The Planets

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The Planets

A planet, as defined by the International Astronomical Union (IAU) in 2006, is a celestial body orbiting a star, is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighboring region of planetesimals.

Under IAU definitions, there are 8 planets in the Solar System – Mercury, Venus, Earth, Mars (called Terrestrial Planets), Jupiter, Saturn, Uranus, Neptune (called Jovian Planets).

Planets are divided into two groups : large, low-density gas giants and smaller, rocky terrestrials.

Planets according to size : Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars and Mercury.

The Solar system also contains 5 Dwarf Planets : Ceres, Pluto (formerly considered to be Solar System’s ninth planet), Makemake, Haumea and Eris.

Mercury Planets

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  • Rotation: 58.65 days.
  • Revolution: 88 days (Fastest revolutionin Solar System).
  • Maximum diurnal range of temperature. Its days are scorching hot and nights are frigid.
  • It has no atmosphere and no satellite.

Venus Planets

  • Also called Earth’s Twin, because it is slightly smaller than earth (500 km less in diameter).
  • Popularly known as Evening star and Morning star. It is seen in the east in the morning and in the west in the evening. It is not possible to see it all over the night.
  • Brightest object after sun and moon (because of 70% albedo, the reflecting power).
  • Closest planet to earth.
  • It is the hottest planet in Solar System. It is because of die Green house effect as its atmosphere contains 90-95% carbon dioxide. The night and day temperatures are almost the same.
  • Rotates backward (clockwise) unlike others.
  • It has no satellite.
  • Slowest rotation in Solar System (257 days). Almost equal rotation and revolution (224.7 days).

Mars Planets

  • Also called Red Planet.
  • Revolution Planets : 687 days
  • Rotation Planets : 24.6 hrs (almost equal to Earth).
  • It has a thin atmosphere comprising of nitrogen and argon.
  • It is marked with dormant volcanoes and deep chasms where once water flowed. Recent explorations have thrown light on the possibilities of existence of life here.
  • The highest mountain here is named ‘Nix Olympia’, which is three times higher than Mount Everest.
  • It has 2 satellites – Phobos and Deimos.

Jupiter Planets

  • Largest of all planets (71% of the total mass of all planets). Called Lord of the Heavens.
  • Jupiter appears to have stopped halfway to becoming a star. It was too massive to solidify as a planet but not massive enough to develop ‘nuclear fusion’ and become a star.
  • It gives off more energy than it receives from the sun, because of the heat inside.
  • Its atmosphere contains hydrogen, helium, methane and ammonia.
  • A great red spot is detected on it. It represents a huge storm- a super hurricane, existing for hundreds of years, without abating. This storm is probably powered by Jupiter’s internal heat.
  • It has the fastest rotation time (9.8 hr) in the Solar System.
  • Revolution : 12 years.
  • It has 63 satellites (Prominent are Europa, Gannymede & Callisto). Gannymede is the largest satellite of Solar System

Saturn Planets

  • Second in size after Jupiter.
  • Revolution: 29 years Rotation: 10.3 hrs
  • Least density of all (30 times less dense than earth).
  • Unique feature is its system of rings (3 well defined). These are separate particles that move independently in circular orbits.
  • 60 satellites (Prominent is Titan).
  • The space probe, Cassini, is on Saturn these days.

Uranus Planets

  • Identified as a planet in 1781 by William Hershel.
  • Seems to rotate from north to south as it is inclined at an angle of 98″ to its orbit.
  • Revolution: 84 years Rotation: 10.8 hrs.
  • Surrounded by a system of 9 faint rings.
  • Has 27 satellites (prominent are Miranda, Ariel, etc)

Neptune Planets

  • Appears as ‘Greenish Star’.
  • Revolution: 165 yrs. Rotation: 15.7 days.
  • Has 5 faint rings.
  • Discovered by J.G Galle of Berlin in 1846.
  • 13 satellites (prominent are Triton and Nereid).

Pluto Planets

  • Pluto is the second largest known Dwarf Planet in the Solar System.
  • Since its discovery by Clyde Tombaugh in 1930, Pluto was considered the ninth planet from the Sun. In August, 2006, the International Astronomical Union (IAU) redefined the term ‘planet’, and classified Pluto, Ceres and Eris as Dwarf Planets.
  • Now, Pluto has been given the number 134340.

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