Solar System

The Solar System consists of the Sun and those celestial objects bound to it by gravity. These objects are the eight planets, their 166 known moons, five dwarf planets, and billions of small bodies. The small bodies include asteroids, icy Kuiper belt objects, comets, meteoroids, and interplanetary dust.

The charted regions of the Solar System are the Sun, four terrestrial inner planets, the asteroid belt, four gas giant outer planets, the Kuiper belt, the scattered disc, and the hypothetical Oort cloud.

A flow of plasma from the Sun (the solar wind) permeates the Solar System. This creates a bubble in the interstellar medium known as the heliosphere, which extends out to the middle of the scattered disc.

In order of their distances from the Sun, the eight planets are:

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune

As of mid-2008, five smaller objects are classified as dwarf plants. Ceres is in the asteroid belt, and four orbit the Sun beyond Neptune: Pluto (formerly classified as the ninth planet), Haumea, Makemake, and Eris.

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.

Discovery and exploration :-

For many thousands of years, humanity,. with a few notable exceptions, did not recognise the existence of the Solar System. They believed the Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Indian mathematician-astronomer Aryabhata and the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system. His 17th-century successors Galileo Galilei, Johannes Kepler; and Isaac Newton developed an understanding of physics which led to the gradual acceptance of the idea that the Earth moves round the Sun and that the planets are governed by the same physical laws that governed the Earth. In more recent times, this led to the investigation of geological phenomena such as mountains and craters and seasonal meteorological phenomena such as clouds, dust storms and ice caps on the other planets.

Structure :-

The relative masses of the Solar planets. Jupiter at 71% of the total and Saturn at 21% dominate the system. Mercury and Mars, which together are less than 0.1 %, are not visible at this scale.

The principal component of the Solar System is the Sun, a main sequence G2 star that contains 99.86 percent of the system's known mass and dominates it gravitationally. Jupiter and Saturn, the Sun's two largest orbiting bodies, account for more than 90 percent 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 (counter-clockwise, as viewed from above the Sun's north pole). There are exceptions, such as Halley's Comet.

Kepler's laws of planetary motion describe the orbits of objects about the Sun. According to Kepler's laws, each abject travels along an ellipse with the Sun at one focus. Objects closer to. the Sun (-with smaller semi-major axes) have shorter years. On an elliptical orbit, a body's distance from the Sun varies aver the course of its year. A body's closest approach to. the Sun is called its perihelion, while its mast distant paint from the Sun is called its aphelion. Each body moves fastest at its perihelion and slowest at its aphelion. The orbits of the planets are nearly circular, but many comets, asteroids and Kuiper belt objects 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 astronomical units (AU) farther out than Mercury, while Saturn is 4,3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus.

Mast of the planets in the Solar System possess secondary systems of their awn. Many are in turn orbited by planetary objects called natural satellites, or moons, same of which are larger than planets. Mast of the largest natural satellites are in synchronous orbit, with one face permanently turned toward their parent. The four largest planets also. possess planetary rings, thin bands of tiny particles that orbit them in unison.

Terminology :-

Informally, the Solar System is sometimes divided into separate regions. The inner Solar System includes the four terrestrial planets and the main asteroid belt. The outer Solar System is beyond the asteroids, including the four gas giant planets. Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.

Dynamically and physically, objects orbiting the Sun are classed into. three categories: planets, dwarf planets and small Solar System bodies.

A planet is any body in orbit around the Sun that has enough mass to form itself into a spherical shape and has cleared its immediate neighborhood of all smaller objects. By this definition, the Solar System has eight known planets: Mercury, Venus, Earth, Mars. Jupiter, Saturn, Uranus, and Neptune. Pluto was demoted from planetary status, as it has not cleared its orbit of surrounding Kuiper belt objects.

A dwarf planet is a celestial body orbiting the sun that is massive enough to be rounded by its own gravity but which has not cleared its neighboring region of planetesimals and is not a satellite. By this definition, the Solar System has five known dwarf planets: Ceres, Pluto Haumea, Makemake, and Eris. Other objects that may become classified as dwarf planets are Sedna, Orcus, and Quaoar. Dwarf planets that orbit in the trans-Neptunian region are called "plutoids."

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

The regions (or zones) of the Solar system: the inner solar system, the asteroid belt. the giant planets (Jovians) and the Kuiper belt.

