Solar System

Image credit: Lunar and Planetary Laboratory, NASA. The planets. From left to right: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Jupiter's diameter is about 11 times that of the Earth's and the Sun's diameter is about 10 times Jupiter's. Pluto's diameter is slightly less than one-fifth of Earth's and it is now classed as a dwarf planet. The planets are not shown at the appropriate distance from the Sun but sizes are to scale. High Resolution

The Solar System is a planetary system containing a central star, the Sun, with gravitationally bound, orbiting members including the Earth, seven other planets with their moons, dwarf planets and their moons, and thousands of other small bodies: asteroids, comets, meteors, and interplanetary dust. The Solar System orbits the core of the Milky Way galaxy, along with billions of other stars.

It is divided into regions. They are, in order of proximity to the Sun: the four inner planets closest to the Sun, an inner belt of asteroids, the four giant outer planets, the Kuiper belt (a belt of asteroids and icy bodies), a region called the scattered disc, the heliopause, which marks the boundary of the Sun's radiation, and a hypothetical region known as the Oort Cloud.

The planets orbit the Sun in the following order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Six of these planets have their own natural satellites (usually termed "moons" after Earth's Moon). In addition, the four giant planets are encircled by planetary rings of dust and other particles.

There are five dwarf planets: Pluto, the largest known Kuiper belt object; Haumea and Makemake, also in the Kuiper belt; Ceres, the largest object in the asteroid belt; and Eris in the scattered disc.

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Terminology

Planet

According to the International Astronomical Union (IAU), objects orbiting the Sun are divided into three classes: planets, dwarf planets, and small solar system bodies.

A planet is any body

• (a) in orbit around the Sun
• (b) that has enough mass to form itself into a spherical shape
• (c) that has cleared its immediate neighborhood of all smaller objects.

There are eight known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto was originally classified as a planet but is now classified as a dwarf planet.

Dwarf planet

A dwarf planet meets two of the three IAU planetary requirements:

• (a) it is in orbit around the Sun
• (b) it has enough mass to form itself into a spherical shape

Given their much smaller mass, dwarf planets are not required to clear their neighborhood of other celestial bodies.

There are five known dwarf planets: Pluto, Ceres, Haumea, Makemake and Eris. Other objects that may in future be classified as dwarf planets include Sedna, Orcus, and Quaoar.

Small solar system bodies

The remainder of the objects in orbit around the Sun are small solar system bodies (SSSBs).

Natural satellites

Natural satellites, or moons, are those objects in orbit around planets, dwarf planets and SSSB's, rather than the Sun itself.

Distances and zones

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 (AU). One AU is the approximate distance between the Earth and the Sun or roughly 149 598 000 km (93,000,000 mi). Pluto is roughly 38 AU from the Sun while Jupiter lies at roughly 5.2 AU. One light year, the best known unit of interstellar distance, is roughly 63,240 AU.

Informally, the Solar System is sometimes divided into separate zones. The inner Solar System includes the four terrestrial planets and the main asteroid belt. Some define the outer Solar System as comprising everything beyond the asteroids. Others define it as the region beyond Neptune, with the four gas giants considered a separate "middle zone".

Layout and structure

The principal component of the Solar System is the Sun, a main sequence G2 star that contains 99.86% of the system's known mass and dominates it gravitationally. Jupiter and 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 and kuiper belt objects are usually at significantly greater angles to it.

All of the planets and most other objects also orbit with the Sun's rotation in a counter-clockwise direction as viewed from a point above the Sun's north pole. There are exceptions, such as Halley's Comet. Objects travel around the Sun following Kepler's laws of planetary motion. Each object orbits along an ellipse with the Sun at one focus of the ellipse. The closer an object is to the Sun the faster it moves. The orbits of the planets are nearly circular, but many comets, asteroids and objects of the Kuiper belt follow highly elliptical orbits.

To cope with the vast distances involved, many representations of the Solar System show orbits the same distance apart. In reality, with a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. For example, Venus is approximately 0.33 AU farther out than Mercury, while Saturn is 4.3 AU out from Jupiter and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a correlation between these orbital distances (see Bode's Law) but no such theory has been accepted.

Formation

The Solar System is believed to have formed according to the nebular hypothesis, first proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon Laplace. 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 and 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, and in range of a number of nearby supernovae explosions. The shock wave from these supernovae may have triggered the formation of the Sun by creating regions of overdensity in the surrounding nebula, allowing gravitational forces to overcome internal gas pressures and cause collapse.

