what is the solar system and what are its members | Knowledge of Physics

 What is solar system?

The Sun with other astronomical bodies such as eight planets, dwarf planets, natural satellites, comets, meteors and small dust particles scattered between these planets, form a group, called the solar system. In this system, the Sun lies at the center and other bodies revolve around the sun on their fixed path, called the orbit. The Sun, being larger than any other bodies of solar system, holds all members together due to its gravitational pulling. 

Fig.1 The Solar System, Sun and Planets

Members of Solar System:

The most considerable members of the Solar system are listed as follow:

1. the Sun 
2. Eight planets
3. comets
4. the Moon, natural satellite of earth etc.

Some important features of these members are listed here as given below.

1. The Sun

• The Sun is the major member of solar system and it is located at the center of the system.
• All other members revolve around the sun due to the gravitational pull of the sun.
• Sun is major source of heat and light that sustain life on earth.
• About 99% mass of solar system is occupied by the mass of the Sun.
• Its diameter is about 13,446 km.
• Just as planets revolve around the sun, the sun also revolve around the center of galaxy named  Milky Way, called galactic center.
• Solar gravity is almost 28 times that of earth, that is, (`g_s = 28 \times g_e`). Where `g_s` is solar gravity and `g_e` is earth gravity whose value is 9.98 `ms^-{2}`.
• Solar surface is called photosphere
• The temperature at the solar surface is `6000^o`C and that at the center is `1,560,000^o`C.
• The instrument used for observing solar surface is called  Helioscope.
• High energy is generated in the sun due to nuclear fusion reaction, in which hydrogen atoms combine together to form helium atom with the release of huge amount of energy.

Nuclear Fusin Reaction:

`1H^2 + 1H^3 = 2He^4 + on^1` + Energy(E) -----(1)

Where, `1H^2` = Deuterium Atom of Hydrogen, where 1 denotes atomic number and 2 denotes atomic mass of deuterium.

`1H^3` = Tritium atom of Hydrogen

`2He^4` = Helium atom

`on^1` = neutron, where o denotes to electrically neutral nature of neutron and 1 denotes its mass.

• Another star closer to the Sun is called Alpha Centauri.
• The galaxy very near to Milky Way galaxy is Andromeda.

2. The planets

There are eight planets in the solar system. They are 

i) Mercury

ii) Venus

iii) Earth

iv) Mars

v) Jupiter

vi) Saturn

vii) Uranus

viii) Neptune

Trick to Remember the names: My Very Energetic Mom Just Served Us Noodles. This trick helps you to remember name of planets from close to far from the sun.

Short Description of eight planets:

i) Mercury

• it is the smallest and closest planet to the sun
• it is also called morning star because of its visibility from the earth in the morning time.
• its diameter is about 4,847 km
• it is about `57.3 \times 10^6` km away from the sun
• it takes 88 days to orbit around the sun, which shows that mercury is the planet of the shortest year.
• surface facing towards the sun has temperature `420^o`C and the surface opposite to it has temperature of `273^o`C. So, it is second hottest planet. The first hottest planet is Venus.
• It has no natural satellites. It does not have atmosphere either. This shows that no life is possible on this planet.
• One day Mercury equals 59 days on earth.
• First spacecraft that reached on this planet is Mariner-10, which was launched by NASA on  November 3, 1973.

ii) Venus

• it is the second closest planet from the sun.
• it is is about `97.3 \times 10^6` km far from the sun
• its diameter is 12, 104 km
• it comes closer to the earth than any other planets
• its temperature reaches up to `464^o`C, so is called the hottest planet of solar system
• its surface is covered with thick layer of clouds that keeps it hotter than any other planets.
• Its atmosphere is mainly composed of carbon dioxide(`CO_2`) gas and small amount of water vapor(`H_2O`)
• it takes 225 days to orbit the sun
• One day on Venus equals 243 days on earth .

