Module 2: Solar System (Part 1)

  • The solar system is located in an outer spiral arm of the vast Milky Way galaxy.
  • The time taken by the solar system to make a rotation around the center of Milky Way galaxy is called a Cosmic Year or Galactic Year. It takes our solar system about 230 million years to complete one orbit around the galactic center.
  • Solar system consists of:
  • The sun, eight planets revolving around it, their satellites and dwarf planets.
  • Interstellar objects such as asteroids, meteoroids and comets.
  • Interplanetary dust particles, electrically charged gases etc.
  • The Sun is the center of solar system. It is also the largest body in solar system, which accounts for more than 99% of the mass of the solar system.

The Sun

  • The Sun is the center of our solar system and makes up 99.8 percent of the mass of the entire solar system.
  • Sun is the major source of energy in the solar system.
  • Sun exerts huge gravitational pull to keep the planets rotate around it in elliptical orbit.
  • Sun is approximately 460 crore years old and it is expected to glow in a similar manner for next 5 billion years or so on.
  • The sun has a surface temperature of about 55000C (99320F) and increases to 20 million 0C (36 million 0F) in the interior.
  • Sun has a diameter of 1,392, 684 km. It is 109 times wider and 3,30,000 times massive than earth.
  • Hydrogen and Helium are the main component of sun. Hydrogen makes up about 92% of all atoms in the sun while Helium makes up about 7.8%.
  • Energy is generated by nuclear fusion at the sun’s core, as Hydrogen converts to Helium.
  • As the gases heats up, atoms break apart into charged particles, turning the gas into plasma. 99% of matter is in plasma state in the sun.
  • Light of sun reach earth in 8 minute 20 seconds (500 seconds). The light of Proxima Centauri, the nearest star to the sun reaches in about 4.3 light years.
  • Sun rotates in the opposite direction to Earth with the sun rotating from west to east instead of east to west like earth


  • Distance from Earth → 1,49,59,78,70,700 km
  • Diameter → 1,392,684 km
  • Surface Temperature → 55000C – 60000C
  • Core Temperature →  15 million °C – 20 million °C
  • Rotation speed →  25.38 days (w.r.t equator), 33 days (w.r.t. poles)
  • Mass →  1.99 × 1030 kg
  • Time taken by sunlight to reach Earth →  8 minute 20 seconds (500 seconds)

  • Solar Winds : Solar wind is a stream of fast moving charged particles (protons, electrons and atoms of heavier metals) from the upper layer of sun, corona.
  • Sun spots: Sun spots are dark, cooler spots on the surface of Sunspots have a temperature of about 35000C where the surrounding hotter areas have temperature of 55000C.
  • Aurora: An aurora is a luminous phenomenon visible in Earth’s sky, predominantly seen in high altitudes. It is caused by the collision between air molecules and the charged particles from the sun that is captured by earth’s magnetic field. Aurora in northern hemisphere is called Aurora Borealis or Northern Light and in Southern hemisphere is called Aurora Australias or Southern Light.
  • The brief period between sunrise and full daylight is called dawn, and that between sunset and complete darkness is called twilight.
  • The atmosphere of the sun is composed of three layers: the photosphere, chromosphere and corona.
  • Photosphere is the deepest layer and the visible part of the sun.
  • The layer in the sun above the photosphere is called chromosphere. Its temperature varies from 60000C to about 20,0000C.
  • The corona is the outer most layer of the sun, starting at about 2000 km above the photosphere. The temperature in corona is about 500,0000C.
  • The corona is visible with naked eye only during a total solar eclipse.
  • Coronagraph is a telescope that can see things very close to the sun. It is used to study the sun’s outer atmosphere, corona.
  • Helioseismology is the study of the interior of the sun using observations of waves on the Sun’s surface.
  • Solar and Heliospheric Observatory (SOHO) mission is a joint project of NASA and European Space Agency launched in 1995 to study the internal structure of the sun, its extensive outer atmosphere and the origin of the solar wind.
  • Aditya L1 is the first Indian space mission to study the sun. The satellite will be launched during 2019 – 2020 timeframe by PSLV.

