Why is Pluto considered to be a planet and not just a huge Comet?
825827
read http://en.wikipedia.org/wiki/Pluto\n\npluto is neither a planet nor a comet.\n\npluto does orbit the sun, is ball-shaped, does not have an isolated orbit (a bunch of other similar bodies have similar orbits.), and is not a satellite so it is not a planet.\n\ni have been waiting for this since i was about ten.  i knew that pluto didn't fit the pattern set by the major bodies in the solar system so it was an anomaly.  it just felt "out of place".  now that astronomers have found hundreds of other bodies with similar orbits, classifying "134340 pluto" as a planet is even more irrational.  i feel somewhat satisfied, but i don't know how long this will drag on tho.  this was the right thing to do, believe me.  this does not change anything about pluto or the solar system.  this just corrects the mistake of classifying pluto as a planet initially.\n\nmany astronomers consider pluto and charon to be a binary system, but two small bodies orbit that system.  they are called nix and hydra.\n\nincidentally, "134340 pluto" was never a moon of neptune.  neptune did capture triton. this is why triton has a retrograde orbit.
1107840
Because there are still arguments whether Pluto is actually a planet, "Sedna" is larger than Pluto. But I believe the argument stems from sheer distance from the Sun, it is more of a rogue body.
112225
The tenth planet is like really great, thanks for asking.  He spins around and around and makes one revolution around the sun about once every several thousand thousand years.  Therefore you might call this tenth planet (he doesn't mind) a chunk of ice, and not really planet-like at all.  But wait, you're saying, Pluto is a chunk of ice and also a planet!  Well, Pluto actually (he's telling me now) doesn't even consider himself a planet, as he's really just a large ice-chunk in the Kuiper Belt.  The tenth planet (we'll call him "Frank", that's what he prefers) is a little peeved that Pluto gets a thousand-brother belt to wear and all he's got is the occasional through-comet.  We can't help you, Frank, I'm sorry.  We're like, a billion miles away.
1183831
THere is an argument among astromoners as to whether classify the bodies beyond pluto as planets... consider that in the asteroid belt there are many objects orbiting the sun... do we call them all planets?\n\nCurrently there are officially only 9 planets:\nMercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
89889
July 29, 2005: "It's definitely bigger than Pluto." So says Dr. Mike Brown of the California Institute of Technology who announced today the discovery of a new planet in the outer solar system.\n\nThe planet, which hasn't been officially named yet, was found by Brown and colleagues using the Samuel Oschin Telescope at Palomar Observatory near San Diego. It is currently about 97 times farther from the sun than Earth, or 97 Astronomical Units (AU). For comparison, Pluto is 40 AU from the sun\nThe planet's temporary name is 2003 UB313. A permanent name has been proposed by the discoverers to the International Astronomical Union, and they are awaiting the decision of this body before announcing the name. \nAm I right U_EVERYONE_K
952763
Neptune and Pluto, however Pluto is no longer a planet.
945846
This issue is very similar to the asteroid belt issue some 150 years ago.  When the first asteroid, Ceres, was found, it was considered a planet.  Only after 15 other "planets" were found in the same orbit regime did astronomers downgrade these asteroids to "minor planets" some 50 years after Ceres' discovery.\n\nIt's unclear if Pluto could ever clear its orbit...it crrently has only 7% of the mass of its orbital neighborhood (as opposed to, say, Earth, which contains some 99.9999% the mass of its orbital neighborhood), so it's only cleared a tiny fraction of its orbit throughout the entire age of the Solar System, 4.5 billion years.\n\nMoreover, other objects in Pluto's orbit (and pluto itself, in fact) are trapped in a 3:2 orbital resonance with Neptune.  This means these "plutinos" are in a very stable orbit, unlikely to be scattered away unless something very big and unexpected comes along.