Planetary scientists use the terms gas, ice, and rock to describe the various classes of substances found throughout the Solar System. Rock is used to describe compounds with high melting points (greater than roughly 500 K), such as silicates. Rocky substances are prevalent in the inner Solar System, forming most of the terrestrial planets and asteroids. Gases are materials with low melting points such as atomic hydrogen, helium, and noble gases; they dominate the middle region, comprising most of Jupiter and Saturn. Ices, like water, methane, ammonia, and carbon dioxide, have melting points up to a few hundred Kelvin. Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie beyond Neptune's orbit. The term volatiles refers collectively to all materials with low boiling points (less than a few
hundred Kelvin), including gases and ices; depending on the temperature, volatiles can be found as ices, liquids, or gases in various places in the Solar System.

Sun :-

The Sun is the Solar System's parent star, and far and away its chief component. Its large mass gives it an interior density high enough to sustain nuclear fusion, which releases enormous amounts of energy, mostly radiated into space as electromagnetic radiation such as visible light.

The Sun is classified as a moderately large yellow dwarf, but this name is misleading as, compared to stars in- our galaxy, the Sun is rather large and bright. Stars are classified by the Hertzsprung-Russell diagram, a graph which plots the brightness of stars against their surface temperatures. Generally, hotter stars are brighter. Stars following this pattern are said to be on the main sequence; the Sun lies right in the middle of it. However, stars brighter and hotter than the Sun are rare, while stars dimmer and cooler are common.

It is believed that the Sun's position on the main sequence puts it in the "prime of life' for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion. the Sun is growing brighter, early in its history it was 75 percent as bright as it is todyay.

The Sun is a population I star; it was born in the later stages of the universe's evolution. It contains more elements heavier than hydrogen and helium ("metals" in astronomical

parlance) than older population II stars. Elements heavier than hydrogen' and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, while stars born later have more. This high metallicity is thought to have been crucial to the Sun's developing a planetary system, because planets form from accretion of metals.

Inner Solar, System :-

The inner Solar System is the traditional name for the region comprising the terrestrial planets and asteroids. Composed mainly of silicates and metals, the objects of the inner Solar System huddle very closely to the Sun; the radius of this entire region is shorter than the distance between Jupiter and Saturn

Inner planets :-

The inner planets are Mercury, Venus, Earth, and Mars.

The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring' systems. They are composed largely of minerals with high melting points. such as the silicates which form their crusts and mantles, and metals such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have substantial atmospheres; all have impact craters and tectonic surface features such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets which are closer to the Sun than Earth is (i.e. Mercury and Venus).

Mercury :-

Mercury (0.4 AU) is the closest planet to the Sun and the smallest planet (0.055 Earth masses). Mercury has no natural satellites, and its only known geological features besides impact craters are lobed ridges or rupes, probably produced by a period of contraction early in its history. Mercury's almost negligible atmosphere consists of atoms blasted off its surface by the solar wind. Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the young Sun's energy.

Venus :-

Venus (0.7 AU) is close in size to Earth, (0.815 Earth masses) and like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere and evidence of internal geological activity. However, it is much drier than Earth and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C, most likely due to the amount of greenhouse gases in the atmosphere. No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would. prevent depletion of its substantial atmosphere, which suggests that its atmosphere is regularly replenished by volcanic eruptions.

Earth:-

Earth (1 AU) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only planet known to have life. Its liquid hydrosphere is unique among the terrestrial planets, and it is also the only planet where plate tectonics has been observed. Earth's atmosphere is radically different from those of the other planets, having been altered .by the presence of life to contain 21% free oxygen. It has one natural satellite, the Moon (Latin: Luna), the only large satellite of a terrestrial planet in the Solar System.

Mars :-

Mars (1.5 AU) is smaller than Earth and Venus (0.107 Earth masses). It possesses a tenuous atmosphere of mostly carbon dioxide. Its surface, peppered with vast volcanoes such as Olympus Mons and rift valleys such as Valles Marineris, shows geological activity that may have persisted until very recently. Its red color comes from rust in its iron-rich soil. Mars has two tiny natural satellites (Deimos and Phobos) thought to be captured asteroids.

Asteroid belt:-

Asteroids are mostly small Solar System bodies composed mainly of rocky and metallic non-volatile minerals.

