There are eight planets in the solar system. They rotate around the Sun in the following order, starting from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. In addition to the full size planets there are 5 dwarf planets that also make their way around the Sun: Ceres, Pluto, Haumea, Makemake, Eris.

All of the major planets orbit the Sun in one direction. The orbits of all major planets are ellipses that are very close to the circles, and the planes of their orbits are inclined to the ecliptic plane at small angles. The masses of all the planets put together, make up only 0.0013 of the Sun. In addition to these planets, a large number of smaller bodies, called minor planets or asteroids, is moving mostly between Mars and Jupiter. The number of known asteroids is constantly growing as a result of new discoveries, and currently there are more than 1,600 different bodies known to the scientists.

Planets in the solar system can be easily divided into two groups based on their size. The first group consists of relatively small planets that are closest to the Sun: Mercury, Venus, Earth and Mars. This group is commonly known as the Earth group. The second group consists of the largest planets in the Solar System: Jupiter, Saturn, Uranus and Neptune. This group is also known as the Jupiter group. The asteroid belt separates these groups. Pluto stands alone in this grouping, as it is insufficiently explored.

Looking at the physical characteristics of the major planets, we can easily divide them into two groups. For example, the average density of planets in the first group is 4.5 g/cm3 and an average density of planets in the Jupiter group is 1.21 g/cm3. Judging by the density, we can say that the planets in the Earth group are solid. The densities of the planets in the Jupiter group, however, are similar to the density of the Sun and cannot be considered solid.

There is quite a simple explanation for the strong differences between the four giant planets, and four terrestrial planets.  Pluto is an exception to this scheme; however it looks more like one of the moons of distant planets. Hydrogen-rich planets consist of relatively little altered original substance from which the Solar System was formed. Solid planets of the Earth group on the other hand have lost much of the light gases that form the basis of this substance. Instead of primary atmosphere captured from the gas-dust cloud, from which the Solar System was formed, they have secondary atmospheres that emerged after the planets were formed.

Comparing the rotation periods, we, once again, can notice a big difference between the two groups. Terrestrial planets take more than a day to revolve around their axis – they rotate slowly. Planets of the Jupiter group revolve around their axis much faster – on average the rotation period is less than half a day. Huge Jupiter revolves in only 9h 50m – it is clear that its linear velocity at the equator will be much larger than the corresponding rate of rotation of the Earth (28 times larger). Because of this, planets in the Jupiter group have a higher rate of compression, reaching 0.1 on Saturn, and the planets in the Earth group are almost perfect spheres.

The number of satellites of the planets of both groups is also quite different. The entire first group has only three satellites; the second group has more than 160 moons, and this number is constantly increased by new discoveries.

Thermonuclear reactions started on the Sun around 4.6 billion years ago. Because of this, temperatures of the Solar System planets, especially the interior ones noticeably increased. There were two crucial factors that determined the future look of the planets – their size and distance from the Sun. Small size planets were unable to keep the lightest gases – hydrogen and helium. Planet’s ability to retain hydrogen and helium was the decisive factor during the formation of the Solar System. Another determining factor was the distance from the Sun. The fact is that the closer the planet is to the Sun, the more it is heated, and the more difficult it is to retain light gases on its surface. The combined effect of these factors had a significant influence on the formation of planets in the Solar System and divided them into the two main groups.

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Neptune is the eighth and is the farthest of the gas giants in our Solar System. Planet Neptune was named after the Roman god of the sea. It is the fourth-largest planet by diameter and the third-largest by mass. Neptune is very big and has an equatorial diameter of 49,500 kilometers (30,760 miles). It is that big that if Neptune was hollow, it could contain nearly 60 Earths. Neptune’s mass is seventeen times the mass of Earth, however just 1/19th that of Jupiter. Only Jupiter exceeds Neptune’s surface gravity, making these two gas giants the only planets in the Solar System that have higher surface gravity than Earth. Neptune’s equatorial radius is 24,764 km, which is almost four times that of the Earth.

