Contents
The laws of Physics Topics are used to explain everything from the smallest subatomic particles to the largest galaxies.
Solar System : Definition, Planets, Diagram & Facts
‘Solar’ means ‘of the Sun’. So, ‘Solar System’ is the ‘Sun and its family of objects which revolve around it’. The various objects which revolve around the Sun are planets alongwith their satellites, and millions of other smaller objects such as asteroids, comets and meteoroids. We can now define Solar System as follows: 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 (see Figure).
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. Please note that just as planets revolve around the Sun, in the same way, the satellites (or moons) revolve around the planets. When the planets move around the Sun, then their satellites (or moons) also move alongwith them. Asteroids are small rocky bodies which revolve around the Sun between the orbits of the planets Mars and Jupiter (see Figure). Comets and meteoroids are also the minor members of the Solar System which revolve around the Sun.
The orbits of comets and meteoroids have not been shown in Figure 11 to keep the diagram simple. The Sun is the biggest object in the Solar System. Because of its great size, the Sun has a very powerful 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. In other words, the gravitational pull of the Sun keeps all the planets and other objects revolving around it. We will now describe all the members of the Solar System in detail, one by one. Let us start with the Sun.
Sun
The Sun is a star (see Figure). It is the star around which the Earth and other planets revolve. Compared with the millions of other stars, the Sun is a mediumsized 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. In fact, Sun is the nearest star to the Earth. The Sun is a star having a system of planets around it with life on one of its planets called Earth. The diameter of the Sun is about 100 times the diameter of the Earth, and mass of the Sun is more than a million times the mass of the Earth. The Sun is called ‘Surya’ in Hindi.
The Sun is an extremely hot object. The temperature at the surface of the Sun is about 6000°C. The Sun is not a solid object. 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. It is this nuclear energy which makes the Sun shine. The Sun is continuously emitting huge amounts 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. Even our Earth reflects a part of the sunlight falling on it. So, if the Earth is seen from a spaceship or moon, the Earth will also appear as a bright object in space, just like other planets.
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 (Smallest planet; It is nearest to the Sun)
- Venus
- Earth
- Mars
- Jupiter (Biggest planet)
- Saturn
- Uranus
- Neptune (It is farthest from the Sun)
The planets have no light of their own. The planets, however, shine and look like stars in the night sky. 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. In fact, the easiest way to distinguish planets from the stars in the night sky is that 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.
In general we can say that the planets keep changing their positions with respect to stars in the night sky. This is another characteristic to distinguish planets from the stars. 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 8th planet is our own Earth. A planet may be made of rock and metal, or gas. The first four planets, Mercury, Venus, Earth and Mars are much nearer to the Sun (than the other four planets). 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 comparatively small and dense bodies having solid surfaces like our Earth. Earth is the biggest of the four inner planets. The inner planets have very few natural satellites (or moons) (see Table below).
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 (Jupiter, Saturn, Uranus and Neptune) 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 (see Table below).
All the planets revolve around the Sun and also rotate on their axis. The time taken by a planet to complete one revolution around the Sun is called its period of revolution. The time taken by different planets to make one revolution around the Sun (or period of revolution) is, however, different. Actually, as the distance of a planet from the 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. Some important facts and figures about the planets of the Solar System are given below.
Some Facts and Figures about the Planets
Please note that till the year 2006 there were 9 planets in the Solar System. The 9th planet was Pluto. Pluto was the farthest planet from the Sun (beyond Neptune). In 2006, the International Astronomical Union (IAU) adopted a new definition of planet. Pluto does not fit this new definition of planet. So, though Pluto is still revolving around the Sun, it is no longer considered a planet of the Solar System. We will learn the new definition of planet in higher classes. The planets are called ‘Graha’ in Hindi. We will now describe all the planets in detail, one by one. Let us start with the first planet called Mercury.
1. Mercury
Mercury is the first planet from the Sun (see Figure). Mercury is the planet which is nearest to the Sun. Since the planet Mercury is closest to the Sun, therefore, it is very hot during the day. Mercury is the smallest planet of the Solar System. The planet Mercury has a rocky surface which is covered with craters. Mercury is always close to the Sun and usually hidden by the Sun’s glare. Due to this, Mercury is a planet which is quite difficult to see. Mercury planet is, however, 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. 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.
Planet Mercury can be seen as a ‘Morning Star’ or as an ‘Evening Star’ because it lies inside the orbit of the Earth. Please note that actually we cannot use the term ‘star’ for Mercury because it is a planet and not a star. 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. This is due to the fact that 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. 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. Mercury has no satellite. Mercury planet is known as ‘Budh Graha’ in Hindi.
