Why isn't pluto a planet anymore?

Answer 1

because they keep changing the definition of planet. I'm sure they'll change it again. here's the latest from wiki.

http://en.wikipedia.org/wiki/2006_redefinition_of_planet

The 2006 definition of "planet" by the International Astronomical Union (IAU) states that, in the solar system, a planet is a celestial body that:

* is in orbit around the Sun,
* has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape, and
* has "cleared the neighbourhood" around its orbit.

A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet", whilst a non-satellite body fulfilling only the first criterion is termed a "small solar system body" (SSSB). The definition was a controversial one, and has drawn both support and criticism from different astronomers.

According to the definition there are currently eight planets and three dwarf planets known in the solar system. The definition distinguishes planets from smaller bodies, and is not used outside the solar system, where smaller bodies can't be found yet. Exoplanets are covered separately under a complementary 2003 draft guideline for the definition of planets, which distinguishes them from dwarf stars, which are larger.

Pluto was thought to be larger than Mercury. In 1978, the discovery of Pluto's moon Charon radically changed this picture. By measuring Charon's orbital period, astronomers could accurately calculate Pluto's mass for the first time, which they found to be much smaller than expected.[1] Pluto's mass was roughly one twentieth of Mercury's, making it by far the smallest planet, smaller even than the Moon, although it was still over ten times as massive as the largest asteroid, Ceres. Then, in the 1990s, astronomers began finding other objects at least as far away as Pluto, now known as Kuiper Belt Objects, or KBOs.[2] Many of these shared some of Pluto's key orbital characteristics, and are now called plutinos. Pluto came to be seen as the largest member of a new class of objects, and some astronomers stopped referring to Pluto as a planet. Pluto's eccentric and inclined orbit, while very unusual for a planet, also fit in well with the other KBOs. New York City's newly renovated Hayden Planetarium famously did not include Pluto in its exhibit of the planets.[3] Then, starting in 2000, with the discovery of at least three bodies (Quaoar, Sedna and Eris) all comparable to Pluto in terms of size and orbit, it became clear that they all would either have to be called planets or Pluto would have to be reclassified.

Answer 2

Pluto is big enough to be round, it has an atmosphere and a huge satellite Charon plus two others, Nix and Hydra. Pluto orbits the Sun independently, not a larger planet. It's also likely differentiated, meaning it formed hot and the denser materials sank to the center while the icy materials floated upwards forming the crust and mantle. But Pluto is tiny, it's mass is 500 times less than the Earth. It's mass is so low it has neglible effects upon the paths of Uranus and Neptune. The key definition for a planet that was arrived at is a body that's too small to start nuclear fusion in it's core and become a star or a brown dwarf, but big enough to sweep up all the planetesimals near it's orbit. Pluto cannot do that, it's just doesn't have enough mass and hence gravity to sweep up all the rubble and other Kuiper belt object or at least clear a path for itself. That is why it's been demoted to dwarf-planet status. WIth the exception of Eris, Pluto is the biggest KBO known, and thousands of them came together to build up Uranus and Neptune. One of them was captured by Neptune, which became it's biggest moon Triton.

Answer 3

Why is Pluto not a planet? Because it has been found to be smaller than once thought, and if we were to consider Pluto a planet, a number of asteroid in the asteroid belt would have to be considered planets too. Also, Charon, Pluto's 'moon' is almost 1/2 the size of the planetoids, proportionately, it is the largest moon in the solar system, so, for awhile, Pluto and Charon were considers 'double-planets' The universe is amazing, is it not? What we haven't discovered... TO MARS!

Answer 4

August 24, 2006, was a significant day in the history of planetary science. If you read the newspaper headlines, you might think it was famous as the day that Pluto was thrown out of the planetary family. But if you read more about the 2006 meeting of the International Astronomical Union, you will discover that the story is a bit different than that. August 24, 2006, was really the first day that there was an official scientific definition of the word "planet."

Until that date, scientists used the word planet, and many people called themselves planetary scientists (including Heidi Hammel, whose life story can be found here). But no major scientific organization had come up with an official definition. For a long time, they didn't have to, because everyone seemed to agree on what bodies should be included in the list of planets.

But then, the more observations people made of Pluto, the less certain they were about including it in the list. They discovered it was smaller than they thought at first, even smaller than Mercury. They knew its path around the Sun was more elongated than the other planets, so elongated that it is closer to the Sun than Neptune for about 20 years of its 248-year orbital period. They knew that Pluto's orbit was unlike the other planets' and the asteroids, which lie in nearly the same plane as Earth's path. From Mercury to Neptune those bodies never leave a thin disk-shaped region around the Sun, but Pluto's orbit takes it well above and below that disk.

When Clyde Tombaugh discovered Pluto in 1930, he and everyone else called it the ninth planet. But by the time Tombaugh died in 1997, some astronomers were expressing doubts about it. They realized that it was probably one of many large icy bodies in a distant region of the Solar System called the Kuiper (KI-per) Belt. They even suspected that it might not even be the largest Kuiper Belt object, just the first and largest one found up to that time.

That made Pluto's status as a planet the kind of question that most scientists love to disagree and argue about. When scientists argue, it doesn't mean they're angry with each other. It just means they look at the same evidence and draw different conclusions.

Dr. Fred thinks arguments like that are fun, especially when someone disagrees with something that no one has questioned for years, such as the number of planets in the Solar System. When a large Kuiper Belt Object known as Quaoar (kwa-oh-wahr) was discovered in 2002, people started to talk about changing Pluto's status. That's when Dr. Fred decided to include this question in his "Ask Dr. Fred" pages. He kept adding updates as new large bodies were discovered in the outer Solar System, but on day Pluto was "demoted," he knew the old article had to be rewritten completely.

Answer 5

Because now they are finding (what,over a hundred now) other big planet size rocks out there,and instead of constantly adding these to textbooks,they just said...hey Pluto not really as big as the other planets so..........OK OK, its still a planet,its a minor planet

Answer 6

Because Pluto sucks. Earth rules!

Answer 7

They said that it is a dwarf planet because it is way smaller than the other 8 planets

Answer 8

planet definition has changed

pluto will not be follow the rule which is govern by other planet and stars

Answer 9

I belive that whoever decided this is a complete baffoon:
My Thoughts:
Pluto is made of Ice,
Ice melts into water,
Where there is water.....
There is Life !!!!

I dont understand how can they just toss Pluto "out" like that when there is for sure evidence of possible life.
AAAAAHHHHHHHHHHHHHHH!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Answer 10

Because the little planet gremlins came and took it away....