Jupiter

Jupiter is the fifth planet from the Sun and the largest planet within the Solar System.[11] It is a gas giant with mass slightly less than one-thousandth that of the Sun but is two and a half times more massive than all of the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn, Uranus and Neptune. Together, these four planets are sometimes referred to as the Jovian planets.
The planet was known by astronomers of ancient times and was associated with the mythology and religious beliefs of many cultures. The Romans named the planet after the Roman god Jupiter.[12] When viewed from Earth, Jupiter can reach an apparent magnitude of −2.8, making it on average the third-brightest object in the night sky after the Moon and Venus. (Mars can briefly exceed Jupiter's brightness at certain points in its orbit.)
Jupiter is primarily composed of hydrogen with a quarter of its mass being helium; it may also have a rocky core of heavier elements. Because of its rapid rotation, Jupiter's shape is that of an oblate spheroid (it possesses a slight but noticeable bulge around the equator). The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope. Surrounding the planet is a faint planetary ring system and a powerful magnetosphere. There are also at least 63 moons, including the four large moons called the Galilean moons that were first discovered by Galileo Galilei in 1610. Ganymede, the largest of these moons, has a diameter greater than that of the planet Mercury.
Jupiter has been explored on several occasions by robotic spacecraft, most notably during the early Pioneer and Voyager flyby missions and later by the Galileo orbiter. The most recent probe to visit Jupiter was the Pluto-bound New Horizons spacecraft in late February 2007. The probe used the gravity from Jupiter to increase its speed and adjust its trajectory toward Pluto, thereby saving years of travel. Future targets for exploration in the Jovian system include the possible ice-covered liquid ocean on the moon Europa.

Structure


Jupiter is one of the four gas giants; that is, it is not primarily composed of solid matter. It is the largest planet in the Solar System, having a diameter of 142,984 km at its equator. Jupiter's density, 1.326 g/cm³, is the second highest of the gas giant planets, but lower than any of the four terrestrial planets.

Composition

Jupiter's upper atmosphere is composed of about 88-92% hydrogen and 8-12% helium by percent volume or fraction of gas molecules (see table to the right). Since a helium atom has about four times as much mass as a hydrogen atom, the composition changes when described in terms of the proportion of mass contributed by different atoms. Thus the atmosphere is approximately 75% hydrogen and 24% helium by mass, with the remaining one percent of the mass consisting of other elements. The interior contains denser materials such that the distribution is roughly 71% hydrogen, 24% helium and five percent other elements by mass. The atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia.[13][14] Through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found.[15]
The atmospheric proportions of hydrogen and helium are very close to the theoretical composition of the primordial solar nebula. However, neon in the upper atmosphere only consists of 20 parts per million by mass, which is about a tenth as abundant as in the Sun.[16] Helium is also depleted, although only to about 80% of the Sun's helium composition. This depletion may be a result of precipitation of these elements into the interior of the planet.[17] Abundances of heavier inert gases in Jupiter's atmosphere are about two to three times that of the sun.
Based on spectroscopy, Saturn is thought to be similar in composition to Jupiter, but the other gas giants Uranus and Neptune have relatively much less hydrogen and helium.[18] However, because of the lack of atmospheric entry probes, high quality abundance numbers of the heavier elements are lacking for the outer planets beyond Jupiter

Mass

Jupiter is 2.5 times more massive than all the other planets in our Solar System combined — this is so massive that its barycenter with the Sun actually lies above the Sun's surface (1.068 solar radii from the Sun's center). Although this planet dwarfs the Earth (with a diameter 11 times as great) it is considerably less dense. Jupiter's volume is equal to 1,317 Earths, yet is only 318 times as massive.[19][20] A Jupiter mass (MJ) is used to describe masses of other gas giant planets, particularly extrasolar planets.
Theoretical models indicate that if Jupiter had much more mass than it does at present, the planet would shrink. For small changes in mass, the radius would not change appreciably, and above about four Jupiter masses the interior would become so much more compressed under the increased gravitation force that the planet's volume would actually decrease despite the increasing amount of matter. As a result, Jupiter is thought to have about as large a diameter as a planet of its composition and evolutionary history can achieve. The process of further shrinkage with increasing mass would continue until appreciable stellar ignition is achieved as in high-mass brown dwarfs around 50 Jupiter masses.[21] This has led some astronomers to term it a "failed star", although it is unclear whether or not the processes involved in the formation of planets like Jupiter are similar to the processes involved in the formation of multiple star systems.
Although Jupiter would need to be about 75 times as massive to fuse hydrogen and become a star, the smallest red dwarf is only about 30 percent larger in radius than Jupiter.[22][23] In spite of this, Jupiter still radiates more heat than it receives from the Sun. The amount of heat produced inside the planet is nearly equal to the total solar radiation it receives.[24] This additional heat radiation is generated by the Kelvin-Helmholtz mechanism through adiabatic contraction. This process results in the planet shrinking by about 2 cm each year.[25] When it was first formed, Jupiter was much hotter and was about twice its current diameter.[26]

