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Oerdin
03-01-2007, 21:40:08
The Solar System or solar system[1] comprises the Sun and the retinue of celestial objects gravitationally bound to it: the eight planets, their 162 known moons,[2] three currently identified dwarf planets and their four known moons, and thousands of small bodies. This last category includes asteroids, meteoroids, comets, and interplanetary dust.

In broad terms, the charted regions of the Solar System consist of the Sun (astronomical symbol ), four rocky bodies close to it called the inner planets, an inner belt of rocky asteroids, four giant outer planets and second belt of small icy bodies known as the Kuiper belt. In order of their distances from the Sun, the planets are Mercury (), Venus (), Earth (), Mars (), Jupiter (), Saturn (), Uranus (), and Neptune (). Six of the eight planets are in turn orbited by natural satellites (usually termed "moons" after Earth's Moon) and every planet past the asteroid belt is encircled by planetary rings of dust and other particles. The planets other than Earth are named after gods and goddesses from Greco-Roman mythology.

From 1930 to 2006, Pluto (), the largest known Kuiper belt object, was considered the Solar System's ninth planet. However, in 2006 the International Astronomical Union (IAU) created an official definition of the term "planet".[3] Under this definition, Pluto is reclassified as a dwarf planet, and there are eight planets in the Solar System. In addition to Pluto, the IAU currently recognizes two other dwarf planets: Ceres (), the largest object in the asteroid belt, and Eris, which lies beyond the Kuiper belt in a region called the scattered disc. Of the known dwarf planets, only Ceres has no moons.

The principal component of the Solar System is the Sun or Sol, a main sequence G2 star that contains 99.86% of the system's known mass and dominates it gravitationally.[4] Jupiter and Saturn, the Sun's two largest orbiting bodies, account for more than 90% of the system's remaining mass. (The currently hypothetical Oort cloud would also hold a substantial percentage were its existence confirmed).[5]


The ecliptic viewed in sunlight from behind the Moon in this Clementine image. From left to right: Mercury, Mars, SaturnMost objects in orbit around the Sun lie within the ecliptic, a shallow plane which is roughly parallel to the Sun's equator. The planets are very close to the ecliptic while comets and kuiper belt objects are usually at significantly greater angles to it.

All of the planets (and most other objects) also orbit with the Sun's rotation; in a counter-clockwise direction as viewed from a point above the Sun's north pole. There is a direct relationship between how far away a planet is from the Sun and how quickly it orbits. Mercury, the closest to the Sun, travels the fastest, while Neptune, being much farther from the Sun, travels more slowly. Objects orbit in an ellipse around the Sun, so an orbiting object's distance from the Sun varies in the course of its year. Its closest approach to the Sun is known as its perihelion while its farthest point from the Sun is called its aphelion. Although the orbits of the planets are nearly circular (with perihelions roughly equal to their aphelions), many comets, asteroids and objects of the Kuiper belt follow highly elliptical orbits with large differences between perihelion and aphelion. The paths of objects around the Sun travel according to a law of planetary motion discovered by German astronomer Johannes Kepler in the early 1600s. The sun is slightly off to the side of the center of each ellipse at a point called a focus. The focus is actually a point just outside the centre of the Sun called the barycenter of the solar system.

Astronomers most often measure distances within the solar system in astronomical units or AU. One AU is the average distance between the Earth and the Sun or roughly 149 598 000 km (93,000,000 mi). Pluto is roughly 39 AU from the Sun while Jupiter lies at roughly 5.2 AU.

Informally, the Solar System is sometimes divided into separate zones. The first zone, known as the inner Solar System, includes the four terrestrial planets and the main asteroid belt. The outer Solar System is sometimes defined as "everything beyond the asteroids". Alternatively, the term may be used to describe the region beyond Neptune, with the four gas giants considered a separate "middle zone".[6]


The orbits of the bodies in the solar system to scale (clockwise from top left)One common misconception is that the orbits of the major objects within the Solar System (planets, Pluto and asteroids) are equidistant. To cope with the vast distances involved, many representations of the Solar System simplify these orbits by showing them the same distance apart. However, in reality, with a few exceptions, the Solar System is arranged so that the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. For example, Venus is approximately 0.33 AU farther out than Mercury while Jupiter is 1.9 AU from the farthest extent of the asteroid belt and Neptune's orbit is roughly 20 AU farther out than that of Uranus. Attempts have been made to determine a correlation between these distances (see Bode's Law) but to date there is no accepted theory that explains the orbital distances.

