The Solar System- Physics Guide for Class 8

The Solar System Class 8 Science Guide

Information about The Solar System


The Solar System


Class 8


Class 8 Physics

Topics Covered

  • The Solar System
  • Sun
  • Planets
  • Mercury (Budh)
  • Venus (Shukra)
  • Earth (Prithivi)
  • Mars (Mangal)
  • Jupiter (Brihaspati)
  • Saturn (Shani)
  • Uranus (Arun)
  • Neptune (Varun)
  • Asteroids
  • Comets
  • Meteors and Meteorites
  • Artificial Satellites

The Solar System

We are all quite familiar with the solar system. Our solar system consists of the sun and eight planets. The eight planets of the solar system are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Besides planets, the solar system consists of a large number of minor planets or asteroids, a hosts of comets and meteors. The gravitational pull (attraction) between the sun and these celestial bodies keeps all of them revolving around the Sun. 


The solar system is dominated by the Sun, which accounts for almost 99.9% of the matter of the whole solar system.
  • The Sun is also the source of almost all the energy in the solar system.
  • The Earth receives almost all its energy (heat and light) from the Sun. 
  • The Sun is essentially a sphere of hot gases.
  • The temperature of the bright disc (visible), which is the source of energy for us, is about 6,000 K. The disc is called the photosphere.
  • The radius of the Sun is almost 100 times the radius of the earth and its mass is about a million times the mass of the Earth. 


  • The name planet has been given to all those (bright) heavenly bodies that revolve round the sun. They look like stars but they do not twinkle.
  • We now know that planets do not have any light of their own. Their observed brightness is only due to the light of the sun reflected by them.
  • There are eight planets 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 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, therefore, find it convenient to divide the planets into two categories: 
  1. The Terrestrial planets
  2. 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 also known as the 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 the same size as the earth but rotates relatively slowly around its axis.
  • It rotates from east to west while the earth rotates from west to east.
  • 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 the greenhouse effect.
  • Venus is also known as a morning or evening star as it is usually visible only during these times. It also shows phases like the moon.

Earth (Prithivi)

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 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 and 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% 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.
  • From outer space, the earth appears blue and green due to the reflection of light from the water and landmasses on its surface. 
  • It has a thin atmosphere. Though thin, this atmosphere plays a very vital role in protecting and preserving life on the earth. 
  • It protects us from being hit by smaller bodies of the solar system. 
  • It has an ozone layer, which absorbs the ultraviolet radiation from the sun and protects us from its harmful biological effects. 
  • The atmosphere also acts like a natural greenhouse. This keeps the temperature of the earth in the range suitable for the continuation of life.

It is no wonder that earth is the only planet in the solar system that can sustain life on its surface. It fulfills and meets the conditions needed for development and sustenance of life. We can summerise these conditions as: 
  1. the right size and right distance from the sun so that it has the optimum temperature range and gravity. presence of water on its surface.
  2. suitable atmosphere and a blanket of ozone layer.

Mars (Mangal)

  • Mars usually appears 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 amounts of nitrogen, oxygen, water vapour and other gases.
  • Its surface temperature and surface pressure both are 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 the 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)

Jupiter is the largest of all planets. Its volume is 1,300 times more than that of the earth. It shows 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. Till date, Jupiter is known to have 67 moons or satellites. Further investigation may reveal it to have even more moons. • 

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. These rings can be seen clearly only with the help of a telescope.
Saturn is known to have 62 natural satellites or moons of its own. 

Uranus (Arun)

  • Uranus is also a very large planet. It, in fact, is the third largest planet of the solar system.
  • Its diameter is almost four times than that of the earth. That means 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 blue-green 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 27 satellites or moons of Uranus have been discovered.

Neptune (Varun)

  • Neptune 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 14 satellites revolving around it. 
  • e do not have much detailed information about this planet. This is mainly because of its very large distance from the earth as well as from the sun.

Minor Bodies in the Solar System

Apart from the sun, the eight planets and their associated moons, there are also some other minor bodies in the solar system. These are the asteroids, meteors and comets. We now talk briefly about these minor bodies. 


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. 
  • Taken together, 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. 


