Although Uranus is also sometimes visible without a telescope, ancient astronomers were unable to distinguish it from the true stars.
The planets may be divided into groups in several ways. In one scheme Mercury and Venus, the planets that revolve around the sun in orbits smaller in diameter than that of the Earth, are classified as inferior planets. The so-called superior planets are those that revolve around the sun in orbits larger in diameter than the Earth's orbit.
The planets may also be classified into two groups according to their gross physical characteristics. The terrestrial, or Earth-like, planets are close to the sun and are composed primarily of rock and metal. They include Mercury, Venus, Earth, and Mars. The terrestrial planets are also called the inner planets.
The Jovian, or Jupiter-like, planets are very large compared to the terrestrial planets and are much farther from the sun. They are also called the outer planets. They include Jupiter, Saturn, Uranus, and Neptune. These planets are composed mostly of hydrogen and helium in gaseous and liquid form. Pluto, the outermost planet, is usually considered neither a terrestrial nor a Jovian planet. It is composed of ice and rock and is much smaller than the other planets.
Formation and Evolution
Although the origin of the solar system is uncertain, most scientists believe that it began to develop about 4 1/2 billion years ago from a large cloud of gas and dust. The cloud began to contract. As the material within the cloud became compressed, it grew hot. Most of this mass was drawn toward the center of the cloud, eventually forming the sun. The remaining material, less than 1 percent of the original, formed a spinning disk, called the solar nebula, around the center. The planets and satellites evolved from the nebula as it cooled. (See also Solar System, "Past and Future of the Solar System.")
Close to the center, the material in the disk condensed into small particles of rock and metal that collided and stuck together, gradually growing into larger bodies called planetesimals. As they traveled around the center, the largest planetesimals swept up smaller material in their paths, a process known as accretion. Eventually these accreting bodies evolved into the terrestrial planets. The numerous impact craters still evident on the oldest surfaces of some planets are believed to have been created during this phase, when the nascent planets collided with other bodies.
Farther from the center of the disk, cooler temperatures allowed not only rock and metal but also ice and gas to develop. These materials formed small eddies in the spinning disk that evolved into the Jovian planets. Each young planet had its own, relatively cool nebula from which its satellites formed.
As the planets and satellites 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 gradually forming cores. In the case of the terrestrial planets, mantles of rock formed around metal-rich cores and were covered by thin surface crusts. 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 and satellites had a third source of internal heat: the decay of certain radioactive elements within the bodies (see Radioactivity). Since their formation, many of the physical characteristics of the planets 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 shapes the crusts of the terrestrial planets. (See also Continent; Geology; Plate Tectonics; Volcano.)
These bodies have solid surfaces that have preserved a record of their geologic histories. In smaller bodies such as Earth's moon, Mercury, Mars, and the 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 body's internal heat has dissipated, the surface features stabilize and remain largely undisturbed as the body ages. Larger bodies, like the Earth and Venus, lose their heat more slowly. In fact, they are still subject to the forces of volcanism and tectonism. The landforms of the terrestrial bodies that lack atmospheres have been shaped primarily by these volcanic and tectonic activities, combined with cratering caused by impacts that occurred during the solar system's formation. The same is true for those terrestrial bodies that have atmospheres, but their landforms have been modified by the action of wind and, in some cases, water.
The evolution of the Jovian planets cannot be reconstructed by analyzing their surface features--they have no solid surfaces. These planets are so large that much of their internal heat is still being released.
Mercury, the planet nearest the sun, is difficult to observe from the Earth because it rises and sets within two hours of the sun. Consequently, little was known about the planet until the Mariner 10 spacecraft made several flybys in 1974 and 1975.
Mercury's surface has several different types of terrain. Planetary scientists can estimate the age of a surface by the number of impact craters on it; in general, the older the surface, the more craters it has. Some regions on Mercury are heavily cratered, suggesting that they are very old surfaces that were probably formed about 4 billion years ago. Between these regions are areas of gently rolling plains that may have been smoothed by volcanic lava flows or by accumulated deposits of fine material ejected from impacts. These plains are also old enough to have accumulated a large number of impact craters. Elsewhere on the planet are smooth, flat plains with few craters.