The Solar System - the facts

The Solar System ( just the facts - a article written in 2007)

The Solar System or solar system comprises the Sun & the retinue of celestial objects gravitationally bound to it: the eight planets, their 162 known moons, three currently identified dwarf planets & their four known moons, & thousands of small bodies. This last category includes asteroids, meteoroids, comets, & interplanetary dust. In broad terms, the charted regions of the Solar System consist of the Sun, four rocky bodies close to it called the inner planets, an inner belt of rocky asteroids, four giant outer planets & a 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, & Neptune. Six of the eight planets are in turn orbited by natural satellites (usually termed "moons" after Earth's Moon) & every planet past the asteroid belt is encircled by planetary rings of dust & other particles. All the planets, other than the Earth, are named after gods & 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, & Eris, (no symbol), which lies beyond the Kuiper belt in a region called the scattered disc.

Definition of planet Planets & dwarf planets of the solar system. While the size is to scale, the relative distances from the Sun are not.Objects orbiting the Sun are divided into three classes: planets, dwarf planets, & small solar system bodies.

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A planet is any body in orbit around the Sun that a) has enough mass to form itself into a spherical shape & b) has cleared its immediate neighborhood of all smaller objects. There are eight known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune.

A dwarf planet is not required to clear its neighborhood of other celestial bodies. There are three known dwarf planets: Pluto, Ceres, & Eris. Other objects that may become classified as dwarf planets are Sedna, Orcus, & Quaoar.

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 & SSSB's, rather than the Sun itself.

A planet's distance around 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 most often measure distances within the solar system in astronomical units or AU. One AU is the approximate distance between the Earth & 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 & the main asteroid belt. Some define the outer Solar System as comprising everything beyond the asteroids. Others define it as the region beyond Neptune, with the four gas giants considered a separate "middle zone".
Layout & 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 & dominates it gravitationally. Jupiter & Saturn, the Sun's two largest orbiting bodies, account for more than 90% of the system's remaining mass.[b] The currently hypothetical Oort cloud would also hold a substantial percentage were its existence confirmed.

Most objects in orbit around the Sun lie near the ecliptic, a shallow plane parallel to that of Earth's orbit. The planets are very close to the ecliptic while comets & kuiper belt objects are usually at significantly greater angles to it.

All of the planets & 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 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 & 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 & the previous orbit. For example, Venus is approximately 0.33 AU farther out than Mercury, while Saturn is 4.3 AU out from Jupiter & Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a correlation between these orbital distances (see Bode's Law) but no such theory has been accepted.

Formation & evolution of the solar system, Stellar evolution, & Planetary formation

The Solar System is believed to have formed according to the nebular hypothesis, first proposed in 1755 by Immanuel Kant & independently formulated by Pierre-Simon Laplace. 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 & probably birthed several stars. 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, & 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 & cause collapse.

The region that would become the Solar System, known as the pre-solar nebula, had a diameter of between 7000 & 20,000 AU & a mass just over that of the Sun (by between 0.1 & 0.001 solar masses), 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 center, where most of the mass collected, became increasingly hotter than the surrounding disc. As gravity, gas pressure, magnetic fields, & rotation acted on the contracting nebula, it began to flatten into a spinning protoplanetary disk with a diameter of roughly 200 AU & a hot, dense protostar at the center.

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. These discs extend to several hundred AU & reach only a thousand kelvins at their hottest.

After 100 million years, the pressure & 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 fully fledged star.

Hubble image of protoplanetary discs in the Orion nebula, a light years-wide "stellar nursery" likely very similar to the primordial nebula from which our Sun formed.From the remaining cloud of gas & 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 & ten kilometres 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 course of the next few million years.

The inner solar system was too warm for volatile molecules like water & methane to condense, & so the planetesimals which formed there were relatively small (comprising only 0.6% the mass of the disc) & composed largely of compounds with high melting points, such as silicates & 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. Farther out still, beyond the frost line, where more volatile icy compounds could remain solid, Jupiter & Saturn became the gas giants. Uranus & Neptune captured much less material & 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 & dust in the protoplanetary disk into interstellar space & ended the growth of the planets. T-Tauri stars have far stronger stellar winds than more stable, older stars.

The Sun is the Solar System's parent star, & far & 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 & 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 & hotter than the Sun are rare, while stars dimmer & 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 today.

