THE SOLAR SYSTEM

The Solar System consists of the Sun and other bodies revolving around it. The other bodies include the planets and their satellites, comets, asteroids and meteoroids. There are eight known planets and they can be grouped into two — four rocky planets near the Sun and four gas giants away from the Sun. The small inner planets are Mercury, Venus, Earth and Mars. The outer gas giants are Jupiter, Saturn, Uranus and Neptune. Since 1930, Pluto was considered as the ninth planet. But, on August 24, 2006 the Central Assembly of the International Astronomical Union demoted Pluto from the status of a planet by calling it a 'dwarf planet’. All the planets and their satellites revolve round the Sun in elliptical orbits in anti-clockwise direction. The Sun's gravity pulls the planets inwards while the energy of the planet flings it into space. The equal balance of these two forces enables the planets to revolve round the Sun in a definite path.

Planetary Orbit

Planet - The word planet is derived from the Greek word Planetes which means wanderer. The planets mow around the Sun in elliptical orbits. They cannot produce heat or light by themselves. They receive heat and light from the Sun.

Terrestrial planets - Planets like the Earth are called terrestrial planets. They are Mercury, Venus and Mars. Like the Earth they consist of rock and metals. They also have solid surface like the Earth.

Jovian planets - Planets which resemble Jupiter are called Jovian planets. They are Saturn, Uranus and Neptune. They contain hydrogen and helium as that of Jupiter. These four planets are together called gas giants.

Sedna - Scientists of “The Institute of Technology” at California discovered a new spherical body in the Solar System. It is 1700 kilometres in diameter. It revolves round the Sun at a distance of 1300 billion kilometres through a definite path. This celestial sphere has been named `2003V.B.12' and called Sedna.

THE SUN

The Sun is a huge burning ball of gas at the centre of the Solar System. It is the energy giving star to the whole Solar System. The Sun is about 1.4 million kilometers in diameter and 109 times bigger than the Earth. It is a yellow star of about five billion years old, consisting entirely of hydrogen and helium. The visible surface of the Sun called photosphere has a temperature of about 5,500°C. Dark, cool patches caused by magnetic fields which appear on the photosphere are called Sunspots. Other short-lived activity on the surface of the Sun is flares and prominences. Sudden release of high energy radiation and atomic particles are flares, and huge glowing hydrogen clouds are prominences. The inner atmosphere of the Sun is called chromosphere. The outermost surface which covers the Sun with a thin cloud of gas is the corona. Tiny particles that drop out from the corona cause the solar wind. The corona and chromospheres can be seen only during total solar eclipse.

Solar Energy

Solar energy is emitted through a process called nuclear fusion. The four hydrogen nuclei in the Sun join to form a helium nucleus. This process converts a small amount of matter into energy. A large quantity of energy is emitted during nuclear fusion. This energy is liberated as heat, light and other radiations.

Figures of Sun

• Diameter: About 1,392,000 kilometres (109 times that of the earth)
• Mass: 332,946 times that of the earth
• Volume: About 1,300,000 times that of the earth
• Temperature: Surface about 5500°C, Centre about 15,000,000°C.
• Rotation period: 25.38 days at equator
• Age: 4.6 billion years
• Average distance from the earth to the sun: About 150 million kilometres
• Time required by sunlight to reach the earth: About 8 minutes and 20 seconds

THE MERCURY

Mercury is a small terrestrial planet nearest to the Sun. It was named after Hermes, the swift messenger of the Greek gods who is in the same status with the Roman God Mercury. Mercury is the fastest moving planet at an average speed of about 48 kilometres per second and it completes one revolution in 87.97 earth days. But it rotates on its axis very slowly by taking nearly 59 earth days to complete one rotation. The inner core of Mercury contains iron which covers 3,600 kilometres in diameter. The outer mantle of silicate rock is about 600 kilometres thick. The outermost surface is a crust of silicate rock pitted with craters due to the fall of meteorites. The largest crater, Caloris Basin, measures about 1,300 kilometres across. The craters may have been formed when a huge rock of extra large size hit the planet and due to the force mountains are formed around. The cooling and shrinking of the hot core of the planet for billions of years caused many ridges called rupes around the surface of the planet. The surface temperature in Mercury is extreme ranging from —173°C to 427°C. By night the temperature falls down quickly to icy cold. Mercury contains very small quantity of helium and hydrogen and traces of other gases. Due to the extremity in surface temperature and lack of oxygen existence of life in Mercury is impossible.