Internal structure

This cut-away illustrates a model of the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen.Jupiter is thought to consist of a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen.[25] Beyond this basic outline, there is still considerable uncertainty. The core is often described as rocky, but its detailed composition is unknown, as are the properties of materials at the temperatures and pressures of those depths (see below). In 1997, the existence of the core was suggested by gravitational measurements.[25] indicating a mass of from 12 to 45 times the Earth's mass or roughly 3%-15% of the total mass of Jupiter.[24][27] The presence of a core during at least part of Jupiter's history is suggested by models of planetary formation involving initial formation of a rocky or icy core that is massive enough to collect its bulk of hydrogen and helium from the protosolar nebula. Assuming it did exist, it may have shrunk as convection currents of hot liquid metallic hydrogen mixed with the molten core and carried its contents to higher levels in the planetary interior. A core may now be entirely absent, as gravitational measurements are not yet precise enough to rule that possibility out entirely.[25][28]
The uncertainty of the models is tied to the error margin in hitherto measured parameters: one of the rotational coefficients (J6) used to describe the planet's gravitational moment, Jupiter's equatorial radius, and its temperature at 1 bar pressure. The JUNO mission, scheduled for launch in 2011, is expected to narrow down the value of these parameters, and thereby make progress on the problem of the core.[29]
The core region is surrounded by dense metallic hydrogen, which extends outward to about 78 percent of the radius of the planet.[24] Rain-like droplets of helium and neon precipitate downward through this layer, depleting the abundance of these elements in the upper atmosphere.[17][30]
Above the layer of metallic hydrogen lies a transparent interior atmosphere of liquid hydrogen and gaseous hydrogen, with the gaseous portion extending downward from the cloud layer to a depth of about 1,000 km.[24] Instead of a clear boundary or surface between these different phases of hydrogen, there is probably a smooth gradation from gas to liquid as one descends.[31][32] This smooth transition happens whenever the temperature is above the critical temperature, which for hydrogen is only 33 K (see hydrogen).
The temperature and pressure inside Jupiter increase steadily toward the core. At the phase transition region where liquid hydrogen (heated beyond its critical point) becomes metallic, it is believed the temperature is 10,000 K and the pressure is 200 GPa. The temperature at the core boundary is estimated to be 36,000 K and the interior pressure is roughly 3,000–4,500 GPa.[24]

Planetary rings


Jupiter has a faint planetary ring system composed of three main segments: an inner torus of particles known as the halo, a relatively bright main ring, and an outer gossamer ring.[46] These rings appear to be made of dust, rather than ice as is the case for Saturn's rings.[24] The main ring is probably made of material ejected from the satellites Adrastea and Metis. Material that would normally fall back to the moon is pulled into Jupiter because of its strong gravitational pull. The orbit of the material veers towards Jupiter and new material is added by additional impacts.[47] In a similar way, the moons Thebe and Amalthea probably produce the two distinct components of the gossamer ring.[47Orbit and rotationJupiter is the only planet that has a center of mass with the Sun that lies outside the volume of the Sun, though by only 7% of the Sun's radius.[50] The average distance between Jupiter and the Sun is 778 million km (about 5.2 times the average distance from the Earth to the Sun, or 5.2 AU) and it completes an orbit every 11.86 years. This is two-fifths the orbital period of Saturn, forming a 5:2 orbital resonance between the two largest planets in the Solar System.[51] The elliptical orbit of Jupiter is inclined 1.31° compared to the Earth. Because of an eccentricity of 0.048, the distance from Jupiter and the Sun varies by 75 million km between perihelion and aphelion, or the nearest and most distant points of the planet along the orbital path respectively.
The axial tilt of Jupiter is relatively small: only 3.13°. As a result this planet does not experience significant seasonal changes, in contrast to Earth and Mars for example.[52]
Jupiter's rotation is the fastest of all the Solar System's planets, completing a rotation on its axis in slightly less than ten hours; this creates an equatorial bulge easily seen through an Earth-based amateur telescope. This rotation requires a centripetal acceleration at the equator of about 1.67 m/s², compared to the equatorial surface gravity of 24.79 m/s²; thus the net acceleration felt at the equatorial surface is only about 23.12 m/s². The planet is shaped as an oblate spheroid, meaning that the diameter across its equator is longer than the diameter measured between its poles. On Jupiter, the equatorial diameter is 9275 km longer than the diameter measured through the poles.[32]
Because Jupiter is not a solid body, its upper atmosphere undergoes differential rotation. The rotation of Jupiter's polar atmosphere is about 5 minutes longer than that of the equatorial atmosphere; three systems are used as frames of reference, particularly when graphing the motion of atmospheric features. System I applies from the latitudes 10° N to 10° S; its period is the planet's shortest, at 9h 50m 30.0s. System II applies at all latitudes north and south of these; its period is 9h 55m 40.6s. System III was first defined by radio astronomers, and corresponds to the rotation of the planet's magnetosphere; its period is Jupiter's official rotation.[53]

