Formation of the Solar System

Where did the solar system come from? This question has tantalized astronomers for centuries. While we do not yet have a wholly complete answer, a consensus has arisen about the most likely series of events that led to the present-day system of the Sun and planets.

A key piece of evidence about the origin of the solar system is that all the planets orbit the Sun in the same direction and in nearly the same plane. As long ago as the eighteenth century, German philosopher Immanuel Kant and French scientist Pierre-Simon de Laplace independently suggested that this state of affairs could not be a coincidence. They proposed that our entire solar system-the Sun as well as all of the planets, satellites, asteroids, and comets—formed from a vast, rotating cloud of gas and dust called the solar nebula. ln the modern version of their theory, the solar nebula is thought to have had a mass somewhat greater than that of our present-day Sun.

Each part of the nebula exerted a gravitational attraction on the other parts, and these mutual gravitational pulls tended to make the nebula contract. As it contracted, the greatest concentration of matter occurred at the center of the nebula, forming a relatively dense region called the protosun, which eventually developed into the Sun. The planets formed from the much lesser amount of material in the outer regions of the solar nebula. Indeed, the mass of all the planets together is only 0.1 percent of the mass of the Sun.

When you drop a ball, the gravitational attraction of Earth makes the ball travel faster and faster as it falls; in the same way, material falling inward toward the protosun would have gained speed. As this fast-moving material ran into the protosun, the energy of the collision was converted into thermal energy, causing the temperature deep inside the solar nebula to climb. While the protosun's surface temperature stayed roughly constant, the temperature inside the protosun increased even more by means of further contraction. Eventually, after perhaps 100 million years had passed since the solar nebula first began to contract, the central temperature of the protosun reached a few million degrees, and nuclear reactions began in its interior. When this happened, the contraction stopped and a true star(the Sun) was born. Nuclear reactions in the interior of the present-day Sun are the source of all the energy that the Sun radiates into space.

lf the solar nebula had not been rotating at all, everything would have fallen directly into the protosun, leaving nothing behind to form the planets. Instead, the solar nebula must have had an overall slight rotation, which caused its evolution to follow a different path. As the slowly rotating nebula collapsed inward, it would naturally have tended to rotate faster. Figure skaters use this same phenomenon; when a spinning figure skater pulls her arms and legs inward, close to her body, the rate at which she spins automatically increases. As the solar nebula began to rotate more rapidly, it also tended to flatten out, just as a spinning ball of dough flattens out when it is spun rapidly by a pizza chef. The eventual result was a structure with a rotating flattened disk surrounding a newly formed Sun. The planets formed later from this disk, which explains why their orbits all lie in essentially the same plane and why they all orbit the Sun in the same direction.

Naturally, there were no humans to observe these processes taking place during the formation of our solar system. But astronomers have seen disks of material surrounding other stars that formed only recently. These are called protoplanetary disks, or proplyds, because it is thought that planets can eventually form from these disks around other stars. By studying these proplyds, astronomers are able to examine what our solar nebula may have been like some 5 billion years ago.