Why would the International Astronomical Union now represent Pluto as a dwarf planet?

Question

I know that it's a dwarf planet now and it crossed over Neptune or something but I just don't know why they put it like that. I mean the Solar System changed now and some people still answers with 9 planets. For example: There could be a science test and if it asks the question "How many planets are there?" they would still answer 9 planets. I think that's unfair.

Answer 1

The International Astronomical Union decided that Pluto was no longer a planet. The reason is that while Pluto is round, orbits the sun, and has three moons, it has not cleared (via gravity) its own orbit of debris. 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

Because they have discovered a whole slew of other rocky ice balls and rock balls out there in the Kieper belt that are at least as big as Pluto and they would have to call them all planets if Pluto stayed a planet. On a test you would now say eight planets!

Answer 3

It's because there might be several objects past Pluto that are larger than Pluto and still orbiting our Sun. Instead of listing dozens of these objects as planets, they just scrapped Pluto.