image showing the planets and dwarf planets of the solar system to scale 

A study of the planets is always an interesting journey into the world of outer space, millions of kilometers from our earth. We were always told that there are nine planets but now due to the changed definition by the International Astronomy Union, Pluto does not qualify to be a planet. This means now there are only eight planets. However Pluto is very much where it was.

Each planet is unique and is a world by itself. Mankind is all the time probing to know more about them. Over the decades considerable knowledge has been accumulated and planets are no longer the mystery they used to be earlier. The individual planets are detailed below :

1. Mercury : It is the nearest planet to sun and is the eighth largest in size. It has  diameter of 4880 kms and a mass of 3.3e23 kg or 3.3e20 tons. Its orbit around the sun varies from 46 million kms at the perihelion and 76 million kms at the aphelion. The temperature varies from 90K to 700 K or from -183 C to 427 C.   

Mercury's magnetic field has a extremely low strength equal to 1% of earth. It also has  a very thin atmosphere and due to extreme heat the atoms that come in solar winds, escape into space.It is the second most densest planet after earth at 5.43 gms/cm3.

It has no known satellites. There is  a possibility of water ice in the shadows of some craters. It can be seen through binoculars. It moves very quickly through the skies.

In mythology it is the god of commerce, travel and thievery and messenger of the gods. 


2. Venus : It is the goddess of love and beauty and is named Venus because it is the brightest of all the planes. It is the second planet from the sun and is the sixth largest in size, Its diameter is 12,103 kms and mass is estimated to be 4.869e24 kg. Venus is 95% of earth's diameter and 80% of its weight. However its rotation is slower than earth being equal to 243 earth days.

The pressure on Venus's surface is 90 atmosphere equal to the pressure at a depth of one kilometer depth in earth's ocean. Its atmosphere consists of carbon dioxide and has clouds many kilometer thick consisting of sulfuric acid obstructing the view of its surface. Its surface temperatures are as high as 467 deg. Centigrade. This is surprising as Venus is hotter than Mercury although it is twice the distance away from sun than Mercury. Its surface is covered mostly with lava and at some spots volcanoes are still active.

Venus may have had water but it all has now boiled away making it the most dry among the plants. Venus lacks a magnetic field perhaps due to its very slow rotation and also it has no satellites.

It is not considered scientifically important because of its hostile environment.


3. Earth :   It is the third planet from the sun and the only one on which life as we know exists for reasons unknown. Earth is 12,756 kms in diameter and weighs 5.972e24 kgs. The earth's orbit around the sun has an orbital of 149,600,000 km from the sun. It completes it in 365 days.

The earth is the most densest major body in the solar system so far. The earth has an a gravitational field which supports an atmosphere which comprises of nearly 77% nitrogen , 21% oxygen and other gases especially carbon dioxide form the balance. The presence of carbon dioxide in the atmosphere is very important because it helps to raise the temperature at the surface above freezing point  i.e. from -21 C to +14 C as otherwise oceans would freeze and life would cease to exist. This is also known as the green house effect.

Nearly 71%of earth's surface is covered with water, unique among all the planets. Life would be impossible without it. The earth's crust is divided into seven layers with upper most crust being 40 kms in depth. The earth is composed of 34.6% iron, 29.5% oxygen,15.2% silicon, 12.7% magnesium, 2.4% nickel, 1.9% sulfur and ).5% titanium.

The earth has a natural satellite namely the moon. The distance between the two is 384,000 kms. The moon's diameter is 3,476 kms and mass is 7.35e22 kgms.


4. Mars : The fourth planet from the sun is the red colored Mars, also called the red planet. It is 6,794 kms and has a mass of 6.4219e23 kgms. Its orbit is 227,940,000 kms from the sun. It is known since pre-historic times.

Its orbit is elliptical and thus there is a temperature variation of from -133 C to +27 C in summer with average temperature being -55C. It has very interesting terrain. A mountain named Olympus Mons rises 24 kms with a base of 500 kms and a rim of cliff 6 kms high. A bulge 10 kms high and 4000 kms across named Tharsis. There is also a crater Hellas Planitia 2000 kms in diameter and 6 kms deep.

Mars has a very thin atmosphere composed mostly of carbon dioxide (95%) and nitrogen (2.7%). The average pressure on the surface is about less than 1% of that on earth.

Mars has two tiny satellites named Phobos and Deimos having 22 and 12 kms diameter resp. and at a distance of 9000 and 23000 kms from the center of planet resp. they were discovered in 1877.

The first spacecraft to visit Mars was Mariner 4 in 1965. Around 8 spacecrafts have landed on Mars and increased our knowledge of Mars.

For Greeks Mars was the god of War and today is considered to be the best candidate for existence of life outside earth.


5. Jupiter : It is the fifth planet from the sun and is the largest in size. It has a diameter of 1,42,984 kms and a mass of 1.9e27 kgms. It has an orbit of 778, 330,000 kms from the sun.

In Greek and Roman mythology it was the King of the gods and patron of the Roman state. It is known since ancient times and Galileo had seen it and discovered four of its moons in 1610 and has been visited by many spacecrafts.

Jupiter is a gaseous planet and thus does not have a surface. It is about 90% hydrogen and 10% helium with traces of gases like methane, ammonia and water. It has perhaps a solid inner rocky core above which lies the gaseous mixture. 

