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Jovian Planet

Jovian planet

Jovian planet is an obsolete term for the four largest planets in the solar system: Jupiter, Saturn, Uranus, and Neptune. The term refers to the Roman god Jupiter—a form of which is Jovis, hence Jovian—and was intended to indicate that all of these planets were similar to Jupiter. However, astronomers have found notable differences between these planets as a result of the exploration of the solar system in recent decades. Jupiter and Saturn consist primarily of hydrogen and helium, while Uranus and Neptune are mainly rock and ice with minor amounts of gases. All four planets do have some things in common, however:
- They are markedly larger than the terrestrial planets.
- They are further from the Sun.
- They tend to have rings.
- They tend to have many moons, which are small relative to the size of the planet. As a consequence, it was proposed to apply the term Jovian planet just to Jupiter and Saturn, with Uranian planet or ice giant for Uranus and Neptune, but these terms have not caught on. The term gas giant is now commonly used to describe all four planets, even though it doesn't accurately describe Uranus and Neptune. With the discovery of massive extrasolar planets, the term Jovian planet has seen some renewed usage in describing planets with a mass comparable to that of Jupiter. See also Hot Jupiter. Category:Planets Category:Planets of the Solar System ja:木星型惑星

Solar system

The solar system comprises our 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. 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 our system specifically: the word "solar" is derived from the Sun's Latin name, Sol (and the term sometimes appears as Solar System). When talking about another stellar system (or planetary system), including the star(s) and bodies associated with them through gravity, it is usual to shorten it to "the system" (e.g. "the Alpha Centauri system" or "the 51 Pegasi system").

Structure and layout of the solar system

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 our 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, planetesimals or, in the case of Pluto, planets). Objects in orbit round the Sun all lie within the same shallow plane, called the ecliptic, and all 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 (♅/10px), Neptune (♆), and Pluto (♇), though Pluto's status has been thrown into question by the discovery of (see below). 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. 1 AU is the distance between the Earth and the Sun, or 149 598 000 kilometers. Pluto is roughly 38 AU from the Sun, while Jupiter lies at roughly 5.2 AU. For very large distances within the solar system, such as regions beyond Pluto or the orbital circumferences of planets, the terameter (Tm, one milliard kilometers) is sometimes used. Despite the fact that many diagrams (like the image at the top of this article), for practicality's sake, 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 and so on. This relationship is roughly 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 + 0.3\times k

where k=0,1,2,4,8,16,32,64,128. By this formulation, we 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 "works," and many claim it is merely a coincidence. Pluto

Origin and evolution of the solar system

The current hypothesis of solar system formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant. 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, the nebula began to collapse, possiby due to disturbance by a nearby supernova. This explosion sent shock waves into space, which squeezed the nebula, pushing more and more matter inward until gravitational forces overcame its internal gas pressure and it also 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, and a great amount of heat was released. 4×¹H → ⁴He + neutrinos + photons During that time, the protostar turned into the Sun and the protoplanets and planetesimals were transformed into planets. All of the planets formed in a relatively short time of a few million years.

Regions of the solar system

protostar's rotating magnetic field on the plasma in the interplanetary medium (Solar Wind) [http://quake.stanford.edu/~wso/gifs/HCS.html]. (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) miles (16 Tm or 16 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: volcanoes
- 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/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 [http://www.universetoday.com/am/publish/mars_volcanoes_active.html recent evidence] 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) 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 (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 to 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. A member of the traditional nine planets, Pluto's tiny mass (less than 1% of Earth's) and diameter have called this status into question. 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. One particular scattered disc object, originally found in 2003 but confirmed two years later by Mike Brown, has renewed the old debate about what constitutes a planet since, though its size is not yet known, it is almost certainly larger than Pluto. It currently has no name, but has been given the provisional designation , 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, indeed, at 44 degrees, more so than any known object in the solar system. Like Pluto, it is believed to consist largely of rock and ice, and has a [http://www.gps.caltech.edu/%7Embrown/planetlila/moon/index.html 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.

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. 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.

And beyond

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. gravity 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 at this point, we could not be said to have left the solar system, for the Sun's gravity will still hold sway even up to the Oort cloud, the great mass of icy objects, currently hypothetical, believed to be the source for all long-period comets and to surround our 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.

Age of the solar system

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 around 4.6 billion years old.

Galactic orbit of the solar system

The solar system is part of the Milky Way galaxy, a spiral galaxy with a diameter of about 100,000 light years containing approximately 200 billion stars, of which our Sun is rather large and bright. (The vast majority of stars are red dwarfs; our 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 our Sun in space). Estimates place the solar system at between 25,000 and 28,000 light years from the galactic center in the Orion Arm. 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

For many years, our 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 to our own, and theories have had to be revised accordingly. Exoplanets have not been seen by astronomers yet, however we know they exist because of the gravitational tug the planets induce on the star, and hence making the star ‘wobble’. 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. 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 our 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

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 us 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:
- If the Earth is moving, what force keeps the air from flying off into space?
- The Earth is made of heavy rock. Heavy rock moves down. Down in a sphere means the centre. The planets are ephemeral and light, so they are above. How can Earth be a planet?
- If the Earth is mobile, then why do we not observe parallax in the stars (the stars appearing to shift in relation to further objects due to the 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

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, we 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

All attributes below are measured relative to the Earth: Of the other objects, Ganymede has the largest mass (0.02). Note: Although is a minor planet, it is being considered as possibly being a major planet (the tenth in the solar system). See Planet (Table) for a more comprehensive table.

