Planet Neptune: Facts About Its Orbit, Moons & Rings Planet Neptune: Facts About Its Orbit, Moons & Rings By Charles Q. Choi, Space.com Contributor | November 25, 2014 05:14pm ET MORE Neptune’s winds travel at more than 1,500 mph, and are the fastest planetary winds in the solar system. Credit: NASA/JPL Neptune is the eighth planet from the sun. It was the first planet to get its existence predicted by mathematical calculations before it was actually seen through a telescope on Sept. 23, 1846. Irregularities in the orbit of Uranus led French astronomer Alexis Bouvard to suggest that the gravitational pull from another celestial body might be responsible. German astronomer Johann Galle then relied on subsequent calculations to help spot Neptune via telescope . Previously, astronomer Galileo Galilei sketched the planet, but he mistook it for a star due to its slow motion. In accordance with all the other planets seen in the sky, this new world was given a name from Greek and Roman mythology — Neptune, the Roman god of the sea.

Physical characteristics Neptune's cloud cover has an especially vivid blue tint that is partly due to an as-yet-unidentified compound and the result of the absorption of red light by methane in the planets mostly hydrogen-helium atmosphere. Photos of Neptune reveal a blue planet, and it is often dubbed an ice giant, since it possesses a thick, slushy fluid mix of water, ammonia and methane ices under its atmosphere and is roughly 17 times Earth's mass and nearly 58 times its volume, according to a NASA fact sheet . Neptune's rocky core alone is thought to be roughly equal to Earth's mass , NASA says. Despite its great distance from the sun , which means it gets little sunlight to help warm and drive its atmosphere, Neptune's winds can reach up to 1,500 mph (2,400 kph), the fastest detected yet in the solar system . These winds were linked with a large dark storm that Voyager 2 tracked in Neptune's southern hemisphere in 1989. This oval-shaped, counterclockwise-spinning "Great Dark Spot" was large enough to contain the entire Earth, and moved westward at nearly 750 mph (1,200 kph).

This storm seemed to have vanished when the Hubble Space Telescope later searched for it. Hubble has also revealed the appearance and then fading of two other Great Dark Spots over the past decade. Neptune's magnetic poles are tipped over by roughly 47 degrees compared with the poles along which it spins. As such, the planet's magnetic field, which is about 27 times more powerful than Earth's, undergoes wild swings during each rotation. By studying the cloud formations on the gas giant, scientists were able to calculate that a day on Neptune lasts just under 16 hours . Orbital characteristics Neptune's elliptical, oval-shaped orbit keeps the planet an average distance from the sun of almost 2.8 billion miles (4.5 billion kilometers), or roughly 30 times as far away as Earth, making it invisible to the naked eye. Neptune goes around the sun once roughly every 165 Earth years, and completed its first orbit, since being discovered , in 2011. Every 248 years, Pluto moves inside Neptune's orbit for 20 years or so, during which time it is closer to the sun than Neptune.

Nevertheless, Neptune remains the farthest planet from the sun, since Pluto was reclassified as a dwarf planet in 2006. Composition & structure Atmospheric composition (by volume): hydrogen, 80 percent; helium, 19.0 percent; methane, 1.5 percent Magnetic field: Roughly 27 times more powerful than Earth's Composition : The overall composition of Neptune is, by mass, thought to be about 25 percent rock, 60 to 70 percent ice, and 5 to 15 percent hydrogen and helium, according to Tristan Guillot, author of "Interiors of Giant Planets Inside and Outside the Solar System" in the journal Science. Internal structure: Mantle of water, ammonia and methane ices; Core of iron and magnesium-silicate Orbit & rotation Average distance from the sun: 2,795,084,800 miles (4,498,252,900 km).

By comparison: 30.069 times farther than Earth Perihelion (closest approach to the sun): 2,771,087,000 miles (4,459,630,000 km). By comparison: 29.820 times that of Earth Aphelion (farthest distance from the sun): 2,819,080,000 miles (4,536,870,000 km). By comparison: 30.326 times that of Earth (Source: NASA ) Neptune's moons Neptune has 14 known moons , named after lesser sea gods and nymphs from Greek mythology. The largest by far is Triton , whose discovery on Oct. 10, 1846, was in a sense enabled by beer — amateur astronomer William Lassell used the fortune he made as a brewer to finance his telescopes. Triton is the only spherical moon of Neptune — the planet's other 13 moons are irregularly shaped.

It is also unique in being the only large moon in the solar system to circle its planet in a direction opposite to its planet's rotation — this " retrograde orbit " suggests that Triton may once have been a dwarf planet that Neptune captured rather than forming in place, according to NASA. Neptune's gravity is dragging Triton closer to the planet, meaning that millions of years from now, Triton will come close enough for gravitational forces to rip it apart. Triton is extremely cold, with temperatures on its surface reaching about minus 391 degrees F (minus 235 degrees C), making it one of the coldest places in the solar system. Nevertheless, Voyager 2 detected geysers spewing icy matter upward more than 5 miles (8 km), showing its interior appears warm. Scientists are investigating the possibility of a subsurface ocean on the icy moon. In 2010, seasons were discovered on Triton. In 2013, scientists working with SETI caught sight of Neptune's "lost" moon of Naiad using data from the Hubble Space Telescope.The 62-mile-wide (100 km) moon has remained unseen since Voyager 2 discovered it in 1989.

Also in 2013, scientists using the Hubble Space Telescope found the 14th moon, dubbed S/2004 N 1. It is Neptune's smallest moon and is just 11 miles (18 km) wide. It got its temporary name because it is the first satellite (S) of Neptune (N) to be found from images taken in 2004, according to NASA . The rings of Neptune Neptune's unusual rings are not uniform, but possess bright thick clumps of dust called arcs. The rings are thought to be relatively young and short-lived . Earth-based observations announced in 2005 found that Neptune's rings are apparently far more unstable than previously thought, with some dwindling away rapidly, according to an article in the journal Icarus . Research & exploration NASA's Voyager 2 satellite was the first and as yet only spacecraft to visit Neptune on Aug. 25, 1989. The satellite discovered Neptune's rings and six of the planet's moons — Despina, Galatea, Larissa, Naiad, Proteus and Thalassa.

An international team of astronomers relying on ground telescopes announced the discovery of five new moons orbiting Neptune in 2003. Formation of Neptune Neptune is generally thought to have formed with the initial buildup of a solid core followed by the capture of surrounding hydrogen and helium gas in the nebula surrounding the early sun. In this model, proto-Neptune formed over the course of 1 to 10 million years. Additional reporting by Nola Taylor Redd, Space.com Contributor Additional resources Neptune - Educational facts and history of the planet Neptune. Neptune News Neptune Facts Neptune is the eighth planet from the Sun and the fourth largest (by diameter). Neptune is smaller in diameter but larger in mass than Uranus . Planet Profile orbit : 4,504,000,000 km (30.06 AU ) from Sun diameter : 49,532 km (equatorial) History of Neptune In Roman mythology Neptune (Greek: Poseidon ) was the god of the Sea. After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton 's laws. It was therefore predicted that another more distant planet must be perturbing Uranus' orbit.