Parker Solar Probe

  • The Parker Solar Probe is a solar mission launched by NASA on August 12, 2018 to study the outer corona of the Sun.
  • The Parker Solar Probe will be the first spacecraft to fly into the low solar corona.
  • The mission was renamed for Eugene Parker, Department of Astronomy and Astrophysics at the University of Chicago.
  • In the 1950s, Parker proposed a number of concepts about how stars—including our Sun—give off energy. He called this cascade of energy the solar wind, and he described an entire complex system of plasmas, magnetic fields, and energetic particles that make up this phenomenon.
  • This is the first NASA mission that has been named for a living individual.
  • Parker Solar Probe now holds the record for closest approach to the Sun by a human-made object. The previous record was set in April 1976 by the Helios 2 spacecraft.
  • The star nearest to the earth – Sun
  • The star nearest to the sun – Proxima Centauri (4.24 light years away)


  • Planets are the celestial bodies which continuously revolve around the sun.
  • The definite elliptical path along with the planets revolve around the sun is called orbit.
  • Movement of planets around its own axis is called rotation and around the sun is called revolution.
  • The word ‘Planet’ came from the Greek word ‘Planetai’, which means wanderers.
  • There are 8 planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
  • Planets are classified into:
    1. Inner planets and outer planets
    2. Terrestrial and Jovian planets
  • Mercury, Venus, Earth and Mars are called Inner Planets because of their proximity to the sun.
  • Inner Planet has very few moons. Earth is the largest of inner planets.
  • Jupiter, Saturn, Uranus and Neptune are much farther from the sun and are called Outer planets. They have more number of moons.
  • Mercury, Venus, Earth and Mars are also called Terrestrial planets because of their structural similarity with the earth.
  • Jupiter, Saturn, Uranus and Neptune are called Jovian planets because of their similarity with Jupiter.


  • Period of Revolution : 88 days
  • Period of Rotation : 58 days
  • Mercury is nearest to sun and the smallest planet in the solar system.
  • It is the second hottest and second densest body in the solar system.
  • Mercury is also known as “will-O-the wisp”
  • Mercury is named after Roman God of Messenger.
  • It is the fastest revolving planet with period of revolution (length of a year) 88 earth days.
  • Mercury does not have atmosphere due to low escape velocity.
  • The surface of Mercury is dominated by craters and a younger area of dark plains made from flood of lava.
  • The surface of the planet facing sun receives maximum heat and become hotter; and other side will be extremely cold.
  • Mercury has no moons and rings around it.
  • Mercury has only been visited by two spacecraft—NASA’s Mariner 10 and MESSENGER.
  • First space craft to visit Mercury was Mariner-10 (USA, 1973). Messenger flew past Mercury three times before going into orbit around Mercury in 2011.
  • The third mission to Mercury is BepiColombo, a joint mission between the Japan Aerospace Exploration Agency (JAXA) and the European Space Agency. The two spacecrafts of BepiColombo was launched on 20 October 2018 and is expected to reach Mercury in 2025.


  • Period of Revolution : 224.7 days
  • Period of Rotation : 243 days
  • Venus is the hottest planet in the solar system.
  • Unlike other planets, Venus rotates from East to West. It is also known as Morning Star and Evening Star as it is seen in the East in morning and in the west in the evening.
  • Venus is also known as ‘Earth’s twin’. Its size is nearly same as that of the Earth.
  • Venus is named after the Roman goddess of love and beauty.
  • Venus is the second brightest object in the night sky, because it reflects back 70% of sunlight that falls on it.
  • Venus take more time to complete one rotation than one revolution. It has the slowest rotational speed.(It means that a day in Venus is longer than an year)
  • The atmosphere of Venus is mainly made up of carbon dioxide with clouds of sulfuric acid.
  • Venus has high plateaus, folded mountain belts, numerous volcanoes, and relatively smooth volcanic plains.
  • Maxwell Mount is the highest mountain range in Venus.
  • Venera 1 was the first spacecraft to fly by Venus.
  • The first space craft to land on Venus was Venera-7.
  • The first orbiter which produced detailed maps of Venus using radar was Magellan (USA).
  • The Venus Climate Orbiter mission (PLANET-C), or “Akatsuki”, is studying the atmospheric circulation of Venus.


