Definition
The Solar System is the astronomical name for the Sun, the Earth, and the collection of planets and other rocky and icy objects moving around, or orbiting, the Sun.
Basics
The main component of the Solar System is the Sun, which contains 98.6 percent of the system's mass and whose gravity holds everything else in orbit.
The Earth's orbit around the Sun is nearly a perfect circle, but when mapped it is found that the Earth moves around the Sun in a very slightly oval shaped, or elliptical orbit. The other planets in the Solar System also circle the Sun in slightly elliptical orbits. Mercury has a more elliptical orbit than the others, and some of the smaller objects orbit the Sun in very eccentric orbits.
The Eight Planets in their order from the Sun:
- (1) Mercury
- (2) Venus
- (3) Earth
- (4) Mars
- (5) Jupiter
- (6) Saturn
- (7) Uranus
- (8) Neptune
The planets are the biggest objects that go around Sun. It took people many years of looking carefully through telescopes to find the farthest away ones. No one expects to find new planets, but more small objects are found every year. Most of the planets have moons that orbit around them. There are at least 173 of these moons in the solar system.
Pluto had been called a planet since it was discovered in 1930, but in 2006 astronomers meeting at the International Astronomical Union decided for the first time on the definition of a planet, and Pluto didn't fit. Instead they defined a new category of dwarf planet, into which Pluto did fit along with some other objects.
Pluto is now one of five dwarf planets, here they are in order of their distance from the Sun.:
- Ceres
- Pluto
- Haumea
- Eris
- Makemake
Astronomers think they will find more dwarf planets soon.
Before the discovery of Uranus, ancients thought the solar system consisted only of the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn.
Until the 16th and 17th centuries, western scientists thought the Sun and the planets orbited the Earth. This was what ancient civilizations thought too, except for a few individuals of the Greek, Indian and Muslim civilizations.
- Uranus, discovered in 1781
- Ceres, discovered in 1801, recently (2006) defined as a dwarf planet
- Neptune, discovered in 1846
- Pluto, discovered in 1930
- Eris, discovered in 2005
Topics of Interest
The Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. Of the retinue of objects that orbit the Sun, most of the mass is contained within eight relatively solitary planets whose orbits are almost circular and lie within a nearly-flat disc called the ecliptic plane. The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, Jupiter, Saturn, Uranus and Neptune, also called the gas giants, are composed largely of hydrogen and helium and are far more massive than the terrestrials.
The Solar System is also home to two regions populated by smaller objects. The asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. Beyond Neptune's orbit lie trans-Neptunian objects composed mostly of ices such as water, ammonia and methane. Within these regions, five individual objects, Ceres, Pluto, Haumea, Makemake and Eris, are recognised to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets. In addition to thousands of small bodies in those two regions, various other small body populations, such as comets, centaurs and interplanetary dust, freely travel between regions.
The solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which extends out to the edge of the scattered disc. The hypothetical Oort cloud, which acts as the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere.
Six of the planets and three of the dwarf planets are orbited by natural satellites, usually termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other particles.
The Sun is the star at the center of the Solar System. The Sun has a diameter of about 1,392,000 kilometres (865,000 mi) (about 109 Earths), and by itself accounts for about 99.86% of the Solar System's mass; the remainder consists of the planets (including Earth), asteroids, meteoroids, comets, and dust in orbit. About three-fourths of the Sun's mass consists of hydrogen, most of the rest is helium. Less than 2% consists of other elements, including iron, oxygen, carbon, neon, and others.
The interplanetary medium is the material which fills the solar system and through which all the larger solar system bodies such as planets, asteroids and comets move.
A terrestrial planet, telluric planet or rocky planet is a planet that is primarily composed of silicate rocks. Within the solar system, the terrestrial planets are the inner planets closest to the Sun (Mercury, Venus, Earth, Mars, and one terrestrial dwarf planet, Ceres.). The terms are derived from Latin words for Earth (Terra and Tellus), and an alternative definition would be that these are planets which are, in some notable fashion, "Earth-like".
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. The asteroid belt region is also termed the main belt to distinguish it from other concentrations of minor planets within the Solar System, such as the Kuiper belt and scattered disc.
A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System: Jupiter, Saturn, Uranus, and Neptune. Many extrasolar gas giants have been identified orbiting other stars.
A comet is a small solar system body bigger than a meteoroid that, when close enough to the Sun, exhibits a visible coma (fuzzy "atmosphere"), and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, ranging from a few hundred metres to tens of kilometres across.
The Kuiper belt, sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 55 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20–200 times as massive. Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation. While the asteroid belt is composed primarily of rock and metal, the Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water. It is home to at least three dwarf planets – Pluto, Haumea and Makemake.
The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units. These extreme orbits are believed to be the result of gravitational "scattering" by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune. While the nearest distance to the Sun approached by scattered objects is about 30–35 AU, their orbits can extend well beyond 100 AU. This makes scattered objects "among the most distant and cold objects in the Solar System". The innermost portion of the scattered disc overlaps with a torus-shaped region of orbiting objects known as the Kuiper belt, but its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the belt proper.
The Oort Cloud is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The Kuiper belt and scattered disc, the other two known reservoirs of trans-Neptunian objects, are less than one thousandth the Oort cloud's distance. The outer extent of the Oort cloud defines the gravitational boundary of our Solar System.
The vulcanoids are a hypothetical population of asteroids that may orbit the Sun in a dynamically stable zone inside the orbit of the planet Mercury. They are named after the hypothetical planet Vulcan, whose existence was disproven in 1915. No vulcanoids have yet been discovered, and it is not clear if any exist.
Planets beyond Neptune. Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century but culminated at the start of the 20th with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the gas giants, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.
The formation and evolution of the Solar System is estimated to have begun 4.55 to 4.56 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the centre, forming the Sun, while the rest flattened into a protoplanetary disc out of which the planets, moons, asteroids, and other small Solar System bodies formed.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)
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