The region that would become the Solar System, known as the pre-solar nebula, had a diameter of between 7000 and 20,000 AU and a mass just over that of the Sun (by between 0.1 and 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, and rotation acted on the contracting nebula, it began to flatten into a spinning protoplanetary disk with a diameter of roughly 200 AU and 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 and reach only a thousand kelvins at their hottest.

After 100 million years, the pressure and density of hydrogen in the centre of the collapsing nebula became great enough for the protosun to begin thermonuclear fusion. This increased until hydrostatic equilibrium was achieved, with the thermal energy countering the force of gravitational contraction. At this point the Sun became a fully fledged star.

From the remaining cloud of gas and dust (the "solar nebula"), the various planets formed. They are believed to have formed by accretion: the planets began as dust grains in orbit around the central protostar; then gathered by direct contact into clumps between one and ten 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 and methane to condense, and so the planetesimals which formed there were relatively small (comprising only 0.6% the mass of the disc) and composed largely of compounds with high melting points, such as silicates and metals. These rocky bodies eventually became the terrestrial planets. Farther out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the asteroid belt.

Farther out still, beyond the frost line, where more volatile icy compounds could remain solid, Jupiter and Saturn became the gas giants. Uranus and Neptune captured much less material and are known as ice giants because their cores are believed to be made mostly of ices (hydrogen compounds).

Once the young Sun began producing energy, the solar wind (see below) blew the gas and dust in the protoplanetary disk into interstellar space and ended the growth of the planets. T-Tauri stars have far stronger stellar winds than more stable, older stars.

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 today.

Calculations of the ratios of hydrogen and helium within the Sun suggest it is halfway through its life cycle. It will eventually move off the main sequence and become larger, brighter, cooler and 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 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.

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 and frequent solar flares and 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 and Mars do not have magnetic fields, and 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, and planetary magnetic fields (for planets which have them) also provide some protection. The density of cosmic rays in the interstellar medium and 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 and 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, and was probably created by similar collisions within the Kuiper belt.

Inner planets

(PD) Image: NASA
NASA Montage of inner and outer planets

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 solid crusts and semi-liquid 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 the 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 "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 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.

Mercury's sidereal period is 87.97 days per year. Mercury's distance from Sun is 0.31 AU at perihelion (the closest approach to the sun), 0.47 at aphelion (its nearest approach and it has an orbital inclination of 7.0° relative to the solar plane.

Mercury’s surface temperature fluctuates from more than 400°C to -180°C . Planetary rotation means that alternating areas are exposed to the sun’s heat, with the result that the surface of a planet will heat and cool in alternating periods. However, while Mercury does in fact rotate about its axis, Mercury’s rotational and orbital periods are coupled, that is, nearly the same. This means that some places on Mercury’s surface receive 2.5 times more solar radiation than other areas. As the planet closest to the sun it receives the highest ratio of solar radiation. Solar radiation decreases by the inverse square law as it reaches further away from the sun. The Moon, by comparison, which also has no atmosphere but is much further from the sun that Mercury reaches temperatures of only about 110°C

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 110 times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures as high as 470 °C, most likely due to the amount of greenhouse gases, predominantly CO2, in the atmosphere which retain solar radiation. 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, 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

Mars (1.5 AU) is smaller than Earth and Venus (0.107 Earth masses). It possesses a tenuous atmosphere of 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. Mars has two tiny moons, 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 kilometers 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; spacecraft routinely pass through without incident. Asteroids with diameters between 10 and 10^-4 m are called meteoroids.

Ceres

Ceres (2.77 AU) is the largest body in the asteroid belt and 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 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.

Trojan asteroids are located in either of Jupiter's L₄ or L₅ points, (gravitationally stable regions leading and 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.

Outer planets

(PD) Image: NASA
NASA Montage of inner and 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's atmospheres are largely hydrogen and helium. Uranus and Neptune's atmospheres have a higher percentage of “ices”, such as water, ammonia and 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 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 satellites. The four largest, Ganymede, Callisto, Io, 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), famous for its extensive ring system, has similarities to Jupiter, such as its atmospheric composition, but it is far less massive, being only 95 Earth masses. Saturn has fifty-six moons: two, 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 satellites, the largest ones being Titania, Oberon, Umbriel, Ariel and 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

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, 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 of small bodies (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.

Named for Gerard Kuiper who postulated the existence of a belt region in 1951. He was attempting to provide an solution of the origin of some comets.