Figures of Mercury

• Distance from the sun in km: 57.9 million
• Diameter at equator in km: 4878
• Mass with respect to the earth: 0.056 (earth= 1)
• Gravity with respect to the earth: 0.386
• Period of revolution: 87.97 earth days
• Period of rotation: 59 earth days
• Density: 5.42 g/cm3
• Temperature: - 173°C to 427°C
• Satellites: Nil

Spacecrafts to Mercury

Mariner-10 was the first spacecraft that landed on Mercury. It was launched by the United States on March 29, I974. Mariner-10 sent clear and close photographs of Mercury to the Earth. It also found that the magnetic field of Mercury is I/10th when compared to our planet. Some scientists believe that this discovery reveals the existence of iron in Mercury.

MESSENGER was the second and last NASA robotic spacecraft that orbited Mercury in between 2011 and 2015. It studied the geological history, chemical composition and magnetic field of the planet. It was launched on August 3, 2004. It was deorbited and destroyed in April 30, 2015.

THE VENUS

Venus is the second terrestrial planet from the Sun. It is the brightest object after the Sun and Moon because the atmospheric covering reflects sunlight. Its brightness in appearance earned the name Venus, the name of the Roman goddess of love and beauty. The period of rotation of Venus is the longest in the Solar System. Unlike other planets (except Uranus) it spins on its axis in the opposite direction of its revolution and takes about 243 earth days to complete one rotation whereas it takes only 225 earth days to revolve round the Sun. Like our Earth, Venus consists of mountains, volcanoes, craters and highland regions. The chief component of the atmosphere is carbon dioxide along with small quantities of carbon monoxide, water vapor, nitrogen etc. Thick clouds of sulphuric acid envelopes the surface of Venus and sometimes fall as rain. It is the hottest planet in the Solar System having a maximum surface temperature of about 480°C. Tin, lead and zinc melt in this atmosphere. The atmospheric pressure is 90 times higher than that of the Earth. The inner semi solid core contains iron and nickel, and has a diameter of 6,000 kilometres. The rocky mantle has a thickness of 3,000 kilometres and the silicate crust is about 50 kilometres thick. Aphrodite Terra and Ishtar Terra are the two highland regions found in Venus.

Volcanoes on Venus

There are numerous huge volcanoes on Venus ranging from one to 240 kilometers in diameter. Maat Mons having a height of about 9 kilometers is the most known volcano in Venus. Sometimes volcanic eruptions occur in many volcanoes at the same time in Venus covering the whole planet with lava.

Spacecrafts to Venus

Venera-2 and Venera-3, two Soviet spacecrafts discovered Venus in 1966. Venera-7 landed on this planet in 1970. On February 5, 1974, the US spacecraft Mariner-10 sent close-up photographs of Venus to Earth. Venera-9 and Venera-10 of Soviet Russia landed on Venus in 1975. In December 1978 Pioneer Venus- 1 and Pioneer Venus-2 reached Venus and measured the atmospheric density and chemical composition.

Figures of Venus

• Distance from the sun in km : 108.2 million
• Diameter at equator in km : 12104
• Mass with respect to the earth: 0.815 (earth= 1)
• Gravity with respect to the earth: 0.879
• Period of revolution: 225 earth days
• Period of rotation: 243 earth days
• Temperature: 480°C
• Satellites: Nil

THE EARTH

The Earth is the only planet known to have existence of life in it. It is the third planet from the Sun and the largest rocky planet. Mainly the Earth has four layers- The inner core, the outer core, the mantle and the crust. The inner core contains iron and nickel and has a temperature of 4,000°C. The iron and nickel are molten in the outer core. The third layer is the mantle which consists mostly of solid silicate material of iron and magnesium. The outermost layer is the crust of silicate rock called the lithosphere. The crust contains large quantity of different kinds of elements. Earth's atmosphere contains a mixture of many gases like nitrogen, oxygen, carbon dioxide, helium, ozone etc. The atmosphere helps to filter out most of the deadly rays from the Sun, traps enough warmth to prevent extreme cold and stops meteorites from entering Earth's surface. The Earth takes 23.56 hours to rotate once on its axis. It takes 365.26 days to complete one revolution. The only satellite of Earth is the Moon which is as big as the Earth.

