'Yes, Pluto Is a Planet' (sfgate.com)

schwit1 quotes a Washington Post perspective piece by the authors of a new book about Pluto: The process for redefining planet was deeply flawed and widely criticized even by those who accepted the outcome. At the 2006 IAU conference, which was held in Prague, the few scientists remaining at the very end of the week-long meeting (less than 4 percent of the world's astronomers and even a smaller percentage of the world's planetary scientists) ratified a hastily drawn definition that contains obvious flaws. For one thing, it defines a planet as an object orbiting around our sun -- thereby disqualifying the planets around other stars, ignoring the exoplanet revolution, and decreeing that essentially all the planets in the universe are not, in fact, planets.

Even within our solar system, the IAU scientists defined "planet" in a strange way, declaring that if an orbiting world has "cleared its zone," or thrown its weight around enough to eject all other nearby objects, it is a planet. Otherwise it is not. This criterion is imprecise and leaves many borderline cases, but what's worse is that they chose a definition that discounts the actual physical properties of a potential planet, electing instead to define "planet" in terms of the other objects that are -- or are not -- orbiting nearby. This leads to many bizarre and absurd conclusions. For example, it would mean that Earth was not a planet for its first 500 million years of history, because it orbited among a swarm of debris until that time, and also that if you took Earth today and moved it somewhere else, say out to the asteroid belt, it would cease being a planet.

To add insult to injury, they amended their convoluted definition with the vindictive and linguistically paradoxical statement that "a dwarf planet is not a planet." This seemingly served no purpose but to satisfy those motivated by a desire -- for whatever reason -- to ensure that Pluto was "demoted" by the new definition. By and large, astronomers ignore the new definition of "planet" every time they discuss all of the exciting discoveries of planets orbiting other stars.

Comments

Oh dear god, this again?

[JoshuaZ]

Seriously, this again? Categories by nature have fuzzy boundaries. We'll have trouble telling how best to categorize the edge cases. What matters is what categories are most useful for understanding things. If the experts in an area decide to categorize something one way then let them.

To add insult to injury, they amended their convoluted definition with the vindictive and linguistically paradoxical statement that "a dwarf planet is not a planet." This seemingly served no purpose but to satisfy those motivated by a desire -- for whatever reason -- to ensure that Pluto was "demoted" by the new definition.

If I'm following this argument correctly, they are arguing that it is "linguistically paradoxical" to have something of the form "[adjective] x" to not also be in the category labeled "x". But we do this all the time. For example, in math a "skew field" is not necessarily a field https://en.wikipedia.org/wiki/Division_ring, and one can come up with many other examples in STEM fields. This is a natural thing that we do all the time. And the specific reason in this case that dwarf planets were not planets was to avoid then making Eris and Ceres and a a whole bunch of other objects also planets.

I'm fascinated that with everything else going on the world, serious people apparently think that arguing over what we call a planet or not is an important issue.

Re:Oh dear god, this again?

[jythie]

I doubt people care about pluto itself much, but the debate steps on both nationalism (since Pluto was discovered by an American and children loved it) and intellectualism (standards body vs citizen scientists), so it is symbolic of a pair of much deeper social conflicts going on.

Yes, that was actually the point

[Excelcia]

The whole reason for "demoting" Pluto had nothing to do with Pluto itself. It had to do with Eris. When Eris was discovered, it appeared to be (because of its albedo) larger than Pluto. It is, in fact, more massive tham Pluto is though slightly smaller. Because of its size, the discoverer was making a strong case for having it accepted as the solar system's 10th planet. While this gave NASA a huge ole woody, because they were counting the dollar signs of how a newly discovered actual planet would spur public opinion for exploration, it caused significant consternation at the IAU. That's actually understating things. A few members of the IAU were quite literally horrified at the prospect. They were looking at the fact Michael Brown's team had discovered scads of Trans-Neptunian-Objects, that there very well could be half a dozen or more additional ones the size of Pluto/Eris, and they were quite happy with the set of planets we already had, thank-you-very-much. There was a certain historical congruency to it and no upstart finding a TNO that happened to be as large as Pluto was going to get his name listed in the same breath as such nobility as William Herschel. So they decided that in the end, that since they really couldn't call Eris a planet if Pluto was a planet, they did what they thought was the lesser evil. They preserved the sanctity of the rest of the solar system by demoting Pluto.

