So Pluto is no longer a planet because it doesn't clear objects out of it's path...?

and goes through Neptune's orbit. But why is Neptune still a planet if it's got Pluto going through it's orbit?So Pluto is no longer a planet because it doesn't clear objects out of it's path...?
The interpretation of ';cleared the neighborhood around its orbit'; as it applies to the IAU resolution is poorly defined. Pluto and Neptune are in a resonance s.t. Neptune is far away as Pluto approaches perihelion. ';Clearing'; is a non-issue in this instance.


(In fact, Pluto gets closer to Uranus than it ever does to Neptune!)





The reason Neptune is still a planet is that it is explicitly called out. Read footnote 1 to resolution 5A in the source link.





Now, chances are there are other KBOs still out there near Pluto, and by this standard Pluto's orbit is not cleared, however I don't think this is any more empirical than perhaps some modeling of evolution of the kuiper belt. Give it time though, and I bet some non-resonant Pluto crossing objects will be identified.So Pluto is no longer a planet because it doesn't clear objects out of it's path...?
Because it is larger than Pluto and has a gravitational force better than Pluto
That is the ostensible reason given for its demotion into a second division of Dwarf Planets from the Premier League of Planets,. But there is more to this than meets the eye.





The basic problem Pluto has and had from the outset is that it is smaller than 7 moons in the Solar System: Ganymede, Io, Europa and Callisto (the 4 Gallilean moons of Jupiter) Titan (Saturn's largest moon) Triton (Neptune's largest moon) and our own Moon, all of which were discovered before Pluto.





Heirarchical thinking that Planets ';ought'; to be bigger than Moons and ';size-ism'; prejudice doubtless played a part in the recent IAU decision, But only a minor part. Mercury is smaller than the two biggest moons, Ganymede and Titan but it didn't get downgraded, did it?





From the way some people have reacted. anyone would think the IAU were out to ';get'; poor defenceless little Pluto and the discussion is clouded by anthropomorphic sentiments as a result,





Sentimental attachment is hardly a good basis for scientific classification. The discussion is also clouded by the fact that most people seem to be blissfully unaware of the number and variety of objects we now know there to be in the Solar System and similarly unaware of the fact that we (and Ceres) have been here before, 150 years ago.





Whilst there is understandable and widespread dismay at Pluto being demoted in status, people really need to understand the reasons the IAU had to grapple with definitions and categories at this time:





(1) in 1930 we knew of just one body lying beyond the orbit of Neptune. Now we know of more than 1000





(2) we are discovering asteroids at a rate of 5000 a month





(3) we now know of 200+ extra-solar planets orbiting 170+ other stars, some of which we now know to have asteroid belts





It is conceivable the IAU may create more categories in the future in the light of more discoveries, The moment we find an extra-Solar System planet with extra-terrestrial life on it, for example, I would expect Habitable Zone Planet to be a new category and only Earth and Mars of our local 8 planets to be in it.





We already have the distinction between a terrestrial planet (the inner 4 planets) and a gas giant (the outer 4 planets) and are assessing new extra-Solar-System planets in the light of that distinction and a new category name for the informally-named ';hot Jupiters'; (i.e. large planets orbiting near to their star at less than 1 AU distance) of which we know several, may not be far away,





As science expands its knowledge, it needs more concepts and categories with which to describe and classify that knowledge, That is perfectly normal and should neither surprise nor alarm us,





Creating new categories and reclassifying known objects in the light of them has happened before: in the 19th Century when the number of planets was pruned from 11 to 7 out of concern that being consistent and admitting other, newly discovered bodies to the planetary club that were similar to the ones they chose to kick out instead would have meant the number of planets could rapidly start to escalate and mushroom out of control,





To understand what is going on now, it helps to understand what went on then,





The number of bodies in the Solar System known to astronomers has been burgeoning for a long time now, but the general public seems unaware of this, given the way people blithely talk of Ceres (discovered 1801) Charon (discovered 1978) and Xena (discovered 2003) having ';just been discovered';,





