Viewing Pluto-Charon as a prototype binary planet system makes a lot of sense. Why hasn't the IAU considered this? Maybe it's because the IAU planet definition makes no allowance for a binary system. The two planets in a binary by definition haven't "cleared their orbits" of one another. The argument that those of us who continue to view Pluto as a planet do so out of emotion or resistance to change is a straw man argument. Support for dwarf planets being a subclass of planets comes from preference for the geophysical planet definition, in which a planet is any non-self-luminous spheroidal body orbiting a star. Dwarf planets are simply small planets not large enough to gravitationally dominate their orbits. That was the intent of Dr. Alan Stern when he first coined the term "dwarf planet," a term the IAU distorted from its original meaning. Significantly, in astronomy, dwarf stars are still stars, and dwarf galaxies are still galaxies. There is no scientific basis for the argument that Pluto cannot be counted as a planet because that will require many more objects to be also counted as planets. So what? How is there any scientific merit to artificially keeping the number of planets small? No one has a problem with billions of stars or billions of galaxies. No one argues that Jupiter can have only four moons because no one can remember the names of 67. Memorizing is not important for learning. We don't ask kids to memorize the names of all the rivers or mountains on Earth; we ask them to understand what a river and a mountain are. Similarly, we can teach kids the characteristics of each type of planet--terrestrial, gas giant, dwarf planet, etc. Ironically, the demotion of Ceres stands as evidence of a premature erroneous decision, not as a legitimate support for the demotion of Pluto. Ceres was wrongly demoted because 19th century astronomers' telescopes were not powerful enough to resolve it into a disk. Therefore, they couldn't tell that unlike every other object in the asteroid belt (except Vesta and Pallas, which are borderline cases), Ceres is spherical and therefore a small planet with geology and layering. Today, we know the Ceres is spherical and a complex object much more like the larger planets than like any asteroid. The same is true for Pluto. Referring to Pluto as "debris" makes absolutely no sense, as it blurs the important distinction between a complex world layered into core, mantle and crust, with an atmosphere and active geology, and a tiny, shapeless asteroid. Also, it is a misnomer to refer to Pluto as an "iceball," as it is estimated to be 70 percent rock. Pluto is not a comet and will never become a comet. No comet is this rocky or anywhere near Pluto's size. Comets lose mass with every orbit that brings them into the inner solar system. Pluto does not experience any mass loss at perihelion, and it never comes anywhere near the inner solar system. As for the discoverer of Eris, he was for Pluto and Eris being classed as planets before he was against it. His motivation in changing his mind is hardly selfless, as he has sought celebrity status through billing himself as the "plutokiller," getting paid to give talks and promote his book, which is more memoir than science. Interestingly, he was one of three co-discoverers of Eris. One of the co-discoverers, Dr. David Rabinowitz, signed a petition of 300 professional astronomers rejecting the IAU decision. S there is no consensus on the status of these objects even among the discoverer of the one closest in size to Pluto.
The battle over the status of dwarf planets has never subsided. Pluto and Eris are both planets and Kuiper Belt Objects. One does not preclude the other. They are planets because they are large enough to be rounded by their own gravity. They are Kuiper Belt Objects because they are located in the Kuiper Belt. Ceres too is a small planet because it is large enough for its gravity to pull it into a spherical shape. The IAU misappropriated the term "dwarf planet," which was first coined by Dr. Alan Stern, Principal Investigator of NASA's New Horizons mission to Pluto, to indicate a third class of planets which are large enough to be rounded by their own gravity but not large enough to gravitationally dominate their orbits. He never intended for "dwarf planets" to be classed as not planets at all. The IAU did not "have" to do anything other than allow Eris's discoverer to name it while holding off on any additional classification until more information is discovered about remote planets in this solar system and all planets in other solar systems. Significantly, there are quite a few exoplanet systems in which multiple planets orbit the host star in various different planes. Some have orbits far more eccentric than Pluto's, yet they are giant planets the size of Jupiter or larger. According to the IAU definition, none of these objects are planets! Saying there are more differences between Pluto and the eight closer planets to the Sun depends on what aspects one considers. Earth actually has far more in common with Pluto than with Jupiter. Both have surfaces on which we can place rovers and landers. Both have a large moon formed by giant impact; both are geologically differentiated into core, mantle, and crust, and both have nitrogen in their atmospheres. Other than orbiting the Sun, what do Earth and Jupiter have in common? It is premature to pronounce declarations that these faraway objects are definitively not like the other planets or that one is larger than the other. We just do not have enough data at this point to do more than make educated estimates. What we really need to do is send robotic missions like New Horizons to Eris as well as Haumea and Makemake. Yes, that will take time and money, but it is a far better investment than the black holes the endless wars in Iraq and Afghanistan have become. Also, memorization is not important. It is much more important to teach the characteristics of each category of planet than to ask kids to memorize a bunch of names. We don't ask them to memorize the names of rivers or mountains on Earth, so why do so with planets, and why allow a need for convenient memorization to determine how we classify them?
It is a real planet. The fact is, there are more subclasses of planets than just terrestrials and jovians. Dwarf planets are planets too, regardless of the nonsensical decree of the IAU.
