5th Class Science Our Universe Planets


Category : 5th Class

*     Planets


We have studied about the planets of our solar system in previous classes. We will study only these four planets here.


*        Mercury

It is named after the name of the messenger of the Roman gods. Mercury takes 58 days to spin on its axis and 88 days to revolve around the Sun. There is no atmosphere and water on this planet. It is covered in craters. The biggest craters on this planet is calories basin.



*       Venus

It is named after the Roman goddess of love and beauty. It is the most beautiful object in the sky. This planet was first observed by Galileo. Venus is the brightest and hottest planet, because it has atmosphere to trap the sun's light making it hottest, it is called as the morning and evening star. Venus rotates on its axis very slowly but it revolves around the sun fast and hence 1 day on Venus = 243 earth days.

But 1 year on Venus = 225 earth days. It has no moon.


*       Earth

The only planet in the solar system that has life. It has all the essential components of life i.e.

Appropriate warmth from the Sun;

Water, about 71 % of the surface is covered with water;



The first life on the earth appeared about 3 billions years ago. It is also named after Roman goddess "Terra" meaning earth. The temperature of the earth's surface is 60°c and it increases as one goes deeper and deeper.

Earth is magnetic because of the presence of molten iron in the core. The needle of the compass always points towards the magnetic north pole.

The earth rotates on its axis in approximately 23 hours, 56 minutes and 22.7 seconds. It revolves around the sun in 365 days, 6 hours, 9 minutes and 10 seconds.

Object on this planet doesn't fly off because of the force of gravity, which pulls every object towards itself.


*      Gravitation

Every planet in the universe attracts each other with a force proportional to their mass. The earth attracts the object when thrown upwards due to the force of gravity. The force of attraction of the earth is due to the structure of the earth’s core.

Why does all objects fall towards the earth instead of going up?

Why do we feel being pulled down when we jump?

Why it is easier to fail than to jump?

The answer lies in this fact that there is something in the earth which tries all the time to pull all the objects towards itself-

This something in the Earth is called as gravity or gravitational force. Gravitation is a force with which every objects fall towards the surface of the earth.

When we jump we do work against gravity, hence force is applied and energy is spent.

When anything falls, it falls with gravity hence little force is applied and little energy is consumed.


*     How the concept of Gravity came into light

One day sir Isaac Newton was sitting under an apple tree, an apple fell on his head. He started thinking "Why does it happen, why all the objects fall towards ground only". Suddenly he got an idea about gravity and formulated universal law of gravitation. He has a brilliant insight that answered many questions.


*       Mass and Weight

We have already discussed about the mass, now in this chapter we will discuss about weight. Weight can be defined as the product of force with which an object is attracted towards the earth and mass of the object. The unit of weight is kilograms and unit of force is new ton. The force of attraction with which the earth attracts the object is called gravitational force.

Mass is a constant quantity. Mass remains same even if we are in space, but our weight can become zero in space or at some places.

People feel weightless in the space and hence float in the space.


Look at the following picture of astronaut floating in the space:

Astronaut feeling weightlessness in the sky

Weight = Mass x acceleration due to gravity

W = M x g

Value of g = 9.8 on the earth's surface.

Value of g = 9.8 / 6 on the surface of the moon.

A boy weighing 24 kg on the earth would weigh 4 kg on the moon. The weight on the moon reduces to 1/6th times that on the earth.

Sun has strong force of gravity that holds all the eight planets and other heavenly bodies like asteroid of our solar system.

Movement of moons around their planets and movement of satellites and spacecrafts around the earth is due to this force.


*       Relation between Gravitation and Gravity


*      Gravity

The force of gravity decreases as the distance between the two objects increases.


*       Newton's Universal Law of Gravitation

Newton formulated a law about gravitation that holds the force between two bodies. Hence this law is called as universal law of gravitation.


*     This law states that

Every two object in the universe attract each other by a force which is directly proportional to the product of their masses and inversely proportional to the

F is directly proportional to the product of the two masses ,i.e.

F is inversely proportional to the distance between them .i.e.

 and  masses of the two objects.

r - distance between them

G- Gravitation constant

F - Force between the masses



The Sl unit of force is newton and mass is measured in kilograms and distance between The objects is measured in metre. The value of acceleration due to gravity on the surface of the earth is approximately



Planets revolve around the Sun, similarly some heavenly bodies revolve around these planets, such as Moon revolves around the Earth.

