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About Planets
Planets are the large celestial bodies which
revolve around the sun in closed elliptical paths
called orbits. The planets shine because they
reflect the light of the sun which falls on
them.The easiest way to distinguish planets
from the stars in the night sky is that the stars
twinkle at night but the planets do not twinkle
at night.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.

There are eight planets in the solar system -
1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune

Inner Planets
• Mercury,Venus,Earth and Mars are
inner planets. They are made of
rocks and have metallic cores. They
are comparatively small and dense
bodies having solid surfaces like our
Earth. Earth is the biggest of the four
inner planets.

Outer planets
• Jupiter, Saturn, Uranus and Neptune are called
outer planets.The first four outer planets are
very giant. They are made of hydrogen and
helium gases and not of rock and metal.
All the planets revolve around the sun and
also rotate on their axis. Some important facts
and figures about eight planets of the solar
system are given on the next slide.

Some important facts about planets
Name of Diameter Distance Time taken to turn No.of
Planet of planet from sun once on its axis moons
Mercury 4880km 58*106 58 days None
Venus 12100 km 108*106 225 days None
Earth 12760km 150*106 356 1/4 days 1
Mars 6780km 228*106 687 days 2
Jupiter 142800km 778*106 11 ¾ years 28
Saturn 120000km 1427*106 29 ½ years 30
Uranus 50800km 2870*106 84 years 21
Neptune 48600km 4504*106 165 years 8

Mercury
Mercury is the first planet from the earth.During
day time the surface of the mercury heats up to
430° but during night the temperature falls up
to -180° c due to lack of atmosphere. Mercury
has a rocky surface which is covered with
craters.Mercury is sometimes visible from
earth,just after sunrise or sunset. When mercury
is visible just before sunrise in the morning ,it is
called “Morning star” and when it is visible just
after sunset, then it is called “Evening star”.It
can be seen as morning star in the eastern sky in
the month September and October or as
evening star in the western sky in the month
March and April .

Planet mercury show phases like moon. This is due to the
fact that the mercury lies in the orbit of the earth. So, as
mercury revolves around the sun, its sunlit surface is
visible in varying amounts from the earth this produces
phases of mercury. Many features of the mercury and
Earth’s moon are as follows:
1] Mercury and moon both are nearly of the same size and
mass.
2] Mercury and moon both have no atmosphere.
3] Mercury and moon both have rocky surface covered
with craters.
Mercury and moon both show phases when viewed from
the earth.

Venus
Venus is the second planet from the earth Venus is slightly
smaller than the earth.Venus is a rocky planet. The planet
Venus has a dense atmosphere which consists almost
entirely of carbon dioxide gas.The cloudy atmosphere of
Venus is an excellent reflector of sun light. The planet
Venus is the brightest object in the night sky(except the
moon).Being quite near to the sun , the planet Venus is
very hot .The planet Venus also gets heated excessively by
the trapping of the sun’s heat rays by carbon dioxide gas
present in its atmosphere which is also called green house
effect The maximum temperature on the planet is 480°c.

The planet Venus can be seen as a morning star in the
eastern sky or as a evening star in the western sky
The venues shows phases like moon. This is due to
the fact that the 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 the 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. Mercury and Venus are only two
planets which have no satellites or moons revolving
around the sun.

Earth
Earth is the third planet from the earth.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 .The earth has an atmosphere contains gases
like nitrogen,oxygen,carbon dioxide, noble gases ,water
vapor and some ozone .

The earth rotates on its axis.It rotates on an imaginary axis
which phases which through its north and south pole. The
average temperature of the surface of the earth is 15°c
The earth has one natural satellite called moon. The two
major factors which factors which are responsible for the
existence of life on the planet earth are :size of the earth
and distance of the earth from the sun.

The motions of the earth
• Rotation
The earth rotates or spin on an imaginary axis which
phases through its north and south poles.The earth
completes one rotation on its axis in 24 hours which
we call one day. This causes day and night.The earth
rotates on its axis from east to west.The axis of rotation
of earth is slightly tilted with respect to the plane of its
orbit around the sun.

The rotation of the earth causes Day
and Night.
An important consequence of the rotation of the earth on
its axis is that it causes day and night on the earth. As the
earth rotates on its axis, first one half of the Earth is
turned around towards the sun. This part of earth
receives sunlight and has day on it. The part of earth
which is turned away from the sun does not receive any
sunlight, it remains dark and has night.Almost every 12
hours, a day changes into night, and a night changes into
day.The change in the length of a day and night is,
however, caused by the tilt of Earth’s axis.

