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Hey A1, can you give us a Pluto update? I hear there are moves to get it back in the planet club.
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Sorry for the delayed post, it took quite a while to write.
Well sorry to disappoint ya, trixsie, but I'm a proponent to Pluto not being a planet. Initially I had been pro-Pluto-being-a-planet-again, but after I learned more about Planetary Science, and here are a number of reasons Pluto was downgraded from being a planet, and also why it should stay that way.
1 - First, and most importantly the thing that made Pluto only a 'dwarf' planet was the fact that was discovered by Mike Brown (There is a huge story on this guy that would take a while to cover) on January 5th, 2005 from images taken on October 21st, 2003 (no, that was not intentionally quoted from Wikipedia) and, after the discovery of a moon, it was discovered based on the moon's speed and distance from Eris that Eris had a mass about 1.27 that of Pluto, and later found to have a
radius approximately 0.969-0.996 that of Pluto (0.997-1.007 not including Pluto's sparse atmosphere.) Therefore not only is it a similar size (nearly exactly the same) but if it had a similar density/size curve taking into account massive compression (AKA the size assuming it was made of the same stuff but was slightly denser because it had extra gravity pulling it in) would put it at about 1.19 times the size of Pluto; it is, however about 1.182-1.305 times denser than Pluto.
2 - Second, some of you may be thinking 'then why not just make Eris and Pluto both planets?' - well that might be true, except Eris and Pluto just happen to be the largest two of a huge range of objects with varying sizes. The third largest object in the area is Makemake, another dwarf planet with an orbit taking it between 38.5 and 58.1 AU (Pluto goes from 29.7 and 48.9, and Eris goes between 38.3 and 97.7) Makemake has a radius of 0.606-0.624 that of Eris (0.593-0.614 Pluto (with atmosphere) After Makemake (708-722 km) comes a number of similarly large objects, Haumea (values range from 525-700 (spitzer) to 718 (Keck) But most are around 625-700 km; 2007 OR10 (540-740); Quaoar (552-558), etc...
3 - Some may argue that Pluto had been discovered long before the rest, and although it shares a similar mass should be kept as a planet because it was found far before the others (the first[second] object that far out was found in 1992 (~160 km by the way.).) Well, there are a few arguments against this. Firstly, a quote from Mike Brown, which I particularly like, is that if Clyde Tombaugh were to search for Pluto approximately 200 years earlier, the dwarf planet Eris would have been similarly bright, and he would have likely found both at once and our solar system would have had 10 planets. Another argument against this is that the definition of 'planet' has always been optimally been a scientific definition strictly defining a group of objects from another group. That definition has been revised over the years. From antiquity to 1610, that was simply defined as the 5 classical visible 'wanderers' - Mercury, Venus, Mars, Jupiter, and Saturn. However, when it was challenged in the 1500s that the planets didn't all orbit Earth, Earth was added as a 6th planet to the model. A few centuries later, on March 13th, 1781, William Herschel discovered Uranus and made the grand total 7 planets. Less than 20 years later, on January 1st, 1801, the first asteroid 1 Ceres was discovered, after it followed 2 Pallas (March 28th, 1802), 3 Juno (September 1st, 1804), and 4 Vesta (March 29th, 1807) The first two, according to a tradition for naming elements after planets starting with Uranium, got the chemicals Cerium and Palladium, but they were discovering asteroids quicker than they could discover elements, so Juno and Vesta never got any elements. While 11 planets was quite a lot, no new ones were discovered for about 38 years. Many thought that that might be all there was in the solar system to find. however on December 8th, 1845, the 5th asteroid 5 Astraea was discovered, and astronomers began to ponder about the fact if any of these should be considered planets, but were delayed when Neptune, the 13th planet, was discovered on September 23rd, 1846. After that came 6 Hebe (July 1st, 1847) and 7 Iris only a month later (August 13th), and by the end of 1847 the solar system had 16 Planets. By now, They had decided that 16 was simply too many and removed all of the asteroids in the asteroid belt, bringing the total back down to only 8. Not a moment too soon, too. By 1850, the Solar System would have had 18 planets, and by 1860, 57. Imagine having to memorize the names of not 8 or 9, not 12, not 16, but 57 planets! The definition of a 'planet' by that point was any object not in the asteroid belt, and the appearance of the solar system did not change to any great degree until, after much effort, Pluto was discovered on February 18th, 1930. Originally there was not much conflict on its existence as a planet until early guesses of its size - In 1931 guessed to be the same as Earth, to a 1948 guess of 1/10th that of Earth, making it about twice as massive as Mercury. However as the mass estimates went down, in 1976 Pluto only had an estimated mass of 1/100th that of Earth, making it 1/5th as massive as Mercury, and only ~67 times more massive than the dwarf planet Ceres. By then people began to question its status as a planet, but that was a very small group of people, and they did not gain much traction until 1978, when it was much more correctly estimated at a mass of 1/500th of Earth, 1/25th as massive as Mercury, but still ~13 times more massive than Ceres. Pluto was obviously not as large as the other planets, but continued on until a large portion of the Kuiper belt and Eris was discovered before finally in 2006 the definition of a planet was finally, and most recently, redefined. Just as Ceres was debunked from being a real planet by people finding that it just happened to be the largest member of a gigantic number of objects - how many of them count as a planet? - Who decides the threshold? - and it was the same situation with Pluto, except recent studies show that Pluto is actually simply the closest of of a large number of large objects in the Kuiper belt and Oort cloud, in fact there are likely several object larger than Pluto residing out there right now, just beyond the furthest our telescopes can see.
My personal opinion, shared with many other astronomers, is that the current IAU definition of planet, while the idea was correct, was a horrible definition. My personal proposal, if it were to be evaluated, might be that any regularly-shaped object with at least a trace atmosphere would be considered a planet. The requirement for an object to simply be large enough to pull itself into a roughly circular (or ovoid, mind you, Haumea) shape, is so general that as much as 40 or 50 objects in the solar system might qualify - and that's only of the objects we know about. The total might be as many as 500. The requirement of an object to have a trace atmosphere would mean that objects that are roughly circular wouldn't all count - and the definition of 'a roughly circular object' still holding would keep objects like comets from being called 'planets'. This would make Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris planets with many others being likely planets, putting the total somewhere around 14 or 15, but with an upper limit of around 20. 20 is a bit much, but at least it's not 200, right?
TL;DR: read it all.
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