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Sol with its family of planets and debris, shown at their relative sizes. |
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inner solar system 
outer solar system
table of solar planets
terrestrial planets and moons
mars, then & now
asteroid belt
giant planets
jupiter's largest moons
glossary
references
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The Solar System consists of the Sun, eight major planets, more than 150 moons, and hundreds of thousands of rocky and icy objects called asteroids, planetoids, and comets. Our Sun – sometimes known by its Latin name, Sol, which gives us the adjective “solar” – probably formed in a primordial cloud of hydrogen almost five billion years ago, along with a hundred or more other stars (Lada & Lada 2003). For the first few million years of its existence, the Sun was surrounded by an extended disk of gas and dust (see Evolution of Planetary Systems). Condensation, clumping, and accretion within the disk resulted in the system of planets and planetoids that we now observe. Radioactive dating of meteorites establishes that the rocky bodies of the Solar System formed about 4.6 billion years ago. Around this time, the Sun may have emerged from its primordial star cluster to pursue an independent orbit around the Galactic center. Although the Sun is an ordinary main sequence star of spectral class G2, similar to dozens of others in the Solar neighborhood, its family of planets appears to be unique. Not only does the Solar System include four planets – Mercury, Venus, Earth, Mars – that are smaller than any detected around nearby stars; the orbital architecture of the Solar System's giant planets is also anomalous in at least three ways.
The eight principal planets of the Solar System are arranged in two distinct regions, each of which corresponds to a different population. The terrestrial or "Earthlike" planets occupy the inner system, and the giant planets (which can be further divided into gas giants and ice giants) occupy the outer system. For a table of physical and orbital characteristics, see Table of Solar Planets. terrestrial planetsThe four terrestrial planets are located within 1.5 AU of the Sun. (See diagram of the inner solar system.) All are dense rocky bodies with a low percentage of volatile elements. Although all four have gaseous envelopes, Mercury's is so tenuous that it is little different from a vacuum, while the Martian atmosphere is orders of magnitude less dense than Earth's. Only Earth and Venus have relatively extensive atmospheres, albeit of quite dissimilar compositions. The four terrestrial planets are also alike in lacking co-formed satellites. Mercury and Venus have no moons at all; Mars has captured two small asteroids whose orbits are decaying; while Earth has an atypically massive companion, Luna, that was evidently formed from a collision between the proto-Earth and another planetary embryo in the early days of the Solar System. asteroid beltThe four inner planets are bounded by a ring of debris consisting of remnants of our system’s original protoplanetary disk. (See diagram of the Asteroid Belt.) Most of these rocky bodies, known individually as asteroids, are irregularly shaped and rapidly spinning. Although they probably number in the millions, fewer than 30 have diameters larger than 124 miles (200 kilometers). The largest, Ceres, has no near rivals. Yet Ceres’ diameter of only 914 km (567 miles) makes it smaller than Tethys (diameter 1066 km, 661 miles), the fifth-largest moon of Saturn and the fifteenth-largest moon in the Solar System. The asteroids are scattered over a region of space larger than the entire inner solar system. The vast majority orbit in a belt whose inner edge is located around 2 AU and whose outer edge is located around 3.5 AU. The orbital dynamics of these objects are determined primarily by the gravitational influence of Jupiter. giant planetsThe outer Solar System begins at the edge of the Asteroid Belt and is far more expansive than the inner, extending from the orbit of Jupiter, with a semimajor axis of 5.2 AU, to that of Neptune, with a semimajor axis of 30 AU. (See diagram of the outer solar system.) This region is dominated by the giant planets, which range in mass from Uranus (14 times Earth's mass) to Jupiter (318 times Earth's mass). All four planets have thick turbulent atmospheres but lack rocky surfaces. Jupiter and Saturn are composed almost entirely of hydrogen and helium, like stars, while Uranus and Neptune contain more heavy elements. All four probably have dense rocky cores several times as massive as Earth. Although all four giants have a substantial percentage of gaseous elements in their composition, theorists typically draw a distinction between Jupiter and Saturn, the classic gas giants, and Uranus and Neptune, which are considered ice giants because a large portion of their mass exists in the form of ices. kuiper belt and beyondBeyond the orbit of Neptune begins the Kuiper Belt, which covers an enormous region of space lying between 30 and 85 AU from the Sun (Backman 2004), substantially exceeding the area in which the eight planets orbit. This region is occupied by millions of widely scattered bodies made primarily of ice. So far, more than 600 objects with diameters greater than 60 miles (100 kilometers) have been discovered in the Kuiper Belt, including a few extremely large objects such as Eris, Pluto, and Quaoar, which have diameters in excess of 1000 miles (1600 kilometers). As in the Asteroid Belt, however, most of the objects orbiting in the Kuiper Belt range in size from interplanetary glaciers a few miles in diameter down to gigantic snowballs as big as houses. Many nearby stars of all spectral types are probably surrounded by similar belts of rocky and icy planetoids (see also Debris Disk Systems). |
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All text is copyright Raymond Harris 2006-2008. Image credits appear in the accompanying caption. |