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#258255 07/12/06 09:21 PM
Joined: Oct 2004
Posts: 1,111
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Joined: Oct 2004
Posts: 1,111
The solar system began as a cloud of dust and gas. The cloud began to spin and contract. It contracted into a disc with the sun at the center. Planets formed from the disk. Gravity caused rocky, terrestrial planets to form near the sun. Gas giants floated further out. Celestial bodies are round because they are molded by the effects of spinning. Every star in the galaxy may have planets.

The sun is a star. It is average in size. It is 93 million miles away. Its surface is 11,000 degrees. Sunspots are dark because they are cooler. The sun is a hydrogen bomb, shining by nuclear fusion. Hydrogen turning into helium emits energy in the form of light and heat. This energy is stored in fossil fuels: coal, oil and gas.

The sun is a middle-age star and will burn another 5 billion years. It will become a red giant. The planets will be consumed. Earth's oceans will boil away. The atmosphere will go. The sky will turn black.

There are two kinds of objects, those which shine by their own light like stars and those which reflect light like planets and moons.

The planet Mercury is so close to the sun that many astronomers have never seen it. Mercury's surface is cratered like the moon's. Like the moon, Mercury has no atmosphere.

NASA launched the Messenger spacecraft to Mercury. Scientists want to know why it is so dense.

Like Mercury, Venus is between Earth and the sun. For that reason, Venus never strays far from the sun in the morning or evening skies. It is seen in the western sky after sunset. Venus goes through phases which are visible through a small telescope. Venus is brightest in its crescent phase because it is closer to Earth. It gets as bright as -4.5 magnitude.

Venus is about the size of Earth. We might expect similarities. In fact, the surface of Venus is 900 degrees because of the greenhouse effect. Its atmoshere consists of carbon dioxide. It is hell. Still, Venus accompanied by a crescent moon is one of the beautiful sights in nature.


From space, Earth is a blue planet spotted with white cloud tops. Earth is 25,000 miles in circumference and 8,000 miles in diameter. It revolves around the sun every 365 days, the period defined as a year.

Earth's orbit varies over million of years. It stretches and shrinks. This accounts for 7 ice ages.

Earth tilts on its axis 23 1/2 degrees. This tilt causes seasons. The northern and southern hemispheres alternately lean into and away from the sun. When it is summer in the United States, it is winter in Australia.

Earth's moderate distance from the sun is a factor in the evolution of life. It is neither too hot nor too cold. Liquid water can exist. Where there is water, there is life. It rained millions of years to create oceans. Mountains were created by stresses in the earth. Earth's atmosphere came from volcanos. The atmosphere provides pressure and protection from harmful rays. Oxygen is produced by the photosynthesis of plants. Earth's atmosphere extends 300 miles.

Life began in the sea (or so we have read). Four billion years ago, chemicals began showing characteristics of life. Viruses are on the line between the living and nonliving. One-celled organisms developed, microbes. Plants colonized the land. Invertebrates evolved. Vertebrates followed. Fish evolved into amphibians which evolved into reptiles. Dinosaurs lived in the Mesozoic Era between 65 million and 220 million years ago. Tyrannosaurus, brontosaurus, triceratops, stegosaurus, ankylosaurus and duckbills ruled. I did a paper titled "The Other Sciences," which was dinosaur-based. A species is a group of animals whose members interbreed. There are over one million species of animals.

Continents formed one land mass. As continental drift occurred, reptiles evolved into birds and mammals. Some paleontologists believe birds are dinosaurs. There was a golden age of giant mammals in the Cenozoic Era. Mammoths and mastadons became extinct only at the end of the recent Ice Age. Man has existed in some form for 5 million years. He evolved from primates in southeast Africa and spread through Europe and Asia. From Asia, he populated the South Sea islands and walked across the land bridge at the Bering Strait into the Americas. That was 50,000 years ago. Races as we know them came into existence at the end of the Ice Age 20,000 years ago. Civilization was born in Egypt and Mesopotamia. Recorded history spans 5,000 years. Colonization of the Americas by Europeans from the Renaissance forward is the most important human migration in history. World population is currently 6 billion. 300 million are in the United States.

Natural history is understood in terms of the Geological Time Scale. Paleontologists study the fossil record. Fossils are found in sedimentary rocks, those laid down by water. Radio carbon dating determines the age of rocks. To know the age of rocks is to know the age of their fossils.

The Outer Planets (Mars)

The outer planets exhibit retrograde motion. They appear to travel backwards against the stars as the faster Earth overtakes and passes them. Mars is a dramatic example.