Attributes of the largest minor planets

The largest minor planets are smoothly rounded, like 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:

Other facts

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

The solar system in small scales

Scaling down the size of the solar system makes it easier for students to grasp the relative distances. The enormous ratio of interplanetary distances to planetary diameters makes constructing a scale model of the solar system a challenging task. (For example, the distance between the Earth and the Sun is almost 12,000 times the diameter of the Earth.) Several places have built such models.

The solar system in astrology

External links

- [http://solarsystem.nasa.gov/index.cfm NASA's Solar System Exploration site]
- [http://space.jpl.nasa.gov NASA's Solar System Simulator]
- [http://www.jpl.nasa.gov/solar_system NASA/JPL Solar System main page]
- [http://members.aol.com/astroequation/ Astronomical Enigma] Mathematical Order in the orbits of the solar system.
- [http://www.solarviews.com Solarviews]
- [http://celestia.sourceforge.net Celestia] Free 3D realtime space-simulation (OpenGL)
- [http://www.nineplanets.org/ The Nine Planets] Comprehensive solar system site by Bill Arnett
- [http://www.krysstal.com/solarsys_planets.html Planetary data]
- [http://www.solstation.com/habitable.htm Stars and Habitable Planets]
- [http://www.michaelschultz.de/index_en.html Solar System] An interactive planets animation (145 zoom steps and time effects)
- [http://my.execpc.com/~culp/space/timeline.html Timeline of solar system exploration]
- [http://www.anzwers.org/free/universe/index.html An Atlas of the Universe]
- mirror matter [http://uk.arxiv.org/abs/astro-ph/0104251 planets] and other [http://uk.arxiv.org/abs/astro-ph/0110161 mirror objects] in the solar system?
- [http://www.solarsystem.org.uk/ The Virtual Solar System, including a scale model of the system]
- ko:태양계 ms:Sistem suria ja:太陽系 simple:Solar system th:ระบบสุริยะ zh-min-nan:Thài-iông-hē

Saturn (planet)

Saturn is the sixth planet from the Sun. It is a gas giant, the second-largest planet in the solar system after Jupiter. Saturn has a prominent system of rings, consisting of mostly ice particles with a smaller amount of rocky debris. It was named after the Roman god Saturn. Its symbol is a stylized representation of the god's sickle (Unicode: ♄). The Chinese, Korean, Japanese, and Vietnamese cultures refer to the planet as the earth star, 土星, based on the Five Elements.

Physical characteristics

Saturn's shape is visibly flattened at the poles and bulging at the equator (an oblate spheroid); its equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but to a lesser degree. Saturn is also the only one of the Solar System's planets less dense than water, with an average specific density of 0.69. This is only an average value, however; Saturn's upper atmosphere is less dense and its core is considerably more dense than water. Saturn's interior is similar to Jupiter's, having a rocky core at the center, a liquid metallic hydrogen layer above that, and a molecular hydrogen layer above that. Traces of various ices are also present. Saturn has a very hot interior, reaching 12000 K at the core, and it radiates more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism (slow gravitational compression), but this alone may not be sufficient to explain Saturn's heat production. An additional proposed mechanism by which Saturn may generate some of its heat is the "raining out" of droplets of helium deep in Saturn's interior, the droplets of helium releasing heat by friction as they fall down through the lighter hydrogen. Kelvin-Helmholtz mechanism Saturn's atmosphere exhibits a banded pattern similar to Jupiter's (in fact, the nomenclature is the same), but Saturn's bands are much fainter and they're also much wider near the equator. Saturn's winds are among the Solar System's fastest; Voyager data indicates peak easterly winds of 500 m/s (1116 mph).(1) Saturn's finer cloud patterns were not observed until the Voyager flybys. Since then, however, Earth-based telescopy has improved to the point where regular observations can be made. Saturn's usually-bland atmosphere occasionally exhibits long-lived ovals and other features common on Jupiter; in 1990 the Hubble Space Telescope observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters and in 1994 another, smaller storm was observed. The 1990 storm was an example of a Great White Spot, a unique but short-lived Saturnian phenomenon with a roughly 30-year periodicity. Previous Great White Spots were observed in 1876, 1903, 1933, and 1960, with the 1933 storm being the most famous. The careful study of these episodes reveal interesting patterns; if it holds another storm will occur in ~2020.(2) Astronomers using infrared imaging have shown that Saturn has a warm polar vortex, and is the only planet in the solar system known to do so. (1) [http://www.solarviews.com/eng/vgrsat.htm Voyager Saturn Science Summary] (2) Patrick Moore, ed., The 1993 Yearbook of Astronomy, Mark Kidger, "The 1990 Great White Spot of Saturn", 176-215, (New York: W.W. Norton & Company, 1992).

Rotational behavior

Since Saturn does not rotate on its axis at a uniform rate, two rotation periods have been assigned to it, like in Jupiter's case: System I has a period of 10 h 14 min 00 s (844.3°/d) and encompasses the Equatorial Zone, which extends from the northern edge of the South Equatorial Belt to the southern edge of the North Equatorial Belt. All other Saturnian latitudes have been assigned a rotation period of 10 h 39 min 24 s (810.76°/d), which is System II. System III, based on radio emissions from the planet, has a period of 10 h 39 min 22.4 s (810.8°/d); because it is very close in value to System II, it has largely superseded it. While approaching Saturn in 2004, the Cassini spacecraft found that the radio rotation period of Saturn had increased slightly, to approximately 10 h 45 m 45 s (± 36 s). [http://www.nasa.gov/mission_pages/cassini/media/cassini-062804.html] The cause of the change is unknown.

