The solar system comprises the Earth's Sun and the retinue of celestial objects gravitationally bound to it. Traditionally, this is said to consist of the Sun, nine planets and their 158 currently known moons; however, a large number of other objects, including asteroids, meteoroids, planetoids, comets, and interplanetary dust orbit the Sun as well. Astronomers are debating the classification of a potential tenth planet and other trans-Neptunian objects.



Although the term "solar system" is frequently applied to other star systems and the planetary systems which may comprise them, it should strictly refer to Earth's system specifically: the word "solar" is derived from the Sun's Latin name, Sol, and thus the term sometimes appears as Solar System. When talking about another stellar system or planetary system, including the stars and bodies associated with them through gravity, it is usual to shorten it to drop the term "solar" and form names such as "the Alpha Centauri system" or "the 51 Pegasi system".

The Sun (astronomical symbol ??) is a main sequence G2 star that contains 99.86% of the system's known mass. Its two largest orbiting bodies, Jupiter and Saturn, account for more than 90% of the remainder; the Oort Cloud might hold a substantial percentage, but as yet its existence is unconfirmed. The total surface area of the solar system's objects that have solid surfaces and a diameter greater than 1 km is approximately 1.7??109 km2 ??about 11 times the area of the Earth's land masses.

In broad terms, the charted regions of the solar system consist of the Sun and its planetary system: the eight bodies in relatively unique orbits commonly called planets or major planets and two belts of smaller objects, which can be called minor planets, planetoids, meteoroids, or planetesimals. Pluto, the ninth planet, is also considered a member of the outer belt, and its status is currently uncertain, particularly since the discovery of 2003 UB313 (see below). Most objects in orbit round the Sun all lie within the same shallow plane, called the ecliptic plane, and orbit in the same direction. Many are in turn orbited by moons, and the largest are encircled by planetary rings of dust and other particles.

Interplanetary distances

Distances within the solar system are measured most often in astronomical units, or AU. One AU is the mean distance between the Earth and the Sun, or 149 598 000 kilometres. Other units in common use include the gigametre (Gm, one million kilometres) and the terametre (Tm, one billion/milliard kilometres). Pluto is roughly 38 AU (5.9 Tm) from the Sun, while Jupiter lies at roughly 5.2 AU (778 Gm).

Despite the fact that many diagrams represent the solar system as having each orbit the same distance apart, in actuality the orbits are largely arranged geometrically, that is, each is roughly double the distance from the Sun as the one before it. Venus??s distance from the Sun is roughly double that of Mercury, Earth??s distance is roughly double that of Venus, Mars??s double that of Earth etc. This relationship is expressed in the Titius-Bode law, a mathematical formula for predicting the semi-major axes of planets in AU. In its simplest form, it is written:

a= 0.4 + 0.3k

where k=0, 1, 2, 4, 8, 16, 32, 64, 128.

By this formulation, one would expect Mercury's orbit (k=0) to be 0.4 AU, and Mars's orbit (k=4) to be at 1.6 AU. In fact their orbits are 0.38 and 1.52 AU. Ceres, the largest asteroid, lies at k=8.

This law is only a rough guide, and doesn't fit all of the planets; Neptune is far closer than predicted, though Pluto lies at Neptune's predicted orbit. As of now, there is no scientific explanation for why this law applies, and many claim it is merely a coincidence, falling into the region of uncomfortable science.