Why is pluto a planet?

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) the neighborhood of its own orbit of debris. Instead, they decided to classify it as a "dwarf planet".

There is a precedent for this. When Ceres was first discovered in 1801, it was considered to be a planet. After astronomers discovered several other objects in the same orbit, it was reclassified as an asteroid. And, in 2006, it was again reclassified as "dwarf planet" just like Pluto.

Neither Ceres nor Pluto have enough mass (gravity) to accrete all the junk from their orbits. They will be "dwarf planets" until we humans decide otherwise.

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

At the begginning of this year pluto was redefined as a dwarf planet.

The criteria that I know of for a planet are that it has enough gravity to make itself round and that it orbits a star.

Pluto has an irregular oval shaped orbit and it's not "that round" also compared to the other planets in our solar system it's really small and it's so far away it might be being moved along by not just the gravity of the sun but the gravity of other objects too

Answer 3

It was downgraded earlier this year. It is now just a chunk of rock orbiting the Sun.

Pluto turned out to be much smaller than people believed when it was declared a planet. Subsequent investigation showed it was of a similar size to other chunks of rock out there. It came down to choosing whether to upgrade some other planetoids, or downgrade Pluto. Pluto lost out.

Answer 4

Recently, within the last year, I have heard some things about them changing the definition of a planet and that pluto no longer meets the requirement. but i assure you that pluto is a planet mainly for the reason that it circulates our sun. You see it cannot be a moon because a moon circulates a planet, and a planet a sun. Pluto is indeed small, just like our moon, and it does indeed have characteristics like our moon, but because it circulates our sun, it is a planet based on the definition.

Answer 5

Pluto has been downgraded from Planet to Dwarf Planet since it doesn't meet the new standards for determining a Planet

Pluto
http://en.wikipedia.org/wiki/Pluto

Dwarf Planet defined:
http://en.wikipedia.org/wiki/Dwarf_planet

Answer 6

It's not a planet.

Answer 7

It's not. It's too small and hasn't cleared it's orbit.