Planetary rings

Saturn is probably best known for its planetary rings, which make it one of the most visually remarkable objects in the solar system.

History

The rings were first observed by Galileo Galilei in 1610 with his telescope, but he clearly did not know what to make of them. He wrote to the Grand Duke of Tuscany that "Saturn is not alone but is composed of three, which almost touch one another and never move nor change with respect to one another. They are arranged in a line parallel to the zodiac, and the middle one [Saturn itself] is about three times the size of the lateral ones [the edges of the rings]." He also described Saturn as having "ears." In 1612 the plane of the rings was oriented directly at the Earth and the rings appeared to vanish, and then in 1613 they reappeared again, further confusing Galileo. The riddle of the rings was not solved until 1655 by Christiaan Huygens, using a telescope much more powerful than the ones available to Galileo in his time. In 1675, Giovanni Domenico Cassini determined that Saturn's ring was actually composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division.

Physical characteristics

The rings can be viewed using a quite modest modern telescope or with a good pair of binoculars. They extend from 6,630 km to 120,700 km above Saturn's equator, and are composed of silica rock, iron oxide, and ice particles ranging in size from specks of dust to the size of a small automobile. There are two main theories regarding the origin of Saturn's rings. One theory, originally proposed by Édouard Roche in the 19th century, is that the rings were once a moon of Saturn whose orbit decayed until it came close enough to be ripped apart by tidal forces (see Roche limit). A variation of this theory is that the moon disintegrated after being struck by a large comet or asteroid. The second theory is that the rings were never part of a moon, but are instead left over from the original nebular material that Saturn formed out of. This theory is not widely accepted today, since Saturn's rings are thought to be unstable over periods of millions of years and therefore of relatively recent origin. While the largest gaps in the rings, such as the Cassini division and Encke division, could be seen from Earth, the Voyager spacecrafts discovered the rings to have an intricate structure of thousands of thin gaps and ringlets. This structure is thought to arise from the gravitational pull of Saturn's many moons in several different ways. Some gaps are cleared out by the passage of tiny moonlets such as Pan, many more of which may yet be undiscovered, and some ringlets seem to be maintained by the gravitational effects of small shepherd satellites such as Prometheus and Pandora. Other gaps arise from resonances between the orbital period of particles in the gap and that of a more massive moon further out; Mimas maintains the Cassini division in this manner. Still more structure in the rings actually consists of spiral waves raised by the moons' periodic gravitational perturbations. Data from the Cassini space probe indicates that the rings of Saturn possess their own atmosphere, independent of that of the planet itself. The atmosphere is composed of molecular oxygen gas (O₂) and is thought to be a product of the disintegration of water ice from the rings into its components, oxygen and hydrogen. [http://news.bbc.co.uk/1/hi/sci/tech/4640641.stm]

Dark side of the rings

Compare images from the Cassini spacecraft taken in March and October 2004, and a Pioneer 11 picture from 1979: The side of Saturn's rings that is lit by the Sun looks very different to the backlit side, which is darker overall and appears almost black in the thick B ring. From Earth, we cannot appreciate this because the Earth cannot view Saturn from an angle that displays the backlit side of the rings, and our only views of it are from spacecraft. In 2004, the Cassini spacecraft revealed the first views of the backlit side in 25 years.

Spokes of the rings

1979.]] Until 1980, the structure of the rings of Saturn was explained exclusively as the action of gravitational forces. The Voyager spacecraft found radial features in the B ring, called spokes, which could not be explained in this manner, as their persistence and rotation around the rings were not consistent with orbital mechanics. The spokes appear dark against the lit side of the rings, and light when seen against the unlit side. It is assumed that they are connected to electromagnetic interactions, as they rotate almost synchronously with the magnetosphere of Saturn. However, the precise mechanism behind the spokes is still unknown. magnetosphere.]] Twenty-five years later, Cassini observed the spokes again. They appear to be a seasonal phenomenon, disappearing in the Saturnian midwinter/midsummer and reappearing as Saturn comes closer to equinox. The spokes were not visible when Cassini arrived at Saturn in early 2004. Some scientists speculated that the spokes would not be visible again until 2007, based on models attempting to describe spoke formation. Nevertheless, the Cassini imaging team kept looking for spokes in images of the rings, and the spokes reappeared in images taken September 5, 2005.

Natural satellites

2005 Saturn has a large number of moons. The precise figure will never be certain as the orbiting chunks of ice in Saturn's rings are all technically moons, and it is difficult to draw a distinction between a large ring particle and a tiny moon. Seven of the moons are massive enough to have collapsed into a spheroid under their own gravitation. These are compared to Earth's moon in the table below. Saturn's most noteworthy moon is Titan, the only moon in the solar system to have a dense atmosphere. Due to the tidal forces of Saturn, the moons are currently not at the same position as they were when they were first formed (for a timeline of discovery dates, see Timeline of natural satellites).

Exploration of Saturn

Timeline of natural satellites

Pioneer 11 flyby

Saturn was first visited by Pioneer 11 in September 1979. It flew within 20,000 km of the planet's cloudtops. Low-resolution images were acquired of the planet and few of its moons. Resolution was not good enough to discern surface features, however. The spacecraft also studied the rings; among the discoveries were the thin F-ring and the fact that dark gaps in the rings are bright when viewed towards the Sun, or in other words, they are not empty of material. It also measured the temperature of Titan. [http://spaceprojects.arc.nasa.gov/Space_Projects/pioneer/PN10&11.html]

Voyager flybys

In November 1980, the Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, rings, and the satellites. Surface features of various moons were seen for the first time. Voyager 1 performed a close flyby of Titan greatly increasing our knowledge of the atmosphere of the moon. However, it also proved that Titan's atmosphere is impenetrable in visible wavelengths, so no surface details were seen. The flyby also changed spacecraft's trajectory out from the plane of the solar system. Almost a year later, in August 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings. Unfortunately, during the flyby, the probe's camera stuck and some planned imaging was lost. Saturn's gravity was used to direct the spacecraft's trajectory towards Uranus. The probes discovered and confirmed several new satellites orbiting near or within the planet's rings. They also discovered the small Maxwell and Keeler gaps.

