The Solar System

Question 1 What is a Solar system? Name the different celestial objects which are members of the Solar system?

Question 2 What are planets? How many planets are there in a Solar system?

Question 3 Name all the planets of Solar system?

Question 4 What are inner and outer planets? Name them?

Question 5 How can you distinguish between planets and stars at night?

Question 6 What are the various environmental conditions on Earth which are responsible for the existence and continuation of life?

Question 7 What is the consequence of Rotation of Earth on axis?

Question 8 What is the consequence of motion of tilted Earth around the Sun?

Question 9 Why life is not possible on other planets except Earth?

Question 10 Define the term morning star and evening star?

Question 11 Why planet Mercury shows phases like the Moon?

The Solar System

Solar System consists of the Sun, the eight Planets and their Satellites (or moons), and millions of smaller celestial objects such as Asteroids, Comets and Meteoroids. The Sun is at the centre of the Solar System and all other objects are revolving around it in fixed elliptical paths called orbits.

The eight planets of the Solar System (in order of their increasing distances from the Sun) are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. All these planets are orbiting around the Sun. All the planets (except Mercury and Venus) have natural satellites (or moons) around them.

The satellites (or moons) revolve around the planets. When the planets move around the Sun, then their satellites (for moons) also move along with them.

Asteroids are small rocky bodies which revolve around the Sun between the orbits of the planets Mars and Jupiter. Comets and meteoroids are also the minor members of the Solar System which revolve around the Sun.

The Sun is the biggest object in the Solar System. Because of its great size, the Sun has a gravitational force (or gravitational pull). The gravitational force of the Sun keeps the Solar system together and controls the movements of planets and other members of the Solar System.

 

Sun

The Sun is the the star around which the Earth and other planets revolve. The Sun is a medium-sized star and of average brightness. The Sun appears to be bigger and brighter because it is much more nearer to the Earth than any other star.

The Sun is a star having a system of planets around it with life on one of its planets called Earth.

The temperature at the surface of the Sun is about 6000°C. It is a sphere of hot gases. The Sun consists mostly of hydrogen gas. The nuclear fusion reactions taking place in the centre of the Sun (in which hydrogen is converted into helium), produce a tremendous amount of energy in the form of heat and light.

The Sun is the main source of heat and light energy for all the planets of the Solar System (including the Earth) and their satellites, etc. The planets and other objects in the sky reflect a part of the sunlight falling on them due to which they shine and become visible to us.

Planets

Planets are the large celestial objects (or celestial bodies) which revolve around the Sun in closed elliptical paths called orbits. There are 8 major planets (including the Earth).

The planets of the Solar System are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. 

The planets shine because they reflect the light of the Sun which falls on them. Since the planets are much nearer to us than the stars, they appear to be big and do not twinkle at night.

The stars appear to twinkle at night but the planets do not twinkle at all. The planets move around the Sun from west to east, so the relative positions of the planets in the night sky keep changing day by day.

The planets are very small as compared to the Sun or other stars. Every planet has its own fixed path (called orbit) in which it revolves (or moves) around the Sun. Since all the planets revolve in their separate, fixed paths (or fixed orbits), they do not collide with one another while revolving around the Sun.

Out of the 8 planets of the Solar System, 5 planets, Mercury, Venus, Mars, Jupiter and Saturn can be seen easily with the naked eye, so they were known to the ancient astronomers. The 2 planets, Uranus and Neptune are very far off and have been discovered with the help of a telescope.

The first four planets are called inner planets. The four inner planets (Mercury, Venus, Earth and Mars) are made of rocks and have metallic cores. The four inner planets are small and dense bodies having solid surfaces like our Earth. The inner planets have very few natural satellites (or moons)

The planets outside the orbit of Mars are called outer planets. Thus, Jupiter, Saturn, Uranus and Neptune are called outer planets. The outer planets are much farther off from the Sun than the inner planets. The four outer planets are giant planets (very, very big planets). The four outer planets are made mainly of hydrogen and helium gases, and not of rock and metal. They do not have solid surfaces at all. The outer planets have ring systems around them. The outer planets also have a large number of natural satellites (or moons) around them.

The time taken by a planet to complete one revolution around the Sun is called period of revolution. As the distance of a planet from Sun increases, its period of revolution also increases.

The time taken by a planet to complete one rotation on its axis, is called its period of rotation. The time taken by different planets to rotate once on their axis (or period of rotation), is also different.

 

Mercury

(1) Mercury is the planet which is nearest to the Sun. 

(2) It is very hot during the day.

(3) Mercury is the smallest planet of the Solar System.

(4) The planet Mercury has a rocky surface which is covered with craters.

