The Solar System consists of nine planets. The statement in all textbooks round the world is set to change. Pluto the ninth planet has lost the coveted honour and now there are eight planets in the Solar System. Pluto has been demoted to the status of a “Dwarf Planet”.

                Right from the day of its discovery on 13th March, 1930 Pluto has been considered as a planet. It is not only much smaller than any other planet; its orbit is also more eccentric than any. The members of International Astronomical Union (IAU) took the decision on 24th August 2006 at Prague in Czechoslovakia not to count Pluto  as a planet, after heated deliberations. Apart from dwarf planet, “Small Solar-System Bodies” was coined which includes all other heavenly objects of the Solar System orbiting the Sun such as comets, asteroids, trans-Neptune objects etc.

What is a Planet?

A planet is a celestial body that

(a) is in  orbit around the Sun,

(b has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape,

(c) has cleared the neighborhood around its orbit.

                In accordance with that, we have now eight planets:- Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

What is a Dwarf Planet?:- A dwarf planet is a celestial body that (a) is in orbit around the Sun (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite. IAU thus resolved that Pluto is a dwarf planet by this definition and is recognized as the prototype of a new category of trans-Neptune objects to be called ‘plutonium objects”

Facts about Pluto:-Pluto was the smallest, coldest, and most distant member planet of the Solar System. Discovered on 18th February 1930 by a young 24-year old American Astronomer Clyde Tom Baugh. In Roman mythology, Pluto is the God of underworld. The planet got this name because it is far away from the sun.

                Pluto is about two-third of the diameter of Earth’s Moon, and has rocky core surrounded by a mantle of water ice. The size of Pluto is very small, about 2400Km, even smaller than Saturn’s moon ‘Titan’, Neptune’s Moon ‘Triton” and Earth’s own Moon. Due to its lower density, its mass is about one-sixth that of Moon. Pluto has a bright layer of frozen methane, nitrogen, and carbon monoxide on its surface. It has large quantities of ices of nitrogen and simple molecules containing carbon , hydrogen and oxygen. Due to its peculiar composition it is quite different from Neptune, Uranus, Saturn and Jupiter.

                While Pluto is close to the Sun, these ices  rise and temporarily form a thin atmosphere, with a pressure  one-millionth that of Earth’s atmosphere. Pluto’s low gravity (about 6% that of earth) causes the atmosphere to be much more extended in altitude than that of our planet. Due to the low gravity of Pluto, a man weighing 70 kg in earth would weigh about 6 kg in Pluto. Because Pluto’s orbit is so elliptical, Pluto grows much colder during the part of each orbit when it is traveling away from the Sun. During this time the bulk of the planet’s atmosphere freezes. Its surface temperature is about        - 233oC.

Controversial Factors Regarding Pluto

1.Pluto is extremely small size as compared to other planets.

2. It has a highly elliptical path, unlike the other planets, all of which has almost circular orbit.

3.There is also a physical mismatch. The first four planets near the Sun:- Mercury, Venus, Earth, and Mars have rocky hard structure, while the next four planets in the outer part are made of gases, lighter and huge in size. On the other hand, Pluto consists of rocky and solidified gases, resembling more a comet.

Comparison Between Neptune and Pluto:- Astronomers had observed that Neptune was deviating from its path due to some invisible force. They thought that there could be some bigger object or planet that was deflecting it. In 1930 a careful sky survey resulted in the discovery of Pluto. It was thought that Pluto could be causing Neptune to deviate from its path. But it was highly improbable that a small object like Pluto could pull a heavy planet like Neptune, which is 20 times larger than Pluto and 9000 times more in mass.

KBOs and Oort cloud:- It is important to note that during the 1930s there was nothing known about Kuiper Belt and Kuiper Belt Objects (KBOs). Pluto and its moon Charon orbit the sun in region where there may be a population of hundreds or thousands of similar bodies that were formed early in solar system history. These objects are referred to interchangeably as trans-Neptunian objects, Edgeworth-Kuiper Disk objects (KBOs) or ice dwarves. This region is known as the Kuiper Belt. It is a disk-shaped volume of space extending from about 5 to 15 billion km out and spread along the elliptical plane of orbit of the planets of the solar system.

             There is another region further beyond known as the Oort cloud, which was discovered by a famous Dutch astronomer Jan Oort. The Oort cloud is an immense spherical cloud surrounding the planets of our solar system from about 5 trillion km and extending out to about one to two light years from the Sun. Comets are dirty balls of ice and rock that orbit the Sun in highly elliptical orbits, taking them relatively close and then very far away from the Sun. These comets are divided into two categories depending on their furthest distance from the Sun:- “short-period” and “long-period”. Kuiper belt is the source of “short-period” comets – that takes less than 200 years to orbit the Sun (such as Halley’s with a orbit period of 76 years) and “long-period” comets (period of orbit more than 200 years, sometimes thousands or even millions of years).

Decision by IAU:-More than 2500 astronomers attended the 26th General Assembly of the International Astronomical Union in Prague. The vote involved just 424 astronomers who remained for the last day of the meeting, on 24 August 2006, which accounted for only about 10% of the membership. The elasticity of that definition caused a revolt amongst some 3000 members of the IAU. Brown and the IAU voting members in Prague were convinced that cashiering Pluto was the right choice; the feeling was not unanimous among astronomers in the US. Under the rejected definition, Brown estimated the solar system could end up with 53 planets or more, many of them smaller than earth’s moon.

                However, large group of astronomers have their sentiments attached to Pluto and they were not in favour of it being downgraded. This resulted in heated discussions in various forums. While many supported the decision, many others were not prepared to accept the move. Some are expecting the astronomy community to overturn the decision. Few other astronomers criticized the decision. Few others criticized the decision as ambiguous.

               In a move that surprised many legal observers, one of Pluto’s leading public-interest groups even sued the International Astronomical Union. Of course, it is not known where the venue for the case would be or precisely which judge would hear the case. The complaint alleged that the IAU violated key provisions of the Administrative Procedure Act, federal constitutional law, and certain Plutonian legal principles that are too complicated to set forth here.

                According to the IAU, more dwarf planets are expected to be announced in the coming months and years. Currently, a dozen candidate dwarf planets are on the IAU’s dwarf planet watch list, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.