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Chapter: 9th Science : Universe

The Solar System

The Sun and celestial bodies which revolve around it form the solar system. It consists of large number of bodies such as planets, comets, asteroids and meteors. The gravitational force of attraction between the Sun and these objects keep them revolving around it.

The Solar System

The Sun and celestial bodies which revolve around it form the solar system. It consists of large number of bodies such as planets, comets, asteroids and meteors. The gravitational force of attraction between the Sun and these objects keep them revolving around it.     


1. The Sun

The Sun is sometimes referred to by its Latin name Sol or by its Greek name Helios. The ancient Greeks grouped the Sun together with the other celestial bodies which moved across the sky, calling them all planets. But the Sun is a medium sized star, a very fiery spinning ball of hot gases. Three quarters of the Sun has hydrogen gas and one quarter has helium gas. It is over a million times as big as the Earth. Hydrogen atoms combine or fuse together to form helium under enormous pressure. This process, called nuclear fusion releases enormous amount of energy as light and heat. It is this energy which makes Sun shine and provide heat. The Sun is situated at the centre of the solar system. The strong gravitational fields cause other solar matter, mainly planets, asteroids, comets, meteoroids and other debris, to orbit around it. The Sun is believed to be more than 4.6 billion years old.

Formation of the Sun

At the time of the Big Bang, hydrogen gas condensed to form huge clouds, which later concentrated and formed the numerous galaxies. Some of the hydrogen gas was left free and started floating around in our galaxy. With time, due to some changes, this free-floating hydrogen gas concentrated and paved way for the formation of the Sun and solar system. Gradually, the Sun and the solar system turned into a slowly spinning molecular cloud, composed of hydrogen and helium molecules, along with dust. The cloud started to undergo the process of compression, as a result of its own gravity. Its excessive and high-speed spinning ultimately resulted in its flattening into a giant disc.


The Sun rotates on its axis. Since the Sun is primarily made of very hot gas, the surface at the equator rotates once every 25.4 days. The rotation near the poles takes around 36 days.

Energy output

Most of the energy emitted by the Sun is visible light and a form of radiation known as infrared rays, which we feel as heat.


It is a common misconception that Sun emits yellow color radiation and it is not true. The radiation coming from the Sun contains all the colors. But the yellow is most intense among all the colors.

Sunlight is scattered by the molecules when it passes through the Earth's atmosphere. The scattering of light depends on the color. Blue and violet are scattered more and red is scattered less.


2. Planets

A planet revolves around the Sun along a definite curved path which is called an orbit. It is elliptical. The time taken by a planet to complete one revolution is called its period of revolution. The period of revolution increases as the distance of the planet from the Sun increases. Thus the period of revolution of the Earth is 365.30 days whereas that of Neptune is 164.80 years.

Besides revolving around the Sun, a planet also rotates on its own axis like a top. The time taken by a planet to complete one rotation is called its period of rotation. The period of rotation of the Earth is 23 hours and 56 minutes and so the length of a day on Earth is taken as 24 hours. Table 3.1 tells about the length of a day on each planet. A day on the planet mercury is 59 Earth days, i.e., 59 × 24 = 1416 hours. Jupiter rotates so fast that a day lasts only less than 10 hours.

The planets are spaced unevenly. The first four planets are relatively close together and close to the Sun. They form the inner solar system. Farther from the Sun is the outer solar system, where the planets are much more spread out. Thus the distance between Saturn and Uranus is much greater (about 20 times) than between the Earth and the Mars.

The four planets grouped together in the inner solar system are Mercury, Venus, Earth and Mars. They are called inner planets. They have a surface of solid rock crust and so are called terrestrial or rocky planets. Their insides, surfaces and atmospheres are formed in a similar way and form similar pattern. Our planet, Earth can be taken as a model of the other three planets.

The four large planets Jupiter, Saturn, Uranus and Neptune spread out in the outer solar system that slowly orbit the Sun are called outer planets. They are made of hydrogen, helium and other gases in huge amounts and have very dense atmosphere. They are known as gas giants and are called gaseous planets. The four outer planets Jupiter, Saturn, Uranus and Neptune have rings whereas the four inner planets do not have any rings. The rings are actually tiny pieces of rock covered with ice. Now let us learn about each planet in the solar system.


Mercury is a rocky planet nearest to the Sun. It is very hot during day but very cold at night. It moves around the Sun faster than any other planet – one year being only 87.97 Earth days and rotates very slowly. One day is equal to 58.65 days. Mercury can be easily observed thorough telescope than naked eye since it is very faint and small. It always appears in the eastern horizon or western horizon of the sky.