Day and Night

The Earth rotates on its axis while orbiting around the Sun. When it rotates, one side faces the Sun and the other side is away from the Sun. The side towards the Sun gets direct sunlight and it is the day. The other side is darkened due to the lack of sunlight and it is the night. Gradually the portion getting sunlight moves to darkness and the dark portion moves to sunlight. Thus the rotation of the Earth on its axis causes day and night.

Blue Planet Earth is a hydrosphere. About 70% of the total surface of Earth is covered with water. Earth is the only planet which has water in it in liquid form, and so called a watery planet. The distance between the Earth and the Sun is suitable to keep the water in liquid form. When viewed from space, the Earth seems as a cloudy blue colored sphere. So the Earth is known as blue planet.

Seasons

The Earth revolves round the Sun while spinning on its axis. The axis tilts 23.4° from the vertical Seasons occur due to the tilt and revolution of the Earth When the northern part of the Earth faces the Sun it has summer. At the same time the southern part is away from the Sun and it has winter. In a period of six months the southern part comes towards the Sun and gets summer, and the northern part that goes far away from the Sun gets winter. Thus the seasons change all through the year.

Geoid

The shape of the Earth is not exactly like a sphere. It is bulged at the centre and flattened at the poles. The speed of rotation is lower at the poles than at the equator. Because of the irregularity in shape the Earth is called a geoid t by the scientists.

Figures of Earth

• Distance from the sun in km: 149.6 million
• Diameter at equator in km: 12,756.32
• Diameter at the poles in km: 12,713.54
• Period of revolution: 365.26 days
• Period of rotation: 23 hours and 56 minutes
• Temperature: -89.6°C to 58t
• Age: About 4.6 billion years
• Satellites: One (the Moon)

THE MOON

The Moon is the only natural satellite of the Earth. It takes the same 27.3 days to revolve round the Earth as well as to rotate on its axis. The Moon is a non-luminous body and it reflects the sunlight. The Moon mainly contains solid rock. It is dry and lifeless with no atmosphere or water. The surface is covered with dust, and large craters can be seen filled with solidified lava forming dark regions called maria or seas.

Origin of the Moon

Scientists put forth some theories about the origin of the Moon as follows:

The collision theory: This theory states that an external body in space hit the Earth, and a piece of solid material from Earth's mantle got separated and it orbited the Earth. This material gradually became the Moon.

The capture theory: This theory says that once the Moon was a planet revolving around the Sun. The gravity of the Earth caught it, and it became the satellite of the Earth.

The third moon formation theory: It states that the Earth and the Moon was formed at the same time in space as neighboring bodies. These two bodies were formed from the gas and dust particles left behind after the formation of the Sun.

The escape theory: This theory says that once the Earth and the Moon were a single body. The gravity of the Sun formed a bulge on the Earth and later this bulge detached from the Earth to form the Moon.

Phases of Moon

The Moon is a non-luminous body. Only the light of the Sun reflects on the Moon. The shape of the Moon is seen changed while it revolves round the Earth. This change occurs due to the changes in position with respect to the Earth and the Sun. When the position of the Moon comes in between the Earth and the Sun the side facing the Earth will be dark and it is called a New Moon. When the position of Earth comes in between the Sun and the Moon, the side of the Moon towards the Earth is seen fully illuminated and it is called a Full Moon. As the Moon travels from New Moon to Full Moon the illumination gradually increases and it is the Waxing of the Moon. When it moves from Full Moon to New Moon the lighted portion gradually decreases and it is the Waning of the Moon. When the Moon seems larger than half of a Full Moon it is called Gibbous and when it seems smaller than half of a Full Moon it is called a Cresent. The Moon changes its shape step by step in about a month's time.

Figures of Moon

• Distance from the earth in km.: 384403
• Diameter in km.: 3476
• Mass with respect to the earth: 1/81
• Gravity with respect to the earth: 1/6
• Volume with respect to the earth: 1/50
• Period of revolution: 27 days 7 hours 43 minutes
• Period of rotation: 27 days 7 hours 43 minutes
• Surface area: 38,000,000 sq.km
• Age: 460 crore years
• Speed of revolution around the earth (km/hr.): 3700

Eclipses

The Earth and the Moon are revolving constantly around the Sun by rotating on their own axis. And so sometimes the Sun, the Earth and the Moon fall in a straight line. This is called an Eclipse. When the Earth comes in between the Sun and the Moon, it blocks the rays of the Sun from reaching the Moon and the shadow of the Earth falls over the Moon. This is known as Lunar Eclipse. When the Moon comes in between the Sun and the Earth it blocks the Sun's rays from falling to the Earth. The Moon covers the Sun. The Sun and the sky seems to be dark for a few minutes. This is Solar Eclipse. It lasts only upto 7 minutes and 30 seconds.