In a decision passed by the International Astronomical Union General Assembly on August 24, 2006, the objects in the Solar System other than the Sun and natural satellites were divided into three separate groups: planets, dwarf planets and small solar system bodies.

Under this classification, a planet is any body in orbit around the Sun that a) has enough mass to form itself into a spherical shape and b) has cleared its immediate neighborhood of all smaller objects. Eight objects in the Solar System currently meet this definition; they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Dwarf planet was a second and new classification. The key difference between planets and dwarf planets is that while both are required to orbit the Sun and be of large enough mass that their own gravity pulls them into a nearly round shape, dwarf planets are not required to clear their neighborhood of other celestial bodies. Three objects in the solar system are currently included in this category; they are Pluto (formerly considered a planet), the asteroid Ceres, and the scattered disc object Eris. The IAU will begin evaluating other known objects to see if they fit within the definition of dwarf planets. The most likely candidates are some of the larger asteroids and several Trans-Neptunian Objects such as Sedna, Orcus, and Quaoar.

The current hypothesis of Solar System formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon Laplace.[8] The nebular theory holds that 4.6 billion years ago (a date determined via radiometric dating of meteorites),[9] the Solar System formed from the gravitational collapse of a gaseous cloud called the solar nebula. It had a diameter of between 7000 and 20,000 AU [10] and was 23 times the mass of the Sun. [citation needed] Over time, a disturbance (possibly a nearby supernova) [11] squeezed the nebula, pushing matter inward until gravitational forces overcame the internal gas pressure and it began to collapse. As the nebula collapsed, conservation of angular momentum meant that it spun faster, and became warmer. As the competing forces associated with gravity, gas pressure, magnetic fields, and rotation acted on it, the contracting nebula began to flatten into a spinning protoplanetary disk with a diameter of roughly 200 AU [12] and a gradually contracting protostar at the center.[13] Studies of young, pre-fusing solar mass stars, called T Tauri stars, show that these discs extend to several hundred AU and are rather cool, reaching only a thousand kelvins at their hottest.[14]

From this cloud and its gas and dust, the various planets formed. The currently accepted method by which the planets formed is known as accretion, in which the planets began as dust grains in orbit around the central protostar, which initially formed by direct contact into clumps between one and ten kilometres in diameter, which in turn collided to form larger bodies (planetesimals), of roughly 5 km in size gradually increasing by further collisions by roughly 15 cm per year over the course of the next few million years.[15]

The inner solar system was too warm for volatile molecules like water and methane to condense, and so the planetesimals which formed there were relatively small (comprising only 0.6% the mass of the disc) [12] and composed largely of compounds with high melting points, such as silicates and metals. These rocky bodies eventually became the terrestrial planets. Farther out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the asteroid belt. [16]

Oerdin
03-01-2007, 21:40:30
Man... With a post that long it is like he never left.

Special Ed
03-01-2007, 21:47:05
Yaaay!

Qaj the Fuzzy Love Worm
04-01-2007, 06:42:29
You didn't include your attributions.

MOBIUS
04-01-2007, 09:10:55
Sorry, I thought that was a bit short...

Now go and write that out a hundred times.

MoSe
04-01-2007, 10:09:33
where are the bibliographic references?

___
unless that's what you technically call "attribution".... (?)
I mean, the [#] references.

Drekkus
04-01-2007, 10:30:16
That's just a copy past. You have to type it yourself to be truely Darkstarian

Nav
04-01-2007, 10:38:49
copy past = repeat same mistakes ??

Oerdin
04-01-2007, 20:28:15
Yep.