Cassini orbiter

On July 1, 2004, the Cassini-Huygens spacecraft performed the SOI (Saturn Orbit Insertion) maneuver and entered into orbit around Saturn. Before the SOI, Cassini had already studied the system extensively. In June 2004, it had conducted a close flyby of Phoebe sending back high-resolution images and data. The orbiter completed two Titan flybys before releasing the Huygens probe on December 25, 2004. Huygens descended onto the surface of Titan on January 14, 2005, sending a flood of data during the atmospheric descent and after the landing. As of 2005, Cassini is conducting multiple flybys of Titan and icy satellites. The primary mission ends in 2008 when the spacecraft has completed 74 orbits around the planet. :For the latest information and news releases, see [http://saturn.jpl.nasa.gov Cassini website].

Best viewing of Saturn

2008 While it is a rewarding target for observation for most of the time it is visible in the sky, Saturn and its rings are best seen when the planet is at or near opposition (the configuration of a planet when it is at an elongation of 180° and thus appears opposite the Sun in the sky.) In the opposition on January 13, 2005, Saturn appeared at its brightest until 2031, mostly due to a favourable orientation of the rings relative to the Earth. Saturn appears to the naked eye in the night sky as a bright, yellowish star varying usually between magnitude +1 and 0 and takes approximately 29 and a half years to make a complete circuit of the ecliptic against the background constellations of the zodiac. Optical aid (a large pair of binoculars or a telescope) magnifying at least 20X is required to clearly resolve Saturn's rings for most people.

Appearance

Saturn in fiction and film

Saturn is a popular setting for science fiction novels and films, although the planet tends to be used as a pretty backdrop rather than as an important part of the plot.
- In Voltaire's Micromégas (1752), the eponymous hero arrives at Saturn first (Uranus and Neptune were unknown then). Saturn's citizens are « only a thousand fathoms high », have 72 senses and live for about 15,000 years. Micromégas forms a close friendship with the secretary of the Academy of Saturn, who accompanies him to Earth.
- The unwitting adventurers in Jules Verne's Off on a Comet (1877) pass within 415,000,000 miles of Saturn while riding on a comet. The book describes Saturn as having 8 satellites and 3 rings. It contains a black and white illustration showing what night might look like from the surface of the planet. The rings are brightly illuminated by the sun, and an elliptical shadow is cast on them by the planet. The drawing shows the surface of Saturn as a rocky, desolate, solid surface.
- In H. P. Lovecraft's Cthulhu Mythos (1928–), Saturn was known as Cykranosh in the Hyperborean Era, both Tsathoggua and Atlach-Nacha came to Earth from there, and Tsathoggua's paternal uncle Hziulquoigmnzhah still resides there.
- In Isaac Asimov's short story The Martian Way (1952), Martian colonists use a chunk of ice from Saturn's rings to bring water to the dry world.
- Kurt Vonnegut's novel The Sirens of Titan (1959) is partly set on Titan, Saturn's best known moon.
- In the Star Trek universe (1966–), Saturn is used for the Starfleet Academy Flight Range.
- In Arthur C. Clarke's novel version of 2001: A Space Odyssey (1968), a spacecraft visits the Saturnian system. Clarke's later novel Imperial Earth (1976) takes place partially at a human colony on Titan.
- Douglas Trumbull's film Silent Running (1972) features an ark-like spacecraft traveling through the Saturnian system.
- In the sixth book of the Yoko Tsuno comic book series (Les Trois soleils de Vinéa, 1976), a small part of the action takes place on a Vinean space station in orbit around Saturn. Saturn's moon Titan is also briefly mentioned and depicted. Other Saturnian moons are visible but not named.
- The film Saturn 3 (1980) is mostly set on one of Saturn's moons, but also features a journey through the planet's rings.
- The science fiction anime series The Super Dimension Fortress Macross (1982–1983) has one episode that takes place in Saturn's rings, and the beginning of the movie adaptation The Super Dimension Fortress Macross: Do You Remember Love? takes place near the moon Titan and Saturn's rings.
- An episode of the cartoon series Transformers from 1985, "The God Gambit," reveals that humanoid aliens have a thriving civilization on the moon Titan. In a later episode from 1986, "Money is Everything," which takes place in the year 2006, Titan has been terraformed by humans.
- Warhammer 40,000's universe (1987) places the headquarters of the Grey Knights and Ordo Malleus in Saturn's moons, owing to their defensive capability.
- Tim Burton's film Beetlejuice (1988) is partly set on a fictional Saturn, populated by giant sandworms.
- The Citadel research and mining space station, setting of the computer game System Shock (1994), is in orbit of Saturn for most of the game.
- Stephen Baxter's novel Titan (1997) is focused on the moon Titan, but contains vivid depictions of a journey through the Saturnian system.
- In Michael McCollum's novel The Clouds of Saturn (1998), SparrowHawk pilots Larson Sands and Halley Trevanon fight against the Northern Alliance during a time when the Sun has flared out of control and boiled Earth's oceans away.
- In the sci-fi anime Cowboy Bebop (1998), in the year 2068 a war was fought on Titan.
- In the anime Bishoujo Senshi Sailor Moon, Sailor Saturn is a guardian representing the planet. Her birth is thought to bring destruction to the world, as she's known as the sailor of death and rebirth. On her forehead is the planet's symbol.
- Ben Bova's novel Saturn (2003) is about a spacecraft traveling toward the planet, although Saturn itself does not figure greatly in the story.