2. Venus
Venus is the second planet from the Sun (see Figure). Venus is the closest planet to the Earth. The rotation of Venus on its axis is somewhat unusual.
This is because 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. This is opposite to what happens on the Earth. Venus is a rocky planet. The planet Venus has a dense atmosphere which consists almost entirely of carbon dioxide gas. Venus is the brightest planet in the night sky. In fact, the planet Venus is the brightest object in the night sky (except the Moon). So, we can easily recognise the planet Venus in the night sky by its brightness. The planet Venus appears very bright because its cloudy atmosphere reflects 75 percent of the light which it receives from the Sun. In fact, Venus reflects more sunlight than any other planet of the Solar System and hence it appears to be the brightest planet.
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). The nearness to the Sun along with its heating caused by the greenhouse effect makes Venus the hottest planet of the Solar System. Please note that though Mercury is the nearest planet to the Sun, it is not the hottest planet. The hottest planet is Venus.
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. Planet Venus can be seen as a ‘Morning Star’ or as an ‘Evening Star’ because it lies inside the orbit of the Earth (just like Mercury).
The planet Venus also shows phases like the Moon (see Figure). This is due to the fact that 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. There is no evidence of life on the planet Venus. 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. Venus has no satellite. Please note that Mercury and Venus are the only two planets of the Solar System which have no satellites (or moons) revolving around them. Venus planet is called ‘Shukra Graha in Hindi.
3. Earth
Earth is the third planet from the Sun. The two planets which lie between the Sun and the Earth are Mercury and Venus. 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 (see Figure). Earth is the only planet in the Solar System on which life is known to exist. The two major factors which are responsible for the existence of life on Earth are : Distance of the Earth from the Sun, and Size of the Earth.
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. The 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 :
(i) The Earth has an atmosphere (which contains many gases including oxygen and carbon dioxide). The Earth’s atmosphere plays an important role in maintaining life on the Earth. For example, the Earth’s atmosphere 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.
(ii) The Earth has large quantities of water. In fact, Earth is the only planet to have lots of water. This water helps in the evolution and maintenance of life on Earth. There can be no life without water.
(iii) The Earth has a suitable temperature range for the existence of life. The Earth is neither too hot nor too cold.
(iv) 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.
The Earth has two types of motion :
(i) the Earth rotates on its axis, and
(ii) the Earth revolves around the Sun. The Earth rotates (or spins) on an imaginary axis which passes through its North and South Poles (see Figure). 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 not perpendicular to the plane of the Earth’s orbit. The axis of rotation of Earth is slightly tilted with respect to the plane of its orbit (or path) around the Sun. In fact, the axis of Earth is tilted at an angle of 23.5 degrees to the perpendicular plane (see Figure). The Earth rotates on its axis in the tilted position and it also revolves around the Sun in the same tilted position throughout.
An important 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. When the Earth moves around the Sun in its orbit, it remains tilted to its orbit throughout. An important consequence of the motion of tilted Earth around the Sun is that it causes different seasons on the Earth (such as summer, autumn, winter and spring). The Earth has one natural satellite called Moon. Earth planet is called ‘Prithvi Graha’ in Hindi.
4. Mars
Mars is the fourth planet from the Sun. It is the first planet beyond Earth.
Mars is also called the red planet because its surface appears red (see Figure). The red colour of Mars is used to distinguish it from other planets of the Solar System. Mars is visible from the Earth for most part of the year.
Please note that 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 he outside the Earth’s orbit around Sun. 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.
Of all the planets, Mars is most like the Earth. 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. The planet Mars appears to have the right ingredients for life. The scientists think that though the atmosphere of Mars contains much less oxygen and water vapour (than that of the Earth), but it could be enough to support some primitive forms of life. Though no evidence of the existence of any form of life on Mars has been found so far but investigations are still going on. Mars has two natural satellites. Mars planet is called ‘Mangal Graha in Hindi.
5. Jupiter
Jupiter is the fifth planet from the Sun (see Figure). Jupiter is the biggest planet of the Solar System. It is almost twice as large as rest of the planets put together. 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. Jupiter is so large that about 1300 Earths can be placed inside this giant planet. Because of its very big size, Jupiter can be seen easily in the night sky.
Being very far off from the Sun, Jupiter receives much less heat and light from the Sun as compared to the Earth and Mars. Even then, 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. We can easily recognise Jupiter as it appears quite bright in the sky. Jupiter rotates very rapidly on its axis.