Calculations of the ratios of hydrogen & helium within the Sun suggest it is halfway through its life cycle. It will eventually move off the main sequence & become larger, brighter, cooler & redder, becoming a red giant in about five billion years.

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 & helium ("metals" in astronomical parlance) than older population II stars. Elements heavier than hydrogen & helium were formed in the cores of ancient & 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.

Interplanetary medium Along with light, the Sun radiates a continuous stream of charged particles (a plasma) known as the solar wind. This stream of particles spreads outwards at roughly 1.5 million kilometres per hour, creating a tenuous atmosphere (the heliosphere) that permeates the Solar System out to at least 100 AU (see heliopause). This is known as the interplanetary medium. The Sun's 11-year sunspot cycle & frequent solar flares & coronal mass ejections disturb the heliosphere, creating space weather. The Sun's rotating magnetic field acts on the interplanetary medium to create the heliospheric current sheet, the largest structure in the solar system.

Earth's magnetic field protects its atmosphere from interacting with the solar wind. Venus & Mars do not have magnetic fields, & the solar wind causes their atmospheres to gradually bleed away into space. The interaction of the solar wind with Earth's magnetic field creates the aurorae seen near the magnetic poles.

Cosmic rays originate outside the solar system. The heliosphere partially shields the Solar System, & planetary magnetic fields (for planets which have them) also provide some protection. The density of cosmic rays in the interstellar medium & the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic radiation in the solar system varies, though by how much is unknown.

The interplanetary medium is home to at least two disclike regions of cosmic dust. The first, the zodiacal dust cloud, lies in the inner Solar System & causes zodiacal light. It was likely formed by collisions within the asteroid belt brought on by interactions with the planets. The second extends from about 10 AU to about 40 AU, & was probably created by similar collisions within the Kuiper belt.

Terrestrial planet The four inner or terrestrial planets have dense, rocky compositions, few or no moons, & no ring systems. They are composed largely of minerals with high melting points, such as the silicates which form their solid crusts & semi-liquid mantles, & metals such as iron & nickel, which form their cores. Three of the four inner planets (Venus, Earth & Mars) have substantial atmospheres; all have impact craters & tectonic surface features such as rift valleys & volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets which are closer to the Sun than the Earth is (i.e. Mercury & Venus).
Mercury (0.4 AU) is the closest planet to the Sun & the smallest planet (0.055 Earth masses). Mercury has no natural satellites, & its only known geological features besides impact craters are "wrinkle ridges", 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 & thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, & that it was prevented from fully accreting by the young Sun's energy.
Venus (0.7 AU) is close in size to Earth (0.815 Earth masses), & , like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere & evidence of internal geological activity. However, it is much drier than Earth & 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 (1 AU) is the largest & densest of the inner planets, & the only one known to have current geological activity. Earth is the only planet known to have life. Its liquid hydrosphere, unique among the terrestrial planets, is probably the reason Earth is also the only planet where plate tectonics has been observed, because water acts as a lubricant for subduction. Earth's atmosphere is radically different from the other terrestrial planets, having been altered by the presence of life to contain 21 percent free oxygen. Earth has one satellite, the Moon; the only large satellite of a terrestrial planet in the Solar System.
Mars (1.5 AU) is smaller than Earth & Venus (0.107 Earth masses). It possesses a tenuous atmosphere of carbon dioxide. Its surface, peppered with vast volcanoes such as Olympus Mons & rift valleys such as Valles Marineris, shows geological activity that may have persisted until very recently. Mars has two tiny moons (Deimos & Phobos) thought to be captured asteroids.

Asteroids are mostly small solar system bodies composed mainly of rocky & metallic non-volatile minerals. The main asteroid belt occupies the orbit between Mars & Jupiter, between 2.3 & 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 kilometers to microscopic. All asteroids save the largest, Ceres, are classified as small solar system bodies, but some asteroids such as Vesta & 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; spacecraft routinely pass through without incident. Asteroids with diameters between 10 & 10-4 m are called meteoroids.

Ceres (2.77 AU) is the largest body in the asteroid belt & its only 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 nineteenth 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 & 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.
Trojan asteroids are located in either of Jupiter's L4 or L5 points, (gravitationally stable regions leading & trailing a planet in its orbit) though the term is also sometimes used for asteroids in any other planetary Lagrange point as well. Hilda asteroids are those Trojans whose orbits are in a 2:3 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.

