Solar system

The solar system comprises the Sun and the retinue of celestial objects gravitationally bound to it: nine planets and their 158 currently known moons, as well as asteroids, meteoroids, planetoids, comets, and interplanetary dust. Astronomers are debating the classification of a potential tenth planet and other trans-Neptunian objects.

The principal component of the solar system is the Sun (astronomical symbol ☉); a main sequence G2 star that contains 99.86% of the system's known mass and dominates it gravitationally. Because of its large mass, the Sun has an interior density high enough to sustain nuclear fusion, releasing enormous amounts of energy, most of which is radiated into space in the form of electromagnetic radiation, including visible light. The Sun's two largest orbiting bodies, Jupiter and Saturn, together account for more than 90% of the system's remaining mass. (The Oort cloud too might hold a substantial percentage, but as yet its existence is unconfirmed).

), Neptune (), and Pluto (). Many planets have moons orbiting them, and the largest are encircled by planetary rings of dust and other particles. The planets (with the exception of Earth) are named after gods and goddesses from Greco-Roman mythology.

Most objects in orbit round the Sun lie within the same shallow plane, called the ecliptic plane, which is roughly parallel to the Sun's equator. The major planets, with the exception of Pluto, lie very close to the plane, while comets and kuiper belt objects often lie at significant angles to it. The majority of solar system objects also orbit in the same direction in which the Sun rotates. Although no major planet's orbit is a true circle, all save Pluto have roughly circular orbits.

Layout and distances

Astronomers most often measure distances within the solar system in astronomical units, or AU. One AU is the mean distance between the Earth and the Sun, or roughly 149 598 000 kilometres. Other units in common use include the gigametre (Gm, one million kilometres) and the terametre (Tm, one billion/milliard kilometres). Pluto is roughly 38 AU (5.9 Tm) from the Sun, while Jupiter lies at roughly 5.2 AU (778 Gm).

All objects in orbit around the Sun have elliptical orbits. An ellipse is a circle stretched in one dimension, therefore a planet's distance from the Sun varies in the course of its year. It's closest approach to the Sun is known as its perihelion, while its farthest point from the Sun is called its aphelion. Though the majority of major planets follow nearly circular orbits, with perihelions roughly equal to their aphelions, Pluto and the objects of the Kuiper belt follow highly elliptical orbits, with their perihelions and aphelions widely spaced apart.

There is a direct relationship between how far away a planet is from the Sun, and how quickly it orbits. Mercury, which is closest to the Sun, not only has the smallest orbital circumference but also travels the fastest, while Pluto, since it is much farther from the Sun, travels more slowly.

By and large, the planets within our solar system are arranged so that each is roughly double the distance from the Sun as the one before it. Venus is roughly twice as far from the Sun as Mercury, Earth is roughly double the distance as Venus, Mars double that of Earth etc. This relationship is expressed in the Titius-Bode rule, a mathematical formula for predicting the semi-major axes of planets in AU. In its simplest form, it is written:

\mathbf{a} = 0.4 + 0.3k

(where k=0, 1, 2, 4, 8, 16, 32, 64, 128)

By this formulation, one would expect Mercury's orbit (k=0) to be 0.4 AU from the Sun, and Mars's orbit (k=4) to be 1.6 AU. In fact, the actual figures are 0.387 and 1.524 AU. Ceres, the largest asteroid, lies at k=8.

This rule is only a rough guide, and doesn't fit all of the planets. After Uranus, Neptune has to be skipped; Pluto lies at the next predicted distance. There is no scientific explanation for this rule, and many claim it is merely a coincidence, falling in the region of uncomfortable science.

See also: Kepler's laws, Titius-Bode Law

Sun

The Sun is the solar system's parent star, and far and away its chief component. It is classed as a moderately large yellow dwarf; however, this name is misleading, as on the scale of stars in our galaxy, the Sun is rather large and bright. The Sun is placed near the middle of the Hertzsprung-Russell diagram, but stars larger and hotter than it are rare, whereas stars dimmer and cooler than it are very common. The vast majority of stars are red dwarfs, though their inherent dimness means they are under-represented in star catalogues, as we can observe only those few that are very near the Sun in space.

