Why is Pluto not considered a planet? What is it then?

Answer 1

One of the nation's leading science museums has quietly shaken up the universe by suggesting that Pluto is not necessarily a planet at all but just a lump of ice.

After a tumultuous week of clashing over the essence of the cosmos, the International Astronomical Union stripped Pluto of the planetary status it has held since its discovery in 1930. The new definition of what is — and isn't — a planet fills a centuries-old black hole for scientists who have labored since Copernicus without one.

Although astronomers applauded after the vote, Jocelyn Bell Burnell — a specialist in neutron stars from Northern Ireland who oversaw the proceedings — urged those who might be "quite disappointed" to look on the bright side.

"It could be argued that we are creating an umbrella called 'planet' under which the dwarf planets exist," she said, drawing laughter by waving a stuffed Pluto of Walt Disney fame beneath a real umbrella.

"Many more Plutos wait to be discovered," added Richard Binzel, a professor of planetary science at the Massachusetts Institute of Technology.

The decision by the prestigious international group spells out the basic tests that celestial objects will have to meet before they can be considered for admission to the elite cosmic club.

For now, membership will be restricted to the eight "classical" planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Much-maligned Pluto doesn't make the grade under the new rules for a planet: "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 ... nearly round shape, and has cleared the neighborhood around its orbit."

Pluto is automatically disqualified because its oblong orbit overlaps with Neptune's.

Instead, it will be reclassified in a new category of "dwarf planets," similar to what long have been termed "minor planets." The definition also lays out a third class of lesser objects that orbit the sun — "small solar system bodies," a term that will apply to numerous asteroids, comets and other natural satellites.

Experts said there could be dozens of dwarf planets catalogued across the solar system in the next few years.

NASA said Thursday that Pluto's demotion would not affect its US$700 million New Horizons spacecraft mission, which earlier this year began a 9 1/2-year journey to the oddball object to unearth more of its secrets.

"We will continue pursuing exploration of the most scientifically interesting objects in the solar system, regardless of how they are categorized," Paul Hertz, chief scientist for the science mission directorate, said in a statement.

The decision on Pluto at a conference of 2,500 astronomers from 75 countries was a dramatic shift from just a week ago, when the group's leaders floated a proposal that would have reaffirmed Pluto's planetary status and made planets of its largest moon and two other objects.

That plan proved highly unpopular, splitting astronomers into factions and triggering days of sometimes combative debate that led to Pluto's undoing. In the end, only about 300 astronomers cast ballots.

Now, two of the objects that at one point were cruising toward possible full-fledged planethood will join Pluto as dwarfs: the asteroid Ceres, which was a planet in the 1800s before it got demoted, and 2003 UB313, an icy object slightly larger than Pluto whose discoverer, Michael Brown of the California Institute of Technology, has nicknamed Xena.

Charon, the largest of Pluto's three moons, is no longer under consideration for any special designation.

Brown, who watched the proceedings from Cal Tech, took Thursday's vote in stride — even though his discovery won't be christened a planet.

Answer 2

Discovery of the Week
Wait, No . . . 8 Planets



Time to update your solar system (again!)

Friends, our solar system has eight planets, not nine as you've been told all your life--or 12 as a committee of experts suggested last week. So ruled the International Astronomical Union (IAU) after a huge debate over the definition of "planet" at its meeting in Prague this week.

Last week, a crack IAU committee had proposed to fill a glaring hole in astronomy--the lack of an official scientific definition for "planet"--with a definition that would have raised the number of planets from nine to 12, adding the asteroid Ceres, Pluto's moon Charon, and a distant object called 2003 UB313. But the IAU's members mutinied and adopted a different definition.

Under the new definition, Pluto is not a planet, and neither are the other erstwhile candidates. So, official solar system, take 2: what is the new official definition of "planet"? And what are Pluto and the others, if not planets?


* *

"Planets" Rule Their Space

According to the new definition, approved by the IAU, a planet in our solar system is a celestial body that meets three criteria:

It orbits the sun.


It's round. More technically, it "has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape."


It "has cleared the neighborhood around its orbit." Basically, it dominates its part of space.
That last criterion revokes Pluto's planetary license--little Pluto doesn't dominate anything. The only planets are Mercury (the smallest), Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

"Dwarfs" Don't Rule Their Space

So what's Pluto? The IAU has ruled that it's a "dwarf planet." According to the IAU, a dwarf planet in our solar system is a celestial body that meets four criteria:

It orbits the sun.


It's round. (Same as above.)


It's not a satellite.


It has not "cleared the neighborhood around its orbit."
Under this definition, Ceres and 2003 UB313 are "dwarf planets," too. Charon, which was slated for promotion from "Pluto's largest moon" to "planet" will apparently have to rest content with its former title. (The rejected proposal would have revised the definition of "satellite" and made Pluto and Charon a "double planet," since they orbit each other.)

Though demoted to dwarf, Pluto did win official recognition as "the prototype of a new category of trans-Neptunian objects." The crack IAU committee had recommended that these objects be called "plutons," but geologists protested that they already use that word--it's a kind of rock formation. "Whoops," said the astronomers. For now, the new plutonic category lacks a nifty name.

Miscellaneous. Etc. Et al.

What about "trans-Neptunian objects" that aren't round like 2003 UB313, or asteroids other than Ceres, or other assorted hunks in space? The IAU has ruled that these objects--things that orbit the sun, that aren't satellites, and that don't qualify either as "planets" or "dwarf planets"--are officially (and none too poetically) named "small solar-system bodies."

