Solar System

Solar System is a System or an Assembly of various Celestial Bodies. These Celestial bodies include the Sun (also known as the average star) and the Planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune (also known as the solitary planets). It also includes the Satellites of the Planets; numerous Comets, Asteroids, and Meteoroids and the Interplanetary Medium (vast reaches of highly tenuous gas). All these objects are bound to Sun because of the Sun's gravity. The Sun is the most prominent part of the Solar System. The Sun contains 99.86 % of the Solar System's known mass. The Sun's four largest orbiting bodies also known as the Gas Giants, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small Inner Planets surrounded by a belt of rocky Asteroids, and four Gas Giants surrounded by the outer Kuiper Belt of icy objects. 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 known as the 'Gas Giants', are composed largely of hydrogen and helium and are far more massive than the Terrestrials Planets (planet that is primarily composed of silicate rocks). The eight solitary Planets orbit around the Sun and have orbits which are almost circular and lie within a nearly flat disc called the 'Ecliptic Plane'. 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. The Solar System's location in the galaxy is very likely a factor in the evolution of life on Earth.

Formation and Evolution of Solar System

Before the discovery of Solar System, people didn't believe in Solar System's existence and believed that Earth was stationary and the other celestial bodies revolved around it. 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 around the Sun and that the Planets are governed by the same physical laws that governed the Earth. The first semblance of a modern theory was proposed by the German philosopher Immanuel Kant in 1755. In the early 20th century, several scientists came to a conclusion that there were deficiencies in Nebular hypothesis. The Americans 'Thomas Chrowder Chamberlin' and 'Forest Ray Moulton' as well as 'James Jeans' and 'Harold Jeffreys' of Great Britain independently developed variations on the idea that the Planets were formed catastrophically i.e., by a close encounter of the Sun with another star. The basis of this model was that material was drawn out from one or both Stars when the two bodies passed at close range, and this material later joined together to form Planets. However this theory had a flaw as the implication that the formation of Solar Systems in the Milky Way Galaxy must be extremely rare, because sufficiently close encounters between Stars would occur very seldom.

The next significant development took place in the mid-20th century as scientists acquired a more-mature understanding of the processes by which Stars themselves must form and of the behaviour of gases within and around stars. They realized that hot gaseous material stripped from a stellar atmosphere would simply dissipate in space; it would not condense to form Planets. Hence, the basic idea that a Solar System could form through stellar encounters was unjustified. Furthermore, the growth in knowledge about the Interstellar Medium (the gas and dust distributed in the space separating the stars) indicated that large clouds of such matter exist and That Stars form in these clouds. Planets must somehow be created in the process that forms the Stars themselves. This awareness encouraged scientists to reconsider certain basic processes that resembled some of the earlier notions of Kant and Laplace. The prevalent approach to the origin of the Solar System treats it as part of the general process of Star formation.

The Solar System was formed from the gravitational collapse of a giant molecular cloud 4.6 billion years ago. This initial cloud was likely several light-years across and probably birthed several stars. A region named 'Pre-Solar Nebula' collapsed leading to conservation of angular momentum which made it rotate faster, this region eventually became Solar System over a passage of time. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc. As the contracting Nebula rotated, it began to flatten into a spinning Protoplanetary Disc or Proplyd with a diameter of roughly 200 AU and a hot, dense Protostar at the centre. At this point in its evolution, the Sun is believed to have been a T Tauri Star. Studies of T Tauri Stars show that they are often accompanied by discs of pre-planetary matter with masses of 0.001–0.1 solar masses, with the vast majority of the mass of the Nebula in the Star itself. Planets were formed by the growth of this Disc. Within 50 million years, the pressure and density of hydrogen  in the centre of the Protostar became great enough for it to begin thermonuclear fusion.  The temperature, reaction rate, pressure, and density 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 Main Sequence Star'.  

Major Components of Solar System

Asteroids - These are small celestial bodies composed of rock and metal that move around the Sun (mainly between the orbits of Mars and Jupiter). Some of the Asteroids are 951 Gaspra, 243 Ida, 9969 Braille, 5535 Annefrank etc.

Comets - A relatively small extraterrestrial body consisting of a frozen mass that travels around the sun in a highly elliptical orbit. Some of the Comets are 1P/Halley, 2P/Encke, 3D/Biela etc.