622174
astronomers are trying to define the word planet but it isn't easy. many astronomers favor the reclassification of the bodies in the solar system into three groups, the rocky terrestrials, the gas giants, and the ice dwarfs. there are hundreds of bodies that orbit the sun farther than neptune known. these include bodies like pluto, quaoar, sedna, 2003 EL61, and 2003 UB313 (the official name of xena). this is also called the kuiper belt. pluto was the first kuiper belt object discovered, and 2003 UB313 is about five percent bigger than pluto.\n\n\nread this:\nhttp://en.wikipedia.org/wiki/Kuiper_belt
75743
Twertyto's answer is the best! When I was just learning about astronomy I asked the same question. Someone told me a very simple answer that just might work for you:  A planet reflects light and a star creates it. That sounds simple enough for simpler times! But now welcome to the 21st century!!!\nBrown dwarfs are kind of the "animal" that bridges these two seemingly different objects. Astronomy is having a hard (but fun) time right now trying to decide exactly what a planet is- from the very large to the very small. Is an object 75 times the size of Jupiter a planet? 79 times? 81 times? No one seems to know. Do we call them planets or officially name them Brown Dwarfs. On the other side of the issue, is Pluto a planet? They just discovered a larger than Pluto object and several almost-the-size-of-Pluto objects out past Pluto's orbit. Are all of these planets? If Pluto's not a planet, but now has been discovered to have at least 3 moons (two new one's were just discovered recently), then what do we call it? \nThis has been a surprisingly tough answer for Astronomers as well, so not only is it a great question, but you- and everyone else- might have to stick around for a while for the difinitive answer!!!
1037864
There are a large number of balls of ice and rock of various sizes scattered around the solar system. Some are small enough that they are irregularly shaped. Some have highly elliptical orbits. Some are very, very small.  So part of the difficulty is that there is not an agreed upon definition of what a 'planet' really is. By one definition, Pluto is not a planet, but rather is the first of a class of medium sized balls of ice and rock to be discovered at the edges of our system (the Kuiper belt objects). There are many others, some of which are comparable in size. Furthermore, these objects are far enough away from the sun that they don't reflect much light back to us, so they are very hard to locate.\n\nOn the other hand, we can estimate the distances to some fairly bright galaxies and determine that they are billions of light years away. That means that the light from these galaxies has taken a billion years to get to us. That, in turn, means that we are seeing now what happened there billions of years ago. This allows us to say what the universe was like at that time with some confidence.
1071916
no, astronomers are trying to define what a planet is, but it isn't easy.  many astronomers favor the reclassification of the bodies in the solar system into three groups, the rocky terrestrials, the gas giants, and the ice dwarfs.  there are four rocky terrestrials and four gas giants, but there are hundreds of known ice dwarfs. this is called the kuiper belt.  kuiper belt objects are bodies like pluto, quaoar, sedna, 2003 UB313, and 2003 EL61.  2003 UB313 (the official name of xena) is about five percent bigger than pluto, but pluto was the first kuiper belt object discovered.\n\nread this page:\nhttp://en.wikipedia.org/wiki/Kuiper_belt
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http://en.wikipedia.org/wiki/Pluto\n\non 24 august 2006, the international astronomical union reclassified pluto.\n\npluto does orbit the sun, is ball-shaped and is not a satellite, but it does not have an isolated orbit (a bunch of other similar bodies have similar orbits.) so it is not a planet.\n\ni have been waiting for this since i was about ten when i learned that pluto didn't fit the pattern set by the major bodies in the solar system so it was an anomaly. it just felt "out of place". now that astronomers have found hundreds of other bodies with similar orbits, classifying "134340 pluto" as a planet is even more irrational. i feel somewhat satisfied, but i don't know how long this will drag on tho. many planetary astronomers are satisfied that the definition is rigorous enuf. i can accept that the definition is flawed, but i can not accept that "134340 pluto" is a planet.