For a little while at least. The definitional fight over "planet" may not be finished. Alan Stern, head of NASA's New Horizon's mission to Pluto, has already called the new definition "a farce" and claimed that "it won't stand." Others have pointed out that only a fraction of the professional astronomers gazing up at the sky today weighed in on "planets" in Prague. The IAU has spoken, and schoolbooks will be revised. But astronomers say they'll continue to explore strange new words and seek out new definitions.

Steve Sampson
August 24, 2006

Answer 3

First, remember it has been called a planet since it was discovered in 1930. There's really no universally agreed-upon of what should be called a planet and what should not, so it is partly a matter of definition. The main reasons it has been called a planet are that, like the other bodies which we call planets, Pluto is roundish and orbits the sun. The main reasons that many astronomers no longer want to call it a planet are that it's so small (even smaller than our moon) and that it's path around the sun is so freakish - in fact, Pluto sometimes comes nearer the sun than does the 8th planet, Neptune. There are some other issues among astronomers that are still subjects of argument, concerning how to classify planets. The link below, to a website at NASA, gives a readable review of the subject.

Answer 4

Scientists have finally decided on a definition of what a planet is. I guess previously they didn't have one. The definition says a planet is a blah deblahblah blah blah, and has cleared the area surrounding it's orbital path. Every 9(?) years Pluto's elliptical path brings it closer to the sun the Neptune, therefore it has NOT cleared the area surrounding it's orbital path. Also I believe it says that it has to be a circular path. There is some other stuff to but that's the gist of why Pluto is no longer a planet.

Answer 5

The definition of planet is an object formed by the sun, that revolves around the sun in an elliptical orbit and does not create its own light. Pluto was not formed by the sun. It is really just an asteroid. After the press conference Pluto was classified as a dwarf planet

Answer 6

Nothing about Pluto has changed since it's discovery in 1930. The criteria which denotes a planet, has changed recently, after a lot of discussion about what constitutes a planet ....now. Sometime in the future, the criteria could change again and bring Pluto back into the fold.
Pluto is still what it was way back in 1930 when discovered, a ball of regolith, mixed loosly with ice and possibly a few other things such as captured asteroids, meteors, and comets. Sure it is a dwarf by other earth like standards, but if Jupiter's scientists looked towards earth, do you think earth would be declassified from a planet too, from their perspective?
...jj

Answer 7

cuz some rejects in the science world noticed that there are more planets around Planet Pluto. Named it Planet Xena and others. But they didnt want to add them to part of our solar system even tho they are. So the rejects instead took away pluto from the system. Poor pluto.

Answer 8

Well: Pluto is a dwarf planet in the solar system. It was classified as a planet until it was determined on August 24, 2006 that it is not a true planet according to the redefinition of the term "planet". It is the prototype of a yet-to-be-named family of trans-Neptunian objects.[1][2]

From its discovery in 1930 until 2006, Pluto was considered the ninth planet of the Solar System by the International Astronomical Union (IAU). After contemporary observations, recent discoveries, and currently available scientific information were evaluated and debated, however, the IAU decided to reclassify Pluto as a dwarf planet, requiring that a planet must "clear the neighborhood around its orbit."[3]

Pluto has an eccentric orbit that is highly inclined with respect to the planets and takes it closer to the Sun than Neptune during a portion of its orbit. It is smaller than several natural satellites or moons in our solar system (see the list of solar system objects by radius). Pluto itself has a large moon named Charon (the ratio of the size of Charon to that of Pluto is the largest such ratio known in our Solar System, but see below); two smaller moons named Nix and Hydra were discovered in 2005.

The New Horizons spacecraft, which lifted off from Cape Canaveral, Florida on January 19, 2006, is expected to become the first spacecraft from Earth to fly by Pluto; ETA is July 14, 2015.

Pluto's astronomical symbol is a P-L monogram, ♇. This represents both the first two letters of the name Pluto and the initials of Percival Lowell, who had searched extensively for a ninth planet and who had founded Lowell Observatory, the place from where, after initiating several earlier searches, Pluto was eventually discovered. (Another symbol sometimes used for Pluto is an astrological symbol and not an astronomical one. It resembles that of Neptune, ♆, but has a circle in place of the middle prong of the trident.)

Pluto and its satellite Charon have often been considered a binary planet because they are more nearly equal in size than any other planet/moon combination in the Solar System, and because the two bodies orbit a point not within the surface of either. Under the rejected planet definition proposal, since they orbit each other around a center of mass that is outside either body, they would have been officially considered a binary planet system.

Answer 9

pluto is considered a planetoid because it is too small and not round enough to be a planet. it doesnt have a strong enough gravitational pull to squish into a round ball. they have been trying to decide this for a long time i remember doing a report on pluto in elementary school in the mid 90s and part of it included that they weren't sure whether or not it really was a planet.

Answer 10

Pluto' is a dwarf planet in the solar system, and was classified as a true planet until 2006. It is the prototype of a yet to be named family of Trans-Neptunian objects.[1][2] From its discovery in 1930 until 2006, it was considered the ninth and smallest of the planets of the Solar System by the International Astronomical Union (IAU). However, after contemporary observations, recent discoveries and currently available scientific information were evaluated and debated, the IAU decided, on August 24, 2006, to reclassify Pluto as a dwarf planet, requiring that a planet must "clear the neighbourhood around its orbit."[3

Answer 11

The International Astronomical Union votes to redefine the term "planet", demoting Pluto (pictured) to the status of a "dwarf planet", and leaving the solar system with eight planets.