  • The name comet means a 'hairy star'.
  • Comets are relatively small and icy celestial bodies revolving round the sun.
  • When a comet is near the sun, some of the 'ice' in it turns into gas. The gas and loose dust, freed from the ice, create a long illuminated tail that streams behind the comet.
  • Approximately 2,000 comets have been observed over the years.
  • Comets are visible only when they are near the sun. This is because the intense solar heat vaporises parts of their icy matter and creates their characteristic illuminated tail.
There are a few periodic comets that appear again and again at regular (nearly constant) intervals. The best known of such 'periodic comets' is the Halley's comet. It appears (approximately) after every 76 years. It was last seen in the year, 1986. It is, therefote, next expected to 'pass by' the earth in the year, 2062. 

Meteors and Meteorites

Meteors are relatively small solid bodies revolving round the sun. Some of them, by chance, may enter into the earth's atmosphere from outer space. When that happens, they get intensely heated up due to the friction resulting from their rapid motion through the earth's atmosphere. This makes them shine brilliantly like fireballs. Their view then consists of luminous heads followed by a comet like tail of light that may persist for several minutes. We call them as shooting or falling stars. They exist but for a short while because the intense heat vaporises them off very quickly. 
  • There are some meteors that are so large that a part of them is able to reach the surface of the earth. We call such large meteors as meteorites
  • Study of meteorites, found on the earth, has helped scientists to know more about the nature of the materials from which the celestial bodies, in our solar system, may have been formed. 

Artificial Satellites

Celestial bodies, like the earth's moon, revolving around a planet, are called its natural satellites. Artificial satellites are man-made objects which can be made to revolve around the earth. They are, however, much closer to the earth than the moon. 
  • Artificial satellites perform various tasks, such as transmitting radio, telephone and television signals, sending back information for weather forecasting, military surveillance and for locating mineral resources.
  • Satellites, that are being put to the maximum use, are the geostationary satellites.
  • A geostationary satellite appears to remain fixed with respect to a particular point on the earth.
  • For such satellites, the time period of their revolution around the earth, equals the time period of rotation of the earth around its own axis (i.e. nearly 24 hours).
  • It is these satellites that have made global audio-visual communication such a practical, and now routine, reality.
  • These satellites stay in the same position relative to the surface of the earth; hence the broadcasting station does not lose contact with the receiver. 
Satellites have revolutionised communication by making worldwide telephone links and live broadcast a common occurrences and daily feature of our life. A satellite receives a microwave signal from a ground station on the earth (the up link). Its amplifiers first amplify it. Thereafter, its transmitters re-transmit the (amplified) signal back to a receiving station on earth at a different frequency (the down link). 

Some important Points

  • Till the year 2006, Pluto was regarded as the ninth planet of the solar system. However, in the year 2006, the International Astronomical Union (IAU), in its meeting held at Prague on 26 August, 2006, decided that Pluto is to be no longer regarded as a planet as it does not satisfy the basic characteristic of the definition of a planet. We, therefore, now talk of only eight planets. Pluto is now known as a dwarf planet and is thought to be one of the objects in the keeper belt.
  • The Keeper Belt is a region of space, in the solar system (shaped like an ellipse) and is located approximately 30-50 A.U. (Astronomical Units) from the sun. The Keeper Belt objects are mostly composed of frozen water (ice) and ammonia.
  • Days go by quickly on Jupiter. They last just 9 hours 50 minutes as it rotates very fast on its axis. But it takes a long time to go round the Sun. Hence, a year on Jupiter is much longer than that on the earth. One 'Jupiter Year' equals nearly twelve earth years. But for the Sun, Jupiter is the biggest and the heaviest member of the solar family. Its mass, in fact, is more than double (nearly 2.5 times) of the sum total of the masses of all the other planets taken together.
  • Aryabatta was India's first artificial satellite launched in 1975. Bhaskra, Rohini, Apple and INSAT are some of the other Indian satellites.
  • Geostationary satellites move in orbits parallel to the equatorial plane of the earth. We, therefore, call their orbits as equatorial orbits. Such satellites can be 'located' only at certain specific places above the earth. Their 'height' above the earth, is nearly 36,000 km. We need a minimum of three geostationary satellites to have a 'round the globe' communication. We also have artificial satellites that move in polar orbits, i.e. orbits passing over the poles of the earth. Such satellites are, relatively, much nearer to the earth. They are used mainly for weather forecasting and for 'remote sensing' purposes.
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