The Nine Planet Astronomy's Introducing and It's Great History-
Astronomy is the oldest of the natural sciences, dating back to antiquity, with its origins in the religious, mythological, and astrological practices of pre-history: vestiges of these are still found in astrology, a discipline long interwoven with public and governmental astronomy, and not completely disentangled from it until a few centuries ago in the Western World . In some cultures astronomical data was used for astrological prognostication.
Ancient astronomers were able to differentiate between stars and planets, as stars remain relatively fixed over the centuries while planets will move an appreciable amount during a comparatively short time.
Planetary-sized objects to scale:
Top row: Uranus and Neptune; second row: Earth, white dwarf star Sirius B, Venus; bottom row (reproduced and enlarged in lower image) – above: Mars and Mercury; below: the Moon, dwarf planets Pluto and Haumea.
Top row: Uranus and Neptune; second row: Earth, white dwarf star Sirius B, Venus; bottom row (reproduced and enlarged in lower image) – above: Mars and Mercury; below: the Moon, dwarf planets Pluto and Haumea.
A planet (from Greek πλανήτης, alternative form of πλάνης "wanderer") is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.
The term planet is ancient, with ties to history, science, mythology, and religion. The planets were originally seen by many early cultures as divine, or as emissaries of the gods. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized, and remains disputed by some scientists.
The planets were thought by Ptolemy to orbit the Earth in deferent and epicycle motions. Though the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. By careful analysis of the observation data, Johannes Kepler found the planets' orbits to be not circular, but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some share such features as ice-caps and seasons. Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology.
Planets are generally divided into two main types: large, low-density gas giants, and smaller, rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System. In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four gas giants, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites. Additionally, the Solar System also contains at least five dwarf planets and hundreds of thousands of small Solar System bodies.
Since 1992, hundreds of planets around other stars ("extrasolar planets" or "exoplanets") in the Milky Way Galaxy have been discovered. As of December 2010, over 500 known extrasolar planets are listed in the Extrasolar Planets Encyclopaedia, ranging from the size of terrestrial planets somewhat larger than Earth to gas giants larger than Jupiter.
20th Century-
| Mercury | Venus | Earth | Mars | Jupiter | Saturn | Uranus | Neptune |
However, in the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth, the object was immediately accepted as the ninth planet.
Further monitoring found the body was actually much smaller: in 1936, Raymond Lyttleton suggested that Pluto may be an escaped satellite of Neptune, and Fred Whipple suggested in 1964 that Pluto may be a comet. However, as it was still larger than all known asteroids and seemingly did not exist within a larger population, it kept its status until 2006.
| Mercury | Venus | Earth | Mars | Jupiter | Saturn | Uranus | Neptune | Pluto |
In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12. This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).
The discovery of extrasolar planets led to another ambiguity in defining a planet; the point at which a planet becomes a star. Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as "brown dwarfs". Brown dwarfs are generally considered stars due to their ability to fuse deuterium, a heavier isotope of hydrogen. While stars more massive than 75 times that of Jupiter fuse hydrogen, stars of only 13 Jupiter masses can fuse deuterium. However, deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.
21st Century-
| Mercury | Venus | Earth | Mars | Jupiter | Saturn | Uranus | Neptune |
With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There was particular disagreement over whether an object should be considered a planet if it was part of a distinct population such as a belt, or if it was large enough to generate energy by the thermonuclear fusion of deuterium.
A growing number of astronomers argued for Pluto to be declassified as a planet, since many similar objects approaching its size had been found in the same region of the Solar System (the Kuiper belt) during the 1990s and early 2000s. Pluto was found to be just one small body in a population of thousands.
Some of them including Quaoar, Sedna, and Eris were heralded in the popular press as the tenth planet, failing however to receive widespread scientific recognition. The announcement of Eris in 2005, an object 27% more massive than Pluto, created the necessity and public desire for an official definition of a planet.
Acknowledging the problem, the IAU set about creating the definition of planet, and produced one in August 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris).