Mars
Mars is the fourth planet from the sun.Mars is also called
the Red planet because its surface appears red.The red
color of Mars is caused by the presence of hydrated iron
oxide compound on its soil.
Of all of 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.

Since the planet Mars is very far off from the Sun, so it is
quite cold planet. The surface temperature on Mars
varies from a minimum of, -87°c to a maximum of
17°c. It has no water. The atmosphere of mars is so
thin that it allows the deadly ultra violet rays of the sun
to reach its surface.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 called ‘phobos’ and ‘Deimos’.

Jupiter
Jupiter is the fifth planet from the earth.Jupiter is the
biggest planet of the solar system.The diameter of
Jupiter is 11 times the diameter of the Earth and mass
is about 318 times that of the Earth.Because of its very
big size, Jupiter can be seen easily in the night
sky.Jupiter is made mainly of hydrogen and helium
gases. Jupiter has a very deep atmosphere . The
atmosphere of the Jupiter consists of helium,
hydrogen, methane and ammonia.The surface of the
Jupiter is mainly made of liquid hydrogen.

Jupiter is the first of the gas type planets. Below the
surface, increasing pressure changes this liquid this
liquid hydrogen to solid state.Jupiter appears to be
the brightest object in the night sky. Jupiter’s bright
appearance is due to the fact that is it has a thick,
cloudy atmosphere which reflects most of the
sunlight falling on it.Jupiter is a very cold planet, its
surface temperature being,-180°c. Jupiter has 28
satellites .It has also some faint rings around it.

Saturn
Saturn is the sixth planet from the sun.Saturn is somewhat
smaller in size and mass than Jupiter.Saturn is the second
biggest planet in the solar system.The distance of Saturn is
almost double that of Jupiter The chemical composition of
Saturn is very similar to that of Jupiter.Thus, Saturn is
made up mainly of hydrogen and helium gases.The
atmosphere of Saturn consists of hydrogen, helium,
methane and ammonia gases.Saturn has no solid
surface.The surface of Saturn is liquid hydrogen.Below the
surface is frozen hydrogen.The most distinguish feature of
Saturn is the system of colorful rings which surround it.

Three distinct sets of rings of Saturn are visible from 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 telescope.The
presence of a well-developed system of rings around
Saturn makes it unique in the solar system.Rings have
now been discovered around other planets –Jupiter,
Uranus and Neptune

Uranus
Uranus is the seventh planet from the sun.Uranus was
the first planet to have been discovered with the help
of a telescope.Uranus was discovered with the William
Herschel in the year 1781.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.The
distance of Uranus from the sun is almost double that
of the Saturn.After Jupiter and Saturn, Uranus is the
third biggest planet in the solar system.

After Jupiter and Saturn, Uranus is the third biggest
planet in the solar system.
Uranus is made up of mainly of hydrogen and helium.It
is surrounded by a thick atmosphere of hydrogen,
helium and methane. Uranus is an extremely cold
planet, its surface temperature being,-190°C. 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.

Neptune
Neptune is the eighth planet from the sun.It lies beyond
the Uranus. Neptune is the second planet which is
discovered with the help of telescope.Neptune was the
first planet to be discovered as a result of predictions.
Neptune was discovered in 1846 as a result of the effect
of its gravitational pull on Uranus. The astronomers
noticed that the planet Uranus was not moving in its
orbit around the sun exactly as expected. The Uranus
had a slightly,irregular orbit as if ,something, was pulling
it out of its orbit due to its gravitational force.

In order to explain the deviation of Uranus from its normal
orbit , as an astronomer Le Verrier predicated the existence
of another planet beyond Uranus in June 1846. He thought
that this distant planet was disturbing the orbit of Uranus
due to its gravitational force. By making mathematical
calculations,Le Verrier even predicated the mass and size of
the unknown planet correctly.Le Verrier asked the
astronomers all over the world to search for the new
planet with their telescope .Johann Galle discovered this
planet in September1846.It was named Neptune.It was
Newton's Law of gravitation which formed the basis of Le
Verrier’s calculations which led to the discovery of planet
Neptune.

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