  • Period of Revolution : 365 days 5 hours 48 minutes 45.51 seconds
  • Period of Rotation : 23 hours 56 minutes 4.09 seconds
  • Earth is known as ‘Blue Planet’ due to presence of water.
  • It is the largest inner planet in the solar system.
  • Shape of earth is called Oblate Spheroid or Geoid.
  • It is the third nearest planet to the sun and ranks fifth in size.
  • Earth is the only planet that supports life due to favourable atmosphere.
  • Our atmosphere protects us from incoming meteoroids, most of which break up in our atmosphere before they can strike the surface as meteorites.
  • It is the densest planet in the solar system.
  • Moon is the only natural satellite of Earth.
  • The escape velocity of earth is 11.2 km/s.
  • Earth day is observed on 22nd April every year.


  • Period of Revolution : 687 days
  • Period of Rotation : 24 hours 37 minutes 22.63 seconds
  • Mars is also known as ‘Fossil Planet’ and ‘Rusty Planet’.
  • It is also known as ‘Red Planet’ due to the presence of Iron Oxides.
  • It has a thin atmosphere comprising of Nitrogen and Argon.
  • Mars is almost half the size of earth and mass of that of earth.
  • It is visible from the earth for most part of the year.
  • Three huge volcanoes, one more than 28 km high exists at Mars.
  • Mars is named after the Roman god of war.
  • Olympus Mons, the largest volcanic mountain in the solar system is located at Mars.
  • Phobos and Deimos are the two satellites of Mars. They were discovered by Asaph Hall in 1877.
  • Smallest satellite in solar system – Deimos
  • The satellite also known as ‘black moon’ – Phobos
  • First successful space craft launched to observe Mars was Mariner-4 (1964).
  • Mariner 9 was the first spacecraft to orbit around Mars (Nov 14, 1971). It was the first ever space craft orbited around another planet.
  • First space craft to land on the surface of Mars was Viking-1 (USA, launched on 20th August 1975).
  • In 1997, Mars Pathfinder lands and dispatches Sojourner, the first wheeled rover to explore the surface of another planet.
  • Curiosity is a rover designed to explore Gale Crater on Mars. It was launched on November 26, 2011 and landed in 2012.
  • The space probe sent to Mars by NASA on November 2013 to study the upper atmosphere of Mars was MAVEN (Mars Atmosphere and Volatile Evolution).
  • Exo Mars Trace Gas Orbiter is an atmospheric research orbiter built in collaboration between European Space Agency (ESA) and Russia. It was launched on October 19, 2016 to gain a better understanding of Methane and other trace gases in Martian atmosphere.
  • In May 2018, NSA launched InSight, a Mars lander aimed to study the interior of the planet Mars.
  • The rocket that launched InSight also launched a separate NASA technology experiment: two CubeSats called Mars Cube One, or MarCO. Its goal is to test new miniaturized deep space communication equipment. A CubeSat is a type of miniaturized satellite for space research. These two MarCO Cubesats were the first CubeSats to leave earth orbit.


  • Pathfinder is an American Robotic spacecraft launched on 4th December 1996 and landed on the surface of Mars on 4th July 1997.
  • It had a wheeled robotic rover (space exploration vehicle) named Sojourner.
  • Sojourner, the first robot that landed on Martian surface, is also the first rover to operate outside Earth-moon system.
  • The spot where Sojourner was landed is now named Carl Sagan Memorial Station.


  • Curiosity is a robotic rover launched by NASA on 26th November 2011.
  • It landed safely on Mars on 6th August 2012.
  • The main goal was to determine if Mars was ever able to support microbial life.
  • Curiosity landed on Gale Crater. The landing of Curiosity is also known as “Seven Minutes of Terror”.