Saturn in various cultures

Chinese and Japanese culture designate the planet Saturn as "Earth Star." This is based on Five Elements which was traditionally used to classify natural elements. In Hebrew, Saturn is called 'Shabbathai'. Its Angel is Cassiel. Its Intelligence, or beneficial spirit, is Agiel (layga), and its spirit (darker aspect) is Zazel (lzaz). See: Kabbalah.

External links

- [http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html NASA's Saturn fact sheet]
- [http://saturn.jpl.nasa.gov/home/index.cfm NASA's Cassini mission to Saturn]
- [http://hubblesite.org/newscenter/newsdesk/archive/releases/2001/15/image/a Change of seasons on Saturn]
- [http://www.affs.org/html/studies_on_the_rings_of_saturn.html Theoretical description of the rings of Saturn]
- [http://www.vias.org/spacetrip/saturn_1.html A Trip Into Space] Description and photos of Saturn
(moon navigator) | Saturn | Pan | ...
- zh-min-nan:Thó·-chheⁿ ko:토성 ms:Zuhal ja:土星 simple:Saturn (planet) th:ดาวเสาร์

Uranus (planet)

Uranus (pronounced either or ) is the seventh planet from the Sun. It is a gas giant, the third largest by diameter and fourth largest by mass. It is named after Uranus, the Greek god of the sky, and progenitor of the other gods. Its symbol is either ♅ (Unicode U+2645, mostly astrological) or Unicode (mostly astronomical). NASA's Voyager 2 is the only spacecraft to have visited the planet and no other visits are planned. Launched in 1977, Voyager made its closest approach to Uranus on January 24, 1986, before continuing on its journey to Neptune. The Chinese, Korean, Japanese, and Vietnamese cultures have since named the planet sky king star, 天王星.

Physical characteristics

Composition

Uranus is composed primarily of rocks and various ices, with only about 15% hydrogen and a little helium (in contrast to Jupiter and Saturn which are mostly hydrogen). Uranus (like Neptune) is in many ways similar to the cores of Jupiter and Saturn minus the massive liquid metallic hydrogen envelope. It appears that Uranus does not have a rocky core like Jupiter and Saturn but rather that its material is more or less uniformly distributed. Uranus' cyan color is due to the absorption of red light by atmospheric methane. The surface temperature of Uranus's cloud cover is approximately 55 K (-218 °C or -360 °F).

Axial tilt

One of the most distinctive features of Uranus is its axial tilt of ninety-eight degrees. Consequently, for part of its orbit one pole faces the Sun continually whilst the other pole faces away. At the other side of Uranus' orbit the orientation of the poles towards the Sun is reversed. Between these two extremes of its orbit the Sun rises and sets around the equator normally. At the time of Voyager 2's passage in 1986, Uranus' south pole was pointed almost directly at the Sun. Note that the labelling of this pole as "south" is actually in some dispute. Uranus can either be described as having an axial tilt of slightly more than 90°, or it can be described as having an axial tilt of slightly less than 90° and rotating in a retrograde direction; these two descriptions are exactly equivalent as physical descriptions of the planet but result in different definitions of which pole is the North Pole and which is the South Pole. One result of this odd orientation is that the polar regions of Uranus receive a greater energy input from the Sun than its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles, although the underlying mechanism which causes this is unknown. The reason for Uranus' extreme axial tilt is also not known. It is speculated that perhaps during the formation of the planet it collided with an enormous protoplanet, resulting in the skewed orientation. It appears that Uranus' extreme axial tilt also results in extreme seasonal variations in its weather. During the Voyager 2 flyby, Uranus' banded cloud patterns were extremely bland and faint. Recent Hubble Space Telescope observations, however, show a more strongly banded appearance now that the Sun is approaching Uranus' equator. By 2007 the Sun will be directly over Uranus's equator.

Magnetic Field

Uranus' magnetic field is odd in that it is not centered on the center of the planet and is tilted almost 60° with respect to the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus. Neptune has a similarly displaced magnetic field, suggesting that this is not necessarily a result of Uranus' axial tilt. The magnetotail is twisted by the planet's rotation into a long corkscrew shape behind the planet. The magnetic field's source is unknown; the electrically conductive, super-pressurized ocean of water and ammonia once thought to lie between the core and the atmosphere now appears to be nonexistent.