The first Kuiper Belt Objects (KBOs) were discovered by Dave Jewitt (University of Hawaii) and Jane Luu (UC Berkeley)in 1992. Their first find was an object 44 AU from the Sun outside the orbit of Pluto, now designated 1992 QB1.

There are an estimated 70,000 "trans-Neptunians" objects with diameters larger than 100 km in the radial zone which extends outwards in a ring from the orbit of Neptune at 30 AU to 50 AU orbiting the Sun in the ecliptic plane of the solar system.

The Kuiper Belt extends between 4.5 to 7.5 billion km (2.8 billion to 4.6 billion miles), or 30 and 50 AU from the Sun with a total mass of only a tenth or even a hundredth the mass of the Earth. Many Kuiper belt objects have multiple satellites and most have orbits that take them outside the plane of the ecliptic.

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.

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, 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, and are often almost perpendicular to it. 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."

Eris

Eris (68 AU average) is the largest known scattered disc object and 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) and an aphelion of 97.6 AU, and 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 and Neptune. The largest known Centaur, 10199 Chariklo, has a diameter of between 200 and 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.

Heliosphere

The heliosphere is divided into two separate regions. The solar wind travels at its maximum velocity out to about 75-90 AU, or three times the orbit of Pluto.

Solar Wind Loses Power, Hits 50-year Low.

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 and drops below the speed of sound. Here the wind slows, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath that looks and 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 ions and the galactic ions make contact at about 110 AU and the beginning of interstellar space.

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'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 and 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 and other astronomers consider it to be the first in an entirely new population, one which also may include the objects 2000 CR|105, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years, and 2000 OO|67, which has a perihelion of 21 AU, an aphelion of over 1000 AU, and 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.

Oort cloud

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 and to surround the Solar System at around 50,000 AU, and 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, and can be perturbed by infrequent events such as collisions, the gravitational effects of a passing star, or the galactic tide.

Boundaries

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 and 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 and 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 and radius of the galaxy are not accurately known, estimates place the solar system at between 25,000 and 28,000 light years from the galactic center and 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, and 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 and 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 and 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 and 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) and 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 and 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) and 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.

Discovery and exploration

See also Geocentric model, Heliocentrism

For most of human history, 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 and his predececessors, such as the Indian mathematician-astronomer Aryabhatta and 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, and 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, and therefore could be material worlds in their own right, with such earthly phenomena as craters, weather, geology, seasons and ice caps.

Telescopic observations

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, and that Jupiter had four satellites in orbit around it. Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn. Giovanni Domenico Cassini later discovered four more moons of Saturn, the Cassini division in Saturn's rings, and 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.

New planetary discoveries

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 and 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, and 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 reclassified in 2006 as one of three then-designated dwarf planets.

In 1992, astronomers David Jewitt of the University of Hawaii and Jane Luu of the Massachusetts Institute of Technology discovered 1992 QB|1. 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 and its large moon Charon were deemed a part.

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

Observations by spacecraft

Since the start of the space age, a great deal of exploration has been performed by robotic spacecraft missions that have been organized and 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 and, in the case of landers, perform tests of the soils and atmospheres of some.

The first successful probe to fly by another solar system body was Luna 1 (part of Project Luna), 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 and Saturn in 1980 – 1981. Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989. The Voyager probes are now far beyond Neptune's orbit, and are on course to find and study the termination shock, heliosheath, and 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 en route 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, and 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 and 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, increasingly distant 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), and 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 and does not count as a landing.

Notes

1. ^{^}Capitalization of the name varies. The IAU, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects (Solar System). However, the name is commonly rendered in lower case (solar system) including in the Oxford English Dictionary, Merriam-Webster's 11th Collegiate Dictionary, and Encyclopædia Britannica.
2. ^{^}The mass of the Solar System excluding the Sun, Jupiter and Saturn can be determined by adding together all the calculated masses for its largest objects and using rough calculations for the mass of the Kuiper Belt (estimated at roughly 0.1 Earth mass) and the asteroid belt (estimated to be 0.0005 Earth mass)

See also

• List of solar system objects: By orbit—By mass—By radius—By name
• Attributes of the largest solar system bodies
• Astronomical symbols
• Geological features of the solar system
• Numerical model of solar system
• Table of planetary attributes
• Timeline of discovery of solar system planets and their natural satellites
• Solar system model
• Space colonization
• Solar System in fiction
• Celestia - Space-simulation on your computer (OpenGL)
• Family Portrait (Voyager)