The Sun lies on the main sequence of the H-R diagram, which means, according to current theories of stellar evolution, that it is in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion, and been forced, as older red giants must, to fuse more inefficient elements such as helium and carbon. The Sun is growing increasingly bright as it ages. Early in its history, it was roughly 75 percent as bright as it is today. Calculations of the ratios of hydrogen and helium within the Sun suggest it is roughly halfway though its life cycle, and will eventually begin moving off the main sequence, becoming larger, brighter and redder, until, about five billion years from now, it too will become a red giant.

The Sun is a population I star, meaning that it is fairly new in galactic terms, having been born in the later stages of the universe's evolution. As such, it contains far more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars such as those found in globular clusters. Since elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, the first generation of stars had to die before the universe could become enriched with them. For this reason, the very oldest stars contain very little "metal", while stars born later have more. This high "metallicity" is thought to have been crucial in the Sun's developing a planetary system, since planets form from accretion of metals.

The Sun radiates a continuous stream of charged particles, a plasma known as solar wind, ejecting it outwards at speeds of over 2 million kilometres per hour, creating a very tenuous "atmosphere" (the heliosphere), that permiates the solar system for at least 100 AU. This environment is known as the interplanetary medium. Small quantities of cosmic dust (some of it arguably interstellar in origin) are also present in the interplanetary medium and are responsible for the phenomenon of zodiacal light. The influence of the Sun's rotating magnetic field on the interplanetary medium creates the largest structure in the solar system, the heliospheric current sheet.

Earth's magnetic field protects its atmosphere from interacting with the solar wind; however, Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space.

Inner planets

The four inner or terrestrial planets are characterised by their dense, rocky composition, lack of primary atmospheres, and few or no moons or ring systems. They are composed largely of minerals with high melting points such as silicates to form the planets' solid crusts and semi-liquid mantles, and metallic dust grains such as iron, which forms their cores. All have impact craters and many possess 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).

The four inner planets are:

Mercury

Mercury (0.4 AU), the closest planet to the Sun, is also the least massive of the inner planets, at only 0.055 Earth masses. It has no atmosphere, no natural satellite, and, to date, no observed geological activity save that produced by impacts. 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 Sun's gravity. The upcoming MESSENGER probe should aid in resolving this issue.

Venus

Venus (0.7 AU), the first truly terrestrial planet, is of comparable mass to the Earth (0.815 Earth masses), and, like Earth, possesses a thick silicate mantle around an iron core, as well as a substantial atmosphere and evidence of one-time internal geological activity, such as volcanoes. However, It is much drier than Earth and its atmosphere is 90 times as dense and is composed overwhelmingly of carbon dioxide and sulfuric acid. Unlike Earth, evidence suggests that Venus's crust is not divided into tectonic plates but instead comprises a single, very thick rind. Distribution of impact craters suggests that Venus's surface features are all of the same, relatively young age, suggesting that they are periodically erased by sudden, massive volcanism. However, recent computer remodelling suggests the resurfacing could have been as gradual as 2 billion years. Venus has no natural satellite.

Earth and Moon

The largest and densest of the inner planets, Earth (1 AU) is also the only one to demonstrate unequivocal evidence of ongoing geological activity. Its liquid hydrosphere, unique among the terrestrials, is probably the reason why Earth is also the only planet where multi-plate tectonics has been observed, since water acts as a lubricant for subduction. Its atmosphere is radically different from the other terrestrials, having been altered by the presence of life to contain 21 percent free oxygen. Its satellite, the Moon, is sometimes considered a terrestrial planet in a co-orbit with its partner, since its orbit around the Sun never actually loops back on itself when observed from above. The Moon possesses many features in common with other terrestrial planets, though it lacks an iron core.

Mars

Mars (1.5 AU), at only 0.107 Earth masses, is less massive than either Earth or Venus. It possesses a tenuous atmosphere of carbon dioxide. Its surface, peppered with vast volcanoes and rift valleys such as Valles Marineris, shows that it was once geologically active and recent evidence suggests this may have been true until very recently. Mars possesses two tiny moons (Deimos and Phobos) thought to be captured asteroids.

Asteroids

Asteroids are objects smaller than planets that are composed in significant part of rocky, 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 the remnants of a small terrestrial planet that failed to coalesce due to the gravitational interference of Jupiter. It contains tens of thousands (possibly millions) of asteroids over 1 km across, though they can be as small as dust. Despite their large numbers, the total mass of the main asteroid belt is unlikely to be more than a thousandth of that of the Earth. In contrast to its various depictions in science fiction, the main belt is very sparsely populated; several probes have passed through it without incident. Asteroids with a diameter of less than 50 m are called meteoroids. The largest asteroid, Ceres, has a diameter of almost 1000 km; large enough to be spherical, which would make it a planet by some definitions of the word.