THE MARS

The fourth and the outermost rocky planet from the Sun, Mars is also called the red planet. Mars' surface contains abundant quantity of iron. The dampness and warmth in the atmosphere caused the rusting of iron. The rusted iron and the dusty red soil in the surface give reddish colour to the planet. The blood-red colour of Mars resembles war and so it was given the name of Mars, the Roman god of wars. Due to the residues of dust particles in the atmosphere the sky of Mars looks pink in colour. The atmosphere of Mars is much thinner than the Earth's, with icy caps at the north and south poles. Only a few clouds and morning mists can be seen. The atmosphere mainly contains carbon dioxide. It also contains small amount of water vapour, oxygen, nitrogen, carbon monoxide, neon, argon, xenon and krypton. Mars has many plains formed of volcanic lava, large craters, basins, channels, canyons and huge volcanoes. Olympus Mons, which is 25 kilometers high and 600 kilometers in diameter, is the largest known volcano in the Solar System. The canyons were formed by the movements of the surface crest. The Valles Marineris is an enormous canyon system that stretches for about 4000 kilometres along the surface of the planet. Mars takes 687 earth days to revolve round the Sun and 24 hours and 37 minutes to rotate on its axis, which tilts 24° from the perpendicular. In the 19th century astronomers believed that there were signs of life on Mars. Later it has been proved that the canals and dark patches found out there, are optical illusions and blown off dust that covered the planet. The channels in Mars show that there was abundance of flowing, water millions of years ago and now dried up. Phobos and Deimos are the two satellites of Mars found out by an American astronomer Asaph Hall in 1877.

Spacecrafts to Mars

In 1965 Mariner-4 the US spacecraft sent photographs of the surface of Mars towards the Earth. Mariner-4 and Mariner-9 transmitted photographs detecting the meteor craters on the surface of the planet. Viking-1 and Viking-2 of US landed on Mars during the year 1976 and sent to Earth close-up photographs of the surface features of Mars. The ISRO of India launched the maiden Mars Orbiter Mission (Mangalyaan) on November 5, 2013 and it was successfully inserted into Mars orbit on 24 September 2014.

Figures of Mars

• Mars Distance from the sun in km: 227.9 million
• Diameter at equator in km: 6796
• Mass with respect to the earth: 0.107 (earth= 1)
• Gravity with respect to the earth: 0.38
• Period of revolution: 687 earth days
• Period of rotation: 24 hours and 37 minutes
• Density (g/cubic cm): 3.94
• Temperature: -143°C to 17°C
• Satellites: 2

THE JUPITER

Jupiter is the largest planet in the Solar System. So it was named after the ruler of all Greek-Roman Gods, Jupiter. This gas giant is the fifth planet from the Sun. The diameter of Jupiter is eleven times that of the Earth. It is the most massive planet of about 318 times as that of the Earth. But it has a low density. Jupiter consists of a small rocky core covered with an inner mantle of metallic hydrogen. The outer mantle contains liquid hydrogen, helium and traces of some other metals. Jupiter is the fastest rotating planet to take nearly 10 hours to complete one rotation. The speed of rotation is very high so that belts and zones are formed from the clouds and they surround the planet parallel to the equator. Dark warm cloud layers are called belts and bright cooler cloud layers are called zones. Due CO the irregular flow of the fluids the clouds unite at some regions and form storm systems like Red Spots and White Ovals. The most distinctive storm thus formed is the Great Red Spot which rises about eight kilometers from the upper layer of clouds. Jupiter has a thin, faint ring consisting of tiny dust particles. Among the 16 known satellites of Jupiter the largest four are Ganymede, Callisto, Io, and Europa. Ganymede is the largest satellite in the Solar System. Ganymede and Callisto have cratered rocky surface with ice. Io has active volcanoes containing sulphurous materials which appear as red, orange and yellow spots. The icy, smooth Europa shows traces of water contents.