Extrasolar planet definition-
In 2003, The International Astronomical Union (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated the following working definition, mostly focused upon the boundary between planets and brown dwarves:
Comparison of Eris, Pluto, Makemake, Haumea, Sedna, Orcus, 2007 OR10, Quaoar, and Earth (all to scale)
- Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same isotopic abundance as the Sun) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.
- Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located.
- Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).
This definition has since been widely used by astronomers when publishing discoveries of exoplanets in academic journals. Although temporary, it remains an effective working definition until a more permanent one is formally adopted. However, it does not address the dispute over the lower mass limit, and so it steered clear of the controversy regarding objects within the Solar System. This definition also makes no comment on the planetary status of objects orbiting brown dwarfs, such as 2M1207b.
Now the discovery that there is a dearth of cosmic bodies whose mass lies within a particular range could provide a clean dividing line between planets and brown dwarfs, which are heavier than planets but lighter than stars.
Objects are traditionally classed as planets if they have less than about 13 times the mass of Jupiter, and as brown dwarfs if they are heavier. Uncertainties in the measurement of mass make it hard to classify borderline objects this way. But when Johannes Sahlmann of the Geneva Observatory in Switzerland and colleagues surveyed brown dwarfs and planets orbiting stars, they found a dearth of objects between 25 and 45 times Jupiter's mass, but plenty of objects outside this range (arxiv.org/abs/1012.1319).
This dividing line could have arisen because the heavier and lighter objects form in different ways. Planets, on the light side of the divide, may form from leftover gas and dust in discs swirling around newborn stars, while stars and brown dwarfs form from the collapse of clumps of gas.
Other surveys found hints of a poorly populated "trough" at around this mass range, says Stanimir Metchev of the State University of New York in Stony Brook, who was not involved in the study. He predicts, however, that a few borderline objects will fall within this range.
Former classifications-
The table below lists Solar System bodies formerly considered to be planets:
| Body (current classification) | Notes | |||
|---|---|---|---|---|
| Star | Dwarf planet | Asteroid | Moon | |
| Sun | The Moon | Classified as planets in antiquity, in accordance with the definition then used. | ||
| Io, Europa, Ganymede, and Callisto | The four largest moons of Jupiter, known as the Galilean moons after their discoverer Galileo Galilei. He referred to them as the "Medicean Planets" in honor of his patron, the Medici family. | |||
| Titan, Iapetus, Rhea, Tethys, and Dione | Five of Saturn's larger moons, discovered by Christiaan Huygens and Giovanni Domenico Cassini. | |||
| Ceres | Pallas, Juno, and Vesta | The first known asteroids, from their discoveries between 1801 and 1807 until their reclassification as asteroids during the 1850s. Ceres has subsequently been classified as a dwarf planet in 2006. | ||
| Astrea, Hebe, Iris, Flora, Metis, Hygeia, Parthenope, Victoria, Egeria, Irene, Eunomia | More asteroids, discovered between 1845 and 1851. The rapidly expanding list of planets prompted their reclassification as asteroids by astronomers, and this was widely accepted by 1854. | |||
| Pluto | The first known Trans-Neptunian object (i.e. minor planet with a semi-major axis beyond Neptune). In 2006, Pluto was reclassified as a dwarf planet. | |||
The planet has seen some drastic changes within its eco systems in the past few decades; from rising sea levels to extremes in seasonal weather patterns, and from diminishing wildlife to the movement of ocean currents, it is believed that climate change is exacting its influence on the world.
For the experienced skippers of the Velux 5 Oceans, these changes have been observed at first hand and their commentary is alarming.
In the past 12 years Derek Hatfield and Brad Van Liew have racked up four round the world voyages between them, with the current edition of the Velux 5 Oceans adding two more to their tally.
The twos commitment to the sport means they have spent years of their lives dedicated to the sea and have seen the changes brought by global warming close-up. Derek and Brad are no scientists but instead can offer unique personal observations on the environment they love and live for – and they have a grave warning.
