Our solar system began to develop about 4.6 billion years ago from a large cloud of gas and dust called the solar nebula. The gravity of the cloud began pulling the clouds matter inward. As the cloud contracted, it began spinning faster and faster and it flattened into a disk. As the material within the cloud compressed, it grew hot. This caused the dust in the cloud to become gaseous. Most of the clouds mass was drawn toward the center, eventually forming the Sun. The planets developed from the remaining material—the disk of gas spinning around the forming Sun—as it cooled. This explains why the planets orbit the Sun in nearly the same plane and in the same direction. Gases in the cooling disk condensed into solid particles, which began colliding with each other and sticking together. Larger objects began to form. As they traveled around the disk, they swept up smaller material in their paths, a process known as accretion. The larger gravity of the more massive objects also allowed them to attract more matter. Over time, much of the matter clumped together into larger bodies called planetesimals. Ultimately, they formed larger protoplanets, which developed into the planets. The inner and outer planets developed so differently because temperatures were much hotter in the regions near the developing Sun. Close to the center of the solar nebula, the material in the disk condensed into small particles of rock and metal. These particles eventually clumped together into the planetesimals that formed the rocky, dense inner planets. Farther from the developing Sun, the cooler temperatures allowed not only rock and metal to develop but also gas and the ices of such abundant substances as water, carbon dioxide, and ammonia. The availability of these ices to the forming outer planets allowed them to become much larger than the inner planets. Eventually, the outer planets grew massive enough for their gravity to be able to attract and retain even the lightest elements, hydrogen and helium. These are the most abundant elements in the universe, so the planets were able to grow enormous. They also developed compositions fairly similar to that of the Sun. Each young outer planet had its own relatively cool nebula from which its regular satellites formed. The irregular satellites are generally thought to be asteroids or other objects that were captured by the planets strong gravity. Ancient craters mark the surface of the far side of Earths Moon, shown in an image taken by the … F.J. Doyle/National Space Science Data Center Collisions between the forming planets and large planetesimals probably had dramatic effects. The numerous impact craters on the oldest surfaces of some inner planets and some moons are believed to have been created from such collisions. Astronomers think that Earths Moon originally may have formed from material scattered by a violent collision of Earth and a protoplanet about the size of Mars. This material may have settled into orbit around Earth and accreted to form the Moon. A protoplanet also may have slammed into the developing Mercury and stripped away much of its outer rocky mantle. This would explain why Mercurys core takes up such a large percentage of the planets interior. Other protoplanets may have crashed into Venus, greatly slowing its rotation, and Uranus, knocking the planet nearly on its side. As the planets accreted, their interiors grew hot and melted. In a process known as differentiation, heavier materials sank to the centers, generating more heat in the process and, in many planets, gradually forming cores. In the inner planets, the sinking of the heavier materials displaced lighter rocky materials upward, forming mantles of rock. The most buoyant materials rose to the top and solidified into surface crust. Lighter elements escaped from the interiors and formed atmospheres and, on Earth, oceans. In addition to the heat generated by accretion and differentiation, the planets had a third source of internal heat: the decay of certain radioactive elements in their interiors. Since the planets formation, many of their physical characteristics have been determined by the manner in which the bodies generated and lost their internal heat. For example, the release of internal heat accounts for the volcanic and tectonic activity that has shaped the crusts of the inner planets. In smaller bodies such as Mercury, Earths Moon, and many satellites of the outer planets, the internal heat escapes to the surface relatively quickly. As a result, the surface initially undergoes rapid, violent changes. Then, when most of the bodys internal heat has dissipated, the surface features stabilize and remain largely undisturbed as the body ages. Larger bodies such as Earth and Venus lose their heat more slowly. The outer planets are so large that much of their internal heat is still being released. Scientists developed these theories about the formation of the planets based on observations of the solar system. The discovery of planets outside the solar system has challenged some of the details. For example, astronomers have discovered enormous gaseous planets that are closer to their stars than Mercury is to the Sun. This seems to contradict the idea that huge planets can form only in the regions far from the central star. Perhaps these planets initially developed farther away from the star, or perhaps the theories about solar system formation need adjusting in certain ways. The idea that solar systems develop from contracting, spinning clouds of gas and dust, however, is still believed to be correct. Astronomers have observed such disks surrounding several young stars (
Posted on: Mon, 31 Mar 2014 22:18:10 +0000
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