What facts made them conclude that Pluto isn't a Planet?

Answer 1

There were no new facts. The IAU simply chose to change the definition of a planet to exclude Pluto.
"From its discovery by Clyde Tombaugh in 1930, Pluto was considered the ninth planet from the Sun. In the late 20th and early 21st century, many similar objects were discovered in the outer solar system, most notably the Trans-Neptunian object Eris which is slightly larger than Pluto. On August 24, 2006 the IAU redefined the term "planet", and classified Pluto, Ceres, and Eris as dwarf planets.[1] Pluto is also classified as the prototype of a family of trans-Neptunian objects.[2][3] After the reclassification, Pluto was added to the list of minor planets and given the number 134340.[4][5]"

Answer 2

At the International Astronomers Convention in Prague this year one of the most important things they discussed was the definition of a planet. The thing is recently they discovered some new planets in our system and they all look kinda alike Pluto - a ice planet orbiting around the sun on an outer orbit. But studies have shown that those planets as Pluto consist only from ice. So they had to either change the definition of a planet so they include and Pluto and the other 2 or 3 planets that are out there or change so neither of them gets in this list. So they concluded that Pluto as the other planets are more likely to be asteroids out of ice and rock that orbit around the sun than planets.

Answer 3

Its orbit is also a factor, too. Technically speaking, all the planets have elliptical orbits, but the ones they kept are near-circular. Pluto's orbit is more eccentric and even crosses Neptune's.

I should correct the answer below, too: Pluto is, indeed, big enough for its own gravity to pull it into a spheroid. Its diameter is about 2400 km. The largest main belt asteroid, Ceres, has a diameter of less than 1000 km and has pulled itself roughly into a sphere.

By the rules, a planet must be large enough to pull itself into a sphere. Pluto qualifies under that rule, but it doesn't qualify because it doesn't dominate its orbit.

Answer 4

It is too small. A number of large asteroids have recently been discovered orbiting even farther from the Sun than Pluto. One of them is even bigger than Pluto. There may be thousands of such objects. They are more properly considered another asteroid belt than new planets. At least nobody wanted to add so many new planets that were really small and in strange orbits too. Even stranger than Pluto's orbit,

In short, they had to draw the line between planet and asteroid somewhere, and Pluto is on the wrong side, based on the latest discoveries.

Answer 5

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 6

Also, the other eight planets orbit in the same plane around the sun. Pluto orbits in a different plane.

Answer 7

I think it is because it is way too small to be a planet. It's was smaller than our moon. Pluto is even smaller than the US, I think.

Answer 8

A PLANET HAS ALWAYS BEEN DEFINED AS BEING BIG ENOUGH FOR IT'S OWN GRAVITY TO CRUSH IT INTO A SPHERE. PLUTO IS MORE LIKE A POTATO