Interplanetary Dust - These are microscopic particles in the Interplanetary medium.

Meteoroids - Any of the small solid extraterrestrial bodies that hits the Earth's atmosphere. Some of these have been Peekskill Meteorite, H6 Monomict Breccia Meteorite etc.

Natural Satellites - An object that orbits a Planet or other body larger than itself and which is not man-made. Some of these are Earth's moon, Jupiter's Galilean Moons etc.

Planets - Any celestial body (other than comets or satellites) which revolve around a Star. Some of these are Mercury, Venus, Earth etc.

Stars - It is a celestial body composed of hot gases that radiates energy derived from thermonuclear reactions in the interior. Some of these are Sun, Alzir, Baham etc.

Features of Solar System

• 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 are relatively close to the Sun; the radius of this entire region is shorter than the distance between Jupiter and Saturn.

The Inner Solar System consists of:

Inner Planets - These include Mercury, Venus, Earth, Mars and Asteroid Belt (the region of interplanetary space between Mars and Jupiter where most Asteroids are found).

• Outer Solar System - The outer region of the Solar System consists of  the 'Gas Giants' and their large 'Moons'. Many short period Comets, including the Centaurs, also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain more ices (such as water, ammonia, methane, often called ices in planetary science) than the rocky denizens of the inner Solar System, as the colder temperatures allow these compounds to remain solid.

The Outer Solar System consists of:

1.  Outer Planets - These include Jupiter, Saturn, Uranus, Neptune and Comets. The four outer Planets, or Gas Giants (sometimes also known as Jovian Planets), collectively make up 99% of the mass known to orbit the Sun.

2.  Trans-Neptunian Region - The area beyond Neptune is known as the Trans-Neptunian Region (TNO). This area is largely unexplored.

Trans-Neptunian Region is composed of:

A)  The Kuiper Belt - It 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. Though it contains at least three Dwarf Planets, it is composed mainly of small Solar System bodies. There are estimated to be over 100,000 Kuiper Belt objects with a diameter greater than 50 km, but the total mass of the Kuiper Belt is thought to be only a tenth or even a hundredth the mass of the Earth. The Belt include Dwarf Planets like Pluto and Haumea and Makemake. Many Kuiper Belt objects have multiple Satellites, and most have orbits that take them outside the plane of the ecliptic.

The Kuiper Belt can be roughly divided into:

1. The Classical Belt - The Classical Belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 AU to 47.7 AU. Members of the classical Kuiper Belt are classified as 'Cubewanos,' after the first of their kind to be discovered, (15760) 1992 QB1, and are still in near primordial, low-eccentricity orbits.
2. The Resonances - Resonances are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits, or once for every two). The first resonance begins within the orbit of Neptune itself.

B)  Scattered Disc - It 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. 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. As of 2009, over 100 Scattered Disc objects have been identified, including 2007 UK126 (discovered by Schwamb, Brown, and Rabinowitz), (84522) 2002 TC302 (NEAT), Eris (Brown, Trujillo, and Rabinowitz), Sedna  (Brown, Trujillo, and Rabinowitz) and 2004 VN112 (Deep Ecliptic Survey). It is considered that the Scattered Disc formed when Kuiper belt objects (KBOs) were scattered into eccentric and inclined  orbits by gravitational interaction with Neptune and the other outer Planets.  The amount of time for this process to occur remains uncertain. According to one theory a period equal to the entire age of the Solar System resulted in the Disc while the other theory that the scattering took place relatively quickly, during Neptune's early migration epoch (an arbitrarily fixed date that is the point in time relative to which information). Some astronomers consider the Scattered Disc to be merely another region of the Kuiper Belt, and describe Scattered Disc objects as 'Scattered Kuiper Belt Objects'.

C)  Oort Cloud - It is a hypothetical Cloud of up to a trillion icy objects that is believed to be the source for all long-period Comets and to surround the Solar System at roughly 50,000 AU (around 1 light-year  (LY)), and possibly to as far as 100,000 AU (1.87 LY). It is believed to be composed of Comets which were ejected from the inner Solar System by gravitational interactions with the outer Planets. Oort Cloud objects move very slowly, and can be perturbed by infrequent events such as collisions, the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way

• Kepler's laws of planetary motion describe the orbits of objects about the Sun. According to Kepler's laws, each object travels along an ellipse with the Sun at one focus. Objects closer to the Sun (with smaller semi-major axes) travel more quickly, as they are more affected by the Sun's gravity. On an elliptical orbit, a body's distance from the Sun varies over the course of its year. A body's closest approach to the Sun is called its 'Perihelion', while its most distant point from the Sun is called its 'Aphelion'. The orbits of the Planets are nearly circular, but many Comets, Asteroids and Kuiper Belt objects follow highly elliptical orbits.