Percival Lowell studied Mars from his observatory in Flagstaff, Arizona. He saw what he mistook for canals built by Martians.

Mars gets its red color from dust storms. Its surface contains rust (iron oxide). Its polar caps are made of dry ice (frozen carbon dioxide). Mars has a huge volcano known as Olympus Mons and a canyon called Valles Marineris.

Mars' atmosphere is too thin for water. There is evidence that water flowed in the past. If so, life may have evolved. The way to know is to go there and bring back rocks to see if they contain fossils. A journey to Mars will take a year, 6 months to get there and 6 months to return.

The Viking spacecraft landed on Mars in 1976. It found chemicals said to mimic life. 3 essentials for life are water, nutrients and energy. Scientists study the Atacama desert in Chile, the driest place on earth, to learn about Mars.

The Mars Science Laboratory will be launched in 2009 and land in 2010. It will try to determine whether microbes evolved on Mars. There may have been a zone of life in the early solar system extending from Venus to the asteroids.

The Martian moons are Phobos and Deimos. In Homer's Iliad, they were Fear and Panic and attended the god of war.

The asteroid belt is between Mars and Jupiter. Jupiter's influence kept this collection of rocks from coalescing into a planet. Ceres and Vesta are asteroids.

Difference in size means a difference in gravity. Large planets like Venus and Earth hold atmospheres. Small worlds like Mercury and the moon do not. Medium-size Mars holds a thin atmosphere. Worlds with atmospheres do not have many craters because atmospheres vaporize meteors and erode craters. Earth has a few craters. Venus and Mars have more. Mercury and the moon are heavily cratered.

Gas Giants

Voyager 1 flew by Jupiter and Saturn and was deflected. Voyager 2 went on to Uranus and Neptune. Voyager 2 spent 12 years (1977-89) on its Grand Tour. All the gas giants were found to have rings.

Jupiter is a failed star. If it were larger, nuclear reactions would have begun and it would shine by its own light. It is made of hydrogen and helium, the most common elements.

Jupiter has bands because it rotates so fast that its clouds are stretched into patterns. The Great Red Spot is a huge storm. There is now a Red Spot Jr. The Galileo probe reached Jupiter in 1995.

Jupiter has 60 moons. Io, Europa, Ganymede and Callisto are the largest, named for Jupiter's lovers. They were first seen by Galileo, who invented the telescope in 1610. Io is interesting because it has the only volcanos in in the solar system beyond Earth.

Saturn is a butterscotch ball of gas light enough to float in water. Saturn's rings are its glory. There are 7 main rings. They are made of rock and ice. As Saturn orbits the sun every 29 years, we see its rings open at the top, edge-on, open at the bottom and edge-on again.

Saturn has 31 moons. Titan is larger than the planet Mercury and is the only moon in the solar system with an atmosphere. Its atmosphere is orange and dense.

The major gap in Saturn's rings was discovered by Cassini. Titan was discovered by Huygens.

The Cassini-Huygens spacecraft is the first to orbit Saturn. The probe descended into Titan's atmosphere and sent back data. The reason for studying its atmosphere is that it is thought to be similar to that of early Earth. Scientists want to know how life developed. $3.3 billion was spent on Cassini. It is mind-boggling, the money spent on these projects.

Uranus was knocked on its side. Modern astronomy is explained in terms of collisions. Consider that dinosaurs were killed off by an asteroid. If the ancients thought the heavens benign, today's universe is a violent place. Uranus is a green, featureless pool ball. It was discovered by William Herschel in 1781.

Voyager 2 photographed Neptune's Great Dark Spot in the blue methane.

Small, rocky Pluto at the edge of the solar system breaks the rules. Its orbit is erratic, bringing it inside Neptune. Pluto may have been a moon dislodged from somewhere else. It has a companion, Charon. Clyde Tombaugh discovered Pluto in 1930. NASA launched the New Horizons spacecraft in January, 2006. It will take nearly 10 years to reach Pluto at a distance of 3 billion miles. Pictures will arrive in 2015. I plan on being here.

The Voyagers have left the solar system and are on their way to the stars. They contain records of earth sounds, languages and music.

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#258256 07/13/06 09:16 AM
Joined: Mar 2006
Posts: 622
Joined: Mar 2006
Posts: 622
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. Its 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). [1] 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.