Solar System-
Planets and dwarf planets of the Solar System. (Sizes to scale, distances not to scale)
The terrestrial planets: Mercury, Venus, Earth, Mars (Sizes to scale, distances not to scale)
The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune (Sizes to scale, distances not to scale)
According to the IAU's current definitions, there are eight planets and five dwarf planets in the Solar System. In increasing distance from the Sun, the planets are:
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Jupiter is the largest, at 318 Earth masses, while Mercury is smallest, at 0.055 Earth masses.
The planets of the Solar System can be divided into categories based on their composition:
- Terrestrials: Planets that are similar to Earth, with bodies largely composed of rock: Mercury, Venus, Earth and Mars. At 0.055 Earth masses, Mercury is the smallest terrestrial planet (and smallest planet) in the Solar System, while Earth is the largest terrestrial planet.
- Gas giants (Jovians): Planets with a composition largely made up of gaseous material and are significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Jupiter, at 318 Earth masses, is the largest planet in the Solar System, while Saturn is one third as big, at 95 Earth masses.
- Ice giants, comprising Uranus and Neptune, are a sub-class of gas giants, distinguished from gas giants by their significantly lower mass (only 14 and 17 Earth masses), and by depletion in hydrogen and helium in their atmospheres together with a significantly higher proportion of rock and ice.
- Dwarf planets: Before the August 2006 decision, several objects were proposed by astronomers, including at one stage by the IAU, as planets. However in 2006 several of these objects were reclassified as dwarf planets, objects distinct from planets. Currently five dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto, Haumea, Makemake and Eris. Several other objects in both the Asteroid belt and the Kuiper belt are under consideration, with as many as 50 that could eventually qualify. There may be as many as 200 that could be discovered once the Kuiper belt has been fully explored. Dwarf planets share many of the same characteristics as planets, although notable differences remain – namely that they are not dominant in their orbits. By definition, all dwarf planets are members of larger populations. Ceres is the largest body in the asteroid belt, while Pluto, Haumea, and Makemake are members of the Kuiper belt and Eris is a member of the scattered disc. Scientists such as Mike Brown believe that there may soon be over forty trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition.
Planetary attributes-
| Name | Equatorial diameter[a] | Mass[a] | Orbital radius (AU) | Orbital period (years)[a] | Inclination to Sun's equator (°) | Orbital eccentricity | Rotation period (days) | Named moons[c] | Rings | Atmosphere | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Terrestrials | Mercury | 0.382 | 0.06 | 0.39 | 0.24 | 3.38 | 0.206 | 58.64 | 0 | no | minimal |
| Venus | 0.949 | 0.82 | 0.72 | 0.62 | 3.86 | 0.007 | −243.02 | 0 | no | CO2, N2 | |
| Earth[b] | 1.00 | 1.00 | 1.00 | 1.00 | 7.25 | 0.017 | 1.00 | 1 | no | N2, O2 | |
| Mars | 0.532 | 0.11 | 1.52 | 1.88 | 5.65 | 0.093 | 1.03 | 2 | no | CO2, N2 | |
| Gas giants | Jupiter | 11.209 | 317.8 | 5.20 | 11.86 | 6.09 | 0.048 | 0.41 | 49 | yes | H2, He |
| Saturn | 9.449 | 95.2 | 9.54 | 29.46 | 5.51 | 0.054 | 0.43 | 52 | yes | H2, He | |
| Uranus | 4.007 | 14.6 | 19.22 | 84.01 | 6.48 | 0.047 | −0.72 | 27 | yes | H2, He | |
| Neptune | 3.883 | 17.2 | 30.06 | 164.8 | 6.43 | 0.009 | 0.67 | 13 | yes | H2, He | |
| Dwarf planets | |||||||||||
| Ceres | 0.08 | 0.000 2 | 2.5–3.0 | 4.60 | 10.59 | 0.080 | 0.38 | 0 | no | none | |
| Pluto | 0.19 | 0.002 2 | 29.7–49.3 | 248.09 | 17.14 | 0.249 | −6.39 | 3 | no | temporary | |
| Haumea | 0.37×0.16 | 0.000 7 | 35.2–51.5 | 282.76 | 28.19 | 0.189 | 0.16 | 2 | |||
| Makemake | ~0.12 | 0.000 7 | 38.5–53.1 | 309.88 | 28.96 | 0.159 | ? | 0 | ? | ? [d] | |
| Eris | 0.19 | 0.002 5 | 37.8–97.6 | ~557 | 44.19 | 0.442 | ~0.3 | 1 | ? | ? [d] | |
| |||||||||||
Extrasolar planets-
Exoplanets, by year of discovery, through 2010-10-03.