There was a similar definitions crisis in the early 19th century and again in the mid-19th Century as the number of known objects in the Solar System started to grow and grow,





By 1807 the 8 Solar System bodies known to classical astronomy (the Sun, the Earth, our Moon and the 5 classical planets known from antiquity, Mercury, Venus, Mars, Jupiter, and Saturn) (1 star, 6 planets, 1 moon) had grown to 26. Uranus was found in 1781 making 7 planets. There were 4 Jovian moons, 7 Saturnine moons and 2 Uranian moons, 14 in all





And then there was the discovery of the first four asteroids. These were 1 Ceres on January 1, 1801, 2 Pallas on March 28, 1802, 3 Juno on September 1, 1804, and 4 Vesta on March 29, 1807,





What were astronomers to call these new objects? They weren't moons as they rotated around the Sun, so they had to be planets, didn't they? As there was, initially, no other category but moons or planets to put them in.





After 2 Pallas was discovered though, Sir William Herschel (the discoverer of Uranus) coined the term ';asteroid'; meaning ';star-like';), in 1802.





But Ceres was meantime assigned a planetary symbol, and remained listed as a planet in astronomy books and tables (along with 2 Pallas, 3 Juno and 4 Vesta) for about half a century until further asteroids were discovered.





So we now had 1 star, 11 planets and 14 Moons, the beginnings of a distinction between major and minor planets and a sense of unease as to what we would do if more asteroids were discovered as the first four were all disappointingly small in size, so did they really belong in the planetary club? (Similar doubts were expressed about Pluto, right from the outset in 1930,)





38 years pass and then in 1845 the asteroid 5 Astraea is discovered and on September 23, 1846 the planet Neptune and a mere 17 days later on October 10, 1846, Neptune's moon, Triton. (We now have 1 star, 12 Planets 15 Moons and 1 non-planetary Asteroid.)





The pace of discovery then starts to really hot up. Four more asteroids in nine months: 6 Hebe on July 1, 1847, 7 Iris on August 13, 1847, 8 Flora on October 18, 1847, and 9 Metis April 25, 1848





Then on September 16, 1848 an 8th moon of Saturn called Hyperion is discovered,





Plus a further 6 asteroids are found in just over two years: 10 Hygiea on April 12, 1849, 11 Parthenope on May 11, 1850, 12 Victoria on September 13, 1850, 13 Egeria on November 2, 1850, 14 Irene on May 19, 1851 and 15 Eunomia on July 29, 1851.





And on October 24, 1851 a 3rd and a 4th moon of Uranus: called Ariel and Umbriel were discovered.





So now we had 42 objects: 1 star 12 planets 18 moons and 11 asteroids. If the latest asteroids were all to be regarded as planets, making a total of 23 planets (and 10 Hygiea was bigger than 3 Juno, just like Xena is bigger than Pluto), it was likely to start getting silly (by 1868 the number of asteroids was to rise to 107 and Victorian schoolchildren would have needed a massive 115-word mnemonic to remember all the names).





The unease grew to a crisis, a redefinition was clearly necessary and an inevitable decision was taken to regard all 15 asteroids as a separate category from planets and Ceres, Pallas, Juno and Vesta were kicked out of the planetary club, just like Pluto has been kicked out now.





There are some clear parallels between the situation in the 1850s and the situation now, Four under-sized runts had obtained planetary status, with seemingly more to follow as they were discovered, creating an overwhelming feeling among astronomers that the currency would be devalued if all these further objects were to then be automatically awarded planetary status. So they cried Whoa! And called a halt. And created a new category, Just like the IAU has now done,





SO HOW MANY OBJECTS HAVE WE GOT IN THE SOLAR SYSTEM NOW?





Stars: 1





Planets: 8





Moons: over 80 known moons of the dwarf planets, asteroids and other small solar system bodies.