iii) Earth

• it is the third planet from the sun and fifth largest planet in the solar system.
• it is only the planet known to support lives
• average distance between the sun and the earth is `150 \times 10^6` km.
• its diameter is 12,756 km
• it takes almost 23 hours, 56 minutes, and 4 seconds to complete a rotation in its axis of rotation. The direction of earth rotation is from west to east, that's why, sun appears to rise from east and sets from west making us experience day and night within every 24 hours.
• it takes 365 days 6 hours 9 minutes and 9.5 seconds to revolve around the sun. It is called revolution of earth. It causes change in seasons on earth.
• average surface temperature is `15^o`C
• earth is divided into two parts: dry land and oceans
• it is surrounded by band of gases, called the atmosphere, containing mainly nitrogen, oxygen, argon, water vapor gases.
• As other planets, earth revolves around the sun in oval shaped orbit. The change in seasons is caused because earth is tilted at angle of `66\frac{1}{2}^o` on its axis.
• Equatorial line divides earth sphere into two hemispheres: Northern and Sourthern hemispheres. When the northern hemisphere faces towards the sun, most of sun rays fall directly on this region making its temperature high which results here summer season.
• Meanwhile the southern hemisphere facing away from the sun has winter season since it receives only indirect sun rays  
• Half-way between spring and autumn, there is a time called equinox, when all parts of earth have equal length of day and night.
• earth has a single natural satellite, called the Moon. 

iv) Mars

• Mars is fourth planet from the sun
• Due to its red appearance caused by oxides of iron, it is also called red planet.
• its diameter is 6,720 km, half the diameter of earth
• its distance from the sun is `225.6 \times 10^6`km
• it has very thin layer of atmosphere made up of carbon dioxide with small amount of nitrogen adn argon.
• its surface temperature varies from `27^o`C at day time to `-73^o`C at night time. Average temperature becomes almost `-53^o`C.
• this planet has spots under the crust, which may explain volcano activity.
• it also has seasons like those on earth.
• it takes 687 days to revolve around sun in its orbit.
• rotation time is 24 hours 37 minutes in its axis, almost same as that of earth
• It has two natural satellites named Phobos and Deimos.  

v) Jupiter

• it is fifth planet from the sun and is the largest planet in the solar system
• its diameter is 140,800 km 
• it is `768\times10^6` km away from the sun
• it takes 12 years to revolve around the sun
• takes 9 hours 55 minutes to rotate in its axis. It is the planet of the shortest day
• its surface is covered by thick atmosphere containing mainly `H_2, NH_3, CH_4` and He.
• Does not have solid surface, has low density.
• average temperature is `-15^o`C
• It has 80 natural satellites. They are divided into two groups. The first group contains stalittes such as Europa, Ganymede, Callisto, Metis, Adrastea, Amalthea and Thebe. The second group contains satellites such as Leda, Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae and Sinope.
• It has three rings.

vi) Saturn

• It is the sixth planet from the sun and second largest planet of solar system.
• its diameter is 113, 600 km
• its mean distance from the sun is `1600\times10^6` km, almost twice as far as that of Jupiter
• Time of revolution around the sun: almost 30 years
• time of rotation in its axis: 10 hours 14 minutes
• covered with thick layer of atmosphere containing mainly `H_2, NH_3, CH_4` etc.
• average surface temperature is `-188^o`C
• volume is about 700 times that of earth, and mass is about 95 times that of earth
• Has very low density, making it lighter than water
• it has five rings 
• it has 83 known natural satellites. Some of them are Pan, Atlas, Prometheus, Pandora, Epimetheus, Janus, Mimas, Enceladus, Tethys, Telesto, Calypso, Dione, Helene, Rhea, Titan, Hyperion, Lapeteus, Phoebe etc.

vii) Uranus

• It is seventh planet from the sun third largest planet of solar system.
• its diameter is 51,200 km, four times bigger than earth
• its distance from the sun `2870\times10^6` km
• length of revolution around the sun: 84 years
• length of rotation in its axis: 17 hours 14 minutes
• average surface temperatue is `-210^o`C
• mass is about 15 times the mass of earth
• has low density
• has thick layer of atmosphere containing `H_2, CH_4, NH_3`, He etc.
• it has 11 rings and 27 natural satellites. Some of its satellites are Ophelia, Portia, Juliet, Cupid, Belinda, Miranda, Titania, Oberon etc.

