Why isn't Pluto considered a planet anymore?

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

Is Pluto a planet???? Yes and no...

Well for starters, Pluto is just too small. In the neighborhood where Pluto lives? Planets are supposed to be huge. The Jovian planets, Jupiter, Saturn, Uranus and Neptune are 20 to 300 times the size of the Earth, and Pluto is really small compared to the Earth, smaller than our Moon. Kind of stands out.

And Pluto is not made out the same material as the Jovians. The large planets are mostly gigantic spheres of gas, mostly hydrogen and helium. Likely there are no solid surfaces, only denser and denser gas all the way in. Pluto is a small solid world of methane, water, carbon dioxide and ammonia ices, maybe a little rock and with a just hint of atmosphere (that freezes out and falls as snow in her "winter").

And third, Pluto's orbit is the most eccentric (oval shaped) and the most tilted to the plane that the rest of the planets orbit in. Also, Pluto is locked in a resonance with Neptune's orbit and comes closer to the sun than Neptune sometimes.

There were theories that Pluto was a lost moon of Neptune but that was before we discovered she a has one large moon (Charon) half her size (pretty much, this system is a double planet) and recently two other teeny-tiney moons (Nix and Hydra).

Pluto seems like she cant be an ejected moon-she must have formed on her own and seems to be part of an entire army of small icey-dwarf objects that circle just outside Neptune's orbit in what is known as the Kuiper belt. We have no idea of how many or how large these objects may be, hundreds???? NOT "planets" proper, hence the new term "dwarf planet" where Pluto is king.

But... I still think Pluto SHOULD be called a planet because of historical reasons (discovered by an American, financed by Percival Lowell, Tombaugh's life story, etc).

Answer 3

The issue of a clear definition for "planet" came to a head in 2005 with the discovery of the trans-Neptunian object Eris, a body larger than the smallest accepted planet, Pluto. In response, the International Astronomical Union (IAU), recognised by astronomers as the world body responsible for resolving issues of nomenclature, released its decision on the matter. This definition, which applies only to the Solar System, states that a planet is a body that orbits the Sun, is large enough for its own gravity to make it round, and has "cleared its neighbourhood" of smaller objects.

Pluto's official status as a planet has been a constant subject of controversy, fueled by the past lack of a clear definition of planet, since at least as early as 1992, when the first Kuiper Belt Object, (15760) 1992 QB1, was discovered. Since then, further discoveries intensified the debate in the 21st century.

Answer 4

confident, a "dwarf" planet. it fairly is purely a million/6 the mass of our Moon. it fairly is a very interesting subject rely for a undertaking. that is not any longer a planet while you're writing respectable papers describing scientific journey that are difficulty-loose to "actual" planets. yet, while you're doing a undertaking, you're able to do a paragraph explaining why it fairly is termed a "dwarf planet". Then purely proceed to call it a planet (with no need to apply the be conscious dwarf each time) whenever you may desire to. slightly an identical way as while you're speaking of a toy motor vehicle (as against a "actual" motor vehicle). you initiate via explaining that the motor vehicle is a toy. then you easily can purely seek advice from it as "the motor vehicle" with no need to remind the reader each time that it fairly is a toy. of direction, in case you get to a paragraph the place it fairly is significant rigidity the reality that it fairly is a toy, then you easily can say something like "the motor vehicle, being a toy, ...". Pluto is such countless issues: a dwarf planet, a minor planet, a Plutoid, a Trans-Neptunian merchandise, a planetoid (in spite of exhibiting signs and indications of additionally being a comet), and extra.

Answer 5

Recent visitors to the Rose Center for Earth and Space at the American Museum of Natural History in New York City have been surprised to discover that Pluto is no longer considered a planet in the solar system.








"Pluto, being half ice by volume, should assume its rightful status as the King of the Kuiper Belt of comets," says Neil Tyson, director of the Hayden Planetarium at the American Museum of Natural History.

Some members of the International Astronomical Union agree with Tyson and recommend that Pluto be demoted to "object."

A Planet No More?

The scientists' main reason for the switch is Pluto's tiny size; with a diameter of approximately 1,420 miles ( 2,280 kilometers) it is six times smaller than Earth, and smaller than seven of the solar system's moons.

Pluto's orbit is unlike any other planet. It is the only planet to travel an elliptical orbit, and uncharacteristically crosses the orbit of its closest planetary neighbor, Neptune. This means that for 20 years of its 248-year orbit around the sun it is not the most distant object in the solar system.

Despite its unusual characteristics, Pluto's status as a planet was solid until 1992, when David Jewitt and J. Luu of the University of Hawaii discovered a strange object called 1992 QB1. QB1 is a small icy body, similar in size to an asteroid, orbiting one and a half times further from the sun than Neptune.

QB1 was the first clue that more objects than Pluto might populate the distant reaches of the solar system. Since 1992 nearly one hundred objects like QB1 have been found. They are thought to be similar to Pluto in composition, consisting primarily of ice and rock.

This swarm of Pluto-like objects beyond Neptune is known as the Kuiper Belt, named after Gerard Kuiper, who first proposed that such a belt existed.

Astronomers estimate that there are at least 35,000 Kuiper Belt objects (KBOs) greater than 62 miles (100 kilometers) in diameter. Pluto is practically indistinguishable from other objects in the Kuiper Belt. Aside from its large size, the only real difference is Pluto's reflectivity, which makes it much brighter than other KBOs.

Pluto: Space Object

In the "Hall of the Universe" at the American Museum of Natural History, Tyson and his staff group together families of like-objects, rather than using the all-embracing term of planet.

They organize the objects of the solar system into five broad families: the terrestrial planets, which include Mercury, Venus, Earth and Mars; the asteroid belt, the Jovian planets, which include Jupiter, Saturn, Uranus, and Neptune; the Kuiper belt, and the Oort cloud (a ring of dust even further away from the sun than the Kuiper Belt).

With this approach, Tyson believes, "numbers do not matter and memorized facts about planets do not matter. What matters is an understanding of the structure and layout of the solar system."




It's from the national geographic news:)I found some other informations about the subject and I wish they could help:)
Pluto Facts
Pluto is the only planet in the solar system that has not been visited by a space probe.
On September 13, 2000, NASA suspended its Pluto-Kuiper Express (PKE) mission.
In December 2000 NASA announced new proposals for a mission to Pluto. The costs must stay under U.S. $500 million. The Johns Hopkins Applied Physics Lab, which just landed a probe on the asteroid Eros, is planning to make a proposal to NASA.
Because of its great distance from the Earth, the exact size of Pluto is not known. It is believed to have a radius of roughly 1150-1200 miles (1850-1930 km).
Since its discovery in 1930, Pluto has traveled on only about one-quarter of its orbital path around the Sun. As a consequence, its exact orbital parameters are still not known.
The surface temperature on Pluto is between -378 to -396°F (-228 to -235°C).

Answer 6

Mostly because it is too small. Pluto was once though to be larger than Mars, but we now know it is smaller than the Moon.

Answer 7

here is the new definition of a planet courtesy of Wikipedia: http://en.wikipedia.org/wiki/2006_definition_of_planet. I also heard that to be considered a planet one cannot cross into another planet's orbital path.

Answer 8

Simply put it does not fit the accepted definition of a planet... you can read more on wikipedia

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

Answer 9

Because Clyde Tombaugh is dead and NOT around to fight the scientific ninnies that lowered its statice from a planet to a 'drawf planet' whatever the heck that is suppose to be!!!

Answer 10

Because it doesn't follow it own orbit. It may be one of Neptune's moons.