Saturn

Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant. Together, these four planets are sometimes referred to as the Jovian, meaning "Jupiter-like", planets.
Saturn is named after the Roman god Saturn (that became the namesake of Saturday), equated to the Greek Kronos (the Titan father of Zeus) the Babylonian Ninurta and to the Hindu Shani. Saturn's symbol represents the god's sickle (Unicode: ♄).
The planet Saturn is composed of hydrogen, with small proportions of helium and trace elements.[11] The interior consists of a small core of rock and ice, surrounded by a thick layer of metallic hydrogen and a gaseous outer layer. The outer atmosphere is generally bland in appearance, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h, significantly faster than those on Jupiter. Saturn has a planetary magnetic field intermediate in strength between that of Earth and the more powerful field around Jupiter.
Saturn has a prominent system of rings, consisting mostly of ice particles with a smaller amount of rocky debris and dust. Sixty-one known moons orbit the planet, not counting hundreds of "moonlets" within the rings. Titan, Saturn's largest and the Solar System's second largest moon (after Jupiter's Ganymede), is larger than the planet Mercury and is the only moon in the Solar System to possess a significant atmosphere.

Physical characteristics
Due to a combination of its lower density, rapid rotation, and fluid state, Saturn is an oblate spheroid; that is, it is flattened at the poles and bulges at the equator. Its equatorial and polar radii differ by almost 10%—60,268 km vs. 54,364 km.[4] The other gas planets are also oblate, but to a lesser extent. Saturn is the only planet of the Solar System that is less dense than water. Although Saturn's core is considerably denser than water, the average specific density of the planet is 0.69 g/cm³ due to the gaseous atmosphere. Saturn is only 95 Earth masses,compared to Jupiter, which is 318 times the mass of the Earth[13] but only about 20% larger than Saturn

Internal structure
Though there is no direct information about Saturn's internal structure, it is thought that its interior is similar to that of Jupiter, having a small rocky core surrounded mostly by hydrogen and helium. The rocky core is similar in composition to the Earth, but denser. Above this, there is a thicker liquid metallic hydrogen layer, followed by a layer of liquid hydrogen and helium, and in the outermost 1000 km a gaseous atmosphere.[15] Traces of various ices are also present. The core region is estimated to be about 9–22 times the mass of the Earth.[16] Saturn has a very hot interior, reaching 11,700 °C at the core, and it radiates 2.5 times 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.[17]

Atmosphere
The outer atmosphere of Saturn consists of about 96.3% molecular hydrogen and 3.25% helium.[18] Trace amounts of ammonia, acetylene, ethane, phosphine, and methane have also been detected.[19] The upper clouds on Saturn are composed of ammonia crystals, while the lower level clouds appear to be composed of either ammonium hydrosulfide (NH4SH) or water.[20] The atmosphere of Saturn is significantly deficient in helium relative to the abundance of the elements in the Sun.
The quantity of elements heavier than helium are not known precisely, but the proportions are assumed to match the primordial abundances from the formation of the Solar System. The total mass of these elements is estimated to be 19–31 times the mass of the Earth, with a significant fraction located in Saturn's core region.[21]

Orbit and rotation

The average distance between Saturn and the Sun is over 1 400 000 000 km (9 AU). With an average orbital speed of 9.69 km/s,[4] it takes Saturn 10 759 Earth days (or about 29½ years), to finish one revolution around the Sun.[4] The elliptical orbit of Saturn is inclined 2.48° relative to the orbital plane of the Earth.[4] Because of an eccentricity of 0.056, the distance between Saturn and the Sun varies by approximately 155 000 000 km between perihelion and aphelion,[4] which are the nearest and most distant points of the planet along its orbital path, respectively.
The visible features on Saturn rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions (as 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 in the period of the Voyager flybys, has a period of 10 h 39 min 22.4 s (810.8°/d); because it is very close to System II, it has largely superseded it.
However, a precise value for the rotation period of the interior remains elusive. While approaching Saturn in 2004, the Cassini spacecraft found that the radio rotation period of Saturn had increased appreciably, to approximately 10 h 45 m 45 s (± 36 s).The cause of the change is unknown—it was thought to be due to a movement of the radio source to a different latitude inside Saturn, with a different rotational period, rather than because of a change in Saturn's rotation.
Later, in March 2007, it was found that the rotation of the radio emissions did not trace the rotation of the planet, but rather is produced by convection of the plasma disc, which is dependent also on other factors besides the planet's rotation. It was reported that the variance in measured rotation periods may be caused by geyser activity on Saturn's moon Enceladus. The water vapor emitted into Saturn's orbit by this activity becomes charged and "weighs down" Saturn's magnetic field, slowing its rotation slightly relative to the rotation of the planet itself. At the time it was stated that there is no currently known method of determining the rotation rate of Saturn's core.
The latest estimate of Saturn's rotation based on a compilation of various measurements from the Cassini, Voyager and Pioneer probes was reported in September 2007 is 10 hours, 32 minutes, 35 seconds.