Asteroids in the main belt are subdivided into asteroid groups and Asteroid groups and families based on their specific 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, though the term is also sometimes used for asteroids in any other planetary Lagrange point as well.

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

Outer planets

The four outer planets, or gas giants, (sometimes called Jovian planets) are so large they collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn are true giants, at 318 and 95 Earth masses, respectively, and composed largely of hydrogen and helium. Uranus and Neptune are both substantially smaller, being only 14 and 17 Earth masses, respectively. Their atmospheres contain a smaller percentage of hydrogen and helium, and a higher percentage of “ices”, such as water, ammonia and methane. For this reason some astronomers suggested that they belong in their own category, “Uranian planets,” or “ice giants.” The term outer planet should not be confused with superior planet, which designates those planets which lie outside Earth's orbit (thus consisting of the outer planets plus Mars).

Jupiter

Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. Its composition of largely hydrogen and helium is not very different from that of the Sun. Jupiter's atmosphere possesses a number of semi-permanent features, such as cloud bands and the great red spot. Three of its 63 satellites, Ganymede, Io and Europa, share elements in common with the terrestrial planets, such as volcanism and internal heating. Ganymede has a larger diameter than either Mercury or Pluto. Jupiter has a faint, smoky ring. Jupiter's intense gravitational pull attracts many comets, and may have played a role in lowering the number of impacts Earth has experienced in its history.

Saturn

Saturn (9.5 AU), famous for its extensive ring system, has many qualities in common with Jupiter, including its atmospheric composition, though it is far less massive, being only 95 Earth masses. Two of its 49 moons, Titan and Enceladus, show signs of geological activity, though they are largely made of ice. Titan, like Ganymede, is larger than both Pluto and Mercury and is the only satellite in the solar system with a substantial atmosphere.

Uranus

Uranus (19.6 AU) at 14 Earth masses, is the smallest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt lies at over ninety degrees to the ecliptic. Its core is remarkably cold, radiating almost no heat into space. This has led some to speculate that, unlike the similar Neptune, Uranus is undifferentiated and has no core. The lack of internal heat means that Uranus's surface features are relatively bland, with little in the way of cloud bands. Uranus has 27 moons, five of which are relatively large, though none show any evidence of geological activity. Its ring system is dark and insubstantial, and composed of sparse fragments larger than 50 m in diameter.

Neptune

Neptune (30 AU), is slightly larger than Uranus, at 17 Earth masses, and radiates far more internal heat. Its peculiar ring system is composed of a number of dense "arcs" of material separated by gaps. Neptune's largest moon, Triton, is geologically active, with geysers of liquid nitrogen. The heat at Neptune's core drives some of the fastest winds in the solar system. Neptune possesses marked surface features and cloud bands, though they appear far more changeable than those of Jupiter.

Comets

Comets are small bodies (usually only a few kilometres across) composed largely of volatile ices and have highly eccentric orbits, generally having a perihelion within the orbit of the inner planets and an aphelion far beyond Pluto. When a comet approaches the Sun, its icy surface begins to sublimate, or boil away, creating a coma, a long tail of gas and dust which is often visible with the naked eye. Short-period comets exist with apoapses closer than this, however, and old comets that have had most of their volatiles driven out by solar warming are often categorized as asteroids. Long period comets have orbits lasting thousands of years, and are believed to originate in the Oort Cloud (see below). Some comets with hyperbolic orbits may originate outside the solar system.

Centaurs are icy comet-like bodies that have less-eccentric orbits so that they remain in the region between Jupiter and Neptune. The first centaur to be discovered, 2060 Chiron, has been called a comet since it has been shown to develop a coma just as comets do when they approach the sun.

Kuiper belt

The area beyond Neptune, often referred to as the outer solar system or simply the "trans-Neptunian region", is still largely unexplored.

