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Presentation of the solar system (not to scale)
Presentation of the solar system (not to scale)
The solar system comprises the Earth's Sun and the retinue of celestial objects gravitationally bound to it. Traditionally, this is said to consist of the Sun, nine planets and their 158 currently known moons; however, a large number of other objects, including asteroids, meteoroids, planetoids, comets, and interplanetary dust, orbit the Sun as well. Astornomers are debating over the presence of a tenth planet.Although the term "solar system" is frequently applied to other star systems and the planetary systems which may comprise them, it should strictly refer to Earth's system specifically: 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, including the stars and bodies associated with them through gravity, it is usual to shorten it to drop the term "stellar" and form names such as "the Alpha Centauri system" or "the 51 Pegasi system".

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Contents

Structure and layout of the solar system
Age and origin of the solar system
Regions of the solar system
Galactic orbit of the solar system
Planetary system formation
Discovery of the solar system
Exploration of the solar system
Attributes of major planets
Attributes of the largest minor planets
Other facts



Structure and layout of the solar system - Contents

The Sun ( astronomical symbol ☉) is a main sequence G2 star that contains 99.86% of the system's known mass. Its two largest orbiting bodies, Jupiter and Saturn, account for 91% of the remainder; the Oort Cloud might hold a substantial percentage, but as yet its existence is unconfirmed.In broad terms, the charted regions of the solar system consist of the Sun and its planetary system: the 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, meteoroids, or planetesimals. Pluto, the ninth planet, is also considered a member of the outer belt, and its status is currently uncertain, particularly since the discovery of 2003 UB313 (see below). Most objects in orbit round the Sun all lie within the same shallow plane, called the ecliptic plane, and orbit in the same direction. Many are in turn orbited by moons, 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 (/), Neptune (), and Pluto (). Eight of the nine planets are named after or derived from gods and goddesses from Greco- Roman mythology; Earth, a Germanic word, is known in many Romance languages as Terra, the Roman goddess of the Earth.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 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).
This picture illustrating the orbit of Sedna gives a good idea of the layout of the solar system. Clockwise from top left: the inner solar system, the outer planets and the Kuiper belt, the orbit of Sedna and the Oort Cloud.
This picture illustrating the orbit of Sedna gives a good idea of the layout of the solar system. Clockwise from top left: the inner solar system, the outer planets and the Kuiper belt, the orbit of Sedna and the Oort Cloud.
Despite the fact that many diagrams represent the solar system as having each orbit the same distance apart, in actuality the orbits are largely arranged geometrically, that is, each is roughly double the distance from the Sun as the one before it. Venus’s distance from the Sun is roughly double that of Mercury, Earth’s distance is roughly double that of Venus, Mars’s 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:
a= 0.4 + 0.3times k
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, and Mars's orbit (k=4) to be at 1.6 AU. In fact their orbits 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; Neptune is far closer than predicted, though Pluto lies at Neptune's predicted orbit. As of now, there is no scientific explanation for why this law applies, and many claim it is merely a coincidence, falling into the region of uncomfortable science.Scaling down the size of the solar system makes it easier for students to grasp the relative distances. The exponential increases in interplanetary distances, and the enormous ratio of those distances to planetary diameters, make constructing a scale model of the solar system a challenging task. The distance between the Earth and the Sun is almost 12,000 times the diameter of the Earth, while the distance between Saturn and Uranus is over 30 times the distance between Mercury and Venus. Several places have built such models. Claiming to be the largest scale model of the solar system in the world is that at Otford at Kent, England [2]
This illustration shows the approximate sizes of the planets relative to one another and the Sun.
This illustration shows the approximate sizes of the planets relative to one another and the Sun.



Age and origin of the solar system - Contents

Scientists estimate that the solar system is 4.6 billion years old. To calculate this figure, they examine an unstable element, which is subject to radioactive decay. By observing how much this element has decayed, they can calculate how old this element is. The oldest rocks on earth are approximately 3.9 billion years old, however it is hard to find these rocks as the earth has been thoroughly resurfaced. To estimate the age of the solar system, scientists must find rocks from space, such as meteorites – which are formed during the early condensation of the solar nebula. The oldest meteorite was 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. It states the solar system was formed from 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, sent shock waves into space, which squeezed the nebula, pushing more and more of its matter inward until gravitational forces overcame its internal gas pressure and it began to collapse. As the nebula collapsed, it decreased in size, which in turn caused it to spin faster to conserve angular momentum. And 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 pancake shape with a bulge at the center.When the nebula further condensed, a protostar was formed in the middle. This system was heated by the friction of the rocks colliding into each other. Lighter elements such as hydrogen and helium evaporated out of the centre and migrated to the edges of the disc, thus concentrating the heavier elements to form dust and rocks in the centre. These heavier elements clumped together to form planetesimals and protoplanets. In the outer regions of this solar nebula, ice and volatile gases were able to survive, and as a result, the inner planets are rocky and the outer planets were massive enough to capture large amounts of lighter gases, such as hydrogen and helium.After 100 million years, the pressures and densities of hydrogen in the centre of the collapsed nebula became great enough for the protosun to sustain thermonuclear fusion reactions. As a result of this, hydrogen was converted to helium in such reactions, and a great amount of heat was released. The general equation is given as:4times {}^1!mbox{H} longrightarrow {}^4!mbox{He}, {+}, mbox{neutrinos}, {+}, mbox{photons}The output energy and exact isotope of the products and reactants would have varied with each reaction. This is because thermonuclear fusion is often a multi-step reaction with a varying binding energy depending on the circumstances. However, the scale of energy production and helium creation would have generally been massive. During that time, the protostar turned into the Sun and the generation of gravity at the center would have allowed the protoplanets and planetesimals to transform into planets via accretion. All of the planets formed in a relatively short time of a few million years.