Discovery and naming

Uranus was the first planet to be discovered that was not known in ancient times, although it had been observed on many previous occasions but was always mistakenly identified as a star. The earliest recorded sighting was in 1690 when John Flamsteed catalogued it as 34 Tauri. Flamsteed observed Uranus twice again, in 1712 and 1715. Bradley observed it in 1748, 1750 and 1753; Mayer in 1756. Le Monnier observed it four times in 1750, twice in 1768, six times in 1769, and one last time in 1771. He was a victim of his own disorderliness: one of his observations was found consigned on a paper bag used to store hair powder! Sir William Herschel discovered the planet on March 13, 1781, but reported it on April 26, 1781, as a "comet": Account of a Comet, By Mr. Herschel, F. R. S.; Communicated by Dr. Watson, Jun. of Bath, F. R. S., Philosophical Transactions of the Royal Society of London, Volume 71, pp. 492-501. Herschel originally named it Georgium Sidus (George's Star) in honour of King George III of Great Britain. When it was pointed out that sidus means star and not planet, he rebaptised it the Georgian Planet. In any case, this name was not acceptable outside of Britain. Lalande proposed in 1784 to name it Herschel, at the same time that he created the planet's symbol ("a globe surmounted by your initial"); his proposal was readily adopted by French astronomers. Prosperin, of Uppsala, proposed the names Astraea, Cybele, and Neptune (now borne by two asteroids and a planet). Lexell, of St. Petersburg, compromised with George III's Neptune and Great-Britain's Neptune. Bernoulli, from Berlin, suggested Hypercronius and Transaturnis. Lichtenberg, from Göttingen, chimed in with Austräa, a goddess mentioned by Ovid (but who is traditionally associated with Virgo). The name Minerva was also proposed.[http://vesuvius.jsc.nasa.gov/er/seh/hersc.html] Finally, Bode, as editor of the Berliner Astronomisches Jahrbuch, opted for Uranus, after the Greek god; Hell followed suit by using it in the first ephemeris, published in Vienna. Examination of earliest issues of Monthly Notices of the Royal Astronomical Society from 1827 shows that the name Uranus was already the most common name used even by British astronomers by then, and probably earlier. The name Georgium Sidus or "the Georgian" were still used infrequently (by the British alone) thereafter. The final holdout was HM Nautical Almanac Office, which did not switch to Uranus until 1850.

Visibility

The brightness of Uranus is between magnitude +5.5 and +6.0, so it can be seen with the naked eye as a faint star under dark sky conditions. It can be easily found with binoculars. From Earth, it has a diameter of 4". Even in large telescopes no details can be seen on its disc. However, infrared studies of its atmosphere using adaptive optics have yielded interesting data in the years since the Voyager flyby. (1) (1) [http://www.space.com/scienceastronomy/uranus_images_041110.html Space.com: New Images Reveal Clouds on Uranus]

Appearance

Planetary rings

Uranus has a faint planetary ring system, composed of dark particulate matter up to 10 metres in diameter. This ring system was discovered in March 1977 by James L. Elliot, Edward W. Dunham, and Douglas J. Mink using the Kuiper Airborne Observatory. The discovery was serendipitous; they planned to use the occultation of a star by Uranus to study the planet's atmosphere, but when they analysed their observations they found that the star had disappeared briefly from view five times both before and after it disappeared behind the planet. They concluded that there must be a ring system around the planet; it was directly detected when the Voyager 2 space probe passed Uranus in 1986.

Natural satellites

Uranus has 27 known moons. The names for these moons are chosen from characters from the works of Shakespeare and Alexander Pope. The five main satellites are Miranda, Ariel, Umbriel, Titania and Oberon (For a timeline of discovery dates, see Timeline of natural satellites).

Uranus in fiction

- Mr. Vivenair (a pseudonym) published A Journey Lately Performed Through the Air in an Aerostatic Globe, Commonly Called an Air Balloon, From This Terraquaeous Globe to the Newly Discovered Planet, Georgium Sidus in 1784.
- In the Buck Rogers series (1928–), Uranus is portrayed as having biodomes and robots.
- R. R. Winterbotham's "Clouds over Uranus" was published by Astounding in March of 1937
- In the 1962 film [http://www.imdb.com/title/tt0056127/ Journey to the Seventh Planet], astronauts on (sic) Uranus encounter a strange intelligence.
- In the Doctor Who (1963–) serial The Daleks' Master Plan, Uranus is described as being the only location in the universe where the mineral Taranium can be acquired.
- In H. P. Lovecraft's Cthulhu Mythos (1928–), Uranus is known as L'gy'hx and is inhabited by cubical metallic many-legged creatures who worship Lrogg. They entered in religious conflict with the Shan. (Ramsey Campbell's The Insects from Shaggai, 1964)
- In the anime Bishoujo Senshi Sailor Moon (1992), Sailor Uranus is the seventh guardian soldier to appear in the series, and represents the planet. Her attacks are associated with the force of nature (e.g. World Shaking, which is basically an earthquake). Later in the series, her attacks become more of galactic-like (e.g. Space Turbulence). Her talisman is Space Sword, and her image colours are beige and sometimes, gold.
- In the animated series Futurama (1999–2003), in 2620 the name of Uranus was changed to Urectum to get rid of "That Stupid Joke" once and for all.
- In Wayne W. Dyer's Gifts from Eykis (2002), Eykis is a female from Uranus who imparts wisdom and spirituality to Earth.

External links

- [http://nssdc.gsfc.nasa.gov/planetary/factsheet/uranusfact.html NASA's Uranus fact sheet]
- [http://www2.keck.hawaii.edu/news/science/uranus/ Keck pictures of Uranus show best view from the ground] -- Press release with some photographs showing rings, satellites and clouds
- ko:천왕성 ms:Uranus ja:天王星 simple:Uranus (planet) th:ดาวยูเรนัส

Neptune (planet)

