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.

Although the term "solar system" is frequently applied to other star systems, it should strictly refer to Earth's system only: the word "solar" is derived from the Sun's Latin name, Sol, and thus the term sometimes appears as Solar System. 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".

Major features of the solar system (not to scale)

Structure and layout

Scale of planetary orbits
(million kilometres)
The bottom and top of each colored bar represent a planet's nearest approach to the sun and farthest distance from the sun.

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. Its two largest orbiting bodies, Jupiter and Saturn, together account for more than 90% of the remaining mass. (The Oort Cloud too might hold a substantial percentage, but as yet its existence is unconfirmed). 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, mostly in the form of visible light.

In broad terms, the charted regions of the solar system consist of the Sun, eight bodies in relatively unique orbits commonly called planets or major planets and two belts of smaller objects, which can be called minor planets, planetoids, or planetesimals. Pluto, the ninth planet, is also considered a member of the outer belt, and its status as a planet is currently under debate, particularly since the discovery of the larger 2003 UB313. Most objects in orbit round the Sun lie within the same shallow plane, called the ecliptic plane, and orbit in the same direction. Many have moons orbitting them, and the largest are encircled by planetary rings of dust and other particles.

The major planets are, in order, Mercury (☿), Venus (♀), Earth (♁), Mars (♂), Jupiter (♃), Saturn (♄), Uranus (♅/File:X - Uranus B.png), Neptune (♆), and Pluto (♇). The planets (with the exception of Earth) are named after gods and goddesses from Greco-Roman mythology.

Interplanetary distances

Distances within the solar system are measured most often 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).

By and large, a planet 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 law, 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 0.38 and 1.52 AU. Ceres, the largest asteroid, lies at k=8.

This law 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 law, and many claim it is merely a coincidence, falling in the region of uncomfortable science.

Origin and age

See also: Age of the Earth and History of Earth

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, vulcanism 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 theory 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 has been refined over many years and now has a great deal of evidence supporting it.

To briefly summarize, 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 it began to spin faster to conserve angular momentum, 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. 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 jovian protoplanets.

File:Trans-Neptunian object 2003 VB12.Sedna.orbit comparisons.jpg

This image gives an overview of the Solar system's regions. Clockwise from top left: the inner planets and asteroid belt; the outer planets and Kuiper belt; the orbit of Sedna; the Oort cloud

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.

Regions

According to their location, the objects in the solar system are described as being either in the inner solar system, including terrestrial planets and the Main belt of asteroids, the middle region including the giant planets, their satellites and the centaurs, or the outer solar system, comprising the area of the Trans-Neptunian objects including the Kuiper Belt, the Oort cloud, and the vast region in between. This region is occasionally referred to as the solar system's "third zone".[2]

Interplanetary medium

File:Heliospheric-current-sheet.jpg

The heliospheric current sheet results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium [1]. (click to enlarge)

The environment in which the solar system resides is called the interplanetary medium. The Sun radiates a continuous stream of charged particles, a plasma known as solar wind, which forms a very tenuous "atmosphere" (the heliosphere), permeating the interplanetary medium in all directions for at least 16 Tm or 16×10⁹ km into space. Small quantities of dust are also present in the interplanetary medium and are responsible for the phenomenon of zodiacal light. Some of the dust is likely interstellar dust from outside the solar system. 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.

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 formed in the hotter regions close to the Sun, where hydrogen compounds remained gaseous, leaving only those mineral dust grains 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:

The inner planets, from left to right: Mercury, Venus, Earth, and Mars

Mercury

Mercury (0.39 AU), the closest planet to the Sun, is also the smallest and most atypical of the inner planets, having no atmosphere 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, though hypotheses include that it was 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 (0.72 AU), the first truly terrestrial planet, is of comparable mass to the Earth, 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 composed overwhelmingly of carbon dioxide with traces of sulfuric acid. Unlike Earth, Venus's crust is not divided into tectonic plates but instead comprises a single, very thick rind. Surface features on Venus are all of the same, relatively young age, suggesting that they are periodically erased by sudden, massive vulcanism.

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 of the features in common with other terrestrial planets, though it lacks an iron core.

Mars

Mars (1.5 AU), smaller than the Earth or Venus, 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 [3] suggests this may have been true until very recently. Mars possesses two tiny moons thought to be captured asteroids.

Asteroids

Image of the main asteroid belt and the Trojan asteroids.

Asteroids are objects smaller than planets that mostly occupy the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun, and are composed in significant part of rocky, non-volatile minerals.

The main belt

The main asteroid belt 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, [4] 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.[5] Asteroids with a diameter of less than 50 m are called meteoroids. The largest asteroid, Ceres, has a diameter of roughly 1000 km; large enough to be spherical, which would make it a planet by some definitions of the word.