Neptune was first observed by Galle and d'Arrest on 1846 Sept 23 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter , Saturn and Uranus . An international dispute arose between the English and French (though not, apparently between Adams and Le Verrier personally) over priority and the right to name the new planet; they are now jointly credited with Neptune's discovery. Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune's actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location. More than two centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune's motion would have been obvious to him. But, alas, cloudy skies prevented observations on those few critical days.

Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Much of we know about Neptune comes from this single encounter. But fortunately, recent ground-based and HST observations have added a great deal, too. Because Pluto 's orbit is so eccentric, it sometimes crosses the orbit of Neptune making Neptune the most distant planet from the Sun for a few years. Neptune's composition is probably similar to Uranus': various " ices " and rock with about 15% hydrogen and a little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but rather to be more or less uniform in composition. But there is most likely a small core (about the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a small amount of methane. Neptune's blue color is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint. Like a typical gas planet , Neptune has rapid winds confined to bands of latitude and large storms or vortices.

Neptune's winds are the fastest in the solar system, reaching 2000 km/hour. Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates more than twice as much energy as it receives from the Sun. At the time of the Voyager encounter, Neptune's most prominent feature was the Great Dark Spot (left) in the southern hemisphere. It was about half the size as Jupiter's Great Red Spot (about the same diameter as Earth). Neptune's winds blew the Great Dark Spot westward at 300 meters/second (700 mph). Voyager 2 also saw a smaller dark spot in the southern hemisphere and a small irregular white cloud that zips around Neptune every 16 hours or so now known as "The Scooter" (right). It may be a plume rising from lower in the atmosphere but its true nature remains a mystery. However, HST observations of Neptune (left) in 1994 show that the Great Dark Spot has disappeared! It has either simply dissipated or is currently being masked by other aspects of the atmosphere. A few months later HST discovered a new dark spot in Neptune's northern hemisphere.

This indicates that Neptune's atmosphere changes rapidly, perhaps due to slight changes in the temperature differences between the tops and bottoms of the clouds. Neptune also has rings. Earth-based observations showed only faint arcs instead of complete rings, but Voyager 2 's images showed them to be complete rings with bright clumps. One of the rings appears to have a curious twisted structure (right). Like Uranus and Jupiter, Neptune's rings are very dark but their composition is unknown. Neptune's rings have been given names: the outermost is Adams (which contains three prominent arcs now named Liberty, Equality and Fraternity), next is an unnamed ring co-orbital with Galatea, then Leverrier (whose outer extensions are called Lassell and Arago), and finally the faint but broad Galle. Neptune's magnetic field is, like Uranus', oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers. Neptune can be seen with binoculars (if you know exactly where to look) but a large telescope is needed to see anything other than a tiny disk.

There are several Web sites that show the current position of Neptune (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such charts can be created with a planetarium program . Neptune's Satellites Data on the newly discovered moons can be found at JPL and Scott Sheppard's site . Open Issues Neptune's magnetic field is off center and at a large angle to its rotation axis. What processes in the interior generate this oddly shaped field? What accounts for the relative lack of hydrogen and helium in Neptune (and Uranus)? Why are Neptune's winds so strong in spite of the fact that it is so far from the Sun and has a relatively weak internal heat source? What happened to the Great Dark Spot? Can we design a useful Neptune orbiter mission cheap enough to be funded? Neptune Facts: Interesting Facts about Planet Neptune • The Planets Interactive Solar System Model Neptune Facts Neptune is the eighth planet from the Sun and last of the known planets. While it is the third largest planet with respect to mass, it is only the fourth largest in terms of diameter. Due to its blue coloration, Neptune was named after the Roman god of the Sea.

Size of Neptune compared with the Earth Side by side comparison of the size of Neptune vs Earth Facts about Neptune It takes Neptune 164.8 Earth years to orbit the Sun. On 11 July 2011, Neptune completed its first full orbit since its discovery in 1846. Neptune was discovered by Jean Joseph Le Verrier. The planet was not known to ancient civilizations because it is not visible to the naked eye. The planet was initially called Le Verrier after its discoverer. This name, however, quickly was abandoned and the name Neptune was chosen instead. Neptune is the Roman God of the Sea. In Greek, Neptune is called Poseidon. Neptune has the second largest gravity of any planet in the solar system – second only to Jupiter. The orbit path of Neptune is approximately 30 astronomical units (AU) from the Sun. This means it is around 30 times the distance from the Earth to the Sun. The largest Neptunian moon, Triton, was discovered just 17 days after Neptune itself was discovered. Neptune has a storm similar the Great Red Spot on Jupiter. It is commonly known as the Great Dark Spot and is roughly the size of Earth.

Neptune also has a second storm called the Small Dark Spot. This storm is around the same size as Earth’s moon. Neptune spins very quickly on its axis. The planets equatorial clouds take 18 hours to complete one rotation. The reason this happens is that Neptune does not have a solid body. Only one spacecraft, the Voyager 2, has flown past Neptune. It happened in 1989 and captured the first close-up images of the Neptunian system. It took 246 minutes – four hours and six minutes – for signals from Voyager 2 to reach back to Earth. The climate on Neptune is extremely active. In its upper atmosphere, large storms sweep across it and high-speed solar winds track around the planet at up to 1,340 km per second. The largest storm was the Great Dark Spot in 1989 which lasted for around five years. Like the other outer planets, Neptune possesses a ring system, though its rings are very faint. They are most likely made up of ice particles and grains of dust with a carbon-based substance coating them. Neptune has 14 known moons.

The largest of these moons is Titan – a frozen world which spits out particles of nitrogen ice and dust from below its surface. It is believed that Titan was caught by the immense gravitational pull of Neptune and is regarded as one of the coldest worlds in our solar system. Neptune has an average surface temperature of -214°C – approximately -353°F. More information and facts about Neptune When scientific discoveries are made there is often a debate (sometimes heated) as to who deserves credit. The discovery of Neptune is one such example. Shortly after the discovery of the planet Uranus in 1781, scientists noticed that its orbit had significant fluctuations that were not expected. To solve this mystery, they proposed the existence of another planet whose gravitational field would account for such orbital variances. In 1845, the English astronomer John Couch Adams completed his calculations as to the position of this unknown planet. Although he submitted his findings to the Royal Society (the leading English scientific organization), his work was met with little interest. However, a year later the French astronomer Jean Joseph Le Verrier made known his calculations that were strikingly similar to those of Adams.

As a result of the two men’s independent estimates being so close, the scientific community took notice and began its search for the planet in the region of the sky Adams and Le Verrier had predicted. On September 23, 1846, the German astronomer Johann Gall observed the new planet near to where Adam’s calculations had forecasted and even closer to those of Le Verrier. Le Verrier was initially given credit for the discovery. As a result, an international dispute arose, with one faction championing Adams and the other Le Verrier. This conflict, however, was not shared between the two men themselves. Eventually, the campaign for each side cooled, and both men were given credit. Until the Voyager 2 spacecraft fly-by in 1989, little was known about Neptune. This mission provided new information about Neptune’s rings, number of moons, atmosphere and rotation. Additionally, Voyager 2 discovered significant features of the moon Triton. There are no official planetary missions scheduled to Neptune in the near future. Atmosphere Neptune’s upper atmosphere is composed of 80% hydrogen (H2), 19% helium and trace amounts of methane.

