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World Geography / Universe and Solar System

Celestial Bodies in Solar System

The Sun, 8 planets, 5 dwarf planets, more than 200 moons or satellites, meteoroids, asteroids and comets like celestial bodies form the solar system. The Sun is located at the centre. The planets, the moons and other celestial objects of the solar system revolve around the sun in specified orbits. The surrounding areas around all the revolving objects are also part of the solar system.

Structure of the Solar System

The structure of the Solar system, from Sun to Outer space, consists of Inner Planetary zone, Asteroid zone, Outer Planetary zone, Kuiper Belt region, Scattered Disc region, Farthest region and Interstellar region. The Inner planetary region has 4 planets and Outer planetary region has 4 planets.

  • Inner Planetary Zone

    • The planets in this zone are known as rocky or terrestrial planets. Here, the planets in order from the sun are Mercury, Venus, Earth and Mars. These planets are made up of solid rocks. Their interior is made up of Iron, Nickel and Cobalt.

    • Mercury is the closest planet to the sun and has negligible atmosphere. It is located at a distance of 0.4 Astronomical Units (AU is the distance between Sun and Earth equivalent to 150 million Km) from the Sun.

      Mercury is the smallest planet among all the planets and has no natural satellites. It has large Iron core (similar to earth) and its surface is wrinkled.

    • Venus is at 0.7 AU distance from the Sun. It is regarded as Earth's twin as it has 81.5% of mass of earth, has almost equal density and Composition is similar to that of Earth. But Earth is the densest planet in the solar system. Venus has substantial atmosphere and is extremely dry but is highly dense in nature, 90 times the atmosphere of earth.

      It is the hottest planet in the solar system and the surface temperature is about 4000C. It has no moons and has lot of volcanoes. It has large scale of greenhouse gases (CO2). It is also the brightest planet in solar system.

      Though orbit wise Venus is closer to the earth, Mercury is considered as the planet closest to earth as it revolves very fast and on an average it remains closer to the earth most of the time.

    • Mars is at 1.5 AU distance from the Sun. It is much smaller than Earth and has 0.107 times mass of earth. It has atmosphere made up of CO2.

      Large number of active volcanoes are present here. It has deep narrow valleys. It has two moons which are nothing but captured asteroids.
  • Asteroid Belt

    Asteroid Belt is in between 2.3 to 3.3 AU from the Sun. It is made of small bodies and remnants of solar system, the sizes of which range from few 100 km in diameter to small dust particles.

    The largest asteroid is Ceres, which is being classified as dwarf planet nearest to the Sun. Other bigger bodies are Vesta, Hygiea, etc. Very small asteroids are known as meteoroids.

  • Outer Planetary Zone

    • This zone has 4 planets. Here, the planets in order from the sun are Jupiter, Saturn, Uranus and Neptune and they are also known as Jovian planets as their structure is similar to Jupiter. These are huge in size and mass and are known as gas giant planets.

      These 4 planets together comprise 99% of mass orbiting around the sun. Because of gaseous composition, density is lower for these planets.

    • Jupiter is the largest planet in the solar system. It is at a distance of 5.2 AU from the Sun. It has huge number of cloud bands. Its composition is having Hydrogen and Helium similar to the composition of Sun.

      Its mass is around 318 times the mass of earth and it is 2.5 times larger than all other planets together. Its surface is known for "Great Red Spots".

      It has largest number of moons, 63 in number and one among them is Ganymede, the largest moon in the solar system. Ganymede itself is larger than Mercury. Other moons of Jupiter are Callisto, Io, Europa, etc.

    • Saturn remains at a distance of 9.5 AU from the Sun. It is the second largest planet in the solar system. Its mass is around 95 times the mass of earth. Its composition is similar to that of planet Jupiter.

      Here, gases are arranged in the form of rings. Saturn has 56 known moons and the largest one is Titan, which is also larger than Mercury. Titan is the only moon having substantial atmosphere.