They are subdivided into 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 [6] which may have been the source of Earth's water.

Other asteroids

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 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. Their large sizes and distances from the Sun meant they could hold on to much of the hydrogen and helium too light for the smaller and hotter terrestrial planets to retain. Jupiter and Saturn are true giants, at 318 and 95 Earth masses, 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).

File:Gas giants large.jpg

From bottom: Jupiter, Saturn, Uranus and Neptune (sizes not to scale).

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. 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.[7]

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 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 undiffrentiated 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. Neptune has more internal heat than Uranus, and the heat at its core drive 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 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. 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. 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 tail, or coma, just as comets do when they approach the sun.[8]

The Kuiper belt

Artist's rendering of the Kuiper Belt and hypothetical Oort cloud.

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. [9] Many Kuiper belt objects have multiple sattelites and most have orbits that take them outside the plane of the ecliptic.

Pluto

File:Plutonianmoons.jpg

Pluto and its three known moons

Pluto, (38 AU average) the solar system's smallest planet, is considered to be part of the Kuiper Belt population. As such, some scientists contend it should be considered a large comet, rather than a planet. Like other objects in the Kuiper belt, it 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. If Pluto were placed close to the Sun, it would develop a tail, as 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. Like the Earth/Moon, Pluto and Charon are often considered a double planet.

Kuiper belt objects with Pluto-like orbits are called Plutinos. Other Kuiper belt objects have resonant orbits and are grouped accordingly. The remaining Kuiper belt objects, in more "classical" orbits, are classified as Cubewanos.

The scattered disc

Orbits of scattered disc objects; the blue band is the classical Kuiper belt; the green band comprises the resonant KBOs, including Pluto.

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. 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.

2003 UB313 ("Xena")

File:Xenaandgabrielle.jpg

2003 UB₃₁₃ and its moon

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 2003 UB313, and has been nicknamed "Xena" by its discoverers, after the television character. It 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 2004 XR190. Like Pluto, it is believed to consist largely of rock and ice, and has a moon [10]. 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. The Sun's gravity, however, is believed to hold 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 halfway 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 is or may be lying at its outer edge. [11]

Sedna

Sedna, the newly discovered Pluto-like object with a gigantic, highly elliptical 10,500-year orbit that takes it from about 76 to 928 AU, has too distant a perihelion to be a scattered member of the Kuiper Belt and could be the first in an entirely new population. 2000 CR105, which has a parehelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years, is also believed to be a member of this population. [12] Some astronomers have termed this region the "Inner Oort cloud," part of a 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. [13]

The heliopause

The Voyagers entering the heliosheath

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.[14] 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. [15]

Minor planets

Main article: Minor planet

A minor planet is an object in orbit around the Sun that is smaller than any of the major planets yet larger than meteoroids (that is, greater than 50m across), the vast majority of which are located within the asteroid belt, the Kuiper belt and the scattered disc. The largest minor planets (also known as planetoids) are smoothly rounded, like major planets, because their gravity overcomes material strength that keeps smaller bodies in non-spherical shapes. Before the discovery of 2060 Chiron and the trans-Neptunian objects, the term "minor planet" was a synonym for asteroid, but many people now prefer to restrict the use of "asteroid" to refer to rocky bodies of the inner solar system. Most trans-Neptunian objects are icy, like comets, although those we can detect at that distance are much larger than comets.

Several asteroids, in the strict sense, are large enough to be spherical. The largest known trans-Neptunian objects are much larger than the large asteroids. (Natural satellites of major planets also range smoothly from small non-spherical objects to large spherical ones, and the largest are more massive than both Pluto and 1 Ceres, the largest asteroid), and two are larger (though less massive) than Mercury. The largest satellite is Ganymede with a mass of 2 percent that of the Earth.

The total surface area of the solar system's objects that have solid surfaces and a diameter greater than 1 km is approximately 1.7×10⁹ km² —about 11 times the area of the Earth's land masses.

Tables of planetary attributes

See also: table of planets in the solar system.

All attributes below are measured relative to the Earth

Major planets

^* See Earth article for absolute values.