Jupiter is the first of the gas-type planets. 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. Moreover, Jupiter is a very cold planet. Jupiter has 28 satellites (or moons). It has also some faint rings around it. If we observe the planet Jupiter through a telescope, we can also see four of its large satellites (or moons). Jupiter planet is called ‘Brihaspati Graha’ in Hindi.
6. Saturn
Saturn is the sixth planet from the Sun (see Figure). Saturn is somewhat smaller in size and mass than Jupiter. But after Jupiter, Saturn is the second biggest planet of the Solar System. The chemical composition of Saturn is very similar to that of Jupiter. Thus, Saturn is also made up mainly of hydrogen and helium. One interesting thing about Saturn is that it is the least dense among all the planets of the Solar System. The density of Saturn is even less than that of water. Since Saturn is lighter than water, so if we imagine it in a large pool of water, then Saturn will float on water. It will not sink in water.
The most distinguishing feature of Saturn is the system of colourful rings which surround it (see Figure). 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. The rings of Saturn cannot be seen with naked eyes, they can be observed only with the help of a telescope.
The presence of a well-developed system of rings around Saturn makes it unique in the Solar System. Being far off from the Sun, Saturn is an extremely cold planet. So, no life can exist on Saturn. Saturn has the maximum number of satellites (or moons) of all the planets in the Solar System. Saturn has 30 satellites (or moons). Planet Saturn is called ‘Shani Graha’ in Hindi.
7. Uranus
Uranus is the seventh planet from the Sun (see Figure). It can be seen only with the help of a large telescope. In fact, Uranus was the first planet to have been discovered with the help of a telescope. Though the diameter of 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. After Jupiter and Saturn, Uranus is the third biggest planet of the Solar System. Like Venus, Uranus also rotates on its axis from east to west. The most remarkable feature of Uranus is that it has highly tilted 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 (see Figure).
Uranus is made up mainly of hydrogen and helium. Uranus is an extremely cold planet. It is also surrounded by an atmosphere of poisonous gases. Due to these reasons, no life can exist on the planet Uranus. The planet Uranus has 21 satellites (or moons). It has also some rings around it. Uranus planet is called ‘Indra Graha’ in Hindi.
8. Neptune
Neptune is the eighth planet from the Sun (see Figure). It lies beyond Uranus. Neptune is the outermost planet of the Solar System. It is the most distant planet from the Sun. Thus, the planet Neptune is farthest from the Sun. It is also farthest from the Earth. Neptune is the second planet which was discovered with the help of a telescope. The planet Neptune can be seen as a tiny blue-green speck even by using the most powerful telescope on the Earth. 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. Neptune has 8 satellites (or moons). It has also some rings around it. Neptune planet is called ‘Varun Graha’ in Hindi. We will now solve a problem taken from the NCERT Science book.
Example Problem.
The radius of Jupiter is 11 times the radius of the Earth. Calculate the ratio of the volumes of Jupiter and the Earth. How many Earths can Jupiter accommodate ? (NCERT Book Question)
Solution:
By saying that the radius of Jupiter is 11 times the radius of the Earth, we mean that if the radius of the Earth is taken to be 1 (in any units), then the radius of Jupiter will be 11 (in the same units). Jupiter and
Earth are sphere-shaped objects (like balls). The formula for calculating volume of a sphere is \(\frac{4}{3}\)πr3 where π (pi) is a constant and r is the radius of the sphere. We will now calculate the volumes of Jupiter and Earth one by one and then find out the ratio of their volumes.
(i) Volume of Jupiter = \(\frac{4}{3}\)πr3
= \(\frac{4}{3}\) × π × (11)3 (for Jupiter, r = 11)
= \(\frac{4}{3}\) × π × 1331 ……. (1)
(ii) Volume of Jupiter = \(\frac{4}{3}\)πr3
= \(\frac{4}{3}\) × π × (1)3 (for Jupiter, r = 1)
= \(\frac{4}{3}\) × π × 1 ……… (2)
Let us divide the volume of Jupiter by the volume of Earth to obtain the ratio of their volumes. So,
\(\frac{\text { Volume of Jupiter }}{\text { Volume of Earth }}\) = \(\frac{4 \times \pi \times 1331 \times 3}{3 \times 4 \times \pi \times 1}\)
= \(\frac{1331}{1}\)
Thus, the ratio of the volumes of Jupiter and Earth is 1331 : 1. Since the volume of Jupiter is 1331 times the volume of Earth, therefore, Jupiter can accommodate 1331 Earths.