The solar system

THE SOLAR SYSTEM

The sun and all bodies in orbit around it make up the solar system. The centre of the solar system is the sun. The other members of the solar system include the eight planets, their moons or satellites, a large number of asteroids, thousands of comets and innumerable meteors. These other members of the solar system also revolve around the sun and are bound to it by strong gravitational forces.

SUN:

The sun is the star closest to the earth at a distance of 150 million km from the earth. Light from the sun takes 8minutes 20 second to reach the earth. The diameter of the sun is over 109 times the diameter of the earth and it is 333,000 times as heavy as the earth. The sun is made up of hydrogen gas that is continuously being converted into helium at extremely high temperature. The centre of the sun is thus extremely hot at about 14 million degrees Celsius, while its surface temperature is about 6000 degrees Celsius. Compared to the other stars the sun looks enormous to us as it is so close to the earth. The next nearest star, Alpha Centauri, is several times bigger than the sun, but it appears tiny as it is much away from us. Sometimes, from some areas on the surface of the sun, hot gases shoot outward.

These are called prominences. There are also some darker, relatively cooler patches (of about 4000°C) on the surface of the sun. These spots are called sunspots and are of interest to scientists as they are found to interfere with radio and wireless transmission from the earth. They also produce change in weather on the earth.

PLANETS:

The planet are those (bright) heavenly bodies that revolve round the sun. They look like stars but they do not twinkle. Their observed brightness is only due to the light of the sun reflected by them. There are eight planets now in our solar system. They move in elliptical shaped paths called orbits around the sun. The eight planets of our solar system, in increasing order of their distances from the sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune . The planets, relatively nearer to the sun have features that are quite different from those which are `far off'. We can divide the planets into two categories :

(i) The terrestrial planets, and

(ii) The Jovian planets.

Mercury, Venus, Earth and Mars are the terrestrial planets. They have solid and rocky surfaces.

The Jovian planets are Jupiter, Saturn, Uranus and Neptune. These planets are very large in size and are made up largely of gases.

• Mercury (Budh): Mercury lies closer to the sun than any other planet. It is a dry, hot and virtually airless planet. It has craters like the moon, but its interior is similar to that of the earth. Like the earth, its interior contains iron and other heavy elements. Mercury is much smaller in size than the earth. It is occasionally visible just before sunrise or immediately after sunset. Hence, it is often known as morning or evening star. Being close to the sun, it takes only 88 earth days to go once around the sun.

• Venus (Shukra): Venus is the brightest object in our sky after the sun and the moon. Its bright appearance is due to its cloudy atmosphere which reflects almost three-fourth of the sunlight falling on it. Venus is almost of the same size as the earth but rotates relatively slowly around its axis. It has no moon or satellite of its own. Venus is even hotter than mercury though it is relatively farther away from the sun. This is because of the high percentage of carbon dioxide in its atmosphere. This gas traps most of the sun's heat falling on it. This is due to an effect called the greenhouse effect. Venus is also known as morning or evening star as it is usually visible only during these times.

• Earth (Prithvi): The earth is a very unique and special planet of our solar system. Like the other planets, the earth not only revolves round the sun but also rotates about an (imaginary) axis of its own. The portion of the earth facing the sun at any time has day, the other portion facing away has night. As the earth rotates on its axis, day & night follow one another. The axis of rotation of the earth is known to be tilted with respect to its orbit. It completes one revolution around the sun in nearly 365.24 days. When the northern hemisphere is tilted towards the sun, it is summer there. At that time, it is winter in the southern hemisphere. The reverse happens when the northern hemisphere is tilted away from the sun. Autumn and spring occur when the earth is in between these two extreme positions in its orbit.

• Mars (Mangal): Mars usually appear reddish in colour, hence it is also often known as the red planet. Its surface resembles a cold, high altitude desert. Its atmosphere consists primarily of carbon dioxide, along with small amount of nitrogen, oxygen, water vapour and other gases. Its surface temperature and surface pressure are both very low. These conditions make it unlikely for water to exist in a liquid state on this planet. The diameter of mars is only a little more than half of that of earth. Its mass is, however, only one-tenth of that of earth. Mars, therefore has a low average density as compared to the earth. Mars has two natural satellites or moons named Phobos and Deimos.

• Jupiter (Brihaspati): It is the largest of all planets. Its volume is 1,300 times more than that of the earth. !t has its own colourful bands. These are believed to be due to its strong atmospheric currents and the dense cloud cover around it. Jupiter consists mainly of hydrogen and helium in gaseous form. Its cloud cover is made up of methane in gaseous form, with some ammonia in crystalline form. Jupiter is known to have 28 moons or satellites.

• Saturn (Shani): Saturn is quite similar to Jupiter in size, mass and composition. It is the second largest planet of the solar family. It is distinguished by its very unique and special system of rings. These rings give it a beautiful appearance. There are three distinct rings surrounding this planet. These rings can be seen clearly only with the help of a telescope. Galileo was the first to observe these rings with the telescope. Saturn is known to have 30 natural satellites or moons of its own. This planet has the largest number of moons amongst all the planets.

•  Uranus (Arun):  This is also a very large planet. It, infact is the third largest planet of the solar system. Its diameter is almost four times than that of the earth. It can contain as many as (nearly) 64 earths in it. Hydrogen and methane have been detected in the atmosphere of this planet. This planet is observed to have bluegreen colours. This is believed to be because of the presence of methane gas in its cold, clear atmosphere. Its northern hemisphere remains in a four-decade long period of darkness because of the way the planet rotates. So far 21 satellites or moons of uranus have been discovered.

• Neptune (Varun): It is very far away from the sun and is visible only through a telescope. It has been named after the Roman sea god Neptune. Neptune has 8 satellites revolving around it.

ASTEROIDS:

These are very small planets or planetoids that are found mainly in a belt between the orbit of Mars and Jupiter. Each asteroid has its own orbit and the orbits of all of them are spread over a large distance, forming a band. Astronomers have discovered more than 500 asteroids which are larger than 48 km in diameter. Ceres is the largest of the asteroids discovered till now.

COMETS:

The name comet means a 'hairy star'. Comets are relatively small and icy celestial bodies revolving round the sun. When a comet comes near the sun, some of the 'ice' in it turns into gas. The gas and loose dust, free from ice, create a long illuminating tail that streams behind the comet. The tail of the comet could be as long as 800 million kilometres. Comets are visible only when they are near the sun. This is because the intense solar heat then vaporises parts of their icy matter. There are a few `periodic comets 'that appear again and again after a regular (nearly, constant) interval. One of such periodic comet is the Hailey's comet. It appears approximately after every 76 years. It was last seen in the year 1986. It is, therefore, next expected to `pass by' the earth in the year 2062.

METEORS:

Meteors are pieces of stony or metallic rock widely scattered throughout the solar system. They travel at high speeds around the sun. When meteors come close enough to the earth, they are pulled towards the earth by its gravitational force. As the meteor passes through the earth's atmosphere, it gets burnt due to the immense heat produced by friction with the atmosphere, leaving a brilliant trail of light behind it. Such a meteor is also called a shooting star. Most of the meteors are so small that they burn up completely in the upper atmosphere, but some of the larger meteors may land on the surface of the earth producing craters. Such meteors which land on the earth are called meteorites.