Jupiter radiates more energy than it receives from the sun. The deep core temperature is said to be around 20,000 c and heat is generated due to the slow gravitational compression of the planet. Its magnetic field is much stronger than that of earth. it also has rings like Saturn;s but much smaller and fainter.

Jupiter has 63 known satellites. The major four moons are Io with a radius of 1815 kms, Europa with 1569 kms, Ganymede with 2631 kms and Callisto with a radius of 2400 kms.

Jupiter was first visited by Pioneer 10 in 1973 and thereafter five spacecrafts have visited it. After Sun,moon and Venus it is the fourth brightest object in the sky.


6. Saturn : It is the sixth from the Sun and second largest in size. It is 1,20,536 kms in diameter and a mass of 5.68e26 kgms with an orbital of 1,429, 400,000 kms around the sun.

It is also known since pre-historic times and Galileo saw it through a telescope in 1610 but got confused by its shape. It is also a gaseous planet with the least density among all planets equal to 70% of water.

The interior of Saturn is similar to Jupiter consisting of a rocky core, liquid metallic hydrogen and a gaseous hydrogen layer. Its interior temperature is estimated to be around 12,000 K and it also radiates more energy than it receives from the Sun. 

Of the seven rings, two prominent  and one faint outer rings surrounding  the planet can be seen from the earth. The rings are composed of innumerable small particles ranging from a centimeter to to several meters, each in its own independent orbit. The rings are 250,000 kms in diameter are very thin, less than a kilometer thick. They contain water ice and other particles coated with ice.

Saturn also has a strong magnetic field.

Saturn has 53 satellites and all have been named unlike Jupiter. Their radius vary from 10 kms to 2575 for the Titan satellite.

Saturn has been visited by Pioneer 11 in 1979 and thereafter by Voyager 1 and Voyager 2 and in 2004 by Cassini spaceship.

Saturn is the god of agriculture in Roman mythology and  Saturday is named after it.


7. Uranus : The third largest by diameter planet is the seventh in distance from the Sun. It has a diameter of 51,118  kms and a mass of 8.683e25 kgms. Its orbit around the sun is 2,870,990,000 kms.

Uranus was the first modern day discovery of a planet , by Herschel in 1781.

It is made of rock and various ices with about 15% hydrogen and small quantity of helium. Uranus's atmosphere consists of around 83% hydrogen, 15% helium and 2% methane. Like other gaseous planets Uranus also has rings, 13 have been discovered so far. They are faint and consist of fine dust and upto 10 meter in diameter. It also radiates more heat than it receives from the sun.

Only one spaceship has visited it namely  Voyager 2 in 1986.

It has 27 moons, all named and possibility exists it can have more. Its magnetic field is odd in that it is not in the centered in the center but 60 degrees off. It is faintly visible on a clear night but can be spotted by telescopes.


8. Neptune : It is the eighth planet from the Sun and fourth largest in diameter. It has a diameter of 49,532 kms and a mass of 1.0247e26 kgm. Its orbit from sun is 4,504,000,000 kms. In Roman mythology Neptune is the god of the sea. However in modern times it was observed in 1846 by two astronomers very near to the location calculated out by two others.

Neptune's composition is somewhat similar to that of Uranus. It consists of ice and rocks along with 15% hydrogen and a little helium.Its atmosphere is mostly hydrogen, helium and small volumes of methane. The gaseous planet has the highest winds in the solar system reaching 2000 kms/hr speeds. It also emits more nearly twice the energy it receives from the sun, like Jupiter and Saturn. Neptune also has its share of rings and their composition is not yet known. It also has its own  magnetic field. 

Only one spacecraft namely Voyager 2 visited near to it in 1989 and is presently the only source for much of the knowledge about it. It also discovered six of the 13 known satellites of Neptune.


9. Pluto : The ninth planet from the Sun is named Pluto. It is very small and is now officially designated as a dwarf planet . Its diameter is only 2274 kms and mass is 1.27e22 kgms. It orbits 5,913,520,000 kms from the Sun. It is now not recognized as a planet but is still counted as a planet by the common people for we were taught there are nine planets.

Pluto was discovered in 1930. Its orbit is highly eccentric and it rotates opposite to other planets. its surface temperature varies between -235 and 210 C. Not much is known about its composition and atmosphere. 

It has not been visited by a spacecraft. In 2006 a spacecraft named New Horizons has been launched to reach it in 2015 after travelling for nine years.

It has one satellite named Charon whose diameter is 1,206 kms. It was discovered in 1978.

It is expected that in 2015 our knowledge of the dark planet Pluto will increase when New Horizons will reach it.

The above planets are revolving around the Sun a star. More than a thousand planets have been discovered around various stars in the Milky Way. The planets Mercury, Venus, Earth and Mars are categorized as terrestrial because they are composed mostly of rocks  and the planets Jupiter, Saturn, Uranus and Neptune are termed as gas giants as they are composed mostly of gaseous materials and are considerably massive in size than terrestrial planets. 

As it becomes possible to send spacecrafts and even land them on the nearer planets like Mars, mankind's knowledge about them increases. It is a matter of time when perhaps a manned mission may become possible. One has to praise the pioneering advances done by organisations like American NASA which has enhanced our knowledge of the planets but at the same time increased our curiosity still further. In a distant future there exists a possibility that a planet could be found capable of supporting life. It could be colonized and its surface mined for its resources and these are brought to earth. Also producing some materials in a low gravity atmosphere could be also undertaken.

It is all fascinating but into distant future. 

(Picture from Wikimedia)

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