The inner solar system is also dusted with rogue asteroids, many of which cross the orbits of the inner planets.

Gas giant

From top to bottom: Neptune, Uranus, Saturn, & Jupiter (sizes not to scale).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 & Saturn's atmospheres are largely hydrogen & helium. Uranus & Neptune's atmospheres have a higher percentage of “ices”, such as water, ammonia & methane. Some astronomers suggest they belong in their own category, “Uranian planets,” or “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 & 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 & helium. Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands & the Great Red Spot. Jupiter has sixty-three satellites. The four largest, Ganymede, Callisto, Io, & Europa show similarities to the terrestrial planets, such as volcanism & internal heating. Ganymede, the largest satellite in the Solar System, is larger than Mercury.
Saturn
Saturn (9.5 AU), famous for its extensive ring system, has similarities to Jupiter, such as its atmospheric composition. Saturn is far less massive, being only 95 Earth masses. Saturn has fifty-six moons; two, Titan & Enceladus, show signs of geological activity, though they are largely made of ice. Titan is larger than Mercury & 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, & radiates very little heat into space. Uranus has twenty-seven satellites, the largest ones being Titania, Oberon, Umbriel, Ariel & Miranda.
Neptune
Neptune (30 AU), though slightly smaller than Uranus, is denser at 17 Earth masses. It radiates more internal heat, but not as much as Jupiter or Saturn. Neptune has thirteen moons. The largest, Triton, is geologically active, with geysers of liquid nitrogen. Triton is the only large satellite with a retrograde orbit. Neptune possesses a number of Trojan asteroids.

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 & an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate & ionise, creating a coma: a long tail of gas & 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, such as Halley's Comet, 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 may 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 categorized as asteroids.

Kuiper belt
The area beyond Neptune, often called the outer solar system 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 & a mass far smaller than that of the Moon) composed mainly of rock & ice. 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 & 50 AU from the Sun. This region is thought to be the source of short-period comets, such as Halley's comet. It is composed mainly of small solar system bodies, but many of the largest Kuiper belt objects, such as Quaoar, Varuna, (136108) 2003 EL61, (136472) 2005 FY9 & Orcus may be reclassified as dwarf planets. 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. Many Kuiper belt objects have multiple satellites & most have orbits that take them outside the plane of the ecliptic. The Kuiper belt can be roughly divided into the "resonant" belt & the "classical" belt. The resonant belt consists of objects with orbits linked to that of Neptune (e.g. orbiting twice for every three Neptune orbits, or once for every two). The resonant belt actually begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, & 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.

Pluto & 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 & ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion.
It is unclear whether Charon, Pluto's largest moon, will continue to be classified as such or as a dwarf planet itself. Both Pluto & Charon orbit a barycenter of gravity above their surfaces, making Pluto-Charon a binary system. Two much smaller moons, Nix & Hydra, orbit Pluto & Charon.
Pluto lies in the resonant belt, having a 3:2 resonance with Neptune (it orbits twice round the Sun for every three Neptunian orbits). Kuiper belt objects which share this orbit are called Plutinos.

Scattered disc The scattered disc overlaps the Kuiper belt but extends much further outwards. Scattered disc objects are believed to come from the Kuiper belt, having been ejected into erratic orbits by the gravitational influence of Neptune's early outward migration. Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia as far as 150 AU from the Sun. SDOs' orbits are also highly inclined to the ecliptic plane, & are often almost perpendicular to it. Some astronomers consider the scattered disc to be merely another region of the Kuiper belt, & describe scattered disc objects as "scattered Kuiper belt objects."

Eris (68 AU average) is the largest known scattered disc object & caused a debate about what constitutes a planet, since it is at least 5% larger than Pluto with an estimated diameter of 2400 km (1500 mi). It is the largest of the known dwarf planets. It has one moon, Dysnomia. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) & an aphelion of 97.6 AU, & steeply inclined to the ecliptic plane.
Centaurs The Centaurs, which extend from 9 to 30 AU, are icy comet-like bodies that orbit in the region between Jupiter & Neptune. The largest known Centaur, 10199 Chariklo, has a diameter of between 200 & 250 km. The first centaur discovered, 2060 Chiron, has been called a comet since it develops a coma just as comets do when they approach the sun. Some astronomers classify Centaurs as inward scattered Kuiper belt objects along with the outward scattered residents of the scattered disc.