The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. Of the more than 500 extrasolar planets discovered by December 2010, most have masses which are comparable to or larger than Jupiter's, though masses ranging from just below that of Mercury to many times Jupiter's mass have been observed. The smallest extrasolar planets found to date have been discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12.
There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses, such as those orbiting the stars Mu Arae, 55 Cancri and GJ 436. These planets have been nicknamed "Neptunes" because they roughly approximate that planet's mass (17 Earths). Another new category are the so-called "super-Earths", possibly terrestrial planets far larger than Earth but smaller than Neptune or Uranus. To date, about twenty possible super-Earths (depending on mass limits) have been found, including OGLE-2005-BLG-390Lb and MOA-2007-BLG-192Lb, frigid icy worlds discovered through gravitational microlensing, and five of the six planets orbiting the nearby red dwarf Gliese 581. Gliese 581 d is roughly 7.7 times Earth's mass, while Gliese 581 c is five times Earth's mass and was initially thought to be the first terrestrial planet found within a star's habitable zone. However, more detailed studies revealed that it was slightly too close to its star to be habitable, and that the farther planet in the system, Gliese 581 d, though it is much colder than Earth, could potentially be habitable if its atmosphere contained sufficient greenhouse gases. COROT-7b, a planet with a diameter estimated at around 1.7 times that of Earth, (making it the smallest super-Earth yet measured), but with an orbital distance of only 0.02 AU, which means it probably has a molten surface at a temperature of 1000–1500 °C,
Size comparison of HR 8799 c (gray) with Jupiter. Most exoplanets discovered thus far are larger than Jupiter, though discoveries of smaller planets are expected in the near future.
It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. There may also exist a class of hot Jupiters, called Chthonian planets, that orbit so close to their star that their atmospheres have been blown away completely by stellar radiation. While many hot Jupiters have been found in the process of losing their atmospheres, as of 2008, no genuine Chthonian planets have been discovered.
More detailed observation of extrasolar planets will require a new generation of instruments, including space telescopes. Currently the COROT and Kepler spacecraft are searching for stellar luminosity variations due to transiting planets. Several projects have also been proposed to create an array of space telescopes to search for extrasolar planets with masses comparable to the Earth. These include the proposed NASA's, Terrestrial Planet Finder, and Space Interferometry Mission programs, and the CNES' PEGASE. The New Worlds Mission is an occulting device that may work in conjunction with the James Webb Space Telescope. However, funding for some of these projects remains uncertain. The first spectra of extrasolar planets were reported in February 2007 (HD 209458 b and HD 189733 b). The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy.
Inner planets-
Mercury:
Due to its small size (and thus its small gravity), Mercury has no substantial atmosphere. Its extremely thin atmosphere mostly consists of a small amount of helium and traces of sodium, potassium, and oxygen. These gases derive from the solar wind, radioactive decay, meteor impacts, and breakdown of Mercury's crust. Mercury's atmosphere is not stable and is constantly being refreshed because of its atoms escaping into space as a result of the planet's heat.
Venus-
Atmosphere of Venus in UV, by Pioneer Venus Orbiter in 1979
Venus' atmosphere is mostly composed of carbon dioxide. It contains minor amounts of nitrogen and other trace elements, including compounds based on hydrogen, nitrogen, sulfur, carbon, and oxygen. The atmosphere of Venus is much hotter, denser, and deeper than that of Earth.
