s o l a r   s y s t e m

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 hundreds or even thousands of other stars (Lada & Lada 2003, Gaidos et al. 2009).

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 that 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 so far detected around nearby stars; the orbital architecture of the Solar System's giant planets is also unusual in at least three ways.

• The four giant planets travel in almost circular orbits, unlike the eccentric configurations typical of exoplanetary systems.
• The closest of the four has a semimajor axis of 5.2 astronomical units (AU), meaning that it is more distant from its primary star than 95% of known extrasolar planets.
• All four giant planets are widely separated from each other, unlike the tightly packed orbits of most known multiple planetary systems.

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 planets

The four terrestrial planets are located within 1.5 AU of the Sun. (See diagram of the inner solar system.) All are dense rocky bodies whose composition is dominated by iron, with lower percentages of volatile elements. All four have envelopes of gases, but 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 Venus and Earth have relatively extensive atmospheres, although their compositions are quite different. Earth's atmosphere is mostly nitrogen, and its clouds are made of water. The Venusian atmosphere is mostly carbon dioxide, with clouds of sulfuric acid.

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, Phobos and Deimos, whose orbits are decaying; while Earth has an atypically massive companion, Luna, that was evidently formed in a collision between the proto-Earth and another protoplanet in the early days of the Solar System.

asteroid belt

The four inner planets are encircled by a ring of debris consisting of remnants of our system’s original protoplanetary disk. (See diagram of the Asteroid Belt.) Almost all 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 200 kilometers (124 mi). The largest, Ceres, is the only one that has assumed a spherical shape, and it has no near rivals in size or mass. Yet Ceres’ diameter of only 914 kilometers (567 miles) makes it smaller than Tethys (diameter 1066 km, 661 mi), 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 planets

The 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 of water and ammonia.

kuiper belt and beyond