(The asteroid 87 Sylvia has 2 moons for example as does the Kuiper Belt Object KBO 2003 EL61.)





AND another 162 moons orbiting around planets: Mercury has none, Venus has none, Earth has 1, Mars has 2, Jupiter has 63, Saturn has 56, Uranus has 27, Neptune has 13.





Kuiper Belt Objects: over 800 (all discovered since 1992).





Trans-Neptunian Objects: over 1000 (includes the 800+ KBOs) i,e, there are 200+ in the Scattered Disk and the Oort Cloud.





Asteroids: Hundreds of thousands of asteroids have been discovered within the solar system and the present rate of discovery is about 5000 per month. As of July 23, 2006, from a total of 338,186 registered minor planets, 134,339 have orbits known well enough to be given permanent official numbers. Of these, 13,242 have official names.





Current estimates put the total number of asteroids above 1 km in diameter in the solar system to be between 1.1 and 1.9 million





So you can see





(a) why some definitions are needed and why reclassification is necessary





(b) how totally unaware of the state of scientific knowledge the general public is and how uninformed people are when they get excited at tales of ';3 new planets being discovered'; and wonder if there might perhaps be more where those came from,





Finally, these issues need to be seen in the context of the 205 extra-solar planets we now know to exist and the asteroid belts that have now been detected in some of those stellar systems,





Consistency being a desirable thing to achieve in science, whatever definitions and categories the IAU now adopt, they need to be applicable to every star with other objects in orbit around it, throughout the entire universe, That is the context in which Pluto's status is now being discussed,





SO: Pluto should not be a planet, nor should it be just another TNO or small solar system body, It has been given a status intermediate between these two extremes and that is how it should now be seen.
the international astronomical union defined three terms ';planet';, ';dwarf planet';, and ';small solar system body';. this does not change anything about the solar system or pluto. it just corrects the mistake of classifying pluto as a planet initially





i have been waiting for this since i was about twelve. i feel somewhat satisfied. this was the right thing to do, believe me. i don't understand why so many are having such a problem with this. this happened when astronomers discovered many bodies orbiting the sun between mars and jupiter in the early 1800s.





neptune doesn't really have anything to do with this. neptune and pluto never interact.





because pluto orbits the sun, is round, does not have an isolated orbit (a bunch of other similar bodies have similar orbits.), and is not a satellite it is a dwarf planet.





(1) A ';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, and (c) has cleared the neighborhood 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, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.





(3) All other objects orbiting the Sun shall be referred to collectively as ';small solar system bodies';.








look here:


http://www.iau2006.org/mirror/www.iau.or鈥?/a>


http://en.wikipedia.org/wiki/Kuiper_belt
Neptune dominates Pluto's orbit - for every 3 times Neptune orbits the Sun, Pluto orbits 2 times. So Pluto's orbit would be quite different if Neptune was never there to begin with, but Neptune's orbit wouldn't be changed a bit if Pluto had never been there.





For a well written, in depth explanation of this part of the definition of ';planet'; go here: http://www.gps.caltech.edu/~mbrown/eight鈥?/a>
It is no longer a planet because it has an elliptical orbit, instead of concentric just like all of the other planets.
Neptune has the stronger orbit and is larger. Pluto is getting smaller and is the size of its moon. Personally, pluto was the best planet... look at mickey's dog... isn't he the best!!
see how easy that is





it all clear to me now
WHAT I THINK THEY MEAN IS THat


NEPTUNE IS GOING TO CLEAR PLUTO AWAY SOME DAY


I WANT PLUTO TO REMAIN A PLANET


BUT I THINK ITS A MOOT POINT
That's the duhhhhfinition so far.





The problem with that definition is that Earth shares its orbit with some 10,000 other objects and Jupiter shares its orbital path with over 100,000 other objects.





By their definition, neither Earth nor Jupiter are planets.





Not to worry. In three years they'll meet again and screw it up some more or fix the definition, whichever comes first.