Similar to Uranus , the blue coloration of Neptune is due in part to its atmospheric methane, which absorbs light having a wavelength corresponding to red. Unlike Uranus, Neptune is a deeper blue, and, therefore, some other atmospheric component must be present in the Neptunian atmosphere that is not found in Uranus’ atmosphere. Two significant weather patterns have been observed on Neptune. The first, seen during the Voyager 2 fly-by mission, are the Dark Spots. These are storms comparable to the Great Red Spot found on Jupiter . However, a difference between these storms is their duration. Whereas the Great Red Spot has lasted for centuries, the Dark Spots are much more shortly lived as is evident by their disappearance when Neptune was viewed by the Hubble Space Telescope just four years after the Voyager 2 fly-by. The second of the two weather patterns observed by Voyager 2 is the swiftly moving white storm system, nicknamed Scooter. This type of storm system, which is much smaller than the Dark Spots, also appears to be short-lived. As with the other gas giants , Neptune’s atmosphere is divided into latitudinal bands.

The wind speed achieved in some of these bands is almost 600 m/s, the fastest known in the Solar System. Interior The interior of Neptune, similar to that of Uranus, is made of two layers: a core and mantle. The core is rocky and estimated to be 1.2 times as massive as the Earth . The mantle is an extremely hot and dense liquid composed of water, ammonia and methane. The mantle is between ten to fifteen times the mass of the Earth. Although Neptune and Uranus share similar interiors, they are, however, quite distinct in one way. Whereas Uranus emits only about the same amount of heat that it receives from the Sun, Neptune emits nearly 2.61 times the amount of the sunlight it receives. To place this in perspective, the two planets’ surface temperatures are approximately equal, yet Neptune receives only 40% of the sunlight that Uranus does. Additionally, this large internal heat is also what powers the extreme winds found in the upper atmosphere. Orbit & Rotation With the discovery of Neptune, the size of the known Solar System increased by a factor of two.

With an average orbital distance of 4.50 x 109 km, it takes sunlight almost four hours and forty minutes to reach Neptune. Moreover, this distance also means that a Neptunian year lasts about 165 Earth years! Neptune’s orbital eccentricity of .0097 is second smallest behind that of Venus . This small eccentricity means that the orbit of Neptune is very close to being circular. Another way of looking at this is to compare Neptune’s perihelion of 4.46 x 109 km and its aphelion of 4.54 x 109 km and notice that this is a difference of less than two percent. Like Jupiter and Saturn, Neptune rotates very quickly as compared to the terrestrial planets . With a rotational period of a little over 16 hours, Neptune has the third shortest day in the Solar System. The axial tilt of Neptune is 28.3°, which is relatively close to the Earth’s 23.5°. What is amazing is that, even at such a far distance from the Sun, Neptune still experiences seasons (though more subtly) similar to those on Earth as a result of its axial tilt.

Rings Currently, Neptune is known to have thirteen moons. Of these thirteen only one is large and spherical in shape. This moon, Triton, is believed to have originally been a dwarf planet captured by Neptune’s gravitational field, and, thus, not a natural satellite of the planet. Evidence for this theory comes from Triton’s retrograde orbit of Neptune; that is, Triton orbits in the opposite direction that Neptune rotates. With a recorded surface temperature of -235° C, Triton is the coldest known object in the Solar System. Neptune has three major rings&#8212Adams, Le Verrier and Galle. This ring system is much fainter than that of the other gas giants. In fact, some of the rings are so dim that it was believed at one time that they were incomplete. However, images from the Voyager 2 fly-bys show extremely faint rings. Neptune Profile Uranus and Neptune The Jovian Planets: Uranus, and Neptune Uranus has a radius of 25,500 km, and a mass of 14.5 Earth's. It has a density similar to Jupiter: 1.27 gm/cm3.

It also spins rapidly, and has a "day" of 17 hours and 14 minutes. This is the internal rotation period, the cloud layers rotate in about 14 hours. It orbits the Sun at an average distance of 19.2 AU (2.8 billion km), and takes 84 years to complete one trip around the Sun. It was the first planet to be discovered since antiquity, when it was found by William Herschel in 1781. Its name was chosen as Uranus was the father of Cronus (Saturn), and thus grandfather of Zeus (Jupiter). Comparison to Earth: Uranus is very strange in that its rotational axis lies very close to its orbital plane. This means that the seasons on Uranus are very long, with 21 years that have near constant daylight ("summer"), and 21 years of almost perpetual darkness ("winter"). Here is a comparison of the obliquity of the planets in the solar system: How Uranus got to this configuration is a mystery. Presumably, a large body must have crashed-into Uranus and flipped it over (note that it spins backwards compared to the other planets--except Venus).

The disk of Uranus is almost completely featureless. There appears to be a high-altitude haze that blocks our view of the cloud deck. Saturn also has this haze layer, but it is not as thick as that of Uranus (Jupiter doesn't really have this structure). The cloud tops of Uranus are very cold, 49 K! Uranus has a structure similar to Jupiter and Saturn, a big ball of hydrogen and helium gas with a rocky core about equal to one half of an Earth mass. The structure of Uranus is just like the other gas giants. The central pressure is high (8 Mbar), and with a high temperature: 5,000 K. But both Uranus and Neptune are somewhat different from Jupiter in Saturn--they have extensive "ice mantles" : Again, this is not the ice you are familiar with---this ice is formed by high pressure (and the temperature is well above the freezing point of water on Earth): the density of this mantle is near 9 gm/cm3. It is still not completely clear whether Uranus even has a rocky core. As there are models that can explain the density that do not include such an feature.

Here's the atmospheric profile: Even though Uranus is mostly featureless, it has been possible to track clouds, and thus deduce the wind speeds in its atmosphere: At the time of these observations (South pole pointing at Sun), the wind speeds were much higher closer to the pole, than near the equator (note that 100 m/s = 360 km/hr = 250 mph). By false color stretching, we can extract some details, showing that there are bands/zones like the other Jovian planets: And even spots: If you notice, the last two pictures show radically different orientations of Uranus. In 1986, when Voyager 2 flew by Uranus, the south pole was pointed towards the Sun. But by the time of the HST picture in 2006, we were looking at the equator. Remember that Uranus has an orbital period of 84 years, thus the four seasons each last 21 years. It is 20 years between these two pictures, so our view of Uranus has changed rather dramatically!

Here is a small table with times of the Uranian seasons: With the increased sunlight, Uranus has begun to have some storms and show more structure (infrared images): And another: Yes, Uranus has rings (see below). Like Jupiter and Saturn, Uranus emits more heat than it recieves from the Sun, but just barely---about the same as that emitted by the Earth. There may be a layer inside Uranus that acts to trap the internal heat, but that remains unclear.