viii) Neptune

• It is eighth planet from the sun and fourth bigger planet of solar system
• its diameter is 48, 000 km, about 4 times the diameter of earth
• its distance from the sun is `4480\times10^6`km
• mass is about 14 times that of earth
• length of revolution around the sun: 164 years
• length of rotation in its axis: 16 hours
• average surface temperature is `-225^o`C, the coldest planet of solar system
• it has thick layer of atmosphere containing `H_2, CH_4, NH_3`, He etc.
• its rapid rotation produces giant storm system that looks like dark spot in its surface
• it has a ring and 14 natural satellites. They are Naiad, Thalassa, Despina, Galatea, Larissa, Hippocamp, Proteus, Triton, Nereid, Halimede, Sao, Laomedeia, Psamathe and Neso.  
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Trick to Remember planets from larger in size to smaller:

Just Started Using New Essential Version of Mars and Mercury !

From this trick, you can remember that the largest planet of solar system is Jupiter and the smallest one is Mercury. Our Earth is fifth largest planet. Earth is just smaller than Neptune and just larger than Venus.

Types of planets in Solar System: 

1. Terrestrial Planets

Most of terrestrial planets are made up of iron and hard rocks. They are also called earth-like planets.

These planets are Mercury, Venus, Earth and Mars. Having solid forms and smaller size, these planets very high surface density. They have not more than two natural satellites. Mars has two satellites. Earth has only one satellite. However, Mercury and Venus don't have satellites.

2. Jovian Planets

This category of planets contains Jupiter, Saturn, Uranus and Neptune. Most of these planets are made up of gases and clouds. They have high volume  but relatively low densities compared wo Terrestrial planets. They have more than two natural satellites. They are also called Jupiter-like planets.

Comets:

A body of solar system, made  of dust particles and gases, that moves in an orbit around the sun is called comet. 

Comets can be spotted for varying amounts of time from the sun.

One of the major parts of comet is Nucleus, which made of thick solid body.

As the comet approaches near the sun, the nucleus becomes wrapped by a bright reflection of dust and gases. Due to hot solar wind,  a tail flares out for it which is as much long as `160\times10^7`km in length.

Size of comet varies widely as large as planets, however its mass remains very less.  Due to hot and strong solar wind, comets lose particles and brightness with each time it passes nearby the sun. 

The Moon:

Satellites are those bodies in the solar system which revolve around the fixed planets, just as planets revolve around the centrally located Sun. 

Satellites can be of two categories based on its origin or how it is created. These categories involve: Natural Satellites and Artificial Satellites. Those satellites which are naturally created are called natural satellites. Our earth has only one natural satellite, called the Moon. Mars has two natural satellites, Phobos and Deimos. Jupiter and other Jovian planets have more than two natural satellites.

Artificial satellites are those which are creates by human scientists and launched in the sky to learn various inner and outer surface characteristics of planets, and other astronomical bodies. 

Fig. 2 The System of Sun, Earth and Moon

Characteristics of Moon:

• it is only one natural satellite of earth
• it is nearer to us than any other natural satellites
• it appears in earth's sky during every 15 days.
• on different days, various shape of moon can be observed. These shapes are called phases of the moon. 
• it takes about 27.33 days to revolve around the earth once.
• however it takes 29.5 days to present the same phase to the earth.
• the moon's orbit is titled by angle `5\frac{1}{2^o}` to plane of earth's orbit. This is the reason why we can not observe lunar eclipse on every full moon day, solar eclipse on every new moon day. 
• relative motion of earth and moon is in the same direction
• Moon also revolve around the sun once on every 365 days. But it revolves around the earth 12 times on every 365 days(or every year).
• average mass: `7346\times10^19`kg
• average diameter: 3464 km
• average daytime surface temperature `107^o`C, night time temperature falls below `-153^o`C.
• distance from earth: 384,400km
• gravity: `1.63 ms^{_2}`, which almost 6 times less than that of earth(`9.81ms^{-2}` )
• Our sky looks beautiful because of the moon
• But the moon does not have its own light, it reflects sun light back to the earth. This is why the moon is visible from the earth. 

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