The troposphere begins at the surface and extends up to an altitude of 65 kilometres (an altitude at which the mesosphere has already been reached on Earth). At the top of the troposphere, temperature and pressure reach Earth-like levels. Winds at the surface are a few metres per second, reaching 70 m/s or more in the upper troposphere. The stratosphere and mesosphere extend from 65 km to 95 km in height. The thermosphere and exosphere begin at around 95 kilometres, eventually reaching the limit of the atmosphere at about 220 to 250 km.
The air pressure at Venus' surface is about 92 times that of the Earth. The enormous amount of CO2 in the atmosphere creates a strong greenhouse effect, raising the surface temperature to around 470 °C, hotter than that of any other planet in the solar system.
Mars-
The Martian atmosphere is very thin and composed mainly of carbon dioxide, with some nitrogen and argon. The average surface pressure on Mars is 0.6-0.9 kPa, compared to about 101 kPa for Earth. This results in a much lower atmospheric thermal inertia, and as a consequence Mars is subject to strong thermal tides that can change total atmospheric pressure by up to 10%. The thin atmosphere also increases the variability of the planet's temperature. Martian surface temperatures vary from lows of approximately −140 °C (−220 °F) during the polar winters to highs of up to 20 °C (70 °F) in summers.
Pits in south polar ice cap, MGS 1999, NASA
Between 1975 and 1995, Mars warmed by about 0.65 °C (1.2 °F) due to dust storms that made its surface darker so that it absorbed more sunlight. Changes in pits in the layer of frozen carbon dioxide at the Martian south pole observed between 1999 and 2001 suggest the south polar ice cap is shrinking. More recent observations indicate that Mars' south pole is continuing to melt. "It's evaporating right now at a prodigious rate," says Michael Malin, principal investigator for the Mars Orbiter Camera. The pits in the ice are growing by about 3 meters (9.8 ft) per year. Malin states that conditions on Mars are not currently conductive to the formation of new ice. NASA has suggested that this indicates a "climate change in progress" on Mars. One study suggests this may be a local phenomenon rather than a global one.
Colin Wilson has proposed that the observed variations are caused by irregularities in the orbit of Mars.ice age. Other scientists believe the warming may be a result albedo changes from dust storms. The study predicts the planet could continue to warm, as a result of positive feedback. William Feldman speculates the warming could be because Mars might be coming out of an
Gas giants-
The four outer planets of the Solar System are gas giants. They share some atmospheric commonalities. All have atmospheres that are mostly hydrogen and helium and that blend into the liquid interior at pressures greater than the critical pressure, so that there is no clear boundary between atmosphere and body.
Jupiter-
Oval BA on the left and the Great Red Spot on the right
Jupiter's upper atmosphere is composed of about 75% hydrogen and 24% helium by mass, with the remaining 1% consisting of other elements. The interior contains denser materials such that the distribution is roughly 71% hydrogen, 24% helium and 5% 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, possibly underlaid by a thin layer of water.
Jupiter is covered with a cloud layer about 50 km deep. The clouds are composed of ammonia crystals and possibly ammonium hydrosulfide. The clouds are located in the tropopause and are arranged into bands of different latitudes, known as tropical regions. These are sub-divided into lighter-hued zones and darker belts. The interactions of these conflicting circulation patterns cause storms and turbulence. The best-known feature of the cloud layer is the Great Red Spot, a persistent anticyclonic storm located 22° south of the equator that is larger than Earth. In 2000, an atmospheric feature formed in the southern hemisphere that is similar in appearance to the Great Red Spot, but smaller in size. The feature was named Oval BA, and has been nicknamed Red Spot Junior.
Observations of the Red Spot Jr. storm suggest Jupiter could be in a period of global climate change. This is hypothesized to be part of an approximately 70 year global climate cycle, characterized by the relatively rapid forming and subsequent slow erosion and merging of cyclonic and anticyclonic vortices in Jupiter's atmosphere. These vortices facilitate the heat exchange between poles and equator. If they have sufficiently eroded, heat exchange is strongly reduced and regional temperatures may shift by as much as 10 K, with the poles cooling down and the equator region heating up. The resulting large temperature differential destabilizes the atmosphere and thereby leads to the creation of new vortices.