Uranus

Uranus is the seventh planet from the Sun, and the third-largest and fourth most massive planet in the Solar System. It is named after the ancient Greek deity of the sky Uranus (Ancient Greek: Οὐρανός) the father of Kronos (Saturn) and grandfather of Zeus (Jupiter). Though it is visible to the naked eye like the five classical planets, it was never recognized as a planet by ancient observers because of its dimness and slow orbit.[14] Sir William Herschel announced its discovery on March 13, 1781, expanding the known boundaries of the solar system for the first time in modern history. This was also the first discovery of a planet made using a telescope.
Uranus is similar in composition to Neptune, and both have different compositions from those of the larger gas giants Jupiter and Saturn. As such, astronomers sometimes place them in a separate category, the "ice giants". Uranus's atmosphere, while similar to Jupiter's and Saturn's in being composed primarily of hydrogen and helium, contains a higher proportion of "ices" such as water, ammonia and methane, along with traces of hydrocarbons.[10] It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (−224 °C). It has a complex, layered cloud structure, with water thought to make up the lowest clouds, and methane thought to make up the uppermost layer of clouds.[10] In contrast the interior of Uranus is mainly composed of ices and rock.[9]
Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons. The Uranian system has a unique configuration among the planets because its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun. As such, its north and south poles lie where most other planets have their equators.[15] Seen from Earth, Uranus's rings can sometimes appear to circle the planet like an archery target and its moons revolve around it like the hands of a clock, though in 2007 and 2008 the rings appeared edge-on. In 1986, images from Voyager 2 showed Uranus as a virtually featureless planet in visible light without the cloud bands or storms associated with the other giants.[15] However, terrestrial observers have seen signs of seasonal change and increased weather activity in recent years as Uranus approached its equinox. The wind speeds on Uranus can reach 250 meters per second (900 km/h, 560 mph).[16]

Discovery

Uranus had been observed on many occasions before its discovery as a planet, but it was generally mistaken for a star. The earliest recorded sighting was in 1690 when John Flamsteed observed the planet at least six times, cataloging it as 34 Tauri. The French astronomer, Pierre Lemonnier, observed Uranus at least twelve times between 1750 and 1769,[17] including on four consecutive nights.
Sir William Herschel observed the planet on 13 March 1781 while in the garden of his house at 19 New King Street in the town of Bath, Somerset (now the Herschel Museum of Astronomy),[18] but initially reported it (on 26 April 1781) as a "comet".[19] Herschel "engaged in a series of observations on the parallax of the fixed stars",[20] using a telescope of his own design.
He recorded in his journal "In the quartile near ζ Tauri … either [a] Nebulous star or perhaps a comet".[21] On March 17, he noted, "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place".[22] When he presented his discovery to the Royal Society, he continued to assert that he had found a comet while also implicitly comparing it to a planet:[23]
“ The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed. ”
Herschel notified the Astronomer Royal, Nevil Maskelyne, of his discovery and received this flummoxed reply from him on April 23: "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it".[24]
While Herschel continued to cautiously describe his new object as a comet, other astronomers had already begun to suspect otherwise. Russian astronomer Anders Johan Lexell estimated its distance as 18 times the distance of the Sun from the Earth, and no comet had yet been observed with a perihelion of even four times the Earth–Sun distance.[25] Berlin astronomer Johann Elert Bode described Herschel's discovery as "a moving star that can be deemed a hitherto unknown planet-like object circulating beyond the orbit of Saturn".[26] Bode concluded that its near-circular orbit was more like a planet than a comet.[27]
The object was soon universally accepted as a new planet. By 1783, Herschel himself acknowledged this fact to Royal Society president Joseph Banks: "By the observation of the most eminent Astronomers in Europe it appears that the new star, which I had the honour of pointing out to them in March 1781, is a Primary Planet of our Solar System."[28] In recognition of his achievement, King George III gave Herschel an annual stipend of £200 on the condition that he move to Windsor so that the Royal Family could have a chance to look through his telescopes.[29]


Naming
Maskelyne asked Herschel to "do the astronomical world the faver [sic] to give a name to your planet, which is entirely your own, & which we are so much obliged to you for the discovery of."[30] In response to Maskelyne's request, Herschel decided to name the object Georgium Sidus (George's Star), or the "Georgian Planet" in honour of his new patron, King George III.[31] He explained this decision in a letter to Joseph Banks:[28]

Nomenclature


The pronunciation of the name Uranus preferred among astronomers is with stress on the first syllable as in Latin Ūranus;[37] in contrast to the colloquial / with stress on the second syllable and a long a, though both are considered acceptable. Because, in the English-speaking world, ū·rā′·nəs sounds like "your anus", the former pronunciation also saves embarrassment: as Dr. Pamela Gay, an astronomer at Southern Illinois University, noted on her podcast, so as avoid "being made fun of by any small schoolchildren ... when in doubt, don't emphasise anything and just say ūr′·ə·nəs. And then run, quickly.
Uranus is the only planet whose name is derived from a figure from Greek mythology rather than Roman mythology. The adjective of Uranus is "Uranian". Its astronomical symbol is . It is a hybrid of the symbols for Mars and the Sun because Uranus was the Sky in Greek mythology, which was thought to be dominated by the combined powers of the Sun and Mars.[40] Its astrological symbol is , suggested by Lalande in 1784. In a letter to Herschel, Lalande described it as "un globe surmonté par la première lettre de votre nom" ("a globe surmounted by the first letter of your name In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the sky king star