This region's first formation, which actually begins inside the orbit of Neptune, is the Kuiper belt, a great ring of debris, similar to the asteroid belt but composed mainly of ice and far greater in extent, which lies between 30 and 50 AU from the Sun. This region is thought to be the place of origin for short-period comets, such as Halley's comet. Though there are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km, the total mass of the Kuiper belt is relatively low, perhaps barely equalling 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

Astronomers consider Pluto, (38 AU average) the solar system's smallest planet, to be part of the Kuiper Belt population. In fact more and more astronomers are beginning to believe Pluto should no longer be classified as a planet, but only a Kuiper Belt object, since other objects nearly as large and indeed larger then Pluto have been found within the belt. If this is the case then either Pluto should be downgraded from its planet status, or all of these objects need to be classified as planets.

Like other objects in the Kuiper belt, 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. The solar wind is gradually sublimating Pluto's surface into space, in the manner of a comet. If Pluto were placed near the Sun, it would develop a tail, like comets do. Although accepted by the public as a planet since its discovery in 1930, debates about Pluto's identity within the scientific community are still unresolved. Pluto has a large moon (the largest in the solar system relative to its own size), called Charon, as well as two much smaller moons called Nix and Hydra. Like the Earth/Moon, Pluto and Charon are often considered a double planet.

Kuiper belt objects which, like Pluto, possess a 3:2 orbital resonance with Neptune (ie, they orbit twice for every thee Neptunian orbits) are called Plutinos. Other Kuiper belt objects have different resonant orbits (2:1, 4:7, 3:5 etc) and are grouped accordingly. The remaining Kuiper belt objects, in more "classical" orbits, are classified as Cubewanos.

The scattered disc

Overlapping the Kuiper belt but extending much further outwards is the scattered disc. Scattered disc objects are believed to have been originally native to the Kuiper belt, but were ejected into erratic orbits in the outer fringes by the gravitational influence of Neptunes migration (see Origin and evolution, below). Most scattered disc objects have perihelia within the Kuiper belt but aphelia as far as 150 AU from the Sun. Their orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers, such as David Jewitt, consider the scattered disc to be merely another region of the Kuiper belt, and describe scattered disc objects as "scattered Kuiper belt objects."

2003 UB313 ("Xena")

One particular scattered disc object, originally found in 2003 but confirmed two years later by Mike Brown (Caltech), David Rabinowitz (Yale University), and Chad Trujillo (Gemini Observatory), has renewed the old 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 currently has no name, but has been given the provisional designation , and has been nicknamed "Xena" by its discoverers, after the television character. The object has many similarities with 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 is steeply inclined to the ecliptic plane, at 44 degrees, more so than any known object in the solar system except the newly-discovered object , also known as "Buffy". Like Pluto, it is believed to consist largely of rock and ice, and has a moon Whether it and the largest Kuiper belt objects should be considered planets or whether instead Pluto should be reclassified as a minor planet has not yet been resolved.

Farthest regions

The point at which the solar system ends and interstellar space begins is not precisely defined, since its outer boundaries are delineated by two separate forces: the solar wind and the Sun's gravity. The solar wind extends to a point roughly 130 AU from the Sun, whereupon it surrenders to the surrounding envionment of the interstellar medium. It is generally accepted, however, that the Sun's gravity holds sway to the Oort cloud. This great mass of up to a trillion icy objects, currently hypothetical, is believed to be the source for all long-period comets and to surround the solar system like a shell from 50,000 to 100,000 AU beyond the Sun, or almost a quarter the distance to the next star system. The vast majority of the solar system, therefore, is completely unknown; however, recent observations of both our solar system and others have led to an increased understanding of what is or may be lying at its outer edge.

Sedna

Sedna is a large, reddish Pluto-like object with a gigantic, highly elliptical 10,500-year orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper Belt as it has too distant a perihelion 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 object , which has a parehelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years. Some astronomers have termed this region the "Inner Oort cloud," part of a wider disc extending from the scattered disc to the Oort cloud proper. However, others have speculated that Sedna and its compatriots owe their unique orbits to the effects of a star which passed close by the Sun early in its history, or, perhaps more improbably, were once in orbit around a passing brown dwarf but became caught in the Sun's gravitational hold.

The heliopause

The heliosphere expands outward in a great bubble to about 95 AU, or three times the orbit of Pluto. The edge of this bubble is known as the termination shock; the point at which the solar wind collides with the opposing winds of the interstellar medium. 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 sheath, the heliopause, is the point at which the solar wind finally terminates, and one enters the environment of interstellar space. 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.