The heliosphere is divided into two separate regions. The solar wind travels at its maximum velocity out to about 95 AU, or three times the orbit of Pluto. The edge of this region is the termination shock, the point at which the solar wind collides with the opposing winds of the interstellar medium. Here the wind slows, condenses & becomes more turbulent, forming a great oval structure known as the heliosheath that looks & behaves very much like a comet's tail, extending outward for a further 40 AU at its stellar-windward side, but tailing many times that distance in the opposite direction. The outer boundary of the heliosphere, the heliopause, is the point at which the solar wind finally terminates, & the beginning of interstellar space.

The shape & 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's (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.

No spacecraft have yet passed beyond the heliopause, so it is impossible to know for certain the conditions in local interstellar space. How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A dedicated mission beyond the heliosphere has been suggested.

Inner Oort cloud

90377 Sedna 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 & takes 12,050 years to complete. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper Belt as its perihelion is too distant to have been affected by Neptune's migration. He & other astronomers consider it to be the first in an entirely new population, one which also may include the objects 2000 CR105, which has a perihelion of 45 AU, an aphelion of 415 AU, & an orbital period of 3420 years, & (87269) 2000 OO67, which has a perihelion of 21 AU, an aphelion of over 1000 AU, & an orbital period of 12,705 years. Brown terms this population the "Inner Oort cloud," 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.

The hypothetical Oort cloud is a great mass of up to a trillion icy objects that is believed to be the source for all long-period comets & to surround the Solar System at around 50,000 AU, & possibly to as far as 100,000 AU. 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, & can be perturbed by infrequent events such as collisions, the gravitational effects of a passing star, or the galactic tide.

Boundaries

Hypothetical planet The vast majority of our Solar System is still unknown. Its extent is determined by that of the Sun's gravitational field, which is currently estimated to concede to the gravitational forces of surrounding stars at roughly two light years (125,000 AU) distant. Some astronomers contend that the outer extent of the Oort cloud, by contrast, may not extend farther than 50,000 AU. Despite discoveries such as Sedna, the region between the Kuiper belt & the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. There are also ongoing studies of the region between Mercury & the Sun. Objects may yet be discovered in the Solar System's uncharted regions.

Galactic context

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 spiral arms, known as the Orion Arm or Local Spur. While the orbital speed & radius of the galaxy are not accurately known, estimates place the solar system at between 25,000 & 28,000 light years from the galactic center & its speed at about 220 kilometres per second, completing one revolution every 225-250 million years. This revolution is known as the Solar System's galactic year.

The Solar System's orbit appears unusual. It is both extremely close to being circular, & at nearly the exact distance at which the orbital speed matches the speed of the compression waves that form the spiral arms. Evidence suggests that the Solar System has remained between spiral arms for most of the existence of life on Earth. The radiation from supernovae in spiral arms could theoretically sterilize planetary surfaces, preventing the formation of complex life. The Solar System also lies well outside the star-crowded environs of the galactic center. There, gravitational tugs from nearby stars could perturb bodies in the Oort Cloud & send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. Even at the Solar System's current location, some scientists have hypothesised that recent supernovae may have adversely affected life in the last 35,000 years, by flinging pieces of expelled stellar core towards the Sun in the form of radioactive dust grains & larger, comet-like bodies.

The Solar apex, the direction of the Sun's path through interstellar space, is near the constellation of Hercules in the direction of the current location of the bright star Vega. At the galactic location of the solar system, the escape velocity with regard to the gravity of the Milky Way is at least 500 km/s.

The immediate galactic neighborhood of the Solar System is known as the Local Interstellar Cloud or Local Fluff, an area of dense 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.