Earth has one moon called as Luna. It looks beautiful and bright in the night sky. It has no light of its own, shines by reflected light of the Sun. Moon changes its shape from night to night and it completes one cycle in a month. The changes in the shape of Moon are called phases of Moon.

The phases of Moon are explained below:



Crescent moon in the sky                          Gibbous moon


*       Lunar Eclipse

It occurs when Moon passes into the shadow of the Earth. The Moon may remain eclipsed for about an hour.


Moon has no air and no atmosphere. This is the only celestial body on which human has landed. Neil Armstrong, an American astronaut first landed on the Moon’s surface and took samples of lunar rock.


Neil Armstrong on the surface of the moon


*       Mars

Mars is the fourth planet form the Sun in our solar system. It is named after Roman god of war because of its colour is blood red. The cause of redness of the Mars is due to the presence of iron rich soil and rock. There is no life on Mars but there could have been life long ago as ft has signs of flowing rivers of water. Solar system's biggest volcano Olympus mons exists on this planet.


     Mars                                 Phobos                                 Deimos

Mars has two moons, demos and pnobos. They look like baked potatoes.





     Which one of the following statements is correct about planets?

(a) Planets move around the sun

(b) Planets move around the earth

(c) Planets move around the moon

(d) Planets move around the comet

(e) None of these


Answer: (a)


Planets move around the Sun. Therefore, option (A) is correct and rest of the options is incorrect-



     Which one of the following planets is named after the Roman god of war?

(a) Earth

(b) Uranus

(c) Mars

(d) Venus

(e) None of these


Answer: (C)


Mars is named after the Roman god of war. Therefore, option (C) is correct and rest of the options is incorrect.



*     Outer planets


*       Jupiter

Jupiter is the biggest and fifth planet in the solar system. It can accommodate in it all the planets of the solar system. Jupiter spins on its axis faster than our- planet (72 hours). It takes nearly 12 years to revolve around the Sun. It has very strong magnetic field. Multicolored belts of frozen crystals of water, ammonia and other chemicals are seen on this planets. This planet mainly consists of hydrogen storm in the upper atmosphere that could fit two earths inside it.

There are 16 known moons of Jupiter but about two dozens of moons have been discovered in the last few years. To is Jupiter's closest moon. It smells like rotten egg because of the presence of sulphur from active volcanoes.




Note: Ganymede is the largest moon is the solar system. Callisto moon is fully covered in craters Europa, covered by thick crust ice.



*       Saturn    

Saturn is the second largest planet after Jupiter and sixth planet in the solar system- This planet is surrounded by millions of chunks of ice and rocks that follow an individual orbit around the planet. The rings of Saturn looks very beautiful. Saturn has 18 moons.


*       Titan

Titan is the largest moon of Saturn. Saturn is made up of liquids and gases. It is light enough to float.


*     Uranus

Uranus is the seventh planet in the solar system. This is the only planet in the solar system that was found by an accident. The poles on Uranus are warmer than the equator. This planet is bluish green in colour because of the presence of methane gas. Uranus has 17 moons- Titania, umbriel, ariel and miranda are the moons of this planet. Uranus is named after the Greek god of the sky.


*      Neptune


Neptune is the eight planet in the solar system and has pulling power. It is pulling Uranus. Since it's the last planet in the solar system, it takes approximately 164,8 years to orbit around the sun. There are lots of storm on this planet.

Neptune and its moons are extremely cold places. It has 8 moons recorded.

Neptune' north and south poles look pink because of frozen nitrogen.


*         Triton


Triton,one of the moon of Neptune is one of the coldest place ever recorded.





      Which one of the following is an outer planet?

(a) Jupiter

(b) Mars

(c) Earth

(d) Venus

(e) None of these


Answer: (a)


Jupiter out of the given planets is the only outer planet. Therefore, option (A) is correct and rest of the options is incorrect.



       Ganymede is a satellite of which one of the following planets?

(a) Mars

(b) Jupiter

(c) Saturn

(d) Earth

(e) None of these


Answer: (b)


Ganymede is a satellite of Jupiter. Therefore, option (B) is correct and rest of the options is incorrect..

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