Like the other Jovian planets, it has a magnetic field, but a very strange one: The field has a large tilt/angle with respect to the rotational poles (59 degrees), it is also not centered on the core of Uranus: Like all the Jovian planets, Uranus has a ring system, but here the rings have much less material in them as Saturn (much more than Jupiter!), and they are not quite as spectacular: Uranus now has 27 named moons (from the works of Shakespeare and Pope), several of them quite interesting in their own rite, but none of them are very large (the largest, Oberon, has a radius of 761 km, for a comparison of the satellite systems of the Jovian planets, see the Figure 11.14 in the textbook also shown here:). Identifying the moons: The Uranian moon system is the least massive of the Jovian planets. This might be tied to the strange tilt of the spin axis. If something hit Uranus, then it would have scattered the moons into the solar system, and the small set of moons we see now might have mostly formed from the result of that collision.

Here are images of the five biggest moons Oberon (ρ = 1.63 gm/cm3), Titania (ρ = 1.71 gm/cm3), Ariel (ρ = 1.66 gm/cm3), Umbriel (ρ = 1.39 gm/cm3), and Miranda (ρ = 1.20 gm/cm3), probably mostly water ice: Miranda has a dramatically fractured surface, as if it was stuck together from several large pieces. It is believed that there was enormous geological activity in Miranda's past, when it was stuck in a 3:1 resonance with Umbriel. This caused the orbit to get ellipitical, increasing the tidal heating due to the intense tidal forces exerted on it by Uranus. All of these moons are dominated by water ice, and it is possible that some of them have tenuous atmospheres created by geysers or cryovolcanoes. We had only a brief visit to Uranus 30 years ago, so much of the system remains a mystery. Neptune Neptune is much further from the Sun than Uranus (30 AU), and takes 165 years to complete one orbit.

On July 12th, 2011, Neptune completed the first full orbit since its discovery in 1846. The story of the discovery of Neptune is rather interesting, and you can read more about it here (it is a story with a bit of intrigue/lying/cheating due to the intense British-French rivalry at the time). Summarizing, however, Uranus was not behaving itself. It was not moving as one would expect, and it was proposed that there must be something out there pulling on it. Calculations were made, and the hunt was on. It was eventually discovered by Galle in Germany. Neptune has 13 moons, many fewer than any of the other Jovian planets. Neptune has 17.2 Earth masses, so it is more massive than Uranus. Note, however, that it is smaller in size than Uranus, having a radius of 24,700 km. Thus, it must have a higher density: 1.63 gm/cm3. Neptune has a rotation period of 16 hours.

Strangely, Neptune's atmosphere appears to be much more active than that of Uranus (maybe the haze layer "freezes out" at this distance, where the cloud-top temperature is colder than that of Uranus), and has several large storms similar to those on Jupiter: Close-up on high latitude, high velocity clouds: Additional activity: The "great dark spot", and the "small dark spot", and "scooter" (the white blob between the two spots--it moved around very quickly). A movie of the rotation of Neptune, made with the Hubble Space Telescope. Note that Neptune is far away, and it is hard to see detail. So, as you have noticed, the images we have presented of Uranus and Neptune show them to have a blue-green color. This is real. As we have discussed, the color of an object indicates that it is good at reflecting that color. In the case of Uranus and Neptune, their color derives from the most important absorber in their atmospheres---methane. Here is the spectrum of methane 1 : While Jupiter and Saturn certainly have lots of methane, they are warm enough that there are clouds due to ammonia, water, and sulfur molecules.

But at Uranus and Neptune it is too cold at the cloud layer to have such molecules--methane dominates the atmosphere at these levels. Thus, it is what shapes the spectra of these two planets. Recently, astronomers have created a new class of planet for Uranus and Neptune because they actually are somewhat different than Jupiter and Saturn. While we still call them "Jovian" planets, the preferred terminology is "ice giants", as inside of Uranus and Neptune there is a large region of water, methane and ammonia and is a strange form of "ice": Here's a close-up of Neptune's internal structure: [Note that Neptune's rings are clumpy, and these various clumps have been given French names.] Neptune is more massive than Uranus, and this increases its density and that means that it is slightly smaller than Uranus. Here's a graphic showing the sizes of the Jovian planets (compared to Earth): Neptune generates much more internal heat than Uranus, as it emits 2.6 times as much energy as it recieves from the Sun. This might explain why Neptune appears to have much activity in its atmosphere.

Neptune only receives 40% of the sunlight that Uranus gets, but the atmosphres of both have similar temperatures due to the internal heat emitted by Neptune. "Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. Several possible explanations have been suggested, including radiogenic heating from the planet's core, conversion of methane under high pressure into hydrogen, diamond and longer hydrocarbons (the hydrogen and diamond would then rise and sink, respectively, releasing gravitational potential energy), and convection in the lower atmosphere that causes gravity waves to break above the tropopause" 2 .

Neptune has a very similar magnetic field to Uranus in that it is tilted off of the rotational axis (by 47 degrees), and also offset from the center, but even more so than Uranus: it is located one half of a radius from the center 3 : Neptune's very tenuous ring system, more similar to Jupiter's, than Saturn and Uranus: Neptune has one very large moon (of the thirteen known) named Triton , that is 2,700 km in diameter, and has a density of ρ = 2.06 gm/cm3: Triton is the only large moon in the solar system that orbits its planet "backwards" (retrograde). It also has a large inclination with respect to the equator of Neptune (157 degrees). Because of this, and the expectation that large moons form in the left over accretion disk that forms the parent planet (and thus form near the equator of the planet), it is believed that Triton is a captured "Kuiper belt" object (next class) like Pluto.

The combination of this orbital tilt, along with Neptune's own axial tilt (obliquity), Triton has seasons like those of Uranus, only longer! The internal structure of Triton: The capture of Triton probably disrupted the orbits of any small moons that were orbiting Neptune at the time--this explains the paucity of moons around Neptune. It also explains why Neptune's second biggest moon, Nereid (ρ = 1.5 gm/cm3), has the most eccentric orbit of any moon in the solar system (e = 0.75!), varying in distance from Neptune by a factor of 7! The rest of the moons are small chunks of rock (the largest of these has a mass of 0.0025 of Triton), several are associated with the rings of Neptune. Triton has a tenuous atmosphere that is mostly composed of nitrogen. A surprising discovery were these black plumes seen in the photo below that appear to be liquid nitrogen geysers. The plumes seem to be blowing in a wind from the point of origin: "Triton has a tenuous nitrogen atmosphere, with trace amounts of carbon monoxide and small amounts of methane near the surface.

Like Pluto's atmosphere (next class), the atmosphere of Triton is believed to have resulted from evaporation of nitrogen from the moon's surface. The surface temperature is at least 35.6 K (-237.6 C) because Triton's nitrogen ice is in the warmer, hexagonal crystalline state, and the phase transition between hexagonal and cubic nitrogen ice occurs at that temperature. An upper limit in the low 40s (K) can be set from vapor pressure equilibrium with nitrogen gas in Triton's atmosphere. This temperature range is colder than Pluto's average equilibrium temperature of 44 K (-229 C). Triton's surface atmospheric pressure is only about 0.017 millibar. Turbulence at Triton's surface creates a troposphere (a "weather region") rising to an altitude of 8 km. Streaks on Triton's surface left by geyser plumes suggest that the troposphere is driven by seasonal winds capable of moving material of over a micrometer in size.