There are relatively few stars within ten light years (95 trillion km) of the Sun. The closest is the triple star system Alpha Centauri, which is about 4.4 light years away. Alpha Centauri A & B are a closely tied pair of Sun-like stars, while the small red dwarf Alpha Centauri C (also known as Proxima Centauri) orbits the pair at a distance of 0.2 light years. The stars next closest to the Sun are the red dwarfs Barnard's Star (at 6 light years), Wolf 359 (7.8 light years) & Lalande 21185 (8.3 light years). The largest star within ten light years is Sirius, a bright blue dwarf star roughly twice the Sun's mass & orbited by a white dwarf called Sirius B. It lies 8.6 light years away. The remaining systems within ten light years are the binary red dwarf system UV Ceti (8.7 light years) & the solitary red dwarf Ross 154 (9.7 light years). Our closest solitary sunlike star is Tau Ceti, which lies 11.9 light years away. It has roughly 80 percent the Sun's mass, but only 60 percent its luminosity.
Geocentric model & Heliocentrism
For many thousands of years, people, with a few notable exceptions, did not believe the Solar System existed. The Earth was believed not only to be stationary at the centre of the universe, but to be categorically different from the divine or ethereal objects that moved through the sky. While Nicholas Copernicus & his predececessors, such as the Indian mathematician-astronomer Aryabhatta & the Greek philosopher Aristarchus of Samos, had speculated on a heliocentric reordering of the cosmos, it was the conceptual advances of the 17th century, led by Galileo Galilei, Johannes Kepler, & Isaac Newton, which led gradually to the acceptance of the idea not only that Earth moved round the Sun, but that the planets were governed by the same physical laws that governed the Earth, & therefore could be material worlds in their own right, with such earthly phenomena as craters, weather, geology, seasons & ice caps.

Timeline of solar system astronomy

The first exploration of the solar system was conducted by telescope, when astronomers first began to map those objects too faint to be seen with the naked eye.

Galileo Galilei was the first to discover physical details about the individual bodies of the Solar System. He discovered that the Moon was cratered, that the Sun was marked with sunspots, & that Jupiter had four satellites in orbit around it. Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan & the shape of the rings of Saturn. Giovanni Domenico Cassini later discovered four more moons of Saturn, the Cassini division in Saturn's rings, & the Great Red Spot of Jupiter.

Edmund Halley realised in 1705 that repeated sightings of a comet were in fact recording the same object, returning regularly once every 75-6 years. This was the first evidence that anything other than the planets orbited the Sun.

In 1781, William Herschel was looking for binary stars in the constellation of Taurus when he observed what he thought was a new comet. In fact, its orbit revealed that it was a new planet, Uranus, the first ever discovered.

Giuseppe Piazzi discovered Ceres in 1801, a small world between Mars & Jupiter that was initially considered a new planet. However, subsequent discoveries of thousands of other small worlds in the same region led to their eventual separate reclassification: asteroids.

By 1846, discrepancies in the orbit of Uranus led many to suspect a large planet must be tugging at it from farther out. Urbain Le Verrier's calculations eventually led to the discovery of Neptune. The excess perihelion precession of Mercury's orbit led Le Verrier to postulate the intra-Mercurian planet Vulcan in 1859 —but that would turn out to be a red herring.

Further apparent discrepancies in the orbits of the outer planets led Percival Lowell to conclude yet another planet, "Planet X" must still be out there. After his death, his Lowell Observatory conducted a search, which ultimately led to Clyde Tombaugh's discovery of Pluto in 1930. Pluto was, however, found to be too small to have disrupted the orbits of the outer planets, & its discovery was therefore coincidental. Like Ceres, it was initially considered to be a planet, but after the discovery of many other similarly sized objects in its vicinity it was eventually reclassified as a dwarf planet.

In 1992, astronomers David Jewitt of the University of Hawaii & Jane Luu of the Massachusetts Institute of Technology discovered (15760) 1992 QB1. This object proved to be the first of a new population, which came to be known as the Kuiper Belt; an icy analogue to the asteroid belt of which such objects as Pluto & Charon were deemed a part.

Mike Brown, Chad Trujillo & David Rabinowitz announced the discovery of Eris in 2005, a Scattered disc object larger than Pluto & the largest object discovered in orbit round the Sun since Neptune.

Space exploration Since the start of the space age, a great deal of exploration has been performed by robotic spacecraft missions that have been organized & executed by various space agencies.

All planets in the solar system have now been visited to varying degrees by spacecraft launched from Earth. Through these unmanned missions, humans have been able to get close-up photographs of all of the planets & , in the case of landers, perform tests of the soils & atmospheres of some.

The first successful probe to fly by another solar system body was Luna 1, which sped past the Moon in 1959. Mariner 2 was the first probe to fly by another planet, Venus, in 1962. The first successful flyby of Mars commenced with Mariner 4 in 1964. Mercury was first encountered by Mariner 10 in 1974.