Neptune is the eighth, or, due to Pluto's eccentric orbit, occasionally the ninth planet farthest from the Sun, and the outermost gas giant in our solar system. Although the smallest of the gas giants, Neptune is more massive than Uranus: its stronger gravitational field has compressed it to a higher density. Faint dark rings have been detected around the blue planet, but are less substantial than those of Saturn. When these rings were discovered, it was thought that they might not be complete, but this was disproved by Voyager 2. Neptune also has 2,000 km/h winds of hydrogen, helium, and methane that gives it its blue appearance. At the time of the 1989 Voyager 2 flyby, it had in its southern hemisphere a Great Dark Spot comparable to the Great Red Spot on Jupiter. The Great Dark Spot has since disappeared. Neptune possesses nine confirmed moons, and four awaiting confirmation. Neptune's largest moon, Triton, is notable for its retrograde orbit, extreme cold (38K), and extremely tenuous (14 microbar) nitrogen/methane atmosphere. Neptune is named after the Roman god of the sea. It is represented in Unicode by a stylized version of the god's trident (Ψ). The Chinese, Korean, Japanese, and Vietnamese cultures have since named this planet sea king star, 海王星. Discovered on September 23, 1846, Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989. In 2003, there was a proposal to NASA's "Vision Missions Studies" to implement a "Neptune Orbiter with Probes" mission that does Cassini-level science without fission-based electric power or propulsion. The work is being done in conjunction with JPL and the California Institute of Technology.

Discovery

Galileo's astronomical drawings show that he had first observed Neptune on December 27, 1612, and again on January 27, 1613; on both occasions Galileo mistook Neptune for a fixed star when it appeared very close (in conjunction) to Jupiter in the night sky. Believing it to be a fixed star, he cannot be credited with its discovery. At the time Galileo first observed Neptune on December 28, 1612, it was stationary in the sky because it had just turned retrograde that very day; because it was stationary in the sky and only beginning the planet's yearly retrograde cycle, its motion was far too slight to be detected with Galileo's small telescope. Had Neptune been moving at its regular/average speed when Galileo first observed it in 1612 and 1613, he would have most likely realized that it was a planet and not a fixed star due to Neptune's relatively rapid normal motion along the ecliptic compared to the extremely slow motion of any random fixed star found in the night sky. In 1821, Alexis Bouvard published astronomical tables of the orbit of Uranus. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize some perturbing body. In 1843, John Couch Adams calculated the orbit of an eighth planet that would account for Uranus' motion. He sent his calculations to Sir George Airy, the Astronomer Royal, who asked Adams for a clarification; Adams began to draft a reply but never sent it. In 1846, Urbain Le Verrier, independently of Adams, produced his own calculations but also experienced difficulties in encouraging any enthusiasm in his compatriots. However, in the same year, John Herschel started to champion the mathematical approach and persuaded James Challis to search for the planet. After much procrastination, Challis began his reluctant search in July 1846. However, in the mean time, Le Verrier had convinced Johann Gottfried Galle to search for the planet. Though still a student at the Berlin Observatory, Heinrich d'Arrest suggested that a recently drawn chart of the sky, in the region of Le Verrier's predicted location, could be compared with the current sky to seek the displacement characteristic of a planet, as opposed to a fixed star. Neptune was discovered that very night, September 23, 1846, within 1° of where Le Verrier had predicted it to be, and about 10° from Adams' prediction. Challis later realized that he had observed the planet twice in August, failing to identify it owing to his casual approach to the work. In the aftermath of the discovery, there was much nationalistic rivalry between the French and the British over who had priority and deserved credit for the discovery. Eventually an international consensus emerged that both Le Verrier and Adams jointly deserved credit. However, the issue is now being re-evaluated by historians with the rediscovery in 1998 of the "Neptune papers" (historical documents from the Royal Greenwich Observatory), which had apparently been misappropriated by astronomer Olin Eggen for nearly three decades and were not rediscovered (in his possession) until immediately after his death. After reviewing the documents, some historians now suggest that Adams did not in fact deserve equal credit with Le Verrier.

Naming

Shortly after its discovery, Neptune was referred to simply as "the planet exterior to Uranus" or as "Le Verrier's planet." The first suggestion for a name came from Galle. He proposed the name Janus. In England, Challis put forth the name Oceanus, particularly appropriate for a seafaring people. In France, Arago suggested that the new planet be called Leverrier, a suggestion which was met with stiff resistance outside France. French almanacs promptly reintroduced the name Herschel for Uranus and Leverrier for the new planet. Meanwhile, on separate and independent occasions, Adams suggested altering the name Georgian to Uranus, while Leverrier (through the Board of Longitude) suggested Neptune for the new planet. Struve came out in favor of that name on December 29, 1846, to the Saint Petersburg Academy of Sciences. Soon Neptune became the internationally accepted nomenclature. In Roman mythology Neptune was the god of the sea, identified with the Greek Poseidon. The demand for a mythological name seemed to be in keeping with the nomenclature of the other planets all of which, except for Uranus, were named in antiquity.

Physical characteristics

Orbiting so far from the sun, Neptune receives very little heat — in fact the uppermost regions of the atmosphere are −218 °C (55 K). Because Neptune is a gas giant, there is no solid surface; as one ventures deeper and deeper inside the layers of gas, however, the temperature rises steadily. It is thought that this may be leftover heat generated by infalling matter during the planet's birth, now slowly radiating away into space. Neptune's atmosphere has the highest wind speeds in the solar system, up to 2000 km/h, thought to be powered by this flow of internal heat. The internal structure resembles that of Uranus. There is likely to be a core consisting of (molten) rock and metal, surrounded by a mixture of rock, water, ammonia, and methane. The atmosphere, extending perhaps 10 to 20 percent of the way towards the center, is mostly hydrogen and helium at high altitudes, but has increasing concentrations of methane, ammonia, and water as it approaches and finally blends into the liquid interior. The pressure at the centre of Neptune is millions of times more than that on the surface of Earth. Comparing its rotational speed to its degree of oblateness indicates that it has its mass less concentrated towards the center than does Uranus. Neptune also resembles Uranus in its magnetosphere, with a magnetic field strongly tilted relative to its rotational axis at 47° and offset at least 0.55 radii (about 13,500 kilometres) from the planet's physical center. Comparing the magnetic fields of the two planets, scientists think the extreme orientation may be characteristic of flows in the interior of the planet and not the result of Uranus' sideways orientation. rotation One difference between Neptune and Uranus is the level of meteorological activity. Uranus is visually quite bland, while Neptune's high winds come with notable weather phenomena. The Great Dark Spot, an Earth-sized dark marking resembling the Great Red Spot of Jupiter, disappeared in 1994. However, a newer image of the planet taken by the Hubble Space Telescope on November 2, 1994, revealed that a smaller storm similar to its predecessor had formed over Neptune’s Northern Hemisphere. Unique among the gas giants is the presence of high clouds casting shadows on the opaque cloud deck below.

