Short Notes - The Solar System

Category : UPSC


The Solar System


  • The Sun, the nine planets (Pluto is not a planet now, considered as a dwarf planet) along with their satellites, the asteroids, the comets, the inter planetary dust and the electrically charged gases called plasma, together make up the solar system.
  • Our solar system consists of an average star we call it the Sun, the planets - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. It also includes: the satellites of the planets: numerous comets, asteroids, and meteoroids; and the interplanetary medium.
  • The Sun is an average star. It isn't the hottest, it isn't the coolest, it isn't the oldest. Nor is it brightest, biggest, etc.
  • The Sun accounts for 99.85% of all the matter of the solar system.
  • It is composed mainly of hydrogen and helium.
  • Nuclear fusion in the core of the Sun is source of all its energy.
  • The glowing surface of the Sun is called Photosphere.
  • About it is red coloured Chromosphere and beyond it is Corona (visible during eclipses).
  • The surface of the Sun changes continuously. Bright regions are called Plages and dark spots are called Sun spots which frequentlyl form and disappear.


Sun Statistics

  • Distance from the Earth - 150 mn km
  • Diameter-1391980 km
  • Core temperature - 15000000°C
  • Rotation time - 25 days
  • Age - 5 billion years
  • Composition – H2 - 71%, He - 26.5% and other 2.5%
  • Mass-1.99 x l033kg



Solar Eclipse

  • Solar eclipse is caused when the Moon revolving around the Earth comes in between the Earth and the Sun, thus making a part or whole of the Sun invisible from a particular patrol the Earth.


Lunar Eclipse

  • During the revolution of Earth, when comes between moon and the Sun the shadow of the Earth hides moon either fully or partially. This is called lunar edips.



A planet must meet three criteria:

  • It must orbit the Sun,

(ii) It must be big enough for gravity to squash it into a round ball,

(iii) It must have cleared other objects out of the way in its orbital neighbourhood.

  • The Terrestrial Planets or Inner Planets are the four innermost planet in the solar system, Mercury, Venus, Earth and Mars.
  • The Jovian Planets or Outer Planets are Jupiter, Saturn, Uranus, and Neptune because they are all gigantic compared to Earth, and they have, gaseous nature.
  • Mercury - It is the smallest and the closest planet to the Sun, without moon. Surface is full of craters.
  • Venus - It is the second closest planet to the Sun, known as evening as well as morning star, rotates from east to west. It is the hottest planet. The atmosphere of venus is covered with thick clouds that strongly reflects sunlight.
  • Earth - It is the third planet from the Sun with one moon. Perfect place for life. It consists of 78% nitrogen, 21% oxygen, and 1% other ingredient; envelops it. Moon is the only natural satellite of Earth.
  • Mars - It is the fourth planet from the Sun with two moons (Phobos and Deimos).
  • It is known as the Red Planet because iron minerals in the Martian soil oxidize, or rust, causing the soil and the dusty atmosphere to look red. The planet is characterised by volcanoes, canyon systems, river beds, crated terrains and duncfields.
  • Jupiter - It is the fifth planet, from the Its atmosphere is made up mostly of hydrogen (H2) and helium (He). It has the fastest rotational velocity completing one rotation in less than 10 hours.                            

Jupiter has 67 known satellites and    4 Galilean moons. It has the biggest magnetosphere in the entire solar system.                             

  • Saturn-It is the second largest planet of the solar system and surrounded by rings like structures. These rings are made of premordial dust and ice particles. More over Saturn is a gaseous planet. The planet has 62 prominent moons among which the largest moon is Titan which is the second largest in the entire solar system.
  • Uranus - It is the seventh planet from the Sun. One day on Uranus takes about 17 hours (the time it takes for Uranus to rotate or spin once). Uranus makes a complete orbit around the Sun (a year in Uranian time) in about 84 Earth years. It has 27 moons. It is characterised by usual magnetic and electric field.
  • Neptune - It is the eighth planet from the Sun. Its atmosphere is made up mostly of hydrogen (H2), helium (He) and methane (CH4). Triton is its largest moon. It is having a earth size blemish called as Green dark spot. It has 14 satellites among which Triton and Nereid are the prominent ones.



Pluto (not a planet now.) It is now considered as a dwarf planet. It has slowest orbital velocity and hence, the longest year, Charon, is nearly half its size.


Dwarf planet ? A dwarf planet is a planetary ?  mass object that is neither a planet nor a natural satellite. It shares its orbits around the sun with other objects such as asteroids or comets. It is massive enough for its shape to be in hydrostatic equilibrium under its own gravity, but has not cleared the neighborhood around its orbit.

The first 5 recognised dwarf planets are- ceres, Pluto, Eris, Haumea & Makemake.



Light year- A light-year is a unit of astronomical distance. It is the distance that light can travel in one year. It is approximately 9.5 trillion kilometres (or about 6 trillion miles).


Some facts about planets

  1. Biggest Planet is Jupiter
  2. Biggest Satellite is Ganymede
  3. Blue Planet is Earth
  4. Green Planet is Uranus
  5. Planet is Venus
  6. Brightest Planet outside Solar System is Sirus
  7. Closest Star of Solar System is Proxima Centauri
  8. Coldest Planet is Neptune
  9. Evening Star is Venus
  10. Farthest Planet From Sun is Neptune
  11. Planet with maximum number of satellites is Saturn
  12. Fastest revolution in solar system is by Mercury
  13. Hottest Planet is Venus
  14. Densest Planet is Earth
  15. Fastest Rotation in Solar System by Jupiter
  16. Morning Star is Venus
  17. Nearest Planet to Earth is Venus
  18. Nearest Planet to Sun is Mercury
  19. Red Planet is Mars
  20. Slowest Revolution in Solar System is by Neptune
  21. Slowest Rotation in Solar System is by Venus
  22. Smallest Planet is Mercury
  23. Smallest Satellite is Deimos
  24. Earth's Twin-is Venus
  25. Atmosphere like Earth is on Titan



  • The Moon is the Earth's only natural satellite.
  • The Moon revolves around the Earth in 27 days 7 hours 43 minutes and 11.47 seconds and rotates on its own axis exactly the same time. That is why only one face of the Moon is seen from Earth.
  • The total forces of the Moon and the Sun are in the ratio of 9: 4.
  • Study of the Moon is known as Selenology.


Statistics of Moon

  • Diameter – 3500 km
  • Mass – 7.35x1022 kg
  • Distance from Earth – 384,400 km
  • Circumference – 11,000 km
  • Revolution time – 27.3 days
  • Gravitational pull - 1/6 th of that of the Earth


Asteroids, Meteoroids and Comets


  • Asteroids or Planetoids are rocky bodies up to 800 km in diameter, although most are much smaller in diameter less than a km, i.e. Asteroids are the minor planets which especially belong to the inner solar system.
  • They orbit the Sun in the asteroid belt, which lies between the orbits of Mars and Jupiter.
  • Meteoroids are small fragments of rock and metal travelling through the space.
  • Upon reaching the Earth's surface they are called Meteorites.
  • Comets: They are the smallest units of the cosmic bodies which is made up of frozen gases, rocks and dusts.
  • The tail of the comet always points away from the Sun because of the force exerted by solar wind and the radiation pressure.
  • The most common comet is Halley’s comet which is spotted every 76 years.



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