Orbit and rotation
Uranus revolves around the Sun once every 84 Earth years. Its average distance from the Sun is roughly 3 billion km (about 20 AU). The intensity of sunlight on Uranus is about 1/400 that on Earth.[43] Its orbital elements were first calculated in 1783 by Pierre-Simon Laplace.[25] With time, discrepancies began to appear between the predicted and observed orbits, and in 1841, John Couch Adams first proposed that the differences might be due to the gravitational tug of an unseen planet. In 1845, Urbain Le Verrier began his own independent research into Uranus's orbit. On September 23, 1846, Johann Gottfried Galle located a new planet, later named Neptune, at nearly the position predicted by Le Verrier.
The rotational period of the interior of Uranus is 17 hours, 14 minutes
However, as on all giant planets, its upper atmosphere experiences very strong winds in the direction of rotation. In effect, at some latitudes, such as about two-thirds of the way from the equator to the south pole, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours.

Internal structure
Uranus's mass is roughly 14.5 times that of the Earth, making it the least massive of the giant planets, while its density of 1.27 g/cm³ makes it the second least dense planet, after Saturn. Though having a diameter slightly larger than Neptune's (roughly four times Earth's), it is less massive. These values indicate that it is made primarily of various ices, such as water, ammonia, and methane. The total mass of ice in Uranus's interior is not precisely known, as different figures emerge depending on the model chosen; however, it must be between 9.3 and 13.5 Earth masses. Hydrogen and helium constitute only a small part of the total, with between 0.5 and 1.5 Earth masses. The remainder of the mass (0.5 to 3.7 Earth masses) is accounted for by rocky material
The standard model of Uranus's structure is that it consists of three layers: a rocky core in the center, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope. The core is relatively small, with a mass of only 0.55 Earth masses and a radius less than 20 percent Uranus's; the mantle comprises the bulk of the planet, with around 13.4 Earth masses, while the upper atmosphere is relatively insubstantial, weighing about 0.5 Earth masses and extending for the last 20 percent of Uranus's radius. Uranus's core density is around 9 g/cm³, with a pressure in the center of 8 million bars (800 GPa) and a temperature of about 5000 K.The ice mantle is not in fact composed of ice in the conventional sense, but of a hot and dense fluid consisting of water, ammonia and other volatiles. This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean. The bulk compositions of Uranus and Neptune are very different from those of Jupiter and Saturn, with ice dominating over gases, hence justifying their separate classification as ice giants.
While the model considered above is more or less standard, it is not unique; other models also satisfy observations. For instance, if substantial amounts of hydrogen and rocky material are mixed in the ice mantle, the total mass of ices in the interior will be lower, and, correspondingly, the total mass of rocks and hydrogen will be higher. Presently available data does not allow science to determine which model is correct. The fluid interior structure of Uranus means that it has no solid surface. The gaseous atmosphere gradually transitions into the internal liquid layers.[9] However, for the sake of convenience, a revolving oblate spheroid set at the point at which atmospheric pressure equals 1 bar (100 kPa) is conditionally designated as a "surface". It has equatorial and polar radii of 25 559 ± 4 and 24 973 ± 20 km, respectively. This surface will be used throughout this article as a zero point for altitudes.

Internal heat


Uranus's internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low thermal flux. Why Uranus's internal temperature is so low is still not understood. Neptune, which is Uranus's near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun.[16] Uranus, by contrast, radiates hardly any excess heat at all. The total power radiated by Uranus in the far infrared (i.e. heat) part of the spectrum is 1.06 ± 0.08 times the solar energy absorbed in its atmosphere.In fact, Uranus's heat flux is only 0.042 ± 0.047 W/m², which is lower than the internal heat flux of Earth of about 0.075 W/m².[58] The lowest temperature recorded in Uranus's tropopause is 49 K (−224 °C), making Uranus the coldest planet in the Solar System.
Hypotheses for this discrepancy include that when Uranus was "knocked over" by the supermassive impactor which caused its extreme axial tilt, the event also caused it to expel most of its primordial heat, leaving it with a depleted core temperature.[59] Another hypothesis is that some form of barrier exists in Uranus's upper layers which prevents the core's heat from reaching the surface. For example, convection may take place in a set of compositionally different layers, which may inhibit the upward heat transport.
Atmosphere
Although there is no well-defined solid surface within Uranus's interior, the outermost part of Uranus's gaseous envelope that is accessible to remote sensing is called its atmosphere.Remote sensing capability extends down to roughly 300 km below the 1 bar (100 kPa) level, with a corresponding pressure around 100 bar (10 MPa) and temperature of 320 K.[60] The tenuous corona of the atmosphere extends remarkably over two planetary radii from the nominal surface at 1 bar pressure.[61] The Uranian atmosphere can be divided into three layers: the troposphere, between altitudes of −300 and 50 km and pressures from 100 to 0.1 bar; (10 MPa to 10 kPa), the stratosphere, spanning altitudes between 50 and 4000 km and pressures of between 0.1 and 10–10 bar (10 kPa to 10 µPa), and the thermosphere/corona extending from 4,000 km to as high as 50,000 km from the surface. There is no mesosphere.
Composition
The composition of the Uranian atmosphere is different from the composition of Uranus as a whole, consisting as it does mainly of molecular hydrogen and helium.[10] The helium molar fraction, i.e. the number of helium atoms per molecule of gas, is 0.15 ± 0.03[12] in the upper troposphere, which corresponds to a mass fraction 0.26 ± 0.05.[10][58] This value is very close to the protosolar helium mass fraction of 0.275 ± 0.01,[62] indicating that helium has not settled in the center of the planet as it has in the gas giants.[10] The third most abundant constituent of the Uranian atmosphere is methane (CH4).[10] Methane possesses prominent absorption bands in the visible and near-infrared (IR) making Uranus aquamarine or cyan in color.[10] Methane molecules account for 2.3% of the atmosphere by molar fraction below the methane cloud deck at the pressure level of 1.3 bar (130 kPa); this represents about 20 to 30 times the carbon abundance found in the Sun.The mixing ratio[e] is much lower in the upper atmosphere owing to its extremely low temperature, which lowers the saturation level and causes excess methane to freeze out.[64] The abundances of less volatile compounds such as ammonia, water and hydrogen sulfide in the deep atmosphere are poorly known. However they are probably also higher than solar values.In addition to methane, trace amounts of various hydrocarbons are found in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by the solar ultraviolet (UV) radiation.[66] They include ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), diacetylene (C2HC2H).[64][67][68] Spectroscopy has also uncovered traces of water vapor, carbon monoxide and carbon dioxide in the upper atmosphere, which can only originate from an external source such as infalling dust and comets

