The sun, eight planets, satellites and some other celestial bodies known as asteroids and meteoroids form the solar system. We often call it a solar family, with the sun as its Head.
The Sun The sun is in the center of the solar system. It is huge and made up of extremely hot gases. It provides the pulling force that binds the solar system.
The sun is the ultimate source of heat and light for the solar system but that tremendous heat is not felt so much by us because despite being our nearest star, it is far away from us. The sun is about 150 million km away from the earth.
Planets
There are eight planets in our solar system In order of their distance from the sun, they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
An easy way to memorise the name of the planets in order of their distance from the sun is: MY VERY EFFICIENT MOTHER JUST SERVED US NUTS.
All the eight planets of the solar system move around the sun in fixed paths. These paths are elongated,they are called orbits
Mercury is nearest to the sun It takes only about 88 days to complete one round along its orbit Venus is considered as ‘Earth’stwin’ because its size and shape are very much similar to that of the earth.
Till recently (August 2006), Pluto was also considered a planet However, in a meeting of the International Astronomical Union, a decision was taken that Pluto like other celestial objects (Ceres, 2003 UB313) discovered in recent past may be called dwarf planets.
The Earth
The earth is the third nearest planet to the sun. In size, it is the fifth largest planet. It is slightly flattened at the poles and this the reason why, its shape is described as a Geoid (means an earth-like shape).
Conditions favourable to support life are probably found only on the earth.
The earth is neither too hot nor too cold. It has water and air, which are very essential for our survival.
The air has life-supporting gases like oxygen Because of these reasons, the earth is a unique planet in the solar system. From the outer space, the earth appears blue because its two-thirds surface is covered by water. It is, therefore, called a blue planet.
The Moon
Our earth has only one satellite i.e the moon. The moon moves around the earth in about 27 days and it takes exactly the same time to complete one spin. As a result only one side of the moon is visible to us on the earth
The moon does not have conditions favourable for life. It has neither water nor air. It has mountains,plains and depressions on its surface. These cast shadows on the moon’s surface.
Asteroids
Apart from the stars, planets and satellites, there are numerous tiny bodies which also move around the sun. These bodies are called asteroids. They are found between the orbits of Mars and Jupiter. Scientists are of the view that asteroids are parts of a planet which exploded many years back.
Meteoroids
The small pieces of rocks which move around the sun are called meteoroids Sometimes these meteoroids come near the earth and tend to drop upon it During this process due to friction with the air they get heated up and burn It causes a flash of light Sometimes, a meteor without being completely burnt, falls on the earth and creates a hollow.
Milky Way galaxy
A whitish broad band, like a white glowing path across the sky on a clear starry night is a cluster of millions of stars. This band is the Milky Way galaxy.
Our solar system is a part of this galaxy . In ancient India, it was imagined to be a river of light flowing in the sky Thus, it was named Akash Ganga
A galaxy is a huge system of billions of stars, and clouds of dust and gases and there are millions of such galaxies that make the Universe.
Satellites
Natural Satellite is a celestial body that moves around the planets in the same way as the planets move around the sun.
Human-made Satellite is an artificial body. It is designed by scientists to gather information about the universe or for communication. It is carried by a rocket and placed in the orbit around the earth. Some of the Indian satellites in space are INSAT, IRS, EDUSAT, etc.
OUR SOLAR SYSTEM ( some more facts)
Our solar system consists of the sun (the star), 8 planets, 63 moons, millions of smaller bodies like asteroids and comets and huge quantity of dust-grains and gases.
The Moon: The moon is the only natural satellite of the earth. The formation of moon, as a satellite of the earth, is an outcome of ‘giant impact’ or what is described as “the big splat”. A body of the size of one to three times that of mars collided into the earth sometime shortly after the earth was formed. It blasted a large part of the earth into space.
This portion of blasted material then continued to orbit the earth and eventually formed into the present moon about 4.44 billion years ago.
Out of the eight planets, mercury, venus, earth and mars are called as the inner planets as they lie between the sun and the belt of asteroids the other four planets are called the outer planets. Alternatively, the first four are called Terrestrial, meaning earth-like as they are made up of rock and metals, and have relatively high densities.
The rest four are called Jovian or Gas Giant planets. Jovian means jupiter-like. Most of them are much
larger than the terrestrial planets and have thick atmosphere, mostly of helium and hydrogen.
The difference between terrestrial and jovian planets can be attributed to the following conditions:
The terrestrial planets were formed in the close vicinity of the parent star where it was too warm for gases to condense to solid particles. Jovian planets were formed at quite a distant location.
The solar wind was most intense nearer the sun; so, it blew off lots of gas and dust from the terrestrial planets. The solar winds were not all that intense to cause similar removal of gases from the Jovian planets.
The terrestrial planets are smaller and their lower gravity could not hold the escaping gases.
Evolution of The Earth
The earth was mostly in a volatile state during its primordial stage. Due to gradual increase in density the temperature inside has increased. As a result the material inside started getting separated depending on their densities.
This allowed heavier materials (like iron) to sink towards the centre of the earth and the lighter ones to move towards the surface. With passage of time it cooled further and solidified and condensed into a smaller size. This later led to the development of the outer surface in the form of a crust.
Evolution of Atmosphere and Hydrosphere
There are three stages in the evolution of the present atmosphere. The first stage is marked by the loss of primordial atmosphere. In the second stage, the hot interior of the earth contributed to the evolution of the atmosphere.
Finally, the composition of the atmosphere was modified by the living world through the process of photosynthesis. The early atmosphere, with hydrogen and helium, is supposed to have been stripped off as a result of the solar winds. This happened not only in case of the earth, but also in all the terrestrial planets.
During the cooling of the earth, gases and water vapour were released from the interior solid earth. This started the evolution of the present atmosphere. The early atmosphere largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia and very little of free oxygen. The process through which the gases were outpoured from the interior is called degassing. Continuous volcanic eruptions contributed water vapour and gases to the atmosphere.
As the earth cooled, the water vapour released started getting condensed. The carbon dioxide in the atmosphere got dissolved in rainwater and the temperature further decreased causing more condensation and more rains. The rainwater falling onto the surface got collected in the depressions to give rise to oceans.