Origin and evolution

Using radiometric dating, scientists can estimate that the solar system is 4.6 billion years old. The oldest rocks on Earth are approximately 3.9 billion years old. Rocks this old are rare, as the Earth's surface is constantly being reshaped by erosion, volcanism and plate tectonics. To estimate the age of the solar system scientists must use meteorites, which were formed during the early condensation of the solar nebula. The oldest meteorites (such as the Canyon Diablo meteorite) are found to have an age of 4.6 billion years, hence the solar system must be at least 4.6 billion years old.

The current hypothesis of solar system formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon Laplace. The nebular theory holds that the solar system was formed from the gravitational collapse of a gaseous cloud called the solar nebula. It had a diameter of ± 100 AU and was 2-3 times the mass of the Sun. Over time a disturbance (possibly a nearby supernova) squeezed the nebula, pushing matter inward until gravitational forces overcame the internal gas pressure and it began to collapse. As the nebula collapsed, conservation of angular momentum meant that it spun faster, and became warmer. As the competing forces associated with gravity, gas pressure, magnetic fields, and rotation acted on it, the contracting nebula began to flatten into a spinning protoplanetary disk with a gradually contracting protostar at the center.

Grains of dust (silicates and metals) and ice (hydrogen compounds) condensed from the gas, and began to accrete into larger and larger clumps, forming planetesimals. Inside the frost line, planetesimals were composed of rock and metal, because those are the only grains that can condense at those temperatures, and remained relatively small because they were only 0.6% the mass of the disk. These rocky bodies eventually became the terrestrial planets. The larger icy planetesimals beyond the frost line became massive enough to capture and hold onto helium and then hydrogen gases, which caused them to rapidly grow into the gas giants.

One problem with this hypothesis is that of angular momentum. With the vast majority of the system's mass accumulating at the centre of the rotating cloud, the hypothesis predicts that the vast majority of the system's angular momentum should accumulate there as well. However, the Sun's rotation is far slower than expected, and the planets, despite accounting for less than 1 percent of the system's mass, thus account for more than 90 percent of its angular momentum. One resolution of this problem is that dust grains in the original disc created drag which slowed down the rotation in the centre.

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, which increased until hydrostatic equilibrium was achieved. The young Sun's solar wind then cleared away all the gas and dust in the protoplanetary disk, blowing it into interstellar space, thus ending the growth of the planets. A number of current models suggest that, some 600 million years later, the orbits of the giant planets shifted so that Jupiter and Saturn fell into a 1:2 resonance (that is, for every one orbit of Saturn, Jupiter completed two orbits). This resonance created a strong gravitational pull, which ultimately ejected Neptune out to twice its previous orbital distance. This in turn caused it to disturb a ring of icy debris beyond it, scattering many of its members farther into space, (creating the Kuiper belt and the scattered disc) and sending countless more in toward the Sun, smashing into the terrestrial planets in an event known as the "great bombardment." The effects of this bombardment can still be seen on the Moon's cratered surface.

Future

Barring some unforeseeable accident, such as the arrival of a rogue black hole or star into its territory, it is estimated that the solar system as we know it today will last another billion years or so, whereupon the Sun will claim its first casualty, the Earth. As the Sun brightens a further ten percent beyond today's levels, its radiation output will increase, gradually searing the Earth until its land surface becomes uninhabitable, though life could still survive in the deeper oceans. Within 3.5 billion years, Earth will attain surface conditions similar to Venus's today; the oceans will boil, and all life will be impossible. With the hydrogen reserves within its core spent, the Sun will begin to use those in its less dense upper layers. This will require it to expand to eighty times its current diameter, and, about 7.5 billion years from now, to become a red giant, cooled and dulled by its vastly increased surface area. As the Sun expands, it will swallow the planet Mercury. Earth and Venus, however, are expected to survive, since the Sun will lose about 28 percent of its mass, and its lower gravity will send them into higher orbits. Earth will be left a scorched cinder, its land surface reduced to the consistency of hot clay by sunlight a thousand times more powerful than today's, and its atmosphere stripped away by a now-ferocious solar wind. The Sun is expected to remain in a red giant phase for about a hundred million years.

During this time, it is possible that the watery worlds around Jupiter and Saturn, such as Titan and Europa, might achieve conditions similar to those required for current human life.