And this makes no sense. It is also inconsistent with the use of the term "dwarf" in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies.
Case not closed. The demotion of Ceres was wrong. It is not just another asteroid, as it is large enough to be in hydrostatic equilibrium (rounded by its own gravity) and to be geologically differentiated. Nineteenth century astronomers couldn't resolve Ceres into a disk because their telescopes weren't powerful enough. Today, we know Ceres is round and therefore a small planet rather than a large asteroid. The argument that it is only emotion or tradition supporting Pluto's planet status is a straw man, a nice way of trying to discredit those who disagree with you. The IAU decision of 2006 was done by four percent of its members, most of whom are not planetary scientists, and was rejected by hundreds of professional astronomers in a formal petition led by New Horizons Principal Investigator Dr. Alan Stern. A good definition of planet must take into account not just where an object is, but what it is. The IAU definition does not do that. In fact, according to the IAU definition, if Earth were put in Pluto's orbit, it would not clear that orbit either. A definition that takes the same object and makes it a planet in one location and not a planet in another is not very useful. It make scientific sense to keep the term planet broad to encompass any non-self-luminous spheroidal body orbiting a star. We can then distinguish different types of planets by using subcategories. Dwarf planets should simply be a subcategory for objects large enough to be in hydrostatic equilibrium but not large enough to gravitationally dominate their orbits.
If Earth were in Pluto's orbit, it would not clear that orbit of other KBOs. And the requirement that an object "clear its orbit" to be a planet is arbitrary and does not have consensus among astronomers.
Memorization is not important. Do we ask kids to memorize all 63 of Jupiter's moons? If the solar system has 80 planets, then that is what it has. It's more important for kids to know the characteristics of the different types of planets than to memorize a list.
Pluto is not an asteroid, as it is large enough to be rounded by its own gravity, a characteristic of planets and not of shapeless asteroids, which are simply held together by chemical bonds.
Yes, I think Ceres and all the other spherical KBOs should be considered planets too. It's okay to call them dwarf planets since they do not gravitationally dominate their orbits. However, it makes no sense to say, as the IAU definition does, that dwarf planets are not planets at all! This is one reason the IAU definition didn't fix anything; it only further confused matters and took attention away from the discovery of the new planets Haumea, Makemake, and Eris.
It should be noted that the IAUâ(TM)s controversial demotion of Pluto is very likely not the last word on the subject and in fact represents only one interpretation in an ongoing debate. Only four percent of the IAU voted on this, and most are not planetary scientists. Their decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASAâ(TM)s New Horizons mission to Pluto. Stern and like-minded scientists favor a broader planet definition that includes any non-self-luminous spheroidal body in orbit around a star. The spherical part is important because objects become spherical when they attain a state known as hydrostatic equilibrium, meaning they are large enough for their own gravity to pull them into a round shape. This is a characteristic of planets and not of shapeless asteroids and Kuiper Belt Objects. Pluto meets this criterion and is therefore a planet.
The IAU definition, adopted by only four percent of its members, most of whom are not planetary scientists, makes absolutely no sense, even for our solar system. That is why it was rejected by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA's New Horizons mission to Pluto. Here are the two main reasons the definition is useless: 1) It defines a dwarf planet as not being a planet at all. That is like saying a grizzly bear is not a bear. It is also inconsistent with the use of the term "dwarf" in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies. 2) It defines objects solely by where they are while ignoring what they are. If Earth were in Pluto's orbit, according to the IAU definition, it would not be a planet either. A definition that takes the same object and makes it a planet in one location and not a planet in another is one that begs to be overturned and is basically unusable. A far better planet definition, which Stern and many like-minded scientists support, is that a planet is a non-self-luminous spheroidal body orbiting a star. The spheroidal part is crucial because when an object reaches a certain size, it is pulled by its own gravity into a round shape. This is a characteristic of planets and not of shapeless, inert asteroids, comets, and Kuiper Belt Objects. By this definition, our solar system has 13 planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris. Kudos to the Illinois legislature for not blindly accepting the decree of a tiny group because that group calls itself an "authority," and recognizing that there is another side to this very much ongoing debate.
The "scientific establishment" never reached such a conclusion. Only four percent of the IAU voted on the resolution that demoted Pluto, and most are not planetary scientists. Their decision was rejected by hundreds of professional astronomers in a petition led by Dr. Alan Stern, Principal Investigator of NASA's New Horizons mission to Pluto. This debate is far from over.
This discovery changes nothing regarding Pluto's status. Mike "Pluto is dead" Brown is wrong in his non sequitur argument when he says "this is the last chance that Pluto had." Many in the astronomy community still consider Pluto a planet. There is hardly consensus otherwise, as Brown wants people to think. Eris being larger does nothing to negate the argument that Pluto is the solar system's ninth planet and Eris its tenth planet. Why the tenth planet cannot be larger than the ninth, in Brown's view, is beyond understanding. How can the status of another object, rather than its own characteristics, define what Pluto is? The truth is astronomers remain very divided on this issue, and the debate is not over.