The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.

Asteroids range in size from hundreds of kilometres across to microscopic. All asteroids save the largest, Ceres, are classified as small Solar System bodies, but some asteroids such as Vesta and Hygieia may be reclassed as dwarf planets if they are shown to have achieved hydrostatic equilibrium.

The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter. Despite this, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth. The main belt is very sparsely populated: spacecratt routinely pass through without incident, Asteroid with diameters between 10 and (10)'-4 ill are called meteoroids.

Ceres :-

Ceres (2.77 AU) is the largest body in the asteroid belt and is Classified as a dwarf planet. It has a diameter of slightly under 1000 km, large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the 19th century, but was reclassified as an asteroid in the 1850s as further observation revealed additional asteroids. It was again reclassified in 2006 as a dwarf planet.

Asteroid groups :-

Asteroids in the main belt are divided into asteroid groups and families based on their orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets which may have been the source of Earth's water.

Outer Solar System :-

The outer region of the Solar System is home to the gas giants and their planet-sized satellites. Many short period comets, including the centaurs, also orbit in this region. The solid objects in this region are composed of a higher proportion of volatiles (such as water, ammonia, methane, often called ices in planetary science) than the rock denizens of the inner Solar System.

Outer planets :-

The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn consist overwhelmingly' of hydrogen and helium; Uranus and Neptune possess a greater proportion of ices in their makeup. Some astronomers suggest they belong in their own category, "ice giants." All four gas giants have rings, although only Saturn's ring system is easily observed from Earth. The term outer planet should not be confused with superior planet, which designates planets outside Earth's orbit (the outer planets and Mars).

Jupiter :-

Jupiter (5.2 AU), at 318 Earth masses, masses 2.5 times all the other planets put together. It is composed largely of hydrogen and helium. Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Jupiter has sixty-three known satellites. The four largest, Ganymede, Callisto, 10, and Europa. show similarities to the terrestrial planets, such as: volcanism and internal heating. Ganymede, the largest satellite in the Solar System. is larger than Mercury.

Saturn :-

Saturn. (9.5 AU), distinguished by ~ts extensive ring system, has similarities to Jupiter, such as its atmospheric composition. Saturn is far less massive, being only 95 Earth masses. Saturn has sixty known satellites (and three unconfirmed); two of which, Titan and Enceladus, show signs of geological activity, though they are largely made of ice. Titan is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.

Uranus :-

Uranus (19.6 AU), at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt is over ninety degrees to the ecliptic. It has a much colder core than the' other gas giants, and radiates very little heat into space. Uranus has twenty-seven known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel and Miranda.

Neptune :-

Neptune (30 AU), though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and therefore more dense. It radiates more internal heat, but not as much as Jupiter or Saturn. Neptune has thirteen known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen. Triton is the only large satellite with a retrograde orbit. Neptune is accompanied in its orbit by a number of minor planets, termed Neptune Trojans, that are in 1:1 resonance with it.

Comets :-

Comets are small Solar System bodies, usually only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits. generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye.

Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are believed to originate in the Kuiper belt, while long-period comets, such as Hale-Bopp, are believed to originate in the Oort cloud. Many comet groups, such as the Kreutz Sungrazers, formed from the breakup of a single parent. Some comets with hyperbolic orbits ma originate outside the Solar System, but determining their precise orbits is difficult. Old comets that have had most of their volatiles driven out by solar warming are often categorised as asteroids.

Meteors: When we speak of "shooting stars" we mean meteors. These are not stars at all. They are believed to come from two different sources. The majority of them are small fragments like those in the belt of asteroids between Mars and Jupiter. Another source appears to be the tails of comets, for each time the earth crosses the path of a comet, swarms of meteors are seen. 'Meteors enter the earth's atmosphere with such speed (approximately 30-60 times the speed of sound) that the heat generated from friction with the air causes them to vaporise with a brief flash of brilliantly glowing gas.

Stars: Stars are heavenly bodies shining by their own light. They are very far away from us. Our sun is the star nearest to the earth, being only about 150 million kilometres a,way while the next nearest star (Alpha Centauri) is over 42 million kilometres away. Like the Sun and the Moon, the stars appear to rise and set in the west because the earth rotates from west to east. Among the groups of stars that enable us to find our directions on the earth's surface are the Great Bear in the northern skies and the Southern Cross in the Southern Hemisphere.