Unlike other atmospheres, Triton's has no stratosphere, and instead consists of a thermosphere from 8 to 950 km above the surface, and an exosphere above that. The temperature of Triton's upper atmosphere, at 95 kelvins, is higher than the temperature at the surface due to heat deposited from space. A haze permeates most of Triton's troposphere, believed to be composed largely of hydrocarbons and nitriles created by the action of sunlight on methane. Triton's atmosphere also possesses clouds of condensed nitrogen that lie between 1 and 3 km from the surface." 3 Triton haze layer: Let's go back to that diagram for the triplepoint diagram for methane on Triton: Note that the phase diagram for nitrogen is also plotted here (in green). The surface temperature of Triton is 35 K. The observed atmospheric pressure is 0.017 millibar (= 1.7 x 10-5 bar).

Thus, you could predict that Triton would have a nitrogen atmosphere with a pressure close to 0.01 bar, as that is close to the equilibrium point between solid nitrogen and gaseous nitrogen at the surface temperature of Triton. A short movie on the Voyager spacecraft that toured the Jovian planets (launched in 1977), with Voyager 2 passing by Uranus in 1986, and Neptune in 1989. In the four or five years since this movie, the Voyagers have now left the solar system, and Voyager 1 is now officially in interstellar space ( current locations ). Voyager 2 is the only spacecraft that ever went by Uranus and Neptune, and nothing new is being planned or even talked about. Neptune Facts - Planet Neptune - Neptune For Kids Download Questions about Neptune (all answers found on this page) The discovery of the planet Neptune was one of the most exciting discoveries in astronomy. Neptune cannot be seen without a large telescope and was first seen in 1846 from the observatory in Berlin. BUT, the existence of Neptune had actually been “discovered” a year earlier, in 1845.

Neptune is the 8th planet from the Sun. Uranus , the 7th planet, was first discovered by means of a telescope in 1781. Two astronomers (scientists who study the stars and planets), J.C.Adams in England and Urbain Le Verrier in France, had been puzzled by the shape of Uranus’ orbit. They worked out, using mathematics, that some large, more distant, body must be pulling Uranus towards it at certain points in the orbit. The Berlin observatory, following Le Verrier’s calculations giving the possible position of this object, searched for Neptune and found the planet. They named it Neptune after the Roman God of the Sea . Figure 1: Photograph of Neptune taken from the Voyager spacecraft in 1989. Neptune is the third largest planet in the Solar System, much smaller than the real giants, Jupiter and Saturn , and only a little bigger than Uranus. Neptune has a diameter of 29,297 miles, or 47,150 kilometres. The Earth has a diameter of 7928 miles (12,760 kilometres). Neptune is one of the four “gas giants”.

Like Jupiter, Saturn and Uranus, it is composed only of gas. Neptune is a great ball of hydrogen and helium. Like all the other planets in the Solar System, Neptune moves in an orbit round the Sun at the centre of the system. It takes Neptune 165 of our Earth years to orbit the Sun . The Earth orbits the Sun in 365 days, one year. In 2011 Neptune completed the first orbit of the Sun since its discovery 165 years before in 1846. Like all the other planets, Neptune turns on its own axis as it is orbiting the sun. (Imagine walking round a large pond, but turning round and round as you go.) The Earth turns right round on its own axis in 24 hours, giving us the change from day, when we face the Sun, to night, when our part of the Earth turns away from the Sun. Neptune spins slightly faster on its axis, taking just over 19 hours to turn right round. The Earth is 150 million kilometres from the Sun (93 million miles) and this measurement is taken as the standard for astronomical measurements of distance.

Astronomers say that the Earth is one Astronomical Unit (1 AU) from the Sun. Neptune is unimaginably further. Neptune is 30.1 Astronomical Units from the Sun, a staggering 2793 million miles (4495 million kilometres) from the Sun, and 2700 million miles from the Earth. Figure 2 : Distances of Planets from the Sun in Astronomical Units Figure 1 does not show the distances between the planets to scale. Figure 2 shows the true distances of the planets from the Sun. In the same year that Neptune was first seen, 1846, its first moon was also spotted and named Triton. Triton is a most unusual moon since it orbits Neptune in the opposite direction of Neptune’s own rotation on its axis. All the other major satellites (moons) in the Solar System follow their planets round as they turn. Try orbiting the pond again, turning on your own axis, but this time take a friend to orbit round you.

If you are turning on your axis clockwise, have him go round you anticlockwise (be careful not to get so dizzy you fall in the pond!) Triton is about the same size as our own moon. A smaller moon, Nereid, was discovered by telescope in 1949 and six further moons were discovered in the 1980s by the Voyager spacecraft. All these moons are between Triton and Neptune. We now know that Neptune has 13 moons in total. Since Neptune was the God of the Sea, all the moons are named after less important ancient Greek sea gods, like Triton, or sea nymphs, like Nereid. Useful Websites Planet Saturn: Facts About Saturn’s Rings, Moons & Size Planet Saturn: Facts About Saturn’s Rings, Moons & Size By Charles Q. Choi, Space.com Contributor | November 17, 2014 10:25pm ET MORE Credit: NASA/JPL Saturn is the sixth planet from the sun and the second largest planet in the solar system. Saturn was the Roman name for Cronus, the lord of the Titans in Greek mythology.

Saturn is the root of the English word "Saturday." Saturn is the farthest planet from Earth visible to the naked human eye, but it is through a telescope that the planet's most outstanding features can be seen: Saturn's rings. Although the other gas giants in the solar system — Jupiter, Uranus and Neptune — also have rings, those of Saturn are without a doubt the most extraordinary. Physical characteristics of Saturn Saturn is a gas giant made up mostly of hydrogen and helium . Saturn is big enough to hold more than 760 Earths, and is more massive than any other planet except Jupiter, roughly 95 times Earth's mass. However, Saturn has the lowest density of all the planets, and is the only one less dense than water — if there were a bathtub big enough to hold it, Saturn would float. The yellow and gold bands seen in Saturn's atmosphere are the result of super-fast winds in the upper atmosphere, which can reach up to 1,100 mph (1,800 km/h) around its equator, combined with heat rising from the planet's interior. Saturn spins faster than any other planet except Jupiter, completing a rotation roughly every 10-and-a-half hours.

This rapid spinning causes Saturn to bulge at its equator and flatten at its poles — the planet is 8,000 miles (13,000 kilometers) wider at its equator than between the poles. Saturn's most recent curiosity may be the giant hexagon circling its north pole, with each of its sides nearly 7,500 miles (12,500 km) across — big enough to fit nearly four Earths inside. Thermal images show it reaches some 60 miles (100 km) down into the planet's atmosphere. It remains uncertain what causes it. Other titanic storms appear in Saturn’s atmosphere once every Saturn year (approximately 30 Earth-years), disrupting the temperature and winds of the planet’s skies. Six such storms have been observed on the planet since 1876, but in 2011, NASA’s Cassini spacecraft became the first orbiter to observe one. Like other giant planets, Saturn also has northern and southern lights , caused by particles from the sun.