Saturn-
The outer atmosphere of Saturn consists of about 93.2% hydrogen and 6.7% helium. Trace amounts of ammonia, acetylene, ethane, phosphine, and methane have also been detected. As with Jupiter, 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.
The Saturnian atmosphere is in several ways similar to that of Jupiter. It exhibits a banded pattern similar to Jupiter's, and occasionally exhibits long-lived ovals caused by storms. A storm formation analogous to Jupiter's Great Red Spot, the Great White Spot, is a short-lived phenomenon that forms with a roughly 30-year periodicity. It was last observed in 1990. However, the storms and the band pattern are less visible and active than those of Jupiter, due to the overlying ammonia hazes in Saturn's troposphere.
Saturn's atmosphere has several unusual features. Its winds are among the Solar System's fastest, with Voyager data indicating peak easterly winds of 500 m/s. It is also the only planet with a warm polar vortex, and is the only planet other than Earth where eyewall clouds have been observed in hurricane-like structures.
Uranus-
The atmosphere of Uranus is composed primarily of gas and various ices. It is about 83% hydrogen, 15% helium, 2% methane and traces of acetylene. Like Jupiter and Saturn, Uranus has a banded cloud layer, although this is not readily visible without enhancement of visual images of the planet. Unlike the larger gas giants, the low temperatures in the upper Uranian cloud layer, down to 50 K, causes cloud formation from methane rather than ammonia.
Although in general less storm activity has been observed in the Uranian atmosphere than in those of Jupiter or Saturn, due to its overlying methane and acetylene hazes in its atmosphere making the planet look like a bland, baby blue globe. Images taken in 1997 with the Hubble Space Telescope showed storm activity in that part of the atmosphere emerging from the 25-year long Uranian winter. The general lack of storm activity may be related to the lack of an internal energy generation mechanism for Uranus, a feature unique among the gas giants.
Neptune-
Great Dark Spot (top), Scooter (middle white cloud), and Wizard's eye/Dark Spot 2 (bottom).
The atmosphere of Neptune is similar to that of Uranus. It is about 80% hydrogen, 19% helium, and 1.5% methane. However the weather activity on Neptune is much more active, and its atmosphere is much bluer than that of Uranus. The upper levels of the atmosphere reach temperatures of about 55 K, giving rise to methane clouds in its troposphere, which gives the planet its ultramarine color. Temperatures rise steadily deeper inside the atmosphere.
Neptune has extremely dynamic weather systems, including the highest wind speeds in the solar system, thought to be powered by the flow of internal heat. Typical winds in the banded equatorial region can possess speeds of around 350 m/s, while storm systems can have winds reaching up to around 900 m/s, nearly the speed of sound in Neptune's atmosphere. Several large storm systems have been identified, including the Great Dark Spot, a cyclonic storm system the size of Eurasia, the Scooter, a white cloud group further south than the Great Dark Spot, and the Wizard's eye/Dark Spot 2, a southern cyclonic storm.
Neptune, the farthest planet from Earth, has increased in brightness since 1980. Neptune's brightness is statistically correlated with its stratospheric temperature. Hammel and Lockwood hypothesize that the change in brightness includes a solar variation component as well as a seasonal component, though they did not find a statistically significant correlation with solar variation. They propose that the resolution of this issue will be clarified by brightness observations in the next few years: forcing by a change in sub-solar latitude should be reflected in a flattening and decline of brightness, while solar forcing should be reflected in a flattening and then resumed rise of brightness.