Eventually, the helium produced in the shell will fall back into the core, increasing the density until it reaches the unimaginable levels needed to fuse helium into carbon. The Sun will then shrink to slightly larger than its original radius, as its energy source has fallen back to its core, however, due to the relative rarity of helium as opposed to hydrogen, the helium-fusing stage will only last about 100 million years. Eventually it will have to again resort to its reserves in its outer layers, and will regain its red giant form. This phase lasts only 100 million years, after which, over the course of a further 100,000 years, the Sun's outer layers will fall away, ejecting a vast stream of matter into space and forming a halo known (misleadingly) as a planetary nebula.

This is a relatively peaceful event; nothing akin to a supernova, which our Sun is too small to ever undergo. Earthlings, were we still alive to witness this occurrence, would observe a massive increase in the speed of the solar wind, but not enough to destroy the Earth completely.

Eventually, all that will remain of the Sun is a white dwarf, a hot, dim and extraordinarily dense object; half its original mass but only the size of the Earth. Were it viewed from Earth's surface, it would be a point of light the size of Venus with the brightness of of a hundred current Suns.

As the Sun dies, its gravitational pull on the orbiting planets, comets and asteroids will weaken. Earth and the other planet's orbits will expand. When the sun becomes a white dwarf, the solar system's final configuration will be reached: Mercury will have long since ceased to exist; Venus will lie roughly a third again farther out than Earth is now, and Earth's orbit will roughly equal that of Mars today. Two billion years farther on, the carbon in the Sun's core will crystallize, transforming it into a giant diamond. Eventually, after trillions more years, it will fade and die, finally ceasing to shine altogether.

Galactic context

The solar system is located in the Milky Way galaxy, a barred spiral galaxy with a diameter estimated at about 100,000 light years containing approximately 200 billion stars. The galaxy is a spiral, and our Sun resides in one of the outer spiral arms, known as the Orion Arm or Local Spur. The immediate galactic neighborhood of the solar system is known as the Local Fluff, an area of dense cloud in an otherwise sparse region known as the Local Bubble, an hourglass-shaped region roughly 300 light-years across. The bubble is suffused with high-temperature plasma that suggests it is the product of several recent supernovae.

Estimates place the solar system at between 25,000 and 28,000 light years from the galactic center. Its speed is about 220 kilometres per second, and it completes one revolution every 226 million years. The apex of solar motion--that is, the direction in which the Sun is heading--is near 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 about 1000 km/s.

The solar system appears to have a very remarkable orbit. 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. The solar system appears to have 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 large animal life on land. By remaining out of the spiral arms, Earth may be unusually free to form large animal life on its surface. The solar system also lies well outside the star-crowded environs of the galactic centre. The opposing gravitational tugs from so many close stars within the galactic centre would have prevented planets from forming.

Extrasolar planetary systems

For many years, the solar system had the only planetary system known, and so theories of planetary formation only had to explain one system to be plausible. The discovery in recent years of many extrasolar planets has uncovered systems very different compared to Earth's solar system, and theories have had to be revised accordingly. For instance, many extrasolar planetary systems contain a "hot Jupiter" ; a planet of comparable size to Jupiter that nonetheless orbits very close to its star, at, for instance, 0.05 AU. It has been hypothesised that while the giant planets in these systems formed in the same place as the gas giants in Earth's solar system did, some sort of migration took place which resulted in the giant planet spiralling in towards the parent star. Any terrestrial planets which had previously existed would presumably either be destroyed or ejected from the system. Up to this point, most planets discovered have been gas giants -- however, Earth-like planets such as OGLE-2005-BLG-390Lb have been found using a special technique called Gravitational microlensing, and space-based observatories such as the NASA Terrestrial Planet Finder and Darwin are planned to launch and search for Earth-like planets.

Although the term "solar system" is frequently applied to other star systems, literally, it should strictly refer to Earth's system only: the word "solar" is derived from the Sun's Latin name, Sol, and thus is sometimes written capitalised. When talking about another stellar system or planetary system, it is more accurate to drop the term "solar" and form names such as "the Alpha Centauri system" or "the 51 Pegasi system".

See main article: extrasolar planet

Discovery and Exploration

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 "wandering stars" (planets) that moved through the sky. The conceptual advances of the 17th century, led by Nicolaus Copernicus, Galileo Galilei, Johannes Kepler, and Isaac Newton, 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 laws that governed the Earth, and therefore could be similar to it. The first exploration of the solar system was conducted by telescope, with astronomers learning that the Moon and other planets possessed such Earthlike features as craters, ice caps, and seasons.