Composition & structure Atmospheric composition (by volume): 96.3 percent molecular hydrogen, 3.25 percent helium, minor amounts of methane, ammonia, hydrogen deuteride, ethane, ammonia ice aerosols, water ice aerosols, ammonia hydrosulfide aerosols Magnetic field: Saturn has a magnetic field about 578 times more powerful than Earth's. Chemical composition: Saturn seems to have a hot solid inner core of iron and rocky material surrounded by an outer core probably composed of ammonia, methane, and water. Next is a layer of highly compressed, liquid metallic hydrogen, followed by a region of viscous hydrogen and helium. This hydrogen and helium becomes gaseous near the planet's surface and merges with its atmosphere. Internal structure: Saturn seems to have a core between about 10 to 20 times as massive as Earth. Orbit & rotation Average distance from the sun: 885,904,700 miles (1,426,725,400 km). By comparison: 9.53707 times that of Earth.

Perihelion (closest approach to sun): 838,519,000 miles (1,349,467,000 km). By comparison: 9.177 times that of Earth. Aphelion (farthest distance from sun): 934,530,000 miles (1,503,983,000 km). By comparison: 9.886 times that of Earth. Saturn's moons Saturn has at least 62 moons . Since the planet was named after Cronus, lord of the Titans in Greek mythology, most of Saturn's moons are named after other Titans, their descendants, as well as after giants from Gallic, Inuit and Norse myths. Saturn's largest moon, Titan , is slightly larger than Mercury, and is the second-largest moon in the solar system behind Jupiter's moon Ganymede. ( Earth's moon is the fifth largest.) Titan is veiled under a very thick, nitrogen-rich atmosphere that might be like what Earth's was long ago, before life.

While the Earth's atmosphere extends only about 37 miles (60 km) into space, Titan's reaches nearly 10 times as far. The atmosphere contains a number of hydrocarbons, chemicals that primarily make up fossil fuels on Earth. Methane rain falls from the sky and moves through the moon’s icy crust . A recent study detected propylene , a chemical used to make plastics, in the planet’s atmosphere. These moons can possess bizarre features. Pan and Atlas are shaped like flying saucers, Iapetus has one side as bright as snow and one side as dark as coal. Enceladus shows evidence of "ice volcanism," spewing out water and other chemicals from the 101 geysers spotted at the moon’s southern pole. A number of these satellites, such as Prometheus and Pandora, are shepherd moons , interacting with ring material to keep rings in their orbits. Though scientists have identified many moons, the chaotic system has other small moons constantly being created and destroyed . HubbleSite - Reference Desk - FAQs Reference Desk Frequently Asked Questions What is the solar system? The solar system consists of a central star, the Sun, and all of the smaller celestial bodies that continuously travel around it.

The smaller bodies include eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, which are themselves orbited by more than 140 moons. (Only Mercury and Venus have no moons.) In addition, the solar system contains millions of rocky asteroids and billions of icy comets. All of these objects are held together in a group by the Sun's gravity. How did the solar system form? The planets, asteroids and comets in the solar system are loose objects left over from the formation of the Sun. Originally the gas and dust that would become the Sun was the core of a cloud much larger than the solar system, probably several light-years across. (One light-year is equal to approximately 6,000,000,000,000 miles.) The core was slowly rotating at first, but as it collapsed it spun faster, like a spinning ice-skater pulling in his arms. The rotation prevented the material at the core's equator from collapsing as fast as material at the poles, so the core became a spinning disk. Gas and dust in the disc spiraled gradually in to the center, where it accumulated to form the Sun.

But because dust is denser than gas, some of the dust settled to the mid-plane of the disc. These dust particles stuck together to make clumps, then clumps stuck together to make rocks, then rocks collided to make planets. In the case of the "gas giant" planets, Jupiter, Saturn, Uranus and Neptune, the rocky cores were massive enough to also attract some of the gas. The outer layers of these planets are made up of hydrogen and other gases. So the Sun is the collapsed core of an interstellar gas cloud, and the planets, asteroids and comets are small lumps of dust or ice chunks which stayed in orbit instead of spiraling into the Sun. The planets all formed within a very short period, probably a few million years, about five billion years ago. How old is the solar system? The solar system is about 4.6 billion years old. How big is the solar system? There are no physical boundaries in space. The solar system consists of eight planets orbiting around one star: the Sun. Neptune, the farthest planet from the Sun, orbits approximately 30astronomical units from the Sun. An astronomical unit is a unit of length used by astronomers.

One astronomical unit equals the distance from Earth to the Sun: 93 million miles (149 million km). Some of the comets associated with the solar system travel on orbits that take them much farther from the Sun than Neptune. How many planets are there in our solar system? Our solar system officially has eight planets and one star: the Sun. In order from the Sun out, the planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The discovery of an object larger than Pluto in 2005 rekindled the debate over whether such objects, belonging to the "Kuiper Belt" – a collection of icy bodies located beyond Neptune – should be called planets. Pluto and other large members of the Kuiper Belt are now considered "dwarf planets." Can you see any planets with the "naked eye?" Mercury, Venus, Mars, Jupiter and Saturn can be seen with the unaided eye. That is how they were discovered by ancient civilizations. Uranus and Neptune were discovered using telescopes. Are there differences among the planets in our solar system? Planets come in different sizes, compositions and colors.

The four planets closer to the Sun are called "rocky" planets. They are small in size and similar to Earth in composition. They have no rings and only two of them (Earth and Mars) have moons. The four outer planets, also called "gas giants," are much larger than the rocky planets. They all have rings and have many moons. The gas giants are made up mostly of hydrogen, helium, frozen water, ammonia, methane and carbon monoxide. Which planets are called the "rocky" or "terrestrial" planets? Mercury, Venus, Earth and Mars are called "rocky" or "terrestrial" planets. They are similar to Earth in composition. Heat from the Sun evaporated lightweight elements like hydrogen and helium into interplanetary space. Mostly rock and metal was left in this zone and clumped together to form the inner rocky planets. Which planets are called the "gaseous" planets? Jupiter, Saturn, Uranus and Neptune are called the "gaseous" planets. Jupiter and Saturn contain the largest percentages of hydrogen and helium, while Uranus and Neptune contain the largest shares of ices – frozen water, ammonia, methane and carbon monoxide. Which planets have rings?

The four gas giants, Jupiter, Saturn, Uranus and Neptune, have rings. How thick are Saturn's rings? The Hubble telescope has captured snapshots of Saturn with its rings nearly edge-on to our view. Read more about it . Saturn's rings are incredibly thin. The main rings are generally only about 30 feet (10 meters) thick, though parts of the main and other rings can be several kilometers thick. The rings are made of dusty ice, in the form of boulder-sized and smaller chunks that gently collide with each other as they orbit around Saturn. Saturn's gravitational field constantly disrupts these ice chunks, keeping them spread out and preventing them from combining to form a moon. The rings have a slight pale reddish color due to the presence of organic material mixed with the water ice. Find out more from HubbleSite: Hubble discoveries about Saturn in NewsCenter . How close do Earth and Mars get? Mars and Earth are like two cars on a racetrack as they journey around the Sun. Earth is on the inside track and travels faster than Mars, which is on the outside.