Centaurs :-

The centaurs are icy comet-like bodies with a semi-major axis greater than Jupiter (5.5 AU) and less than Neptune (30 AU). The largest known centaur, 10199 Chariklo, has a diameter of about 250 km. The first centaur discovered, 2060 Chiron, has also been classified as comet (95P) since it develops a coma just as comets do when the approach the Sun. Some astronomers classifY centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.

Trans-Neptunian region :-

The area beyond Neptune, or the "trans-Neptunian region", is still largely unexplored. It appears to consist overwhelmingly of small worlds (the largest having a diameter only a fifth that of the Earth and a mass far smaller than that of the Moon) composed mainly of rock and ice. This region is sometimes known as the "outer Solar System", though others use that term to mean the region beyond the asteroid belt

Kuiper belt :-

The Kuiper belt, the region's first formation, is a great ring of debris similar to the asteroid belt, but composed mainly of ice. It extends between 30 and 50 AU from the Sun. It is composed mainly of small Solar System bodies, but many of the largest Kuiper belt objects, such as Quaoar, Varuna, and Orcus. may be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt

Pluto and Charon :-

Pluto (39 AU average), a dwarf planet, is the largest Known object in the Kuiper belt. When discovered in 1930, it was considered to be the ninth planet: this changed in 2006 with the adoption of a formal definition of planet. Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion.

Pluto and its three known moons :-

It is unclear whether Charon, Pluto's largest moon, will continue to be classified as such or as a dwarf planet itself. Both Pluto and Charon orbit a barycenter of gravity above their surfaces, making Pluto-Charon a binary system. Two much smaller moons, Nix and Hydra, orbit Pluto and Charon. Pluto lies in the resonant belt and has a 3:2 resonance with Neptune, meaning that Pluto orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called plutinos.

Haumea and Makemake :

Haumea (43.34 AU average), and Makemake (45.79 AU average) are the largest known objects in the classical Kuiper belt. Haumea is an egg-shaped object with two moons. Makemake is the brightest object in the Kuiper belt after Pluto. Originally designated 2003 (EL),61 and 2005 (FY),9 respectively, they were granted names (and the status of dwarf planet) in 2008. Their orbits are far more inclined than Pluto's (28° and 29°) and unlike Pluto are not affected by Neptune, being part of the classical KBO population.

Sedna :-

90377 Sedna (525.86 AU average) is a large, reddish Pluto-like object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion and takes 12,050 years to complete. Mike Brown, who discovered the object in 2003, asserts that it cannot be parti of the scattered disc or the Kuiper belt as its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, which also ma include the object 2000 (CR),105, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years. Brown terms this population the "Inner Oort cloud, II as it may have formed through a similar process, although it is far closer to the Sun. Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.

Galaxy :-

The Solar System is located in the Milky Way galaxy, a barred spiral galaxy with a diameter of about 100,000 light-years containing about 200 billion stars. Our Sun resides in one of the Milky Way's outer spirl arms, known as the Orion Arm or Local Spur. The Sun lies between 25,000 and 28,000 light years from the Galactic Centre, and its speed within the galaxy is about 220 kilometres per second, so that it completes one revolution every 225-250 million years. This revolution is known as the Solar System's galactic year.

The Solar System as we know it today will last until the Sun begins its evolution off of the main sequence of the Hertzsprung~Russell diagram. As the Sun bums through its supply of hydrogen fuel, the energy output supporting the core tends to decrease, causing it to collapse in on itself. This increase in pressure heats the core, so it bums even faster. As a result, the Sun is growing brighter at a rate of roughly ten percent every 1.1 billion years.

Around 5.4 billion years from now, the hydrogen in the core of the Sun will have been entirely converted to helium, ending the main sequence phase. At this time, the outer layers of the Sun will expand to roughly up to 260 times its current diameter; the Sun will become a red giant. Because of its vastly increased surface area, the surface of the Sun will be considerably cooler than it is on the main sequence (2600 K at the coolest).

Eventually, the Sun's outer layers will fall away, leaving a white dwarf an extraordinarily dense object, half the original mass of the Sun but only the size of the Earth. The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun to the interstellar medium.