The Solar System

The Solar System or solar system consists of the Sun and the other celestial objects gravitationally bound to it: the eight planets, their 166 known moons,[1] three dwarf planets (Ceres, Pluto, and Eris and their four known moons), and billions of small bodies. This last category includes asteroids, Kuiper belt objects, comets, meteoroids, and interplanetary dust.

In broad terms, the charted regions of the Solar System consist of the Sun, four

terrestrial inner planets, an asteroid belt composed of small rocky bodies, four

gas giant outer planets, and a second belt, called the Kuiper belt, composed of icy objects. Beyond the Kuiper belt is the scattered disc, the heliopause, and ultimately the hypothetical Oort cloud.

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 each of the outer planets is encircled by planetary rings

of dust and other particles. All the planets except Earth are named after

gods and goddesses from Greco-Roman mythology. The three dwarf planets

are Pluto, the largest known Kuiper belt object; Ceres, the largest object in

the asteroid belt; and Eris, which lies in the scattered disc.

Terminology

Objects orbiting the Sun are divided into three classes: planets, dwarf planets, and small Solar System bodies.

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. There are eight known planets: Mercury, Venus, Earth,

Mars, Jupiter, Saturn, Uranus, and Neptune.

On August 24 2006 the International Astronomical Union defined the term

"planet" for the first time, excluding Pluto and reclassifying it under the new

category of dwarf planet along with Eris and Ceres.

A dwarf planet is not required to clear its neighbourhood of other celestial

bodies. Other objects that may become classified as dwarf planets are

Sedna, Orcus, and Quaoar.

From the time of its discovery in 1930 until 2006, Pluto was considered the

Solar System's ninth planet. But in the late 20th and early 21st centuries,

many objects similar to Pluto were discovered in the outer Solar System,

most notably Eris, which is slightly larger than Pluto.

The remainder of the objects in orbit around the Sun are small Solar System

bodies (SSSBs).

Natural satellites, or moons, are those objects in orbit around planets, dwarf

planets and SSSBs, rather than the Sun itself.

A planet's distance from the Sun varies in the course of its year. Its closest

approach to the Sun is called its perihelion, while its farthest distance from

the Sun is called its aphelion.

Astronomers usually measure distances within the Solar System in

astronomical units (AU). One AU is the approximate distance between the

Earth and the Sun, or roughly 149,598,000 km (93,000,000 mi). Pluto is

roughly 38 AU from the Sun while Jupiter lies at roughly 5.2 AU. One light

year, the best known unit of interstellar distance, is roughly 63,240 AU.

Informally, the Solar System is sometimes divided into separate zones.

The inner Solar System includes the four terrestrial planets and the main

asteroid belt. Some define the outer Solar System as comprising

everything beyond the asteroids.[4] Others define it as the region beyond

Neptune, with the four gas giants considered a separate "middle zone".[5]

Layout and structure

The principal component of the Solar System is the Sun, a main sequence G2

star that contains 99.86% of the system's known mass and dominates it gravitationally.[6] Jupiter and Saturn, the Sun's two largest orbiting bodies, account for more than 90% of the system's remaining mass.

Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic. 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 are exceptions, such as Halley's Comet.

Objects travel around the Sun following Kepler's laws of planetary motion. Each object orbits along an approximate ellipse with the Sun at one focus of the ellipse.

The closer an object is to the Sun, the faster it moves. The orbits of the planets

are nearly circular, but many comets, asteroids and objects of the Kuiper belt

follow highly-elliptical orbits.

To cope with the vast distances involved, many representations of the Solar System show orbits the same distance apart. In reality, with a few exceptions, 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 Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a correlation between these orbital distances (see Titius-Bodelaw), but no such theory has been accepted.

Formation

The Solar System is believed to have formed according to the nebular

hypothesis, first proposed in 1755 by Immanuel Kant and independently

formulated by Pierre-Simon Laplace.[7] This theory holds that 4.6 billion years

ago the Solar System formed from the gravitational collapse of a giant

molecular cloud. This initial cloud was likely several light-years across andprobably birthed several stars.[8] Studies of ancient meteorites reveal traces

of elements only formed in the hearts of very large exploding stars,

indicating that the Sun formed within a star cluster, and in range of a

number of nearby supernovae explosions.

The shock wave from these supernovae may have triggered the formation of the Sun by creating regions of overdensity in the surrounding nebula, allowing gravitational forces to overcome internal gas pressures and cause collapse.[9]

The region that would become the Solar System, known as the pre-solar

nebula,[10] had a diameter of between 7000 and 20,000 AU[11] [12] and a mass

just over that of the Sun (by between 0.1 and 0.001 solar masses).[13] As the

nebula collapsed, conservation of angular momentum made it rotate faster.

As the material within the nebula condensed, the atoms within it began to

collide with increasing frequency. The centre, where most of the mass

collected, became increasingly hotter than the surrounding disc.[14] As

gravity, gas pressure, magnetic fields, and rotation acted on the contracting

nebula, it began to flatten into a spinning protoplanetary disk with a

diameter of roughly 200 AU[15] and a hot, dense protostar at the center.[16]

[17]

Studies of T Tauri stars, young, pre-fusing solar mass stars believed to be

similar to the Sun at this point in its evolution, show that they are often

accompanied by discs of pre-planetary matter.[18] These discs extend to

several hundred AU and reach only a thousand kelvins at their hottest.[19]

After 100 million years, the pressure and density of hydrogen in the centre of the

collapsing nebula became great enough for the protosun to begin thermonuclear

fusion. This increased until hydrostatic equilibrium was achieved, with the thermal energy countering the force of gravitational contraction. At this point

the Sun became a full-fledged star.

From the remaining cloud of gas and dust (the "solar nebula"), the various planets formed. They are believed to have formed by accretion: the planets began as dust grains in orbit around the central protostar; then gathered by direct contact into clumps between one and ten metres in diameter; then collided to form larger bodies (planetesimals) of roughly 5 km in size; then gradually increased by further collisions at roughly 15 cm per year over the 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)[22] 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.[23]

Farther out still, beyond the frost line, where more volatile icy compounds

could remain solid, Jupiter and Saturn became the gas giants. Uranus and

Neptune captured much less material and are known as ice giants because

their cores are believed to be made mostly of ices (hydrogen compounds).

Once the young Sun began producing energy, the solar wind (see below)

blew the gas and dust in the protoplanetary disk into interstellar space and

ended the growth of the planets. T Tauri stars have far stronger stellar

winds than more stable, older stars.[26] [27]