Regions of the solar system - Contents

Heliospheric current sheet, the largest structure in the Solar System, results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium (Solar Wind) [1]. (click to enlarge)
Heliospheric current sheet, the largest structure in the Solar System, results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium ( Solar Wind) [1]. (click to enlarge)
According to their location, the objects in the solar system are divided into three zones: Zone I or the inner solar system, including terrestrial planets and the Main belt of asteroids; Zone II, including the giant planets, their satellites and the centaurs, and Zone III, 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.

Interplanetary medium
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 ten billion (10×109) miles (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.

The inner planets
The four inner or terrestrial planets are characterised by their dense, rocky makeup. They formed in the hotter regions close to the Sun, where lighter and more volatile materials evaporated, leaving only those with high melting points, such as silicates, which form the planets' solid crusts and semi-liquid mantles, and iron, which forms their cores. All have impact craters and many possess tectonic surface features, such as rift valleys and volcanoes. The four inner planets are:
Earth is the largest of the inner planets
Earth is the largest of the inner planets
  • Mercury (0.39 AU from the Sun): 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 suggests that it was once a much larger world whose outer mantle was sheared off in early formation by the Sun’s gravity.
  • Venus (0.72 AU): The first truly terrestrial planet, Venus, like the 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. It is much drier than Earth, and its atmosphere is 90 times as dense as Earth’s, however, and composed overwhelmingly of carbon dioxide with traces of sulfuric acid.
  • Earth with Moon (1 AU): The largest of the inner planets, Earth 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 (1.5 AU): Smaller than the Earth or Venus, Mars 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 it may have continued to be so until very recently. Mars possesses two tiny moons thought to be captured asteroids.


The asteroid belt
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 non-volatile minerals. The main belt 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 that of the Earth. 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. The asteroids are thought to be the remnants of a small terrestrial planet that failed to coalesce due to the gravitational interference of Jupiter. 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. 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.

The 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 distance 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 is the largest planet in the solar system.
Jupiter is the largest planet in the solar system.
  • 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. 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.
  • Saturn (9.5 AU), famous for its extensive ring system, shares 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 (19.6 AU) and Neptune (30 AU), while having many characteristics in common with the other gas giants, are nonetheless more similar to each other than they are to Jupiter or Saturn. They 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.” Both planets possess dark, insubstantial ring systems. Neptune’s largest moon Triton is geologically active.
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.

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

The Kuiper belt

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 70,000 Kuiper belt objects with a diameter greater than 100 km, the total mass of the Kuiper belt is relatively low, perhaps equalling or just exceeding the mass of the Earth. Many Kuiper belt objects have orbits that take them outside the plane of the ecliptic.
  • Pluto, the solar system's smallest planet, is considered to be part of the Kuiper Belt population. Like others in the belt, it has a relatively eccentric orbit inclined 17 degrees to the ecliptic and ranging from 29.7 AU from the Sun at perihelion to 49.5 AU at aphelion. It has a large moon (the largest in the solar system relative to its own size), called Charon, and, new observations suggest, two other, much smaller moons. Like the Earth/Moon, Pluto and Charon are often considered a double planet. Although accepted by the public as a planet since its discovery in 1930, Pluto's tiny mass (less than 1% of Earth's) and diameter have regularly sparked debates about Pluto's identity within the scientific community.
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 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.

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.
2003 UB313
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 ~30% larger than Pluto with an estimated diameter of 1864 miles. 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. 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.

Possibility of a new region

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 is also believed to be a member of this population.

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.

Farther 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 Voyagers entering the heliosheath
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. 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.But even this point is not classified as outside the solar system, for the Sun's gravity will still hold sway even up to the Oort cloud, which is a great mass of icy objects, currently hypothetical, 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.


Galactic orbit of the solar system - Contents

The solar system is located in the Local Fluff in the Orion Arm of the Milky Way galaxy, a spiral galaxy with a diameter of 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. 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 unusual 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.