The first probe to explore the outer planets was Pioneer 10, which flew by Jupiter in 1973. Pioneer 11 was the first to visit Saturn, in 1979. The Voyager probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 & Saturn in 1980 – 1981. Voyager 2 then went on to make close approaches to Uranus in 1986 & Neptune in 1989. The Voyager probes are now far beyond Neptune's orbit, & are on course to find & study the termination shock, heliosheath, & heliopause. According to NASA, both Voyager probes have encountered the termination shock at a distance of approximately 93 AU from the Sun.

No Kuiper belt object has yet been visited by a spacecraft. Launched on January 19, 2006, the New Horizons probe is currently enroute to becoming the first man-made spacecraft to explore this area. This unmanned mission is scheduled to fly by Pluto in July 2015. Should it prove feasible, the mission will then be extended to observe a number of other Kuiper belt objects.

In 1966, the Moon became the first solar system body beyond Earth to be orbited by an artificial satellite (Luna 10), followed by Mars in 1971 (Mariner 9), Venus in 1975 (Venera 9), Jupiter in 1995 (Galileo, which also made the first asteroid flyby, 951 Gaspra, in 1991), the asteroid 433 Eros in 2000, & Saturn in 2004 (Cassini–Huygens). The MESSENGER probe is currently en route to commence the first orbit of Mercury in 2011, while the Dawn spacecraft is currently set to orbit the asteroid Vesta in 2011 & the dwarf planet Ceres in 2015. The first probe to land on another solar system body was the Soviet Union's Luna 2 probe, which impacted the Moon in 1959. Since then, moredistant planets have been reached, with probes landing on or impacting the surfaces of Venus in 1966 (Venera 3), Mars in 1971 (Mars 3, although a fully successful landing didn't occur until Viking 1 in 1976), the asteroid 433 Eros in 2001 (NEAR Shoemaker), & Saturn's moon Titan in 2005 (Huygens). The Galileo orbiter also dropped a probe into Jupiter's atmosphere in 1995; since there is no physical surface of Jupiter, it was simply designed to burn up in the atmosphere & does not count as a landing.
An Index with links to almost all our sites.

SOLAR SYSTEM RECORD BREAKERS, facts like the tallest mountain for the planets

A site saying the 10 most famous aliens ever

A Clock saying how many people have landed on each planet in our Solar System

Jokes about Space & Aliens

A List of space associated TV programmes & Movies

A Multiple Choice Quiz on Space

Mars - the facts

The Moon Deimos The Moon Phobos

A Poem on space

Some interesting facts about Space

How Many people have landed on each of our Planets see, in this space clock

A list of Planets that could be colonised by humanity

The Yeti with a UFO above

A Joke site on the Moon

Saturn - the facts

Some plans for colonising Venus The Oort Cloud - the facts

Jupiter - the facts

The Moon - the facts

A Quiz on space related TV & movies

The facts on Space Stations Jodrell Bank the facts

Ganymede the facts The Facts on the Moon Europa

Asteroids - the facts The Kuiper Belt - the facts

The Planet Pluto - the facts Space - the facts

The Milky Way - the facts Star Trek - the facts

The facts on the first Extra Solar planet to be known about by Humans PSR B1257 + 12, the first planet known, of outside our Solar System,
The Galaxy andromeda The Star Eta Carinae
Moon Buggies - the facts
Extrasolar plantes the facts

Present Idea website, A top Class website for people thinking up that Gift Idea
The Planet Mercury - the facts The Planet Neptune - the facts
The Planet Uranus - the facts
More records & amazing facts to do with the Solar System, & space & the Universe

Things to do on the Moon

Some ideas for stories, & movie ideas I have had, including some on space

The History Lounge - Where you can peruse & mull over a massive range of great historical related web sites.
The Entrance to the INTERNET SAFARI, with real animals, most of us had never seen before.
Global Geography, sites like what are the 10 largest English speaking countries, 10 largest Celtic cities, biggest forests, volcanoes,
Which are the 10 most powerful countries in 2008___
SOME FUN CLEVER COOL GAMES__
10 Biggest Banks Histories Famous Gates Famous Walls Quizzes Famous Roads Internet Sea Safari, The Most Powerful Countries Ever