Neptune was discovered on September 23, 1846. It was the first planet found by mathematical prediction and not by empirical observation. Surprising changes in the orbit of Uranus showed the way to Alexis Bouvard to assume that its orbit was subject to gravitational perturbation by an unidentified planet. Neptune was consequently observed by Johann Galle within a degree of the position calculated by Urbain Le Verrier. Neptune’s largest moon, Triton, was discovered soon thereafter, although none of the planet’s remaining 12 moons were sited telescopically until the 20th century.

Visibility of Neptune

Neptune is never observable to the naked eye. It can be seen through a telescope or strong binoculars, which will show Neptune as a small blue disk. Neptune’s appearance is similar to Uranus. Since Neptune is so far away from Earth, the angular diameter of the planet as visible from Earth is only 2.2–2.4 arc seconds. Because of its small size, it is a very challenging job to study it visually. Most telescopic data was fairly restricted until the arrival of Hubble Space Telescope and large ground-based telescopes with adaptive optics. In the radio frequency band, observation of Neptune shows that the planet is a source of both nonstop emission and irregular bursts. Both sources are likely to originate from the planet’s rotating magnetic field. In the infrared part of the spectrum, Neptune’s storms appear bright against the cooler surroundings, permitting the size and shape of these storms to be readily tracked.

Neptune’s Composition and Atmosphere

Neptune and Uranus are similar in make up and both have compositions different from those of the larger gas giants Jupiter and Saturn. Neptune’s atmosphere is however similar to Jupiter’s and Saturn’s. It is composed primarily of hydrogen and helium, along with traces of hydrocarbons and potentially nitrogen. It contains a higher proportion of “ices” such as water, ammonia and methane.

The interior of Neptune, similar to Uranus, is mainly composed of ices and rock. The first two thirds of Neptune consist of a mixture of molten rock, water, liquid ammonia and methane. The outer third is a combination of heated gases comprised of hydrogen, helium, water and methane. Traces of methane in the outermost regions result in planet’s blue appearance.

Neptune’s Great Dark Spot

Like a typical gas planet, Neptune has rapid winds restricted to bands of latitude and large storms or vortices. Neptune’s winds are the strongest and fastest in the solar system, reaching up to 2000 km/hour. Neptune is a dynamic planet with a number of large, dark spots. The largest spot, called the Great Dark Spot, is almost the size of the earth and is similar to the Great Red Spot on Jupiter. Voyager discovered a small, irregularly shaped, eastward-moving cloud scooting around Neptune every 16 hours or so. This scooter as it has been called could be a plume rising over a deeper cloud deck.

Neptune’s Rings

Neptune has a set of four rings, though one much less extensive than those of Saturn. Neptune rings are narrow and very faint, and are made up of dust particles considered to have been made by tiny meteorites colliding into Neptune’s moons. From ground based telescopes the rings looked like arcs but from Voyager 2 the arcs were found to be the bright spots or clumps in the ring system. The real cause of the bright clumps is unknown. The outermost ring is called Adams (which includes three prominent arcs now named Liberty, Equality and Fraternity), after that is an unnamed ring co-orbital with Galatea, followed by Leverrier (whose outer extensions are called Lassell and Arago), and lastly the faint but broad Galle.

Magnetic Field of Neptune

Just like Uranus, the magnetic field of Neptune is highly tilted at 47 degrees from the rotation axis and offset at least 0.55 radii, or about 13,500 km from the planet’s physical centre. Evaluating the magnetic fields of the two planets, scientists assume the extreme orientation may be feature of flows in the interior of the planet and not the result of that planet’s sideways orientation or of any probable field reversals at either planet.

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Uranus is the seventh planet from the Sun and is the third in the series of four gas giants. Uranus has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus revolves outside the orbit of Saturn and inside the orbit of Neptune. British astronomer Sir William Herschel unintentionally discovered Uranus in 1781. It had, in fact, been seen many times earlier, but was ignored as simply another star. It is visible to the naked eye like the other five classical planets, but was never recognized as a planet by ancient spectators because of its dimness and slow orbit. It was also the first planet discovered with a telescope.