    • Uranus remains at a distance of 19.6 AU from the Sun. Its mass is 14 times the mass of earth. It has 27 satellites and the important ones are Titania (largest one), Ariel, Oberon, Umbriel, Miranda, etc. Uranus is the coldest planet in the solar system and is also the lightest of the outer planets.

    • Neptune remains at a distance of 30 AU from the sun. It is the farthest planet from the sun. It is smaller than Uranus in size but has more density and mass. Its mass is 17 times the mass of earth. Both Earth and Neptune are generally called Blue planets in solar system.

      It has 13 satellites and the largest one is Triton (known for geysers of liquid Nitrogen). Triton revolves around Neptune in clockwise direction whereas all others revolve in anti-clockwise direction.
  • Kuiper Belt or Trans Neptunian Belt

    It is largely an unexplored region, where we can find large number of debris. The largest Kuiper Belt object is Pluto. Pluto has highly eccentric orbit (not regular or circular).

    Charon was regarded as moon to Pluto but later it was found that it is also sharing the orbit of Pluto. It is regarded as Pluto-Charon binary system. Pluto is not able to clear its neighbourhood. Other bodies in Kuiper Belt are Makemake (a dwarf planet), Haumea(a dwarf planet), Quaoar, Varuna, Orcus, etc.

  • Scattered Disc Region

    The bodies present in this region were originally in the Kuiper Belt and later moved here. The bodies here have eccentric and erratic orbits. The largest body in this region is Eris (a dwarf planet).

    Of the five dwarf planets in our solar system, Eris is the largest in mass and Pluto is the largest dwarf planet by size. In order to satisfy Eris as dwarf planet, IAU stripped Pluto to dwarf planet. Eris is having a moon named Dysnomia.

  • Farthest Region

    The largest body in this region is Sedna (it is similar to Pluto). The farthest area in this Region is called Termination Shock Region. The farthest point of this region at 95 AU from the Sun. At Termination Shock region, the solar winds collide with Interstellar wind region.

    The solar radiation reach up to here. But recent studies say that Farthest Region extends up to 100000 AU and the Farthest Region is having huge clouds known as Oort cloud region. In Oort Cloud region, lot of icy bodies are present but these are hypothetical.

  • Interstellar Region

    It is the region between one star and another star. The matter that is present here includes gases that are in atomic, ionic and molecular form. This region also has dust as well as cosmic rays.


  • Comets are small heavenly bodies having a bright tail. They appear periodically, e.g. Halley appears once in every 76 years. Periodicity is due to their orbits.

    Comets are icy bodies and when they come close to the Sun, the ice sublimes into hot gas. Comets are responsible for water on the earth.

  • Depending on the source region, there are two types of Comets, (i) Short period comets (e.g. Halley) which reappear in less than 200 years and (ii) Long period comets which reappear above 200 years and it may take thousands of years to reappear.

  • All the Comets do not originate from the same region. Source regions for Comets are (i) Asteroid Belt for Short period comets and (ii) Oort Cloud region for Long period comets.

    Recently a Long period comet Hale-Bopp comet was found and it has a period of more than 2000 years for its reappearance.

Galaxic Neighbourhood of Sun

Closest neighbours to the Sun in Milky Way is Triple Star System. The star nearest to Sun is Proxima Centauri located at a distance of 4.22 Light years and the next closest star to the sun is Alpha Centauri.

Within 10 Light years distance, the largest star from the Sun is Sirius (Blue Dwarf star). Mass of the Sirius is 2 times that of sun. The closest star similar to Sun is Tau Ceti (Twin of Sun).

Difference between Planets and Dwarf Planets

In the structure of solar system, dwarf planets are also major bodies. According to International Astronomical Union (IAU), any celestial body is called a planet in the solar system if

  • There is sufficient mass to form into spherical shape

  • It orbits around the sun in specified orbit

  • It should have enough gravitational pull to clear its neighbourhood of all small bodies and no other body shares its orbit

If only the first two conditions are being satisfied for any object, it will be called dwarf planet. At Prague, Pluto got stripped of its planetary status and classified as dwarf planet. It was not able to clear the neighborhood of small bodies.