Why is Pluto no longer a planet but a ''dwarf 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) 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 if folks insist on calling Pluto a planet, then we are going to have over 100 planets in fifty years.

Discovering Pluto when we did was a fluke...we now realize it's part of a class of bodies (Kuiper Belt Objects) that exist in a swarm beyond Neptune. Frankly, I think it's lucky to even be considered a dwarf planet.

When the first asteroid, Ceres, was discovered, it was also called a planet. But we discovered many more asteroids very soon afterward and quickly demoted Ceres from planethood. If Pluto had been discovered just ten years ago, we would not have called it a planet, since we have already found many bodies very similar to Pluto. Shall we call all of them planets? There is likely to be hundreds or even thousands of them discovered in the coming decades and centuries...just like there are thousands of asteroids.

There is one and only one reason to continue calling Pluto a planet: nostalgia. Get over it...our great-grand children will probably think our attachment to the term was silly.

Answer 3

Pluto no longer meets the criteria and loses its planet status, but becomes the prototype of a distinct class of dwarf planets.

Ceres and 2003 UB313 also have been recognised as dwarf planets. Charon, which was previously in the run for promotion, did not meet the final criteria for a dwarf planet.

Some 2500 astronomers from over 75 countries gathered this week in Prague at the Congress of the International Astronomical Union (IAU) to decide on several issues like a formal definition of a planet. Previously, there was no definition and with the discovery of new objects beyond Pluto there was much need for a clear criterion. The scientists also discussed new research findings in their field.

Louis Friedman, the executive director of the Planetary Society in California said: "The classification doesn't matter. Pluto — and all Solar System objects — are mysterious and exciting new worlds that need to be explored and better understood."

The final draft states: "A planet is a celestial body that

is in orbit around the Sun
has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
has cleared the neighbourhood around its orbit.
A dwarf planet is a celestial body that
is in orbit around the Sun,
has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
has not cleared the neighbourhood around its orbit,
is not a satellite.
All other objects except satellites orbiting the Sun shall be referred to collectively as Small Solar-System Bodies."
Pluto did not meet one of the criteria for planet: its orbit is higly eccentric, causing it to overlap with Neptune's. The IAU has a dozen other objects similar to Pluto on its "watchlist" and is expected to announce new dwarf planets in the coming months and years.

Ever since its discovery by American Clyde Tombaugh in 1930, Pluto has been considered a planet, though its status has been questioned many times after it was discovered to be far less massive than earlier calculations suggested, and because of its many other eccentricities. As a consequence of the vote, many textbooks, encyclopedias and other sources will need rewriting.

Answer 4

The International Astronomical Union (IAU) re-defined the term "planet". Pluto doesn't fit the new definition.

If they hadn't changed the definition, then as we discover more large bodies in the Kuiper belt the number of "planets" would keep expanding on a regular basis.

Answer 5

Pluto is no longer a planet because it it does not have the properties. first of all, pluto and sharon, its moon, revolve around each other. Pluto is also too small.

Answer 6

Because "planet" as such was never fully defined before. It now is. And Pluto does not meet the criterion for a planet, hence the new category.

Answer 7

They came up with new "criteria' for what makes a planet. Personally I think the astronomers just have way too much time on their hands.. Pluto will always be a planet to me .. heck with their expertise! :-P

Answer 8

They changed the definition a few months ago. To be a real planet you have to have your own unique orbit. Pluto doesn't qualify because part of its orbit is shared with Neptune for part of its trajection.

Answer 9

what are you talking about???
pluto is neither a planet or a dwarf planet.

pluto is a dog that hangs out with mickey mouse.

Answer 10

because its way to small to be considered a planet, so they just went with the closest thing, dwarf