NEPTUNE

Neptune is the eighth planet from the sun in our solar system. This giant, frigid planet has a hazy atmosphere and strong winds. This gas giant is orbited by eight moons and narrow, faint rings arranged in clumps. Neptune's blue color is caused by the methane (CH4) in its atmosphere; this molecule absorbs red light. Neptune cannot be seen using the eyes alone. Neptune was the first planet whose existence was predicted mathematically (the planet Uranus's orbit was perturbed by an unknown object which turned our to be another gas giant, Neptune).

SIZE

Neptune is about 30,775 miles (49,528 km) in diameter. This is 3.88 times the diameter of the Earth. If Neptune were hollow, it could hold almost 60 Earths.
Neptune is the fourth largest planet in our Solar System (after Jupiter, Saturn, and Uranus).

MASS AND GRAVITY
Neptune's mass is about 1.02 x 1026 kg. This is over 17 times the mass of the Earth, but the gravity on Neptune is only 1.19 times of the gravity on Earth. This is because it is such a large planet (and the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of its radius squared).
A 100-pound person would weigh 119 pounds on Neptune.

LENGTH OF A DAY AND YEAR ON NEPTUNE


Each day on Neptune takes 19.1 Earth hours. A year on Neptune takes 164.8 Earth years; it takes almost 165 Earth years for Neptune to orbit the sun once.
Since Neptune was discovered in 1846, it has not yet completed a single revolution around the sun.


NEPTUNE'S ORBIT AND DISTANCE FROM THE SUN

Neptune is about 30 times farther from the sun than the Earth is; it averages 30.06 A.U. from the sun. Occasionally, Neptune's orbit is actually outside that of Pluto; this is because of Pluto's highly eccentric (non-circular) orbit. During this time (20 years out of every 248 Earth years), Neptune is actually the farthest planet from the Sun (and not Pluto). From January 21, 1979 until February 11, 1999, Pluto was inside the orbit of Neptune. Now and until September 2226, Pluto is outside the orbit of Neptune.At aphelion (the point in Neptune's orbit farthest from the sun) Neptune is 4,546,000,000 km from the sun, at perihelion (the point in Neptune's orbit closest from the sun) Neptune is 4,456,000,000 km from the sun.
Neptune's rotational axis is tilted 30 degrees to the plane of its orbit around the Sun (this is few degrees more than the Earth). This gives Neptune seasons. Each season lasts 40 years; the poles are in constant darkness or sunlight for 40 years at a time.

TEMPERATURE
The mean temperature is 48 K

DISCOVERY OF NEPTUNE
Neptune's existence was predicted in 1846, after calculations showed perturbations in the orbit of Uranus. The calculations were done independently by both J.C. Adams and Le Verrier. Neptune was then observed by J.G. Galle and d'Arrest on September 23, 1846

The Pluto

Pluto is a dwarf planet (or plutoid) that usually orbits past the orbit of Neptune. It was classified as a dwarf planet in 2006; before that it was considered to be a planet, the smallest planet in our solar system. Pluto is smaller than a lot of the other planets' moons, including our moon. Pluto is the only "planet" in our solar system that has not been visited by our spacecraft yet. We only have blurry pictures of its surface; even the Hubble Space Telescope orbiting the Earth can only get grainy photos because Pluto is so far from us. In 2015, a spacecraft called New Horizons (launched by NASA in 2006) will visit Pluto.


SIZE
Pluto is about 1,413 miles (2274 km) in diameter. This is about 1/5 the diameter of the Earth.
Pluto is smaller than the 8 planets in our Solar System.