When Earth laps Mars about every two years, it comes as close as 35 million miles (56 million km) or as far away as 63 million miles (101 million km) because of Mars’ highly elongated orbit. What is a comet? Comets are small, fragile, irregularly shaped bodies composed mostly of a mixture of water ice (ice composed of H20), dust, and carbon- and silicon-based compounds. They have highly elliptical orbits that repeatedly bring them very close to the Sun and then swing them into space. Comets have three distinct parts: a nucleus, a coma and a tail. The solid core is called the nucleus, which develops a coma with one or more tails when a comet sweeps close to the Sun. The coma is the dusty, fuzzy cloud around the nucleus of a comet, and the tail extends from the comet and points away from the Sun. The coma and tails of a comet appear only when the comet is near the Sun. Find out more from HubbleSite: Hubble discoveries about comets from NewsCenter . How did comets form? Comets are some of the material left over from the formation of the planets.

Our entire solar system, including comets, was created by the collapse of a giant, diffuse cloud of gas and dust about 4.6 billion years ago. Much of the matter merged into planets, but some remained to form small lumps of frozen gas and dust in the outer region of the solar system, where temperatures were cold enough to produce ice. Find out more from HubbleSite: Hubble discoveries about comets from NewsCenter . Why do comets have tails? Comets are lumps of frozen water ice, gas and dust. As a comet approaches the Sun, it starts to heat up. The ice transforms directly from a solid to a vapor, releasing the dust particles embedded inside. Sunlight and the stream of charged particles flowing from the Sun – the solar wind – sweeps the evaporated material and dust back in a long tail. The comet's ingredients determine the types and number of tails. What are the types of comet tails? There are two types of comet tails: dust and gas ion. A dust tail contains small, solid particles that are about the same size found in cigarette smoke. This tail forms because sunlight pushes on these small particles, gently shoving them away from the comet’s nucleus.

Because the pressure from sunlight is relatively weak, the dust particles end up forming a diffuse, curved tail. A gas ion tail forms when ultraviolet sunlight rips one or more electrons from gas atoms in the coma, making them into ions (a process called ionization). The solar wind then carries these ions straight outward away from the Sun. The resulting tail is straighter and narrower. Both types of tails may extend millions of kilometers into space. As a comet heads away from the Sun, its tail dissipates, its coma disappears, and the matter contained in its nucleus freezes into a rock-like material. Where do comets come from? Comets are found in two main regions of the solar system: the Kuiper Belt and the Oort Cloud. There are two types of comets: short-period comets and long-period comets. Short-period comets – comets that frequently return to the inner solar system – probably come from the Kuiper Belt beyond the orbit of Neptune. Astronomers estimate that this belt contains at least 200 million objects, which are thought to have remained essentially unchanged since the birth of the solar system 4.6 billion years ago.

Long-period comets, which can take thousands of years to complete their orbits, are thought to emanate from the Oort Cloud, a vast group of frozen bodies in the outer part of the solar system. The Oort Cloud is thought to extend 50,000 times the distance from the Earth to the Sun. Oort Cloud comets, like their Kuiper Belt brothers, probably originated in the region of the solar system between Jupiter and Neptune, but were ejected from to the Oort Cloud by close encounters with the gravity of the giant planets. Comets are kicked out of the Oort Cloud and the Kuiper Belt by the pull of the gravity of another object – a planet, a star, or another small body. They then begin their journey toward the inner solar system and the Sun. What path do comets follow through the solar system? Planets have nearly circular orbits, but comets have elongated paths around the Sun. A comet is at "aphelion" when its orbit is farthest from the Sun. It is at "perihelion" when it is closest to the Sun. Due to angular momentum, a comet will travel fastest at perihelion and will slow down as it approaches aphelion.

Comets can be classified by their orbital period: that is, the time it takes them to make one complete trip around the Sun. Comets with short and intermediate orbital periods of less than 200 years – like Comet Halley, whose orbital period is 76 years – spend most of their time between Pluto and the Sun. These comets originally formed in the Kuiper Belt, but a gravitational "push" from the planets, especially Jupiter, swung them closer to the Sun. A long-period comet will have an orbital period of more than 200 years. Comet Hale-Bopp, for example, completes an orbit about every 4,000 years. Scientists think that this type of comet spends most of its time way out in the Oort Cloud at the farthest edge of our solar system. What is the difference between a meteor, a meteoroid, a meteorite, an asteroid and a comet? Most of us probably have seen meteors or shooting stars. A meteor is the flash of light that we see in the night sky when a small chunk of interplanetary debris burns up as it passes through our atmosphere. "Meteor" refers to the flash of light caused by the debris, not the debris itself.

The debris is called a meteoroid. A meteoroid is a piece of interplanetary matter that is smaller than a kilometer and frequently only millimeters in size. Most meteoroids that enter the Earth's atmosphere are so small that they vaporize completely and never reach the planet's surface. If any part of a meteoroid survives the fall through the atmosphere and lands on Earth, it is called a meteorite. Although the vast majority of meteorites are very small, their size can range from about a fraction of a gram (the size of a pebble) to 100 kilograms (220 lbs) or more (the size of a huge, life-destroying boulder). Asteroids are generally larger chunks of rock that come from the asteroid belt located between the orbits of Mars and Jupiter. Comets are asteroid-like objects covered with ice, methane, ammonia, and other compounds that develop a fuzzy, cloud-like shell called a coma and sometimes a visible tail whenever they orbit close to the Sun. Find out more from HubbleSite: Hubble discoveries about asteroids , comets , and Kuiper Belt objects from NewsCenter . What is the asteroid belt? The asteroid belt is a zone between the orbits of Mars and Jupiter.

Scientists believe that the asteroids in the asteroid belt never formed a planet because the gravity of nearby Jupiter kept pulling them apart. Today, millions of asteroids probably inhabit the asteroid belt, with many more scattered throughout the solar system. Find out more from HubbleSite: Hubble discoveries about asteroids from NewsCenter . What is Planet X? When astronomer Percival Lowell decided to search for a planet beyond Neptune in the 1930s, he called the object of his search "Planet X." This search led to the discovery of Pluto, but for many years some astronomers believed that another world larger than Pluto must exist undiscovered beyond Neptune. They thought this because Neptune's orbit seemed to be influenced by the gravity of an unseen planet. More recent studies indicate that Neptune's orbit is not influenced by undiscovered bodies, and that a large "Planet X" most likely does not exist. Some of the myths and misconceptions about Planet X are debunked on Phil Plait's Bad Astronomy website: The prospects for new planet discoveries in our solar system are small. However, astronomers are now finding hundreds of planets orbiting other stars. Has Hubble looked for Planet X? Astronomers are always on the lookout for new members of the solar system.