\n\nthis was the right thing to do, believe me. this does not change anything about pluto or the solar system. this just corrects the mistake of classifying pluto as a planet initially.\n\nthis same thing happened has happened before.  in 1800, an astronomer found a body orbiting between the orbits of mars and jupiter and thought it was a planet.  astronomers finally stopped classifying them as planets after they found several other bodies with similar orbits, and no one thinks ceres, pallas, juno, and vesta are planets today.\n\nmany astronomers consider pluto and charon to be a binary system, but two small bodies orbit that system. they are called nix and hydra.\n\nincidentally, "134340 pluto" was never a moon of neptune. neptune did capture triton. this is why triton has a retrograde orbit
730339
1.  Pluto, Sedna, Eris, Ceres, etc. are still planets but they are a specific class of planets ("dwarf planets").  This is similar to the Sun being classifed as a "yellow dwarf" but it's still a star.\n\nNow that I've got that off my chest, Pluto and Sedna are members of the Kuiper Belt, a collection of bodies that extend from about Neptune's orbit out to where the Oort Cloud is supposed to begin.  The Kuiper Belt is approximately co-planar with the ecliptic but extends farther away from the central plane of the solar system.  This definition allows Pluto, whose orbital inclination is significantly larger than average to be included in the Kuiper Belt.\n\nAstronomers are consistently finding more members of the Kuiper Belt through photographic review.  Most are smaller than Pluto, Sedna and Eris, some are similar in size and there is likely to be at least one more body that will be larger.
209097
Pluto is the fartherest planet, there is some discussion about it being a planet or not but for this answer we'll let it be one, from the sun during most of it's orbit. It's 5,913,520,000Km from the Sun.\nThe Main asteriod belt lies between Mars and Jupiter, roughly 374,172,480Km from the sun, the planets run Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune & Pluto.\nThe Kuiper Belt, made up of comets instead of asteriods, lies outside of the orbit of Pluto and some think Pluto is just a comet from this belt. Then going way on out past the Kuiper Belt you run into the Oort Cloud roughly 22,450,348,800,000Km from the Sun and made up of trillions of cometary bodies.
209591
From our small world we have gazed upon the cosmic ocean for thousands of years. Ancient astronomers observed points of light that appeared to move among the stars. They called these objects planets, meaning wanderers, and named them after Roman deities - Jupiter, king of the gods; Mars, the god of war; Mercury, messenger of the gods; Venus, the god of love and beauty, and Saturn, father of Jupiter and god of agriculture. The stargazers also observed comets with sparkling tails, and meteors or shooting stars apparently falling from the sky.\n\nSince the invention of the telescope, three more planets have been discovered in our solar system: Uranus (1781), Neptune (1846), and Pluto (1930). In addition, there are thousands of small bodies such as asteroids and comets. Most of the asteroids orbit in a region between the orbits of Mars and Jupiter, while the home of comets lies far beyond the orbit of Pluto, in the Oort Cloud.\n\nThe four planets closest to the Sun - Mercury, Venus, Earth, and Mars - are called the terrestrial planets because they have solid rocky surfaces. The four large planets beyond the orbit of Mars - Jupiter, Saturn, Uranus, and Neptune - are called gas giants. Tiny, distant, Pluto has a solid but icier surface than the terrestrial planets.\n\nNearly every planet - and some of the moons - has an atmosphere. Earth's atmosphere is primarily nitrogen and oxygen. Venus has a thick atmosphere of carbon dioxide, with traces of poisonous gases such as sulfur dioxide. Mars' carbon dioxide atmosphere is extremely thin. Jupiter, Saturn, Uranus, and Neptune are primarily hydrogen and helium. When Pluto is near the Sun, it has a thin atmosphere, but when Pluto travels to the outer regions of its orbit, the atmosphere freezes and collapses to the planet's surface. In that way, Pluto acts like a comet.\n\nThere are 154 known natural satellites (also called moons) in orbit around the various planets in our solar system, ranging from bodies larger than our own Moon to small pieces of debris. Many of these were discovered by planetary spacecraft. Some of these have moons have atmospheres (Saturn's Titan); some even have magnetic fields (Jupiter's Ganymede). Jupiter's moon Io is the most volcanically active body in the solar system. An ocean may lie beneath the frozen crust of Jupiter's moon Europa, while images of Jupiter's moon Ganymede show historical motion of icy crustal plates. Some planetary moons may actually be asteroids that were captured by a planet's gravity. The captured asteroids presently counted as moons may include Phobos and Deimos, several satellites of Jupiter, Saturn's Phoebe, many of Uranus' new satellites, and possibly Neptune's Nereid.