Numerous bodies orbit the Sun. The eight largest of these are called planets, largely for historical reasons. Earth and five others, Mercury, Venus, Mars, Jupiter, and Saturn, were known in ancient times, and these objects defined the ancient concept of planet. Uranus and Neptune were added to the list after their discoveries in 1781 and 1846. These eight planets naturally subdivide into two sets based on their relative size and density: the small and dense terrestrial planets of Mercury, Venus, Earth, and Mars; and the low density giant planets of Jupiter, Saturn, Uranus, and Neptune. The giant planets can be further subdivided by composition into the giant gaseous planets of Jupiter and Saturn and the giant ice planets of Uranus and Neptune. The subdivisions provide a way of unifying the physics for each set of planets.
Up to August 24, 2006, the Solar System contained nine planets. Pluto, the ninth planet during that time, was the smallest of the planets. At the time of its discovery in 1930, it was the most distant of the planets. But since its discovery, astronomers have come to understand that Pluto is one of a large collection of bodies that compose the Kuiper Belt. Some of the bodies in this region of the Solar System are as large as Pluto. For this reason, Pluto is no longer considered a classical planet; it is a dwarf planet.
Mercury, Venus, Earth, and Mars, the four planets closest to the Sun, are the terrestrial planets. They are roughly five-times the density of the giant planets, being primarily composed of iron, nickel, and silicate compounds. The largest and densest of these planets is Earth. Venus is slight smaller than Earth, but its closeness to the Sun radically alters the composition of its atmosphere from that found on Earth. Specifically, because of the intensity of the solar radiation, Venus cannot hold onto hydrogen; for this reason, Venus has no water, and it is shrouded in a dense atmosphere of carbon dioxide. Both Mars and Mercury are dramatically smaller than Earth. Mercury is the smallest and closest to the Sun of the terrestrial planets; it has a surprisingly high density, the second densest of all planets after Earth. It has only the wisps of an atmosphere. Mars, the terrestrial planet farthest from the Sun, has a slight carbon dioxide atmosphere, and it retains a small amount of water in its atmosphere and as ice on its surface.
Jupiter and Saturn, the two giant gaseous planets of our Solar System, are the largest planets in the Solar System. They have a composition similar to that of the Sun, with the primary constituent hydrogen and the secondary constituent helium. The innermost regions of these planets are composed of hydrogen in a metallic state. The outer layers are composed of molecular hydrogen. Other elements, such as oxygen, nitrogen, and carbon, which are present in small quantities, form clouds that govern the cooling of the gaseous giants.
The interest in Jupiter and Saturn has increased lately because of the discover over the past decade of over 100 gaseous giants orbiting other stars, and because of the similarity of their physics to brown dwarfs—dim objects that are too small to be stars, but large enough to undergo deuterium fusion.
The systems surrounding Jupiter and Saturn have interesting, but quite different, properties. Jupiter has four large satellites among its large flock that are geologically distinct because of the tidal forces exerted on them by Jupiter. Jupiter also has a very active magnetosphere. Saturn, on the other hand, has only one large planet in its flock, Titan; it is interesting because of its very large size and its thick, cold methane atmosphere. The rings, however, are the feature that makes Saturn distinct from all other planets. The rings are interesting for their dynamics and for their effect on Saturn's atmosphere. (continue)
The two outer giant planets of our Solar System, Uranus and Neptune, differ significantly in compositions from the two inner giant planets, Jupiter and Saturn. Uranus and Neptune are no more than 15% hydrogen and helium by mass. These planets are predominately composed of water, methane, and ammonia ices, which is the reason they are referred to as giant ice planets. The composition of these planets is an important clue to how these planets formed. Current theory states that these planets formed by the gradual gravitational capture of numerous small ice asteroids. (continue)
The Kuiper Belt is a system of planetoids that lies outside of Neptune's orbit. Interest in the Kuiper Belt is high because it is a remnant of the solar accretion disk that long ago surrounded the Sun, and it is the source of other classes of objects, such as the short-period comets. Pluto and its somewhat smaller companion Charon are the best studied objects within the Kuiper Belt. Two other bodies within the Kuiper Belt are about the same size as Pluto. All of these bodies are dwarf planets; they are also call plutonian objects. (continue)