Venus is a special planet from the Sun, almost the same size as the Earth. It is the hottest planet in our solar system. After our moon, it is the brightest heavenly body in our night sky. A day on this planet is longer than its year. A day on this planet is 243 Earth days, and a year is only 224.7 Earth days. This planet spins in the opposite direction to all other planet and so unlike Earth, the Sun rises in the west and sets in the east here. Venus can be seen clearly through naked eye. It always appears in the horizon of eastern or western sky.

The Earth

The Earth where we live is the only planet in the solar system which supports life. Due to its right distance from the Sun it has the right temperature, the presence of water and suitable atmosphere and a blanket of ozone. All these have made continuation of life possible on the Earth. It moves around the Sun in 365.25 days and rotation period is 23.93 hours. The axis of rotation of the Earth is not perpendicular to the plane of its orbit. The tilt is responsible for the change of seasons on the Earth. From space, the Earth appears bluish green due to the reflection of light from water and land mass on its surface.

The Earth rotates on its axis from west to east (Fig. 3.10), so the Sun appears to move in its opposite direction that is from east to west. Life on Earth as we know would not be possible without the Sun. The solar energy from the Sun has supported and sustained terrestrial existence on Earth since the beginning of time.


The first planet outside the orbit of the Earth is Mars. It appears slightly reddish and therefore it is also called the red planet. It has two small natural satellites (Deimos and Phobos). A natural satellite of any planet is called moon. One day on this planet is of 24 hours 37 minutes 22 seconds, and one year is 686.98 days, i.e., 687 Earth days.


Jupiter is called as Giant planet. It is the largest of all planets (about 11 times larger and 318 times heavier than Earth). It has 3 rings and 65 moons. Its moon Ganymede is the largest moon of our solar system. Rotating faster than any other planet, Jupiter has the shortest days - one day lasting only 9 hours 55 minutes 30 seconds. One year in Jupiter equals our 11.862 years.


Known for its bright shiny rings, Saturn appears yellowish in colour. It is the second biggest and a giant gas planet in the outer solar system. It rotates very fast - the rotation period being 10.7 hours but revolves slowly around the Sun - the revolution period being 29.46 Earth years. At least 60 moons are present - the largest being Titan. Titan is the only moon in the solar system with clouds. Having least density of all (30 times less than Earth), this planet is so light.


Uranus is a cold gas giant and it is the seventh planet from the Sun in the solar system. It can be seen only with the help of large telescope. It has a greatly tilted axis of rotation. As a result, in its orbital motion it appears to roll on its side. Its revolution period is 84 Earth years and the rotation period is 17.2 hours. Due to its peculiar tilt, it has the longest summers and winters each lasting 42 years.


It appears as Greenish star. It is the eighth planet from the Sun and is the windiest planet. Every 248 years, Pluto crosses its orbit. This situation continues for 20 years. It has 13 moons – Triton being the largest. Triton is the only moon in the solar system that moves in the opposite direction to the direction in which its planet spins.


3. Other bodies of the solar system

Besides the eight planets, there are some other bodies which revolve around the Sun. They are also members of the solar system.


There is a large gap in between the orbits of Mars and Jupiter. This gap is occupied by a broad belt containing about half a million pieces of rocks that were left over when the planets were formed and now revolve around the Sun. These are called asteroids. The biggest asteroid is Ceres – 946 km across. Every 50 million years, the Earth is hit by an asteroid nearing 10 km across. Asteroids can only be seen through large telescope.


Comets are lumps of dust and ice that revolve around the Sun in highly elliptical orbits. Their period of revolution is very long.

When approaching the Sun, a comet vaporizes and forms a head and tail. Some of the biggest comets even seen had tails 160 million (16 crores) km long. This is more than the distance between the Earth and the Sun. Many comets are known to appear periodically. One such comet is Halley’s Comet, which appears after nearly every 76 years. It was last seen in 1986. It will next be seen in 2062.

Meteors and Meteorites

Meteors are small piece of rocks scattered throughout the solar system. Traveling with high speed, these small pieces come closer to the Earth’s atmosphere and are attracted by the gravitational force of Earth. Most of them are burnt up by the heat generated due to friction in the Earth’s atmosphere. They are called meteors. Some of the bigger meteors may not be burnt completely and they fall on the surface of Earth. These are called meteorites.


A body moving in an orbit around a planet is called satellite. In order to distinguish them from the man made satellites (called as artificial satellites), they are called as natural satellites or moons. Satellite of the Earth is called Moon (other satellites are written as moon). It moves around the Earth once in 27.3 days in an approximate circular orbit of radius 3.85 × 105 km. Natural satellites do not make their own light. We can see the Earth’s satellite Moon, because it reflects the light of the Sun. Satellite moves around the planets due to gravity, and the centripetal force. Among the planets in the solar system all the planets have moons except Mercury and Venus.


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