See main articles: Geocentric model, Heliocentrism

Since the start of the space age, a great deal of exploration has been performed by unmanned space missions that have been organized and executed by various space agencies. The first probe to land on another solar system body was the Soviet Union's Luna 2 probe, which impacted on the Moon in 1959. Since then, increasingly distant planets have been reached, with probes landing on Venus in 1965, Mars in 1976, the asteroid 433 Eros in 2001, and Saturn's moon Titan in 2005. Spacecraft have also made close approaches to other planets: Mariner 10 passed Mercury in 1973. 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 Pluto's orbit, and astronomers anticipate that they will encounter the heliopause which defines the outer edge of the solar system in the next few years.

Pluto remains the only planet not having been visited by a man-made spacecraft, though that will change with the successful launch of the New Horizons spacecraft on 19 January 2006. This unmanned mission is scheduled to fly by Pluto in July 2015 and then make an extensive study of as many Kuiper Belt objects as it can.

Through these unmanned missions, humans have been able to get close-up photographs of most of the planets and, in the case of landers, perform tests of their soils and atmospheres.

See main article: Space exploration

Hypothetical planets

The solar system is by no means fully mapped and charted. Much of its territory is still unknown, and many astronomers have hypothesised from indirect observation that other substantial objects could still exist undetected in its farthest reaches.

Vulcanoids

In the 19th century, the astronomer Urbain Le Verrier, credited with the discovery of Neptune, attempted to locate a hypothetical planet within the orbit of Mercury that he believed was causing perturbations in its orbit. This planet, which he named Vulcan after the Roman god of the forge due to its closeness to the Sun, was never observed, and Einstein's reworking of Isaac Newton's laws subsequently resolved the issue of Mercury's orbit. However, a gravitationally stable region does exist between Mercury and the Sun, and some astronomers, notably Alan Stern, contend that a field of small minor planets, the Vulcanoids, should exist within it. However repeated observations of the region have yet to yield any results, and the Vulcanoids, if they exist, must be rather small and few in number. Some conclude that the existence of the Vulcanoids is impossible, as any minor planet within the orbit of Mercury would eventually be destabilised by the Yarkovsky effect; motion by the force of its own heat.

Planet X

In the early 20th century, astronomer Percival Lowell's observation of apparent irregularities in the orbits of Uranus and Neptune led him to conclude that a distant planet, which he called Planet X, must lie beyond them. The Lowell Observatory's long search for this planet ultimately led to the discovery of Pluto. However, Pluto's mass was found to be too small to disturb the other planets' orbits significantly, and subsequent measurements by the Voyager 2 spacecraft showed that earlier calculations of Neptune's mass had been in error, leading to the irregularities observed. Today, few scientists accept Lowell's theory, however, a number of recent observations have reopened the debate on the existence of a "Planet X", even if it would bear little resemblance to that invisioned by Lowell.

The Kuiper Belt has a very sharply defined edge. At around 49 AU, a sharp dropoff occurs in the number of objects observed. This dropoff is known as the "Kuiper Cliff", and as yet its cause is unknown. Some speculate that something must exist beyond the belt large enough to sweep up the remaining debris, perhaps as large as Earth or Mars. This view is still controversial, however.

Physicist Richard A. Muller has theorised that the Sun may be part of a binary star system, with a distant companion named Nemesis. Nemesis was proposed to explain some timing regularities of the great extinctions of life on Earth. The hypothesis says that Nemesis creates periodical perturbations in the Oort cloud of comets surrounding the solar system, causing a "comet shower". Some of them hit Earth, causing destruction of life. This hypothesis is no longer taken seriously by most scientists, mostly because infrared surveys failed to spot any such object, which should have been very conspicuous at those wavelengths.

Dr. John Murray of the Open University and John Matese of the University of Louisiana at Lafayette believe that the motions of long-term comets in the sky suggest the existence of a large, distant planet, or, more likely, a small substellar companion such as a Brown dwarf, in the deep solar system. This hypothetical substellar object is not Nemesis, since its existence is inferred from a different set of data; however there is the possibility that both sets of data could be true for the same object.

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Astronomy | Solar system

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