In broad terms, the charted regions of the solar system consist of the Sun, four rocky bodies close to it called the terrestrial planets, an inner belt of rocky asteroids, four gas giant planets, and an outer belt of small, icy bodies known as the Kuiper belt. One planet, Pluto, is also a member of the Kuiper belt. The major planets are, in order, Mercury (☿), Venus (♀), Earth (♁), Mars (♂), Jupiter (♃), Saturn (♄), Uranus (♅/), 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 extreme 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.


The SunThe 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 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. [2]

Solar wind

The heliospheric current sheetThe 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), within the solar system for at least 16 Tm or 16�109 km. This environment is known as the interplanetary medium. 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. [3]

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.

Origin and evolution
See also: Age of the Earth, History of Earth, and Solar nebula

Artist's conception of a protoplanetary discUsing 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. [4]

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. [5] 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 � 15 billion km 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. 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.

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. [6]

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.

The orbital shift of NeptuneA 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. [7]


The Ring nebula, a planetary nebula similar to what the Sun will eventually becomeBarring 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. [8] [9] [10] [11]

Layout and distances
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". [12]

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).

Kepler's laws
The movement and behaviour of the solar system approximately follow the three laws first discovered by Johannes Kepler in the seventeenth century. These laws give a particularly good approximation to general relativity for the more distant planets in the Solar System. For accurate calculations of planetary motion, e.g. for space probe missions, a full general relativity calculation is required however.

Kepler's 1st lawKepler's first law states that the orbits of the planets are all ellpses with the star at one focus. An elipse is essentially a flattened circle, and a planet's orbit is alligned so that, rather than at the exact centre of a planet's orbit, the Sun lies at one of two points ("foci") where the sum of all distances from them to any point on the orbit is constant. 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.

Kepler's 2nd lawKepler's second law says that a line joining a planet and a star covers the same areas for the same length of time. This essentially means that two triangles fromed between the star and two points in a planet's orbit will have the same area if they both represent the planet's progress over the same length of time. The upshot of this is that, when a planet is nearing aphelion, the portion of its orbit traversed is less than for the same length of time near perihelion, and thus that the planet will move more slowly when far from the Sun than when closer to it.

Kepler's third law says that the squares of the orbital periods of planets are directly proportional to the cubes of the semi-major axis of the orbits. This means that 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.

Titius-Bode law
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:

(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.

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:

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 its outer layers were stripped off by a giant impact, or that it was prevented from fully accreting by the Sun's gravity. The upcoming MESSENGER probe should aid in resolving this issue.

The inner planets, from left to right: Mercury, Venus, Earth, and Mars[edit]
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. 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. [13]

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. [14] 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. [15] 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, 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[16] suggests this may have been true until very recently. Mars possesses two tiny moons thought to be captured 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 Asteroid Belt

Image of the main asteroid belt and the Trojan asteroids.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,[17] 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.[18] 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.

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[19] 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, 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).

From bottom: Jupiter, Saturn, Uranus and Neptune (sizes not to scale).[edit]
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.[20]

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 (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 (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 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.


Comet Hale-BoppComets 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.


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


Pluto and its three known moonsPluto, (38 AU average) the solar system's smallest planet, is considered to be part of the Kuiper Belt population. As such, some astronomers believe Pluto should be considered a giant 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. The solar wind is gradually sublimating Pluto's surface into space, in the manner of a comet.[23] If Pluto were placed near the Sun, it would develop a tail, like comets do. [24] 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.

black: scattered disc; blue: classical Kuiper belt; green: resonant KBOs inc. Pluto.[edit]
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 Neptune (see Origin and evolution, above). 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")
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. [25]

2003 UB313 and its moonThe 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 2004 XR190, also known as "Buffy". [26] Like Pluto, it is believed to consist largely of rock and ice, and has a moon [27] 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 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.[28]


an artist's conception of SednaMike Brown asserts that Sedna, his newly discovered Pluto-like object with a gigantic, highly elliptical 10,500-year orbit that takes it from about 76 to 928 AU, 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. It is believed to be the first in an entirely new population, one which also may include the object 2000 CR105, which has a parehelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years.[29] 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. [30]

The heliopause

The Voyagers entering the heliosheathThe 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. [31] 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. [32]

Galactic context

presumed location of the solar system within our galaxyThe 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. [33]

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.[34] 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 ouside 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.[35]

Extrasolar planetary systems

Our Solar System compared with the system of Upsilon AndromedaeFor 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" [36]; 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.

Artist's impression of the rocky planet OGLE-2005-BLG-390Lb (with surface temperature of -364�F), orbiting its star 20,000 light years (117.5 quadrillion miles) from Earth (found using Gravitational microlensing)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[37] and Darwin are planned to launch and search for Earth-like planets. [38]

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".

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