(5) Mercury is a planet which is quite difficult to see. Mercury planet is visible just before sun-rise or just after sun-set, near the horizon. So, planet Mercury can be seen only at those places where trees and other buildings do not obstruct the view of the horizon.

(6) When planet Mercury is visible just before sun-rise in the morning, it is called a Morning Star and when it is visible just after sun-set in the evening, then it is called an Evening Star. The planet Mercury can be seen either as a Morning Star in the eastern sky just before sun-rise or as an Evening Star in the western sky just after sun-set.

7) Planet Mercury can be seen as a Morning Star or as an Evening Star because it lies inside the orbit of the Earth. Mercury is termed as Morning Star or Evening Star because it is a fairly bright object in the sky and appears like a star. In fact, Mercury shines because it reflects light from the Sun.

Planet Mercury shows phases like the Moon

Mercury lies inside the Earth’s orbit. So, as Mercury revolves around the Sun, its sun-lit surface is visible in varying amounts from the Earth. This produces phases of Mercury.

(8) No life can exist on the planet Mercury because it is extremely hot and has no water on it. Mercury has also no atmosphere to prevent the deadly ultraviolet radiations of the Sun from reaching its surface.

(9) Mercury has no satellite.

Venus

(1) Venus is the second planet from the Sun.

(2) Venus rotates on its axis from east to west (whereas Earth rotates on its axis from west to east). Due to this, on Venus the Sun would rise in the west and set in the east.

(3) Venus is a rocky planet.

(4) The planet Venus has a dense atmosphere which consists almost entirely of carbon dioxide gas.

(5) Venus is the brightest planet in the night sky. The planet Venus appears very bright because its cloudy atmosphere reflects 75 per cent of the light which it receives from the Sun. Venus reflects more sunlight than any other planet of the Solar System and hence it appears to be the brightest planet.

(6) Being quite near to the Sun, the planet Venus is very hot. The planet Venus also gets heated excessively by the trapping of Sun’s heat rays by carbon dioxide gas present in its atmosphere (which is called greenhouse effect).

(7) The planet Venus is also called a Morning Star or an Evening Star. The planet Venus can be seen either as a Morning Star in the eastern sky just before sun-rise or as an Evening Star in the western sky just after sun-set.

(8) The planet Venus also shows phases like the Moon. Venus lies inside the Earth’s orbit. So, as Venus revolves around the Sun, its sun-lit surface is presented to the Earth in varying amounts. This produces the phases of Venus.

(9) Life cannot exist on the planet Venus because it is extremely hot, it has no water and there is no sufficient oxygen in its atmosphere,.

(10) Venus has no satellite.

(11) Venus planet is called Shukra Graha in Hindi.

 

Earth

(1) The Earth is spherical in shape. When viewed from the outer space, the Earth appears to be a blue and green ball due to the reflection of sunlight from water and land on its surface.

(2) Earth is the only planet in the Solar System on which life is known to exist.

(3) The two major factors which are responsible for the existence of life on Earth are :

(a) The distance of Earth from the Sun is such that it has the correct temperature range for the existence of water and survival of life.

(b) Size of the Earth is such that it has sufficient gravitational field to hold on to an atmosphere of many gases (like oxygen and carbon dioxide) which are needed for the evolution and maintenance of life.

The various environmental conditions available on Earth which are responsible for the existence and continuation of life on Earth are as follows:

(1) The Earth has an atmosphere which has sufficient oxygen, the gas we need in order to live. The Earth’s atmosphere also supplies carbon dioxide needed for the preparation of food by photosynthesis by the plants.

(2) The Earth has large quantities of water: Earth is the only planet to have lots of water. This water helps in the evolution and maintenance of life on Earth. 

(3)The Earth has a suitable temperature range for the existence of life. The Earth is nether too hot nor too cold.

(4) The Earth has a protective blanket of ozone layer high up in the atmosphere. This ozone layer absorbs most of the extremely harmful ultraviolet radiations coming from the Sun and prevents them from reaching the Earth and hence protects the living things on the Earth.

(4) The Earth has two types of motion: (1) the Earth rotates on its axis, and (2) the Earth revolves around the Sun.

The Earth rotates (or spins) on an imaginary axis which passes through its North and South Poles.The Earth completes one rotation on its axis in 24 hours which we call one day. The Earth rotates (or spins) on its axis from west to east. The axis of rotation of Earth is slightly tilted with respect to the plane of its orbit (or path) around the Sun. The Earth rotates on its axis in the tilted position and it also revolves around the Sun in the same tilted position throughout.

