why is pluto not a planet any more?

Answer 1

The International Astronomical Union decided that Pluto was no longer a planet. Instead, they decided to classify it as a "dwarf planet".

See the details below.

RESOLUTION 5A
The IAU therefore resolves that "planets" and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

(1) A "planet" [footnote 1] 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 (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) 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 (nearly round) shape [footnote 2] , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects [footnote 3] except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".


Footnote 1: The eight "planets" are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Footnote 2: An IAU process will be established to assign borderline objects into either dwarf planet and other categories.

Footnote 3: These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

RESOLUTION 6A
The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

Answer 2

my dear friend,
astronomers as do union workers need to work to keep their jobs. For astronomers, it means, writing and sucking up rich institutions and people to obtain research grants (that means their salary). Expensive travel to exotic destinations is part of the drill. So, it is a good idea to meet, let say, in Budapest and argue like five days about pluto's definition. Great restaurants, hotel, drinks, women, etc. It's like a fraternity for grown ups. Capish?

In a few years, some odd rock will be discovered and Pluto will be re-evaluated, perhaps it will make it to full planet, perhaps, a minime kind of planet classification will be written, discussed (let's say in Ulan Bator), approved and filed. More grants will be requested to investigate the relationship between dwarfies and minis. and so on. Hey, perhaps you will be by then an astronomer and then you may be directing the whole shebang.

Same with global warming. In the seventies, it was global cooling. Perhaps in thirty years you will write "now it is global cooling, back 30 it was global warming. LOL"

Cheers.

Plus Ça Change, Plus C'est La Même Chose

Answer 3

because due to the definition of a planet, it's too little to be one and doesn't have a precise orbit as the others. now pluto is a dwarf planet. go to wikipedia planet for more info if u want.

Answer 4

its small, and has a weird orbit. and if pluto was a planet we would have to call many more things planets.