MASS AND GRAVITY

Pluto's mass is about 1.29 x 1022 kg. This is about 1/500th of the mass of the Earth. The gravity on Pluto is 8% of the gravity on Earth.
Pluto is the least massive planet in our Solar System (and is now classified as a dwarf planet).
A 100 pound person on Pluto would weigh only 8 pounds.LENGTH OF A DAY AND YEAR ON PLUTO Each day on Pluto takes 6.39 Earth days. Each year on Pluto takes 247.7 Earth years (that is, it takes 247.7 Earth years for Pluto to orbit the Sun once).


PLUTO'S ORBIT
Pluto is 39 times farther from than the sun than the Earth is. Pluto ranges from 2.8 to 4.6 billion miles (4.447 billion to 7.38 billion km) from the Sun. From Pluto, the sun would look like a tiny dot in the sky.
Occasionally, Neptune's orbit is actually outside that of Pluto; this is because of Pluto's highly eccentric (non-circular) orbit. During this time (20 years out of every 248 Earth years), Neptune is actually the farthest planet from the Sun (and not Pluto). From January 21, 1979 until February 11, 1999, Pluto was inside the orbit of Neptune. Now and until September 2226, Pluto is outside the orbit of Neptune.

Orbital Eccentricity
Pluto has a very eccentric orbit; that means that its distance from the sun varies a lot during its orbit around the sun. Sometimes it is even closer to the Sun than the planet Neptune (it was that way from January 1979 to February 11, 1999)! Pluto also rotates about its axis in the opposite direction from most of the other planets.
Orbital Inclination

Pluto's orbit is tilted from the plane of the ecliptic. This angle, its orbital inclination, is 17.15°. This is the largest inclination of any of the planets.

TEMPERATURE ON PLUTO
Pluto is VERY, VERY cold. Its temperature may range from between -396°F to -378°F (-238°C to -228°C, or 35 K to 45 K). The average temperature is -393°F (-236°C = 37 K).

PLANETARY COMPOSITION
Pluto's composition is unknown. It is probably made up of about 70% rock and 30% water. This is determined from density calculations; Pluto's density is about 2,000 kg/m3. There may be methane ice together with frozen nitrogen and carbon dioxide on the cold, rocky surface


ATMOSPHERE
Not much is known about Pluto's atmosphere. It is probably mostly nitrogen with a little carbon monoxide and methane - definitely not breatheable by humans. The atmospheric pressure is probably very low. The atmosphere forms when Pluto is closest to the Sun and the frozen methane is vaporized by the solar heat. When it is farther from the Sun, the methane freezes again. From Pluto, the sky would appear black, even when the Sun (the size of a star) is up.


PLUTO'S MOONS
Pluto has one large moon, named Charon; two minscule moons were discovered in 2005. The tiny moons are called.
Although Charon is small, about 1,172 km (728 miles) in diameter, it about half of the size of Pluto itself. Charon orbits about 19,640 km from Pluto on average. It may be covered by water ice and probably has no atmosphere. Charon is in a synchronous orbit around Pluto. That is, Charon is always over the same spot on Pluto; Charon's orbit takes exactly one Pluto day.
Charon was discovered by Jim Christy in 1978. Charon was named after the mythological demon who ferried people across the mythological river Styx into Hades.
The two tiny moons, Nix and Hydra, are from 30 and 100 miles (45 to 160 km) in diameter, and orbit Pluto about 27,000 miles (44,000 km) from Pluto, more than twice as far as the orbit of Charon.



DISCOVERY OF PLUTO
Pluto was discovered after the 8 planets and was originally considered a planet itself (until 2006). In the early 1900s, Planet "X" was the temporary name given to the then-unknown planet beyond Neptune that disturbed the orbits of Uranus and Neptune. Percival Lowell calculated the rough location of Planet "X's" orbit, but died in 1916 before it was found. This planet was eventually found by the American astronomer Clyde W. Tombaugh in 1930 and named Pluto. He did his observations at the Lowell Observatory in Arizona

Solar Mystery Nears Solution With Data From SOHO Spacecraft

A likely solution to one of the major mysteries of the Sun has emerged from recent observations with the European Space Agency/NASA Solar and Heliospheric Observatory (SOHO) mission.

The new findings seem to account for a substantial part of the energy needed to cause the very high temperature of the corona, the outermost layer of the Sun's atmosphere. Since the corona's temperature was first measured 55 years ago, scientists have lacked a satisfactory explanation for why that temperature is three million degrees while the visible surface of the Sun is only 11,000 degrees Fahrenheit or about 6,000 degrees Celsius.

It is physically impossible to transfer thermal energy from the cooler surface to the much hotter corona, so the energy transfer had to be in the form of waves or magnetic energy, but no measurement to date had found adequate energy to account for the coronal temperature.

"We now have direct evidence for the upward transfer of magnetic energy from the Sun's surface toward the corona above. There is more than enough energy coming up from the loops of the 'magnetic carpet' to heat the corona to its known temperature," said Dr. Alan Title of the Stanford-Lockheed Institute for Space Research, Lockheed Martin Advanced Technology Center, Palo Alto, CA, who led the research. "Each one of these loops carries as much energy as a large hydroelectric plant, such as the Hoover dam, generates in about a million years!"