\n\nFrom 1610 to 1977, Saturn was thought to be the only planet with rings. We now know that Jupiter, Uranus, and Neptune also have ring systems, although Saturn's is by far the largest. Particles in these ring systems range in size from dust to boulders to house sized, and may be rocky and/or icy.\n\nMost of the planets also have magnetic fields which extend into space and form a magnetosphere around each planet. These magnetospheres rotate with the planet, sweeping charged particles with them. The Sun has a magnetic field, the heliosphere, which envelops our entire solar system.\n\nAncient astronomers believed that the Earth was the center of the Universe, and that the Sun and all the other stars revolved around the Earth. Copernicus proved that Earth and the other planets in our solar system orbit our Sun. Little by little, we are charting the Universe, and an obvious question arises: Are there other planets where life might exist? Only recently have astronomers had the tools to indirectly detect large planets around other stars in nearby solar systems.\n\nSo basically the inner plantes have solid ground. The outer are made of gas except pluto.\n-good luck
389936
Nope.  That's my professional (astronomer) opinon.  Here is why:\n\nPluto is a member of a large collection of similar sized objects that orbit the sun in the same region as it.  This is the same situation that Ceres, the largest asteroid is in.  Pluto was simply the first of these icy bodies to be found, we have since found almost 1000 other objects that share orbital space with Pluto.  Many of them are almost the same sice as it, and one (2003 UB313) is in fact larger than Pluto.\n\nNo good defination of planet exists that allows for pluto to be a planet without admitting about 10-100 other objects in as planets as well, unless you simple state that Pluto is a planet in an ad hoc manner.\n\nThe definition that I generally use in my work is that a planet is an object that has cleared it's orbital space of all other small bodies.  All of the main 8 planets satisfy the criteria.  Pluto, 2003 UB313, Ceres and other small bodies do not.
343980
Well, really, we don't actually have a scientific definition of a planet.  Shocking yes.  Embarrassing even more so.  But we are working on it.  The problem we are facing right now is that we have a large collection of "transition objects" out there, things that sit right on the border of "planet or not."\n\nAt the high mass end, it is pretty clear cut, Things that produce internal energy by fusion are stars, things that don't are planets.  The transition between star and planet thus seems to be at about 12 times the mass of Jupiter.\n\nAt the small end things are muddled because of Pluto.  No good definition of planet exists that keeps only 9 planets.  You can make a definition to include Pluto, but then you must include a lot more objects.  This is where we actually focusing on right now.\n\n"gravitationally round" is thrown around a lot.  My professional opinion is that this one one make the cut.  It seems nice, a planet is a planet because it has enough self-gravity to crush itself into a round shape.  The problem with this definition is that it means we would have about 100 planets in out solar system.  The largest asteroid, Ceres is round, as are most of the icy bodies in the Kuiper Belt.\n\nThe one that seems to be picking up steam is "dynamically dominant."  This basically means that a planet "dominates" the space around them.  Take a look at the area around the earth - nothing else similar to it.  Same with Venus, Jupiter, and the rest. (moons and rings are bound to the planet so they don't count).  Ceres (the largest asteroid) is surrounded by many, many other asteroids.  Same goes for Pluto, it is surrounded by many other icy bodies of similar size (including one that is bigger than it).  This definition leaves us with 8 planets.\n\nReally right now the "planets" are just defined by fiat, they are planets because we call them that and nothing more.  As a planetary scientist I feel a little guilty not being able to actually define one of the words in the name of my field, as such I've honestly been spending far more time working on this whole "planet" mess, than on my own research.