The recent declassification of Pluto from being a planet had made a lot of news. Many people ask me about Pluto ' Why astronomers discard Pluto from planet status?' Actually there was no specific definition for planet before this.
For thousands of years people have observed bright objects in the sky that moved in certain paths against the background of fixed stars, but very little was known about them. In fact the name "planet" given to these, comes from the Greek word for "wanderer". The five planets Mercury, Venus, Mars, Jupiter & Saturn could be seen with naked eye from the Earth & were known since antiquity. On the other hand, Uranus, Neptune & Pluto are objects discovered comparatively recently & were also classified as planets. But, today, hosts of newly discovered large objects in the outer regions of our solar system present a challenge to our definition of a "planet".
Making use of the currently available scientific information the International Astronomical Union (IAU), on August 24, 2006, agreed that a "planet" is defined as a celestial body that
1. is in orbit around the Sun.
2. has sufficient mass for its self gravity to overcome rigid body force so that it maintains a hydrostatic equilibrium (leading to nearly round shape). and
3. has cleared the neighbourhood around its orbit (meaning that there are no other bodies of comparable size other than its own satellites or those otherwise under its own gravitational influence.)
Pluto failed the third condition because it shares its orbital neighbourhood with Kuiper Belt objects such as the Plutinos. Moreover, Pluto & its satellite Charon have their center of gravity lying outside the Pluto. Hence it is not qualified to be a planet. But astronomers classified Pluto as "Dwarf Planet". A non satellite body fulfilling only the first two of criteria is classified as a "Dwarf Planet". Other two members of the dwarf planet category are Ceres & Eris.
Now the solar system consists of eight "planets". Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune orbiting in that order of their distances around our star the Sun.
Some astronomers have their strong opinion for Pluto as a planet. I don't know whatever happened with Pluto is right or wrong. But, now at least we can classified Solar System objects with more scientific manner which necessary to understand our planetary system.
AK