Other bodies in the Solar System-
Natural satellites:
Seven of the many natural satellites in the solar system are known to have atmospheres: Europa, Io, Callisto, Enceladus, Ganymede, Titan, and Triton. Ganymede and Europa both have very tenuous oxygen atmospheres, thought to be produced by radiation spliting the water ice present on the surface of these moons into hydrogen and oxygen. Io has an extremely thin atmosphere consisting mainly of sulfur dioxide (SO2), arising from volcanism and sunlight-driven sublimation of surface sulfur dioxide deposits. The atmosphere of Enceladus is also extremely thin and variable, consisting mainly of water vapor, nitrogen, methane, and carbon dioxide vented from the moon's interior through cryovolcanism. The extremely thin carbon dioxide atmosphere of Callisto is thought to be replenished by sublimation from surface deposits.
Titan-
True-color image of layers of haze in Titan's atmosphere.
Titan has by far the densest atmosphere of any moon. The Titanian atmosphere is in fact denser than Earth's, with a surface pressure of 147 kPa, one and a half times that of the Earth. The atmosphere is 98.4% nitrogen, with the remaining 1.6% composed of methane and trace amounts of other gases such as hydrocarbons (including ethane, diacetylene, methylacetylene, cyanoacetylene, acetylene, propane), argon, carbon dioxide, carbon monoxide, cyanogen, hydrogen cyanide and helium. The hydrocarbons are thought to form in Titan's upper atmosphere in reactions resulting from the breakup of methane by the Sun's ultraviolet light, producing a thick orange smog. Titan has no magnetic field and sometimes orbits outside Saturn's magnetosphere, directly exposing it to the solar wind. This may ionize and carry away some molecules from the top of the atmosphere.
Titan's atmosphere supports an opaque cloud layer that obscures Titan's surface features at visible wavelengths. The haze that can be seen in the picture to the right contributes to the moon's Anti-Greenhouse Effect and lowers the temperature by reflecting sunlight away from the satellite. The thick atmosphere blocks most visible wavelength light from the Sun and other sources from reaching Titan's surface.
Triton-
Triton, Neptune's largest moon, has a tenuous nitrogen atmosphere with small amounts of methane. Tritonian atmospheric pressure is about 1Pa. The surface temperature is at least 35.6 K, with the nitrogen atmosphere in equilibrium with nitrogen ice on Triton's surface.
Triton has increased in absolute temperature by 5% since 1989 to 1998. A similar rise of temperature on Earth would be equal to about 11 °C (20 °F) increase in temperature in nine years. "At least since 1989, Triton has been undergoing a period of global warming. Percentage-wise, it's a very large increase," said James L. Elliot, who published the report.
Triton is approaching an unusually warm summer season that only happens once every few hundred years. Elliot and his colleagues believe that Triton's warming trend could be driven by seasonal changes in the absorption of solar energy by its polar ice caps. One suggestion for this warming is that it is a result of frost patterns changing on its surface. Another is that ice albedo has changed, allowing for more heat from the Sun to be absorbed. Bonnie J. Buratti et al. argue the changes in temperature are a result of deposition of dark, red material from geological processes on the moon, such as massive venting. Because Triton's Bond albedoSolar System, it is sensitive to small variations in spectral albedo. is among the highest within the
Pluto-
Artist's conception of the New Horizons spacecraft passing over Pluto, showing its tenuous atmosphere
Pluto has an extremely thin atmosphere that consists of nitrogen, methane, and carbon monoxide, derived from the ices on its surface. The atmosphere is believed to completely freeze and collapse when Pluto moves further from the Sun on its extremely elliptical orbit. Pluto needs 248 years for one complete orbit, and has been observed for less than one third of that time. It has an average distance of 39 AU from the Sun, hence in-depth data from Pluto is sparse and difficult to gather. Temperature is inferred indirectly for Pluto; when it passes in front of a star, observers note how fast the light drops off. From this, they deduce the density of the atmosphere, and that is used as an indicator of temperature.