Planet	Equator diam.	Mass	Orbital radius (AU)	Orbital period (years)	Orbital Incline Angle (°)	Orbital Eccentricity	Day (days)	Moons
Mercury	0.382	0.06	0.387	0.241	7.00	0.206	58.6	none
Venus	0.949	0.82	0.72	0.615	3.39	0.0068	-243	none
Earth*	1.00	1.00	1.00	1.00	0.00	0.0167	1.00	1
Mars	0.53	0.11	1.52	1.88	1.85	0.0934	1.03	2
Jupiter	11.2	318	5.20	11.86	1.31	0.0484	0.414	63
Saturn	9.41	95	9.54	29.46	2.48	0.0542	0.426	49
Uranus	3.98	14.6	19.22	84.01	0.77	0.0472	-0.718	27
Neptune	3.81	17.2	30.06	164.8	1.77	0.0086	0.671	13
Pluto	0.18	0.002	39.5	248.5	17.1	0.249	-6.4	3

Largest minor planets

Minor planet	Equatorial diameter	Mass	Orbital radius (AU)	Orbital period (years)	Day (days)
1 Ceres	0.075	0.000 158	2.767	4.603	0.3781
90482 Orcus	0.066 - 0.148	0.000 10 - 0.001 17	39.47	248	?
28978 Ixion	~0.083	0.000 10 - 0.000 21	39.49	248	?
(55636) 2002 TX300	0.0745	?	43.102	283	?
20000 Varuna	0.066 - 0.097	0.000 05 - 0.000 33	43.129	283	0.132 or 0.264
2003 EL61	~.0768	~0.000 67	43.339	285	?
50000 Quaoar	0.078 - 0.106	0.000 17 - 0.000 44	43.376	285	?
2005 FY9	~0.14	?	45.64	308	?
2003 UB313	0.19	>0.002	67.709	557	?
90377 Sedna	0.093 - 0.141	0.000 14 - 0.001 02	502.040	11500	20

Galactic context

presumed location of the solar system within our galaxy

The solar system is located in the Local Fluff in the Orion Arm of the Milky Way galaxy, a barred spiral galaxy with a diameter estimated at about 100,000 light years containing approximately 200 billion stars, of which the Sun is a rather large and bright one. The vast majority of stars are red dwarfs; 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, although we can observe only those few other red dwarfs that are very near the Sun in space.

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.[16] 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.

Extrasolar planetary systems

File:Upsilon Andromedae.jpg

Our Solar System compared with the system of Upsilon Andromedae

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[17]. For instance, many extrasolar planetary systems contain a "hot Jupiter"[18]; 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 probes such as the NASA Terrestrial Planet Finder[19] and Darwin are planned to launch and search for Earth-like planets.[20].

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.

• 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 pertubations 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.[21] 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. [22]

• 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. [23]

• It has been suggested 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. [24]

• 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. However, Dr. John Murray of the Open University and John Matese of the University of Southwestern Louisiana 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 the same as 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. [25]

See also

• List of solar system objects by mass
• List of solar system objects by radius
• Astronomical symbols
• Definition of planet
• Geological features of the solar system
• Laws of Kepler
• Numerical model of solar system
• Origin of life
• Planetary system
• Planetary nomenclature
• Stellar system
• Table of planetary attributes
• Timeline of discovery of Solar System planets and their natural satellites
• Timeline of solar system astronomy
• Landings on other planets
• Geocentric model
• Titius-Bode law
• Zodiacal light
• Solar system in astrology
• Solar system model

External links

• NASA's Solar System Exploration site
• [26] A site mentioning some records for the Solar System
• NASA's Solar System Simulator
• NASA/JPL Solar System main page
• A Scale Model of our Solar System The user specifies a diameter for the Sun and the scaled sizes of the planets and their orbital diameters are automatically calculated and can then be overlaid on a map of any location on Earth (powered by Google Maps).
• Astronomical Enigma Mathematical Order in the orbits of the solar system.
• Solarviews
• Celestia Free 3D realtime space-simulation (OpenGL)
• The Nine Planets Comprehensive solar system site by Bill Arnett
• MySolarSystem Educational Solar System Website.
• Planetary data
• Stars and Habitable Planets
• Solar System An interactive planets animation (145 zoom steps and time effects)
• Timeline of solar system exploration
• mirror matter planets and other mirror objects in the solar system?
• The Virtual Solar System, including a scale model of the system

• Tenth planet discovered in outer solar system - NewScientistSpace.com
• Astronomers Discover "10th Planet" - Sky & Telescope article
• Tenth Planet Discovered - Astronomy Magazine article
• LARGER THAN PLUTO! 10th Planet Discovered? - Planetary Society
• Tenth planet found! - spaceflightnow.com
• Object Bigger than Pluto Discovered, Called 10th Planet - space.com
• Astronomers detect '10th planet' - BBC News
• You Call That a Planet? How astronomers decide whether a celestial body measures up. - Slate.com
• 'Planet Xena' has a sidekick: Gabrielle - edition.cnn.com
• Sky and Telescope article on the moon of 2003 UB₃₁₃ and the diameter estimate
• The Solar System presented by Utah Skies