Appearance and visibility from Earth

Neptune is never visible with the naked eye. The brightness of Neptune is between magnitudes +7.7 and +8.0, so a telescope or binoculars are required to observe it. With the use of a telescope it appears as a small blue-green disk, similar in appearance to Uranus; the blue-green colour comes from the methane in its atmosphere. With an orbital period of 165 years, Neptune will soon return to the approximate position where Galle discovered it, on three different dates. These are April 11, 2009, when it will be in prograde motion; July 17, 2009, when it will be in retrograde motion; and finally for the last time for the next 165 years, on February 7, 2010. Like all planets in the solar system beyond Earth, Neptune undergoes retrogradation at certain points during its synodic period. In addition to the start of retrogradation, other events within the synodic period include astronomical opposition, the return to prograde motion, and conjunction to the Sun.

Planetary rings

conjunction Neptune has a faint planetary ring system of unknown composition. The rings have a peculiar "clumpy" structure, the cause of which is not currently understood but which may be due to the gravitational interaction with small moons in orbit near them. Evidence that the rings are incomplete first arose in the mid-1980s, when stellar occultation experiments were found to occasionally show an extra "blink" just before or after the planet occulted the star. Images by Voyager 2 in 1989 settled the issue, when the ring system was found to contain several faint rings. The outermost ring, Adams, contains three prominent arcs now named Liberté, Egalité, and Fraternité (Liberty, Equality, and Fraternity). The existence of arcs is very difficult to understand because the laws of motion would predict that arcs spread out into a uniform ring over very short timescales. The gravitational effects of Galatea, a moon just inward from the ring, are now believed to confine the arcs. Several other rings were detected by the Voyager cameras. In addition to the narrow Adams Ring 63,000 km from the centre of Neptune, the Leverrier Ring is at 53,000 km and the broader, fainter Galle Ring is at 42,000 km. A faint outward extension to the Leverrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57,000 km. New Earth-based observations announced in 2005 appeared to show that Neptune's rings are much more unstable than previously thought. In particular, it seems that the Liberté ring might disappear in as little as one century. The new observations appear to throw our understanding of Neptune's rings into considerable confusion.

Natural satellites

Neptune has 13 known moons. The largest by far, and the only one massive enough to be spheroidal, is Triton, discovered by William Lassell just 17 days after the discovery of Neptune itself. Unlike all other large planetary moons, it has a retrograde orbit. It is close enough to Neptune to be locked into a synchronous orbit, and is slowly spiraling inward. Triton is the coldest object that has been measured in the solar system. Neptune's second known satellite, the irregular moon Nereid, has one of the most eccentric orbits of any satellite in the solar system. From July to September 1989, Voyager 2 discovered six new Neptunian moons. Of these, the irregularly shaped Proteus is notable for being as large as a body of its density can be without being pulled into a spherical shape by its own gravity. Although the second most massive Neptunian moon, it is only one quarter of one percent of the mass of Triton. Neptune's innermost four moons, Naiad, Thalassa, Despina, and Galatea, orbit close enough to be within Neptune's rings. The next farthest out, Larissa was originally discovered in 1981 when it had blocked a star. This was attributed to ring arcs, but when Voyager 2 observed Neptune in 1989, it was found to have been caused by the moon. Five new irregular moons discovered between 2002 and 2003 were announced in 2004. :For a timeline of discovery dates, see Timeline of natural satellites

Trojan asteroids

As of 2005, there are two known Trojan asteroids of Neptune which have the same orbital period as Neptune and lie in the elongated, curved regions around the L₄ and L₅ Lagrangian points 60° ahead of and behind Neptune. These are and .In 2005, three more suspected Neptune Trojans were spotted: , , and . Better orbits are required before they can be truly labeled as Neptune Trojans.

Neptune in fiction and film

Neptune has been used as a reference and setting in fiction and films. The first fictional visit of Neptune, portrayed as glacial but nevertheless inhabited, occurred in Spirito gentil (1889). In Olaf Stapledon's 1930 epic novel Last and First Men, Neptune is the final home of the highly evolved human race. Samuel R. Delany's 1976 novel Triton has humanity colonizing several parts of the solar system, including Neptune's largest moon. Neptune was the intended destination of the mining ship Red Dwarf in the books based on the BBC sitcom of that name, but an accident on board sends it into deep space instead. Although used purely as a backdrop, the planet was the setting of the 1997 sci-fi/horror film Event Horizon. The planet was also the home of various alien species and characters. In H. P. Lovecraft's Cthulhu Mythos (1928), Neptune is known as "Yaksh" and is inhabited by curious fungoid creatures (Clark Ashton Smith's The Family Tree of the Gods, 1944). In the anime <