Atmosphere and Temperatures of Uranus

Planet Uranus is composed mainly of rock and a variety of ices, having only about 15% hydrogen and a little helium. Uranus’s rocky core is enveloped in a mantle of gases and ices. Planet’s atmosphere contains methane, which gives the planet its characteristic color. Uranus looks blue because red light is absorbed by methane in the upper atmosphere. There may be colored bands similar to Jupiter’s, however they are out of sight because of the overlaying methane layer.

Uranus is located in the cold outer reaches of the Solar System and its cloud tops have very low temperature of about -210° C (-346°F). It is the planet with the coldest planetary atmosphere in the Solar System. Uranus has a complex, layered cloud structure, with water considered to make up the lowest clouds and methane makes up the uppermost layer of clouds.

On the contrary the interior of Uranus is mostly composed of ices and rock. Observations show that Uranus does not have a rocky core like Jupiter and Saturn, however unlike those planets its mass is more or less evenly distributed. Uranus’ atmosphere has about 83% hydrogen, 15% helium and 2% methane.

Orbit and Rotation of Uranus

It takes Uranus 84 Earth years to complete one rotation around the Sun. The planet needs 17hr 15min to rotate around its axis, which is inclined 98° to the plane of the planet’s orbit around the Sun. The rotational period of the interior of Uranus is 17 hours, 14 minutes. On the other hand, as on all giant planets, its upper atmosphere experiences extremely strong winds in the direction of rotation. At some latitudes, for example about two-thirds of the way from the equator to the south pole, atmosphere can move much faster, making a full rotation in as little as 14 hours.

Magnetic Field

Voyager’s observations discovered that the magnetic field of Uranus is quiet peculiar. The reason behind this is that it does not originate from the planet’s geometric center and it is tilted at 59° from the axis of rotation. Actually the magnetic dipole is moved from the center of the planet towards the south rotational pole by almost one third of the planetary radius. This strange geometry results in a extremely asymmetric magnetosphere, in which the magnetic field strength on the surface in the southern hemisphere can be as low as 0.1 gauss (10 µT), while in the northern hemisphere it can be as high as 1.1 gauss (110 µT) and the usual field at the surface is 0.23 gauss (23 µT).

Rings

Similarly to other gas giants, Uranus has rings, a magnetosphere, and numerous moons. In 1977 American astronomer James L. Elliot discovered the presence of five rings around the equator of Uranus.  Starting from the innermost ring, these rings were named as Alpha, Beta, Gamma, Delta, and Epsilon. These form a 9400 km wide belt extending to 51,300 km from the planet’s centre. In January 1986, four more rings were discovered during the exploratory flight of Voyager 2.

Like Jupiter’s rings, they are very dark, however similarly to Saturn’s rings they are composed of rather large particles up to 10 meters in diameter. All of Uranus’ rings are very faint – the brightest is known as the Epsilon ring.

Moons of Uranus

Uranus has 27 moons.  Unlike other moons in the solar system, which have their names mainly from the classical mythology, Uranus’ moons got their names from the writings of Shakespeare and Pope. Most of Uranus’ moons have almost circular orbits, however the outer 4 are much more elliptical. At least 21 of Uranus moons orbit its equator in the east-west rotation. Oberon and Titania are the largest moons and were discovered by Herschel in 1787. The next two, Umbriel and Ariel, were discovered in 1851 by the British astronomer William Lassell.  Cordelia is the closest of Uranus’ ten ‘inner moons’.

