoplanet hypothesis, was formed independently by Carl von Weizsacker and Gerard Kuiper. The steps in planet formation theorized by the protoplanet hypothesis are shown in the diagram on the left. (A) The solar system begins to form as a rotating cloud, or nebulae, collapses. (B) Instabilities in the nebulae cause dust particles to stick together. The dust particles accrete into billions of planetesimals with diameters of about 10 meters. The planetesimals then collide and form protoplanets. Meanwhile, the protosun in the center of the nebular disk becomes massive and hot enough to "turn on" by fusing hydrogen. (C) The Sun begins to radiate energy and vaporize dust in the inner part of the Solar System. The remaining gas is blown away by solar winds. Despite the protoplanet theory's success in correcting problems with the nebular hypothesis, it did not provide an explanation for the distribution of angular momentum in the Solar System. To explain the transfer of angular momentum from the Sun to the planets, scientists proposed a braking action caused by the Sun's magnetic forces. The magnetic lines of force from the Sun transferred angular momentum from the spinning Sun to the planetary disk. With the addition of this proposal, the protoplanet hypothesis became free of any known faults.

The Future of Our Understanding of Planet Formation Our knowledge on planetary formation is still very incomplete. In fact, it was just April of 1999 when astronomers announced the first discovery of a multi-planet system other than our own. This discovery, which Kant predicted over two hundred years ago, shows that our planetary system is not as unique as it was once thought. Scientists continue to make discoveries that will increase our knowledge on the formation of our Solar System and shape the theories that explain planet formation throughout the universe.