Spacecrafts to Jupiter

The first spacecraft to fly past Jupiter was Pioneer-I0 launched by the United States in 1972. It measured the amount of hydrogen and helium in Jupiter's atmosphere, and the radiation belt. It also found that Jupiter has an extra large magnetic tail. Pioneer-Saturn, another US spacecraft sent on December 2, 1974 took close-up photographs of Jupiter. Voyager-1 launched in March 1979 by the US discovered the thin ring which encircles the planet. Voyager 2, in July 1979 sent detailed pictures of the four main satellites of Jupiter towards the Earth.

Figures of Jupiter

• Distance from the sun in km: 778.4 million
• Diameter at equator in km: 142,984
• Mass with respect to the earth: 317.892 (earth= 1)
• Gravity with respect to the earth: 2.53
• Period of revolution: 12 earth years
• Period of rotation: 9 hours and 55 minutes
• Density (g/cubic cm): 1.33
• Temperature: -157°C
• Satellites: 79 (as of 2018)

THE SATURN

Saturn is the second largest planet in the Solar System and the sixth planet from the Sun. It is nine times larger than the Earth with a diameter of about 1,20,500 kilometers. Saturn was named after the Roman God of agriculture who was the father of Jupiter. This gas giant consists of a rocky core and ice encircled by an inner mantle of liquid metallic hydrogen. The outer mantle contains metallic hydrogen that spreads into the atmosphere of gases. Saturn has a system of brightly colored rings and this planet appears as the most beautiful one in the Solar System. The rings are thought to be made up of bulks of tiny bits of rock and dust surrounding the planet. Saturn completes one rotation in 10 hours and 39 minutes, and it takes 29½ years to revolve round the Sun. Saturn has innumerable moons and 82 satellites of Saturn have been confirmed. Among them Titan is the largest one with a diameter of 5,140 kilometers, and is the only satellite known to have clouds and atmosphere. Some other satellites of Saturn are Enceladus, Tethys, Dione and Mimas. Saturn has the least density among planets in the Solar System.

Rings of Saturn

Saturn has seven rings which surround it at its equator without touching the planet. Each ring is a mass of billions of orbiting pieces of ice and rock. It consists of tiny grains of dust to wide chunks of ice of about 10 metres. The A, B, C and F rings are brighter while the D, G, and E rings are very faint. The gravitational pull of the planet and its satellites help the rings to be held together in orbit.

Spaceship to Saturn

Pioneer-Saturn was the spaceship launched by the US in 1973. In 1979 it reached about 21,000 kilometres near Saturn and detected that the planet has stronger magnetic field than the Earth. In 1977 Voyager-I and Voyager-2 of the US confirmed the presence of seven rings surrounding Saturn.

Observation of Saturn's rings

Saturn's rings were discovered by Galileo in 1610. In 1655 the Dutch scientist Christian Huygens found out that the rings are flat and solid, and inclined to the planet's orbital plane. Years later Cassini discovered the gap between every ring. In 1789, French scientist Pierre Simon Laplace stated that the rings are formed of many small components. James Clerk Maxwell in 1857 confirmed that a large number of tiny particles together form the rings of Saturn.

Figures of Saturn

• Distance from the sun in km: 1429.4 million
• Diameter at equator in km: 120,536
• Mass with respect to the earth: 95.184 (earth= 1)
• Gravity with respect to the earth: 1.07
• Period of revolution: 29½ earth years (approx.)
• Period of rotation: 10 hours and 39 minutes
• Density (g/cubic cm): 0.69
• Temperature: -178°C
• Satellites: 82 (Known)

THE URANUS

Uranus is the third largest planet in the Solar System having a diameter of 51,000 kilometers. It is the seventh planet from the Sun. Uranus was named after the Greek-Roman God who was the father of Saturn. It consists of solid rocky core surrounded by dense mixture of icy and gaseous water, methane, and ammonia. An atmosphere of hydrogen, helium and methane give this planet a bluish green appearance. A spectacular feature of Uranus is that its axis tilts at an angle of 97.9°, so that it revolves round the Sun on its sides like a huge rolling ball. Unlike other planets (except Venus) Uranus spins in the opposite direction. Uranus takes 17 hours and 8 minutes to spin on its axis and about 84 years to revolve round the sun. A system of 11 narrow rings formed from scattered rocks and dust particles are found surrounding the planet. The rings cannot be detected clearly because they are made up of the darkest particles. Uranus has 27 known satellites and most of them are named after Shakespearean characters. The inner ten satellites are dark and small having less than 160 kilometers in diameter. The outer five are larger in diameter between 470 and 1,600 kilometers. Miranda, Arid, Titania, Umbriel and Oberon are the outer satellites. The surface features of these five satellites vary widely. Oberon has cratered surface like the moon whereas Miranda is twisted and jumbled with large ridges and cliffs of about 20 kilometers in height, which breaks up to form huge craters. Uranus is the most featureless planet in the Solar System.