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Pluto is a planet which usually orbits further than the orbit of Neptune. It is very small – smaller than any of the ‘official’ planets and is currently classified as a “dwarf planet”. Pluto is even smaller than seven of the solar system’s moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton). American astronomer Pervical Lowell was the man who first got the hints of Pluto’s existence in 1905 from odd deviations he observed in the orbits of Neptune and Uranus, signifying that an additional world’s gravity was tugging at them from outside. He estimated its location in 1915, however died without finding it. Planet Pluto’s discovery came in 1930 from Clyde Tombaugh at the Lowell Observatory, derived from predictions by Lowell and other astronomers.

Pluto was the single planet to be named by a kid. Pluto was named by an 11-year-old girl, Venetia Burney of Oxford, England, who suggested to her grandfather that this new planet needed to be named after the Roman god of the underworld. Her grandfather then suggested the name on to Lowell Observatory. The name Pluto also tributes Percival Lowell, whose initials are the first two letters of Pluto.

Pluto is about two-thirds smaller than Earth’s moon. Since it is too small, many scientists don’t consider it a planet at all. In 1999, a group of scientists tried to re-classify Pluto as a comet. However, on August 24, 2006, Pluto’s status was officially altered from planet to dwarf planet.

Pluto has not yet been visited by a spacecraft. Hubble Space Telescope was able to resolve only the largest features on its surface for scientists to study. Very little is known about Pluto and its moons as it is so far away. A spacecraft known as New Horizons was launched in January 2006 and if all goes right way it should reach Pluto in 2015.

Pluto’s Moons

Pluto has 3 moons. In 1978, astronomers discovered Pluto had a very large moon which is almost half its size called Charon, which named for the mythological demon who ferried souls to the underworld in Greek mythology. The huge size of satellite Charon sometimes leads scientists to mention Pluto and Charon as a double dwarf planet or binary system. These two are just 12,200 miles away from each other. Charon takes 6.4 Earth days to orbit around Pluto and one Pluto rotation, or a Pluto day, also takes 6.4 Earth days. Due to this Charon hovers over the same spot on Pluto’s surface and the same side of Charon forever faces the Planet Pluto. This phenomenon is also known as tidal locking.

In 2005, scientists discovered two other small moons of Pluto, and called them Nix and Hydra. These are two to three times farther away from Pluto than Charon, and are considered to be just 31 to 62 miles wide.

Formation of Pluto

The theory behind the formation of Pluto and Charon assumes that a nascent Pluto was hit with a glancing blow by another Pluto-sized object. The majority of the combined matter became Pluto, whereas the rest spun off to develop into Charon.

Pluto’s Atmosphere

Pluto’s atmosphere comprises of a thin layer of nitrogen, methane and carbon monoxide gases. Pluto’s elongated orbit is expected to have a most important effect on its atmosphere. When Pluto is away from the Sun, its atmosphere should gradually freeze out and fall to the ground. When Pluto is nearer to the Sun, the temperature of Pluto’s solid surface raises, causing the ices to sublimate into gas. Scientists using the Submillimeter Array have discovered that Pluto’s temperature is roughly about 43 K (−230 °C). In October 2006, scientists declared the spectroscopic discovery of ethane on Pluto’s surface. This ethane is formed from the photolysis of frozen methane on Pluto’s surface and suspended in its atmosphere.

Pluto’s Orbit and Rotation

Pluto’s orbital period is 248 Earth years. Unlike rest of planets, Pluto revolves around the Sun in an orbit which is not circular but is an oval or egg shape. Due to this, Pluto will orbit within of Neptune’s orbit. At times because of this Pluto comes nearer to the Sun than Neptune. In addition, Pluto’s orbit does not lie flat in the same plane like the rest of the planets. Pluto’s orbit is tilted as a result it orbits above and below the other planets’ orbits in our Solar System.

Pluto is very small; and because of its small size it is very sensitive to immeasurably small details of the solar system. Therefore it is really difficult to forecast factors that will gradually disrupt its orbit.  Pluto’s orbit itself is stable, but its position on that orbit is not stable and hard to predict. Pluto’s orbit is kept stable and safe from planetary collision or scattering by several resonances and other dynamical effects.

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