Consequence of the rotation of Earth on its axis is that it causes day and night on the Earth. The Earth rotates (or spins) on its axis and also revolves (or moves) around the Sun in an elliptical orbit. The Earth takes 1 year to complete one revolution around the Sun.

An important consequence of the motion of tilted Earth causes different seasons on the Earth (such as summer, autumn, winter and spring).

(5) The Earth has one natural satellite called Moon.

(6) Earth planet is called ‘Prithvi Graha.

Mars

(1) Mars is also called the red planet because its surface appears red.

(2) Mars is visible from the Earth for most part of the year. Mars (and all other planets beyond Mars) do not appear as Morning Stars or Evening Stars, and they also do not show phases like the Moon. This is because they all lie outside the Earth’s orbit around Sun.

(3) Mars is a small planet having a small mass. Since the planet Mars is very far off from the Sun, so it is quite a cold planet.

(4) Mars is a rocky planet. Mars has a thin atmosphere as compared to the Earth. The thin atmosphere of Mars contains mainly carbon dioxide with small amounts of nitrogen, oxygen, noble gases and water vapour. 

(5) Mars has two natural satellites.

(6) Mars planet is called ‘Mangal Graha’ in Hindi.

Jupiter

(1) Jupiter is the fifth planet from the Sun. 

(2) Jupiter is the biggest planet of the Solar System. It is almost twice as large as rest of the planets put together.

(3) The mass of Jupiter is also more than the combined mass of all other planets. The diameter of Jupiter is 11 times the diameter of the Earth and its mass is about 318 times that of the Earth.

(4) Because of its very big size, Jupiter can be seen easily in the night sky.

(5) Being very far off from the Sun, Jupiter receives much less heat and light from the Sun as compared to the Earth and Mars.

(6) Jupiter appears to be a very bright object in the night sky. Jupiter’s bright appearance is due to the fact that it has a thick cloudy atmosphere which reflects most of the sunlight falling on it.

(7) Jupiter rotates very rapidly on its axis.

(8) Jupiter is made mainly of hydrogen and helium. Life cannot exist on the planet Jupiter because it has poisonous gases (like methane and ammonia) in its atmosphere.

(8) Jupiter is a very cold planet.

(9) Jupiter has 28 satellites (or moons). It has also some faint rings around it.

(10) Jupiter planet is called ‘Brihaspati Graha’ in Hindi.

 

Saturn

(1) Saturn is the sixth planet from the Sun.

(2) After Jupiter, Saturn is the second biggest planet of the Solar System.

(3) Saturn is also made up mainly of hydrogen and helium.

(4) It is the least dense among all the planets of the Solar System. The density of Saturn is even less than that of water.

(5) Saturn has a system of colourful rings which surround it. Three distinct sets of rings around Saturn are visible from the Earth. Saturn is the only planet with a system of well-developed rings encircling it. The rings of Saturn are made up of tiny particles, all orbiting the Saturn like miniature satellites.

(6) The presence of a well-developed system of rings around Saturn makes it unique in the Solar System.

(7) Being far off from the Sun, Saturn is an extremely cold planet. So, no life can exist on Saturn.

(8) Saturn has the maximum number of satellites (or moons) of all the planets in the Solar System. Saturn has 30 satellites (or moons).

(9) Planet Saturn is called ‘Shani Graha’ in Hindi.

Uranus

(1) Uranus is the seventh planet from the Sun .

(2) It can be seen only with the help of a large telescope.

(3) Uranus is almost four times that of the Earth, it appears as a small disc through a telescope. This is because Uranus is very, very far off from the Earth.

(4) Uranus also rotates on its axis from east to west. It has highly tiled axis of rotation. As a result of the highly tilted axis of rotation, Uranus appears to roll on its side while orbiting around the Sun.

(5) Uranus is made up mainly of hydrogen and helium. Uranus is an extremely cold planet.

(6) It is also surrounded by an atmosphere of poisonous gases. Due to these reasons, no life can exist on the planet Uranus.

(7) The planet Uranus has 21 satellites (or moons). It has also some rings around it.

(8) Uranus planet is called Indra Graha’ in Hindi.

Neptune

(1) Neptune is the eighth planet from the Sun.

(2) It is the most distant planet from the Sun.

(3) Neptune is the second planet which was discovered with the help of a telescope.

(4) The planet Neptune can be seen as a tiny blue-green speck even by using the most powerful telescope on the Earth.

(5) Neptune is made up mainly of liquid and frozen hydrogen and helium gases. Neptune is an extremely cold planet. So, no life can exist on the planet Neptune.

(6) Neptune has 8 satellites (or moons). It has also some rings around it.

(7) Neptune planet is called ‘Varun Graha’ in Hindi.