The Solar System

 The Sun:

  • Sun accounts for 99.85% of the mass of the solar system.
  • The Sun takes 250 million years to complete one revolution around its center. This period is called a cosmic year.
  •  The Sun is mainly composed of hydrogen.
  • The light from the Sun reaches Earth in about 8.3 minutes.
  • One light-year in a vacuum is the distance traveled by one light-year in one year in a vacuum at the speed of light. (3× 105 Km/s).
  • The value of the light-year is 9.46 × 1012 or 5.88 × 1012 miles.
  • The astronomical unit represents the mean distance between the Sun and the Earth (1.496 × 108 km).

The Planets:

  • Mercury, Venus, Earth and Mars are called inner planets. They are also known as terrestrial planets (Earth-like planets).
  • Jupiter, Saturn, Uranus, Neptune and Pluto are the outer planets. After Jove, they are also known as Jovian planets, the Greek name for Jupiter.
  • Mercury and Venus are termed inferior planets, while planets outside the Earth's orbit are known as superior planets.
Solar System
Image Source: Wikimedia Commons


  • Mercury is the nearest planet to the Sun (58 million km).
  • There is no atmosphere on Mercury, and it has no satellites.
  • Its days are scorching hot, and nights are frigid.
  • Mercury has a close resemblance to the moon. It is near the same size as the moon.


  • Venus is the nearest planet to Earth.
  • It is also called the morning or evening star.
  • The bright appearance of Venus is due to its cloudy atmosphere.
  • It is most probably the hottest planet.
  • The day and night temperatures are almost the same on Venus.
  • Venus's atmosphere contains 90-95% of carbon dioxide. (Greenhouse effect is seen on this planet).


  • The distance of Earth from the sun is 8 light minutes (1 AU) and from the moon is 1.25 light seconds.
  • Distance of earth from Proxima Centauri as 4.3 light-years.
  • The earth rotates from west to east. one revolution is completed in 365.25 days. Its atmosphere contains mostly nitrogen & oxygen.
  • Earth has one satellite called Moon.


  • Mars has a thin atmosphere comprising nitrogen and argon.
  • Its surface has polar ice caps and dry river beds.
  • Mars is also called the red planet. It has two satellites named Phobos and Deimos.


  • Its bright appearance is due to the thick atmosphere that reflects most of the sunlight falling on it.
  • It is the largest planet in the solar system.
  • Its atmosphere contains hydrogen, helium, methane and ammonia.
  • Its satellites are called Galilean satellites. The four largest satellites are Jo, Europa, Ganymede (the largest moon in the solar system) and Callisto.


  • The golden giant Saturn contains rings composed of thousand of rippling, spiraling brands.
  • Its atmosphere has mostly hydrogen and helium.
  • Titan is the largest moon of Saturn and the second largest of the solar system.


  • The atmosphere of Uranus mostly contains hydrogen and helium. There is no solid surface of Uranus.
  • It is the only planet where one pole or other faces the sun as its orbits.


  • There are five rings of Neptune.
  • The atmosphere is mostly hydrogen.
  • There is no solid surface.
  • The average temperature on Neptune is about -220°C.

Dwarf Planets:

  • The International  Astronomical Union (IAU), the official scientific body for astronomical nomenclature, defines a dwarf planet as a celestial body within the Solar System that satisfies these four conditions:
    1. It is in orbit around the sun.
    2. It has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (near-spherical) shape.
    3. It has not cleared the neighborhood around its orbit.
    4. It is not a satellite. Ceres, Pluto and Eris are the three dwarf planets that are currently recognized by the IAU.
  • Ceres is the smallest dwarf planet in the Solar System and the only one discovered in the main asteroid belt. Its name is derived from the Roman goddess Ceres - goddess of growing planets, the harvest and the motherly love. It was discovered on January 1, 1801, by Giuseppe Piazzi. Ceres is the largest object in the asteroid belt, which mostly lies between Mars and Jupiter.
  • Pluto also designated 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting around the Sun. Considered a planet for decades after the discovery, Pluto has since been recognized as the largest member of a distinct region called the Kuiper belt.
  • Clyde Tombaugh discovered Pluto in 1930.
  • Pluto's atmosphere consists of a thin envelope of nitrogen, methane and carbon monoxide, derived from the ices on its surface.
  • Nasa has launched a robotic spacecraft mission New Horizons to study Pluto.
  • It is expected to be the first spacecraft to fly by and study the dwarf planet Pluto and its moons Charon, Nix and Hydra.
  • Charon is the largest moon of Pluto.
  • Eris also designated 136199 Eris and formerly 2003 UB313, is the largest known dwarf planet in the solar system and the ninth-largest body orbiting the Sun directly.
  • Eris was discovered in 2005 by a Mount Palomar-based team led by Mike Brown.
  • It is a trans-Neptunian object (TNO) native to a region of space beyond the Kuiper belt known as the scattered disc.
  • Eris has one moon, named Dysnomia.

The Moon:

  • The moon is the only satellite of the earth. Its size is approximately one-fourth that of the earth and its weight is about one-eighth of the earth's weight & orbit is elliptical.
  • The maximum distance (apogee) of the moon from the earth is 406,00 km and the minimum distance (perigee) is 364,00 km.
  • It takes 27 days 7 hours. 11 minutes & 47 seconds to rotate on its axis and it also rotates around the earth at exactly the same time.
  • The moon has no atmosphere, no twilight and no sound.
  • The light from the moon takes 1.3 sec to reach the earth.


  • Asteroids are a series of very small planets or fragments of planets lying between the orbits of Mars and Jupiter.
  • Ceres is the largest Asteroid.


  • Comets were formed from the remaining material that was felt during the formation of the outer planets.

Meteors and Meteorites:

  • These are the remains of comets that are scattered in the interplanetary space of the solar system.
  • In contact with the earth's atmosphere, they burn out into ash and are called meteors or shooting stars. Those which do not burn completely and strike the earth in the form of rocks are called 'Meteorites'.

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