• Due to 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 from the Sun than Mercury, while Saturn is 4.3 AU (astronomical unit of length equal to about 149,597,870.7 kilometres (92,955,801 miles) out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a correlation between these orbital distances (for example, the Titius-Bode law),but no such theory has been accepted.

• Most of the Planets in the Solar System possess secondary systems of their own, being orbited by planetary objects called 'Natural Satellites', or 'Moons' (some of which are larger than the planet Mercury), or, in the case of the four Gas Giants, by Planetary Rings; thin bands of tiny particles that orbit them in unison. Most of the largest Natural Satellites are in synchronous rotation, with one face permanently turned toward their parent.

• There are various substances found throughout the Solar System. These are known as Gases, Ice and Rock to describe the various classes of substances found throughout the Solar System.

i)  Rock is used to describe compounds with high condensation temperatures or melting points that remained solid under almost all conditions in the protoplanetary nebula. Rocky substances typically include Silicates and metals such as Iron And Nickel. They are prevalent in the inner Solar System, forming most of the Terrestrial Planets and Asteroids.

ii) Gases are materials with extremely low melting points and high vapour pressure such as molecular hydrogen, helium, and neon, which were always in the gaseous phase in the nebula. They dominate the middle region of the Solar System, comprising most of Jupiter and Saturn.

iii) Ices, like Water, Methane, Ammonia, Hydrogen Sulphide And Carbon Dioxide, have melting points up to a few hundred kelvins, while their phase depends on the ambient pressure and temperature. They can be found as ices, liquids, or gases in various places in the Solar System, while in the nebula they were either in the solid or gaseous phase. Icy substances comprise the majority of the Satellites of the Giant Planets, as well as most of Uranus and Neptune (also known as ice giants) and the numerous small objects that lie beyond Neptune's orbit. Together, Gases and Ices are referred to as 'Volatiles'.

Beyond Solar System

Local Interstellar Cloud - The immediate galactic neighbourhood of the Solar System is known as the 'Local Interstellar Cloud' or 'Local Fluff'. It is an area of denser cloud in an otherwise sparse region known as the 'Local Bubble', an hourglass-shaped cavity in the Interstellar Medium roughly 300 light years across. The Bubble is suffused with high-temperature plasma that suggests it is the product of several recent Supernovae (a star that explodes and becomes extremely luminous in the process). The Solar System entered the Local Interstellar Cloud at some time between 44,000 and 150,000 years ago and is expected to remain within it for another 10,000 to 20,000 years. The cloud has a temperature of about 6000 °C,  about the same temperature as the surface of the Sun. It is very tenuous, with 0.1 atoms per cubic centimetre.

Heliopause - The outer boundary of the Heliosphere, the Heliopause, is the point at which the solar wind finally terminates and is the beginning Of Interstellar Space. The point at which the Solar System ends and Interstellar Space begins is not precisely defined, since its outer boundaries are shaped by two separate forces: the Solar Wind and the Sun's Gravity. The outer limit of the Solar Wind's influence is roughly four times Pluto's distance from the Sun; this Heliopause is considered the beginning of the Interstellar Medium. However, the Sun's Roche sphere, the effective range of its gravitational dominance, is believed to extend up to a thousand times farther.

Heliosphere - The Heliosphere is divided into two separate regions. The solar wind travels at roughly 400 km/s until it collides with the interstellar wind; the flow of plasma in the interstellar medium. The collision occurs at the termination shock, which is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind. Here the wind slows dramatically, condenses and becomes more turbulent,  forming a great oval structure known as the 'Heliosheath'. This structure is believed to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind. The shape and form of the outer edge of the Heliosphere is likely affected by the fluid dynamics of interactions with the Interstellar Medium as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU (roughly 900 million miles) farther than the southern hemisphere. Beyond the Heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through the Milky Way.