The most active programs involve searches for new comets, asteroids and satellites of the outer planets, but a few scientists have undertaken an on-again, off-again search for planets beyond Neptune. Astronomers can rule out an object as big as Jupiter orbiting a little further out than Neptune, since its gravitational effect on Neptuneï¿½s orbit would be pretty obvious. It would also be easily visible because of its brightness. To go undetected, a planet similar in size and composition to the other outer planets would have to be very far from the Sun. We do know of many other objects orbiting the Sun that are similar to Pluto, which belongs to the Kuiper Belt – a field of icy, comet-like bodies located beyond Neptune. In 2005, an object likely larger than Pluto, named Eris, was located in the Kuiper Belt. Find out more on HubbleSite: Planets Planets Mercury | Venus | Earth | Mars | Jupiter | Saturn | Uranus | Neptune |* Pluto Traditional Definition Etymology: Middle English planete, from Old French, from Late Latin planeta, modification of Greek planEt-, planEs, literally, wanderer, from planasthai to wander.

any of the seven celestial bodies: Sun, Moon, Venus, Jupiter, Mars, Mercury, and Saturn that in ancient belief have motions of their own among the fixed stars. any of the large bodies that revolve around the Sun in the solar system. [see IAU resolutions below] a similar body associated with another star. EARTH -- usually used with "the". a celestial body held to influence the fate of human beings a person or thing of great importance : LUMINARY - plan�et�like /-"lIk/ adjective * The IAU [ International Astronomical Union ] officially designates planets within our solar system. STATUS February 2, 2006 "The IAU notes the very rapid pace of discovery of bodies within the Solar system over the last decade and so our understanding of the Trans-Neptunian Region is therefore still evolving very rapidly. This is in serious contrast to the situation when Pluto was discovered. As a consequence, The IAU has established a Working Group to consider the definition of a minimum size for a Planet.

Until the report of this Working Group is received, all objects discovered at a distance from the Sun greater than 40 AU will continue to be regarded as part of the Trans-Neptunian population." UPDATE August 24, 2006 RESOLUTION 5A The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way: (1) A planet1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. (2) A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. (3) All other objects3 orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. 2An IAU process will be established to assign borderline objects into either dwarf planet and other categories. 3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies. Relative sizes of the planets and stars by Dave Jarvis . Smallest Planets Neptune - Neptune's moons and rings | planet | Britannica.com Earth Of Voyager’s six discoveries, all but Proteus orbit Neptune in less time than it takes the planet to rotate. Hence, to an observer positioned near Neptune’s cloud tops, these five would appear to rise in the west and set in the east. Voyager observed two of its discoveries, Proteus and Larissa, closely enough to detect both their size and approximate shape. Both bodies are irregular in shape and appear to have heavily cratered surfaces. The sizes of the other four are estimated from a combination of distant images and their brightnesses, based on the assumption that they reflect about as much light as Proteus and Larissa—about 7 percent.

Proteus, with a mean radius of about 208 km (129 miles), is a little larger than Nereid , with a mean radius of about 170 km (106 miles). Because Proteus is so near to Neptune, however, scientists on Earth had been unable to detect it in the planet’s glare. The other five moons are much smaller, each having a mean radius of less than 100 km (60 miles). Voyager did not observe Nereid at close range, but data from the probe indicate that it has a nearly spherical shape. Voyager detected no large variations in brightness as Nereid rotated. Although the spacecraft was unable to determine a rotation period, the moon’s highly elliptical orbit makes it unlikely that it is in synchronous rotation—i.e., that its rotation and orbital periods are equal. The rotation period of Triton is synchronous, and those of Neptune’s other inner moons are probably synchronous or very nearly so. Planets: Fact or Fiction? Triton is similar in size, density, and surface composition to the dwarf planet Pluto .

Its highly inclined, retrograde orbit suggests that it is a captured object, which perhaps formed originally, like Pluto, as an independent icy planetesimal in the outer solar system. Its original orbit would have been highly eccentric, but tidal interactions between Triton and Neptune—cyclic deformations in each body caused by the gravitational attraction of the other—eventually would have reshaped its path around Neptune into a circle. The process of Triton’s capture and circularization of its orbit would have severely disrupted any previously existing system of moons that had formed along with Neptune from a disk of protoplanetary material. Nereid’s radical orbit may be one consequence of this process (although the possibility that Nereid too is a captured object has not been ruled out). Moons that were in orbit between Proteus and Nereid would have been ejected from the Neptunian system, thrown into Neptune itself, or absorbed by the molten Triton. Even those moons orbiting closer to Neptune would not have escaped some disruption. The present orbits of Naiad through Proteus are probably very different from their original orbits, and these moons may be only fragments of the original bodies that formed with Neptune.

Subsequent bombardment by Neptune-orbiting debris and by meteoroids from interplanetary space may have further altered their sizes, shapes, and orbits. The ring system Ringling Bros. Folds Its Tent Evidence that Neptune has one or more rings arose in the mid-1980s when stellar occultation studies from Earth occasionally showed a brief dip in the star’s brightness just before or after the planet passed in front of it. Because dips were seen only in some studies and never symmetrically on both sides of the planet, scientists concluded that any rings present do not completely encircle Neptune but instead have the form of partial rings, or ring arcs. Neptune’s ring system, captured by Voyager 2 in two long-exposure backlit images made a few hours … Jet Propulsion Laboratory/National Aeronautics and Space Administration Images from Voyager 2, however, revealed a system of six rings, each of which in fact fully surrounds Neptune. The putative arcs turned out to be bright regions in the outermost ring, named Adams, where the density of ring particles is particularly high. Although rings also encircle each of the other three giant planets, none displays the striking clumpiness of Adams.

The arcs are found within a 45° segment of the ring. From leading to trailing, the most prominent are named Courage, Liberté, Egalité 1, Egalité 2, and Fraternité. They range in length from about 1,000 km (600 miles) to more than 10,000 km (6,000 miles). Although the moon Galatea, which orbits just planetward of the inner edge of Adams, may gravitationally interact with the ring to trap ring particles temporarily in such arclike regions, collisions between ring particles should eventually spread the constituent material relatively uniformly around the ring. Consequently, it is suspected that the event that supplied the material for Adams’s enigmatic arcs—perhaps the breakup of a small moon—occurred within the past few thousand years. Connect with Britannica Facebook Twitter YouTube Instagram Pinterest The other five known rings of Neptune— Galle, Le Verrier, Lassell, Arago, and Galatea, in order of increasing distance from the planet—lack the nonuniformity in density exhibited by Adams.

Le Verrier, which is about 110 km (70 miles) in radial width, closely resembles the nonarc regions of Adams. Similar to the relationship between the moon Galatea and the ring Adams, the moon Despina orbits Neptune just planetward of the ring Le Verrier. Each moon may gravitationally repel particles near the inner edge of its respective ring, acting as a shepherd moon to keep ring material from spreading inward. (For fuller treatments of shepherding effects, see Saturn: Moons: Orbital and rotational dynamics ; Uranus: The ring system .) Galle, the innermost ring, is much broader and fainter than either Adams or Le Verrier, possibly owing to the absence of a nearby moon that could provide a strong shepherding effect. Lassell consists of a faint plateau of ring material that extends outward from Le Verrier about halfway to Adams. Arago is the name used to distinguish a narrow, relatively bright region at the outer edge of Lassell. Galatea is the name generally used to refer to a faint ring of material spread all along the orbit of the moon Galatea. Rings of Neptune

Question: Which planet (in terms of its orbit) is between Saturn and Neptune?
Answer: Uranus