Rings of Uranus

The rings of Uranus are formed of the darkest particles. So they cannot be detected clearly. The Rings are Epsilon Ring, Delta Ring, Eta Ring, Beta Ring, Alpha Ring.

Probe to Uranus

During January 1986 Voyager-2 of United States passed 81000 kilometers of Uranus and collected many details including a magnetic field encircling the planet. It also discovered 10 small satellites inside the orbit of Miranda, one of the large satellites of Uranus.

Figures of Uranus

• Distance from the sun in km: 2875 million
• Diameter at equator in km: 51.118
• Mass with respect to the earth: 14.54 (earth= 1)
• Gravity with respect to the earth: 0.91
• Period of revolution: 84 earth years
• Period of rotation: 17 hours and 8 minutes
• Density (g/cubic cm): 1.27
• Temperature: -2I6°C
• Known Satellites: 27

THE NEPTUNE

Neptune is the last and the farthest planet from the Sun at a distance of about 4,500 million kilometers. The bright blue atmosphere is cold and contains hydrogen, helium, and methane. The bluish appearance of this planet has given it the name of the Roman God of the sea. A mantle of ammonia, methane and icy water covers the rocky silicate core. Huge storms swirl around the planet at a speed of about 2000 kilometers per hour. The Great Dark Spot and the Small Dark Spot are the largest. Four narrow rings encircle Neptune. The planet has 14 known satellites. Triton the coldest object in the Solar System with -235°C temperature is the largest satellite of Neptune. Unlike other satellites it orbits in the opposite direction of Neptune's rotation. Neptune completes one rotation in 16 hours 7 minutes and it takes 165 years to complete one revolution.

Rings of Neptune

Neptune has four narrow rings. The outermost ring has an unusual feature among planetary rings. It is faint with three curved and bright segments. The Neptunian rings are thickly filled with dust particles. They are visible only when back lighted by sunlight.

Cloud features of Neptune

The atmosphere of Neptune contains numerous important cloud features. The Great Dark Spot, the Small Dark Spot and the Scooter are the largest among them. The Great Dark Spot and the Small Dark Spot are huge storms that blow at a speed of about 2000 kilometers per hour. A large area of cirrus cloud is called the Scooter.

Spacecraft to Neptune

As Neptune is the farthest planet from the Earth only one spacecraft has been sent to this planet. The probe Voyager 2 launched on August 1977 visited Neptune in 1989 after passing Jupiter, Saturn and Uranus. Voyager 2 collected details of Neptune's rings and satellites. Six new satellites of this planet have been discovered by Voyager 2. The largest among them is only 180 kilometers in diameter (about the same size of a crater in the moon).

Figures of Neptune

• Distance from the sun in km: 4504.3 million
• Diameter at equator in km: 49100
• Mass with respect to the earth: 17.15 (earth= 1)
• Gravity with respect to the earth: 1.14
• Period of revolution: 165 earth years
• Period of rotation: 16 hours and 7 minutes
• Density (g/cubic cm): 1.64
• Temperature: 214°C
• Known Satellites: 14

THE DWARF PLANETS

The General Assembly of the International Astronomical Union (IAU) put forward a new definition for planets in August 2006. According to this a planet must be massive enough to have been pulled into a spherical shape by its own gravity. It must dominate its surrounding bodies by clearing out other celestial bodies from its Orbital path. Also a planet must have a definite orbital path around a star, while not being a star.

Based on these concepts Pluto cannot be considered as a planet and thus it is demoted from the status of a planet to a 'dwarf planet'. Ceres and UB3 I 3, two largest rocky bodies orbiting the Sun between Mars and Jupiter are also included in dwarf planets. Smaller objects such as asteroids and comets are called 'Small Solar System Bodies.' UB3 I 3 is called Xena, yet it has not been given a proper name and more details have not been found out.