Planetary system formation - Contents

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.Most exoplanets have not been explicitly seen by astronomers yet, however they are known to exist because of the gravitational tug the planets induce on the star, and hence making the star "wobble", using the Doppler effect as well as the Rossiter-McLaughlin effect. Astronomers can calculate how massive the planets are by observing how much the star wobbles. Exoplanets can also be observed more directly by their occultation of the stars' discs, which dims them slightly. Other techniques like gravitational lensing allow the magnification of light from distant stellar systems due to the effects of relativity, allowing astronomers to briefly view planets inside them.In October, 1995, astronomers Michel Mayor and Didier Queloz announced the discovery of a massive planet orbiting 51 Pegasi – a Sun-like star in the constellation Pegasus. This planet is about half as massive as Jupiter, and had an orbital period of 4.2 Earth days, due to its closeness to the star (0.05 AU). Since then, over 160 more planets have been identified.Many extrasolar planetary systems contain such a " hot Jupiter": a planet comparable to or larger than Jupiter orbiting very close to the parent star, perhaps orbiting it in a matter of days. 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.There has also been some photographic evidence to suggest that regions in the Orion Nebula, which is 1500 light years from Earth, have star systems forming.


Discovery of the solar system - Contents

The planets out to Saturn were known to ancient astronomers, who observed the wandering of these objects against the apparently fixed pattern of stars. Venus and Mercury were each identified as single objects despite the difficulty of connecting "evening" and "morning stars". It was also identified that the two non-pointlike objects, the sun and the Moon, moved across the same fixed background. However knowledge of the nature of these celestial drifters was entirely speculative and largely incorrect.The nature and structure of the solar system were long misperceived, for at least two reasons:
  • The Earth was considered stationary, and the motion of objects in the sky was therefore taken at face value: the sun was thought to orbit the Earth, for example (This conception of the universe, in which the Earth is at the center, is called the Geocentric model; geos means "Earth" in Greek).
  • Many solar system objects and phenomena cannot be perceived at all without technical aid.
Over the last several hundred years, conceptual and technological advances have helped humans understand the solar system much better.The first and most fundamental of the conceptual advances was the Copernican Revolution, which proposed that the planets orbit the sun—models of the solar system with the sun in the center are called heliocentric; helios meaning "Sun" in Greek). Despite the name, the most striking and then-controversial Copernican realization was not that the sun was central but that the Earth was peripheral, orbital: planets had been considered merely points in the sky, but if the Earth itself was a planet, perhaps the other planets were, like Earth, huge solid spheres.Philosophically, there were a number of objections to heliocentrism:
  • The natural state of heavy, mineral objects like the Earth was believed to be at rest. The planets were believed to be made of a separate, ephemeral, light substance.
  • It was believed that the Earth's motion round the Sun would cause the air to fly off the surface.
  • If the Earth were mobile, astronomers should have been able to observe parallax of the stars, such as the stars appearing to shift in relation to further objects due to the Earth's change in position.
The subsequent invention of the telescope gave the principal technological advance on discovering the solar system, with Galileo's improved version of the telescope rapidly giving benefit in terms of discovering satellites of other planets, especially Jupiter's four major satellites. This showed that all objects in the universe did not orbit the Earth. However, perhaps Galileo's most important discovery was that the planet Venus has phases like the Moon, proving that it must orbit the Sun.Then, in 1687, Isaac Newton devised his law of universal gravitation which explained the force that both kept the Earth moving through the heavens and also kept the air from flying away.Finally, in 1838, astronomer Friedrich Wilhelm Bessel successfully measured the parallax of the star 61 Cygni, proving conclusively that the Earth was in motion.


Exploration of the solar system - Contents

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 launching of New Horizons by NASA in January 2006. It 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. Manned exploration, meanwhile, has only taken human beings as far as the Moon, in the Apollo program. The last manned landing on the Moon took place in 1972, but the recent discovery of ice in deep craters in the polar regions of the Moon has prompted speculation that mankind may return to the Moon in the next decade or so. Manned missions to Mars have been eagerly anticipated by generations of space enthusiasts, and it was hoped that the first manned interplanetary flights would take place in the 1980s, after the successful Apollo program. Europe ( ESA and EU) now plans manned Lunar and Mars missions as part of Aurora Exploration Programme endorsed in 2001. United States followed with similar programme called Vision for Space Exploration in 2004.


Attributes of major planets - Contents

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.

All attributes below are measured relative to the Earth:
* See Earth article for absolute values.
** Soon after its discovery in 1930, Pluto was classified a planet by the International Astronomical Union. However, based on additional discoveries since that time, some astronomers have suggested reconsideration of that decision.
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 -6.5 3
Of the other objects, Ganymede has the largest mass, 2% that of the earth.Note: Although 2003 UB313 is a minor planet, it is being considered as possibly
being a major planet (the tenth in the solar system).
Recent findings indicate the object is 30% larger than Pluto.For a more comprehensive table, see table of planets in the solar system.


Attributes of the largest minor planets - Contents

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 larger than 1 Ceres, the largest asteroid).All attributes below are measured relative to the Earth:
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 0.24 0.004? 67.709 557 ?
90377 Sedna 0.093 - 0.141 0.000 14 - 0.001 02 502.040 11500 20



Other facts - Contents

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 wherever possible.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.The concept of the tenth planet has frequently been explored in science fiction works and conspiracy theories (see also Planet X, and hypothetical planet).
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