"We now appear to be closing in on an explanation as to why the solar corona is over 100 times hotter than the solar surface - - the solution to a 55-year old puzzle," said Dr. George Withbroe, Director of the Sun-Earth Connection Program at NASA Headquarters, Washington, DC. "These results underline the importance of long- term study of the changing conditions on the Sun from the superior vantage point of space."

Energy flows from the loops when they interact, producing electrical and magnetic "short circuits." The very strong electric currents in these short circuits are what heats the corona to a temperature of several million degrees. Images from the Extreme ultraviolet Imaging Telescope (EIT) and the Coronal Diagnostics Spectrometer (CDS) on SOHO show the hot gases of the ever-changing corona reacting to the evolving magnetic fields rooted in the solar surface.

The observations with SOHO's Michelson Doppler Imager (MDI) provided long-duration, highly detailed, and well calibrated time- lapse movies of the magnetic fields on the visible surface or "photosphere" of the Sun. These revealed the rapidly changing properties of what Title calls "the Sun's Magnetic Carpet," a sprinkling of tens-of-thousands of magnetic concentrations. These concentrations have both north and south magnetic poles, which are the "foot points" of magnetic loops extending into the solar corona.

Like field biologists who study the populations and life cycles of animal herds, the SOHO researchers analyzed the appearances and disappearances of large numbers of the small magnetic concentrations on the solar surface. "We find that after a typical small magnetic loop emerges, it fragments and drifts around and then disappears in only 40 hours," Title said. "It's very hard to understand how such a short-lived effect could be driven by the magnetic dynamo layer that is over 100,000 miles beneath the surface of the Sun. This may be evidence that unknown processes are at work in or near the solar surface that continuously form these loops all over the Sun."

Professor Phillip Scherrer of Stanford University is the MDI Principal Investigator. MDI was built at the LM Technology Center and is a project of the Stanford-Lockheed Institute for Space Research.

The new observations were made with several instruments on SOHO, which is stationed about 900,000 miles (1.5 million kilometers) sunward of the Earth in interplanetary space, where it has an uninterrupted view of the Sun and of the solar wind particles blown from the Sun. SOHO is operated from a control center at NASA's Goddard Space Flight Center, Greenbelt, MD. SOHO was launched on Dec. 2, 1995 aboard an Atlas-IIAS expendable launch vehicle from Kennedy Space Center, FL.

International Spacecraft Reveals Detailed Processes on the Sun

NASA released on Wednesday never-before-seen images that show the sun's magnetic field is much more turbulent and dynamic than previously known. The international spacecraft Hinode, formerly known as Solar B, took the images.

Hinode, Japanese for "sunrise," was launched Sept. 23, 2006, to study the sun's magnetic field and how its explosive energy propagates through the different layers of the solar atmosphere. The spacecraft's uninterrupted high-resolution observations of the sun will have an impact on solar physics comparable to the Hubble Space Telescope's impact on astronomy.

"For the first time, we are now able to make out tiny granules of hot gas that rise and fall in the sun's magnetized atmosphere," said Dick Fisher, director of NASA's Heliophyics Division, Science Mission Directorate, Washington. "These images will open a new era of study on some of the sun's processes that effect Earth, astronauts, orbiting satellites and the solar system."

Hinode's three primary instruments, the Solar Optical Telescope, the X-ray Telescope and the Extreme Ultraviolet Imaging Spectrometer, are observing the different layers of the sun. Studies focus on the solar atmosphere from the visible surface of the sun, known as the photosphere, to the corona, the outer atmosphere of the sun that extends outward into the solar system.

"By coordinating the measurements of all three instruments, Hinode is showing how changes in the structure of the magnetic field and the release of magnetic energy in the low atmosphere spread outward through the corona and into interplanetary space to create space weather," said John Davis, project scientist from NASA's Marshall Space Flight Center, Huntsville, Ala.

Space weather involves the production of energetic particles and emissions of electromagnetic radiation. These bursts of energy can black out long-distance communications over entire continents and disrupt the global navigational system.

"Hinode images are revealing irrefutable evidence for the presence of turbulence-driven processes that are bringing magnetic fields, on all scales, to the sun's surface, resulting in an extremely dynamic chromosphere or gaseous envelope around the sun," said Alan Title, a corporate senior fellow at Lockheed Martin, Palo Alto, Calif., and consulting professor of physics at Stanford University, Stanford, Calif.

Hinode is a collaborative mission led by the Japan Aerospace Exploration Agency and includes the European Space Agency and Britain's Particle Physics Astronomy Research Council. The National Astronomical Observatory of Japan, Tokyo, developed the Solar Optical Telescope, which provided the fine-scale structure views of the sun's lower atmosphere, and developed the X-ray Telescope in collaboration with the Smithsonian Astrophysical Observatory of Cambridge, Mass. The X-ray Telescope captured the rapid, time-sequenced images of explosive events in the sun's outer atmosphere.

"By following the evolution of the solar structures that outline the magnetic field before, during and after these explosive events, we hope to find clear evidence to establish that magnetic reconnection is the underlying cause for this explosive activity," said Leon Golub of the Smithsonian Astrophysical Observatory.