One such occultation events happened in 1988. Observations of a second occulation on August 20, 2002 suggest that Pluto's atmospheric pressure has tripled, indicating a warming of about 2 °C (3.6 °F). The warming is "likely not connected with that of the Earth," says Jay Pasachoff. One astronomer has speculated the warming may be a result of eruptive activity, but it is more likely Pluto's temperature is heavily influenced by its elliptical orbit. It was closest to the Sun in 1989 (perihelion) and has slowly receded since. If it has any thermal inertia, it is expected to warm for a while after it passes perihelion. "This warming trend on Pluto could easily last for another 13 years," says David J. Tholen. It has also been suggested that a darkening of surface ice may also be the cause, but additional data and modeling is needed. Frost distribution on the surface of Pluto is significantly affected by the dwarf planet's high obliquity.
In January 2006, NASA launched New Horizons, a spacecraft set to study Pluto. It is expected to pass by Pluto in 2015.
Comets-
Telescopic image of Comet 17P/Holmes in 2007
The coma of a comet is a very large but very tenuous atmosphere of dust and gas around the cometary nucleus. The coma can be larger than the sun. The coma is generally made of ice and dust, but the composition varies depending on the composition of the comet.
Extrasolar planets-
Two planets outside the solar system have been observed to have atmospheres. The first observed extrasolar planetary atmosphere was made in 2001. Sodium in the atmosphere of the planet HD 209458 b was detected during a set of four transits of the planet across its star. Later observations with the Hubble Space Telescope showed an enormous ellipsoidal envelope of hydrogen, carbon and oxygen around the planet. This envelope reaches temperatures of 10,000 K. The planet is estimated to be losing (1-5)×108 kg of hydrogen per second. This type of atmosphere loss may be common to all planets orbiting Sun-like stars closer than around 0.1 AU.
In addition to hydrogen, carbon, and oxygen, HD 209458 b is thought to have water vapor in its atmosphere. another hot gas giant planet. Water vapour has also been observed in the atmosphere of HD 189733 b,
Overview-
Ice core data. Note length of glacial cycles averages ~100,000 years. Blue curve is temperature, green curve is CO2, and red curve is windblown glacial dust (loess). Today's date is on the left side of the graph.
It is sometimes asserted that the length of the current interglacial temperature peak will be similar to the length of the preceding interglacial peak (Sangamonian/Eem Stage), and that therefore we might be nearing the end of this warm period. However, this conclusion is probably mistaken: the lengths of previous interglacials were not particularly regular (see graphic at right). Berger and Loutre (2002) argue that “with or without human perturbations, the current warm climate may last another 50,000 years. The reason is a minimum in the eccentricity of Earth's orbit around the Sun.” Also, Archer and Ganopolski (2005) report that probable future CO2 emissions may be enough to suppress the glacial cycle for the next 500 kyr.
Note in the graphic the strong 100,000 year periodicity of the cycles, and the striking asymmetry of the curves. This asymmetry is believed to result from complex interactions of feedback mechanisms. It has been observed that ice ages deepen by progressive steps, but the recovery to interglacial conditions occurs in one big step.
Orbital mechanics require that the length of the seasons be proportional to the swept areas of the seasonal quadrants, so when the eccentricity is extreme, the seasons on the far side of the orbit can last substantially longer. Today, when autumn and winter in the northern hemisphere occur at closest approach, the earth is moving at its maximum velocity and therefore autumn and winter are slightly shorter than spring and summer.
Today, northern hemisphere summer is 4.66 days longer than winter and spring is 2.9 days longer than autumn. As axial precession changes the place in the Earth's orbit where the solstices and equinoxes occur, Northern hemisphere winters will get longer and summers will get shorter, eventually creating conditions believed to be favorable for triggering the next glacial period.
The arrangements of land masses on the Earth's surface are believed to reinforce the orbital forcing effects. Comparisons of plate tectonic continent reconstructions and paleoclimatic studies show that the Milankovitch cycles have the greatest effect during geologic eras when landmasses have been concentrated in polar regions, as is the case today. Greenland, Antarctica, and the northern portions of Europe, Asia, and North America are situated such that a minor change in solar energy will tip the balance between year-round snow/ice preservation and complete summer melting. The presence of snow and ice is a well-understood positive feedback mechanism for climate.
From Wikipedia-
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