THE PLUTO

Pluto was discovered in 1930 by an American scientist Clyde W. Tombaugh. Since then Pluto was considered a peculiar planet. The innermost core of ice and rock is covered by an icy mantle. The surface is covered with frozen methane and icy water. Pluto has a satellite called Charon which has half the size of its mother body. Pluto rotates in synchrony with the orbit of Charon and so they continuously face each other. Pluto takes 248 years to orbit around the Sun. Within this period, due to the gravitational pull of Neptune it goes into the orbital space of Neptune for about 20 years. Also, Pluto shares its space in the outer Solar System with its satellite Charon and some other heavenly bodies like Xena and its satellite Gabriella.

Figures of Pluto Mantle

• Distance from the sun in km.: 5900.1 million
• Diameter at equator in km.: 2300
• Mass with respect to the earth: 0.002 (earth= 1)
• Gravity with respect to the earth: 0.07
• Period of revolution: 248 earth years
• Period of rotation: About 6 earth days
• Density (g/cubic cm): 2.03
• Temperature:-233 to -223°C
• Satellites: 1

THE CERES

Ceres was discovered by Piazzi in January 1801. He reported it as a comet. Later it was called an asteroid because of the small size, high inclination and sharing the orbit with Pallas, one of the largest asteroids found out after Ceres. In 2006 Ceres was proposed before the International Astronomical Unit to be considered a planet. But according to the new definition of a planet, Ceres cannot be considered a planet and it is included among the dwarf planets with Pluto and Xena. Ceres orbits through the asteroid belt between Mars and Jupiter. It takes 4.6 years to complete one orbit. The surface of Ceres is more or less slight and freezing with a temperature of about -38°C. The innermost layer of Ceres is a rocky core covered with a mantle of ice and water. The mantle is about 120-60 kilometers thick and contains a vast amount of water. The outermost layer is a thin and dusty crust. Two dark features were discovered on Ceres, but it was not yet confirmed as craters.

OTHER CELESTIAL BODIES

ASTEROIDS

Asteroids are tiny, rocky bodies moving around the Sun. Most of the asteroids move between the orbits of Mars and Jupiter, but some of them beyond Neptune. The region in between Mars and Jupiter where the asteroids orbit is called 'asteroid belt'. About 10% of asteroids travel in groups far away from the asteroid belt. The groups of asteroids orbiting close to the Earth are the Amor, Apollo and Aten. Most of the asteroids are very tiny, but some are larger with a diameter of about 1,000 kilometers. The tiny asteroids like Gaspra, Mathilde, Ida etc. are irregular in shape and less than one kilometer in diameter.

COMETS

Comets are formed from huge cloud of dust and frozen gases which travel round the Sun through a distinct gas route. When a comet comes near the Sun, its surface vaporizes to form a huge sphere of dust and gas around a nucleus called coma that shines brightly. A long tail of gas and dust is also formed. The tail lengthens to about 160 million kilometers. The most clearly observed comet is the Halley's Comet which was recognized by the English astronomer Edmond Halley in 1682. It appeared last time in 1986 and supposed to reappear after every 76 years.

Halley's Comet

Halley's Comet passed by the Earth during March-April 1986. Two Soviet probes Vega 1 and Vega 2, and a European satellite Giotto observed the passage of the comet and collected many details about the interior structure and functions. These probes found that the comet has an oblong nucleus of about 15 kilometers long and 8 kilometers broad. Rock dust particles and ice form the comet. A black crust having temperature of about 170°F surrounds the comet. While the comet rotates on its axis, the side which faces the Sun pushes out dust and gases through the crust.

METEOROIDS

Meteoroids are small chunks of rock particles or mixture of stone and iron. When a meteorite enters the atmosphere of the Earth, as a result of the friction it is heated and glows showing a streak of light called meteors or shooting stars. When the Earth passes through the debris left by a comet, meteor shower takes place. During this process most of the meteors burn up in the atmosphere. Some larger meteors will reach the surface of the Earth and they are called meteorites. Meteorites are of two kinds - Stony meteorites and stony iron meteorites.

Impact Craters

The craters formed due to the fall of meteorites are called impact craters. There are about 150 impact craters all over the world. The Woolf Crater in Australia and the Meteor Crater of Arizona in USA are the best known examples of impact craters.