  • Mangalyaan was India’s first interplanetary mission. The official name of Mangalyaan is Mars Orbiter Mission (MOM).
  • India is the 4th country in the world and first Asian country to launch space craft to Mars.
  • Mangalyaan was launched on 5th November 2013 from Sathish Dhawan Space Centre, Sriharikota
  • MOM was launched aboard PSLV C-25, which was an XL variant of the PSLV
  • It carries a color camera and four scientific instruments to study the Martian surface and atmosphere
  • Mangalyaan reached the Mars orbit on 24th September 2014
  • It is the first Indian spacecraft to escape the Sphere Of Influence of Earth and orbit Sun.
  • It was the first Mars mission in the world to succeed Mars Orbit Insertion in first attempt.
  • MOM completed 1000 Earth days in its orbit on June 19, 2017
  • Mangalyaan is world’s least expensive mission to Mars (450 crore).
  • The launch mass of Mangalyaan was1337 kg.
  • The ISRO chairman at the time of Mangalyaan launching was K. Radhakrishnan


  • Period of Revolution : 12 years
  • Period of Rotation : 9 hours 50 minutes 30 seconds
  • Jupiter is the largest planet in the solar system. It is 11 times bigger than earth.
  • It is the fastest rotating planet and it have shortest day and night in solar system
  • It is too massive to solidify as a planet and composed mostly of gas and liquid. It has no solid surface.
  • Jupiter has strongest gravitational force and magnetic force.
  • Jupiter has the highest escape velocity among other planets (60 km/s)
  • Jupiter rotates once about every 10 hours (a Jovian day), but takes about 12 Earth years to complete one orbit of the Sun (a Jovian year).
  • When viewed from Earth, Jupiter is the second brightest planet in the night sky, after Venus.
  • Jupiter is very windy with wind speed ranges from 192 mph to more than 400 mph.
  • Great Red spot is a giant spinning storm which is more than twice the size of earth is the largest anticyclone in the solar system.
  • Little Red Spot is another storm on planet Jupiter with a wind speed of about 380 mph.
  • Jupiter’s atmosphere is made up mostly of hydrogen (H2) and helium (He).
  • Jupiter has highest number of satellites (67) in solar system. Jupiter and its satellites are together known as ‘Mini Solar System’.
  • On Jul. 17, 2018, scientists announced they had discovered 12 new moons orbiting Jupiter. That raised Jupiter’s total number of moons to 79. The 53 of these moons are confirmed and named; the other 26 are awaiting official confirmation of discovery before they are named.
  • In 1610, Galileo Galilei makes the first detailed observations of Jupiter.
  • Nine spacecraft have visited Jupiter. Seven flew by and two have orbited the gas giant.
  • In 1973 Pioneer 10 becomes the first spacecraft to cross the asteroid belt and fly past Jupiter.
  • Galileo spacecraft launched by NASA was the first to orbit the planet Jupiter.
  • Jupiter has a faint ring system that was discovered in 1979 by the Voyager-1 mission
  • Juno is a NASA space probe orbiting the planet Jupiter. It was launched from Cape Canaveral Air Force Station on August 5, 2011 and arrived at Jupiter in 2016. It is conducting an in-depth investigation of the planet’s atmosphere, deep structure and magnetosphere for clues to its origin and evolution.​


  • Period of Revolution : 30 years
  • Period of Rotation : 10 hours 14 minutes
  • Saturn is the second largest planet in the solar system.
  • It is named after the Roman God of agriculture.
  • Like Jupiter, Saturn does not have a solid surface.
  • It is a gigantic ball made up mostly of lighter gases, in which 63% is Hydrogen
  • Saturn has the least density among other planets. Its density is less than water.
  • Saturn is also known as ‘the jewel of the solar system’.
  • Saturn has the most spectacular ring system which is made up of seven rings with several gaps between them. These rings are probably made up of particles of ice or ice-covered rocks. All four giant planets in our solar system have ring systems.
  • Galileo Galilee was the first person to observe Saturn through a telescope.
  • Pioneer 11 of NASA was the first spacecraft to reach Saturn. Voyager 1, Voyager 2 and Cassini – Huygens are the other missions visited Saturn.
  • Saturn has 53 known moons with an additional nine moons awaiting confirmation of their discovery—that is a total of 62 moons.
  • Titan is the largest satellite of Saturn.
  • Saturn cannot support life as we know it, but some of Saturn’s moons have conditions that might support life.
  • Great white spot (Great White Oval) and Dragon Storm are periodic storms on planet Saturn.
  • Few missions have visited Saturn: Pioneer 11 and Voyagers 1 and 2 flew by; But Cassini orbited Saturn 294 times from 2004 to 2017.
  • Pioneer 11 was the first spacecraft to reach Saturn (1979).


  • Cassini – Huygens Mission is a joint project of NASA, European Space Agency (ESA) and Italian Space Agency (ISA) launched in 1997 to study Saturn and its system.
  • Cassini was the fourth space probe to visit Saturn and the first to enter its orbit in 2004.
  • Huygens was separated from Cassini and landed on Titan, a satellite of Saturn in 2005. It is the first spacecraft to make a soft landing on another planet’s satellite.
  • Cassini ended its mission on September 2017.


  • Period of Revolution : 84 days
  • Period of Rotation : 17 hours 12 minutes
  • Uranus is the third largest planet in the solar system.
  • Uranus rotates at a nearly 90 degree angle from the plane of its orbit. This unique tilt make Uranus appear to orbit the sun like a rolling ball.
  • It is also known as ‘Rolling Planet’ or ‘Lying Planet’
  • It was the first planet found with the help of a telescope. It was discovered by William Herschel in 1781.
  • Uranus is one of two Ice giants (the other is Neptune). Most of the planet’s mass is made up of icy materials – water, methane and ammonia – above a small rocky core.
  • Uranus appears in blue – green colour due to the presence of Methane gas in atmosphere.
  • Uranus is one of two planets that rotate from east to west (Venus is the other one). But Uranus is unique in that it rotates on its side.
  • Uranus has about 27 satellites.
  • Uranus has 13 known rings. The inner rings are narrow and dark and the outer rings are brightly colored.
  • Uranus is named after the primal Greek god of sky.
  • Voyager 2 is the only spacecraft to fly by Uranus. No spacecraft has orbited this distant planet.
  • In 2011, New Horizons passed the orbit of Uranus on its way to Pluto, becoming the first spacecraft to journey beyond Uranus’ orbit since Voyager 2.


  • Period of Revolution : 165 years
  • Period of Rotation : 16 hours 6 minutes
  • Neptune was discovered by Johann Gottfried Galle using the calculations by Urbain Le Verrier and John Couch Adams in 1846.
  • It was the first planet to get its existence predicted by mathematical calculation.
  • The first person to observe Neptune through a telescope was Galileo Galilee.
  • Neptune is named after Roman god of the sea.
  • It has the second largest gravity among other planets (first is Jupiter)
  • Great Dark Spot and Small Dark Spot are large storms found in Neptune.
  • Neptune is the coldest planet having a surface temperature of -2140C.
  • The windiest planet in our solar system is Neptune.
  • Neptune is the planet that take maximum time for revolution (or planet which has longest year).
  • Neptune has 13 moons (and one more awaiting confirmation of discovery), which are named after sea gods and nymphs in Greek mythology. Triton is Neptune’s largest moon
  • The three rings of Neptune are named Liberty, Equality and Fraternity.
  • Neptune’s atmosphere is composed of Hydrogen, Helium and trace amounts of Methane.
  • Voyager 2 is the only spacecraft to have visited Neptune. No spacecraft has orbited this distant planet.


  • Largest planet –  Jupiter
  • Smallest planet – Mercury
  • Hottest planet – Venus
  • Coldest planet – Neptune
  • Fastest rotating planet – Jupiter
  • Slowest rotating planet –  Venus
  • Fastest revolving planet –  Mercury
  • Slowest revolving planet –  Neptune
  • Farthest planet to the sun – Neptune
  • Closest planet to the sun – Mercury
  • Planet with highest density – Earth
  • Planet with lowest density  – Saturn
  • Planet with longest day – Venus
  • Planet with shortest day  – Jupiter
  • Highest escape velocity – Jupiter
  • Lowest escape velocity  – Mercury
  • Planet with highest weight – Jupiter
  • Planet with lowest weight – Saturn
  • Windiest Planet – Neptune

Planets in the Solar System in the decreasing order of size:

1. Jupiter 2. Saturn 3. Uranus 4. Neptune 5. Earth 6. Venus 7. Mars 8. Mercury

Planets in the solar system in the decreasing order of distance from the sun:

1. Neptune 2. Uranus 3. Saturn 4. Jupiter 5. Mars 6. Earth  7. Venus 8. Mercury


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