Of course the definition has holes and is not even self-consistent. The whole purpose of it was to demote Pluto and all other similar objects and to ensure that it was worded such that no other TNO could ever be labelled a planet, by nature of being out in the area where nothing is "cleared".

What people need to ask themselves is this: does the fact that the IAU had its collective head up its ass tell me what I consider to be a planet? The answer for me personally is a big whopping no. And when it comes up in conversation, usually after I name all 9 traditional planets (sometimes adding Eris) and get "corrected" that, hey, don't I know Pluto isn't a planet any more, that's when I let the poor smarty-pants have it with both barrels. Not really their fault, but anyone toeing the IAU party line and not willing to think for themselves deserves to get it.

The IAU isn't an official body with any authority other than what they have taken on themselves. And if they are going to let historical politics cloud scientific thinking, they certainly aren't going to speak for me. I'd like to see some textbook publishers take a stand too.

Re:Yes, that was actually the point

[Excelcia]

First of all your question assumes facts not in evidence. You are stating as a fact within your question that that Pluto has few similarities to the other eight planets. While it's difficult to nail down Pluto's geology precisely, best evidence suggests that Pluto has a rocky/metalic core, a probably liquid mantle, and a solid crust. How much other differentiation there are between layers (an outer/inner mantle, outer/inner core) is pure guesswork right now, but as it stands it has quite similar enough geology to the other eight planets for me to recognize it as belonging.

Secondly the way your question is worded also suggests that, to you, somehow the number of planets in the solar system is a problem. Who cares if it's just Eris or a dozen more? If they are large enough for gravity to pull them into a sphere, if they are geologically and structurally similar to the other planets, how is the number of them even relevant? That's not evidence-based science at all. That's letting your comfort-level with the result determine what you accept as results.

Re:Yes, that was actually the point

[careysub]

The whole reason for "demoting" Pluto had nothing to do with Pluto itself. It had to do with Eris....

Everything you say is true, but it gets much, much worse.

They concocted this contrived definition of "planet" not long before we started finding lots more planets, thousands of them.

The IAU needs to sit down with all the exo-planet astronomers, and planetologists (like geologists, but not restricted to Earth), and think this through from the beginning, and come up with consistent, science-based classifications, and let the chips fall where they may - not use a pre-determined "correct" number of planets for the Solar System.

Of course the very term "planet" is of ancient origin and referred to those permanent naked eye lights in the sky that weren't stars. There were only the original set until William Herschel discovered Uranus (well, Earth too - after Copernicus). When asteroids were first discovered, they were planets too until astronomers realized they were dozens of them, make that hundreds of them, make that thousands of them. So the term never had a firm scientific definition.

I think a more complex classification system is called for myself. It is true that Pluto and Eris are a new class of body - Kuiper Belt Objects, just as asteroids were and are, but that doesn't mean they can't also be planets. Now that we know of some 4,500 confirmed or candidate planets (which will expand to tens of thousands in a decade or so) it is ridiculous to use those ancient six plus two planets as the standard to define what the term means.

Re:Yes, that was actually the point

[jouassou]

Being large enough for gravity to deform it into a ball is one possible definition, and looking at it's geological structure is another. The latter would likely disqualify the gas giants, but you could probably make several definitions for "gas planet" and "rock planet", for instance. But I'd still call the IAU definition a science-based alternative as well.

There is a gap of several orders of magnitude in the so-called planetary discriminant between typical "planets" (50-50,000) and "dwarf planets" (<0.03), which indicates that there really is a qualitative difference between these. Personally, I don't see the problem with having a two-tier classification scheme where something deformed into a ball classifies as a dwarf planet, and a dwarf planet that is dominant in its region of space classifies as a large planet. It's also a definition that fits with the historical notion that moons and asteroids are not "planets" (even though e.g. Ganymede is larger than Mercury).

Re: So...

[jd]

Pluto is layered, for a start. Ceres is much more homogeneous.

Pluto has liquid water and a liquid core. Ceres has no liquid water and has no core at all.

Pluto is stable, Ceres is slowly evaporating in the solar winds.

Pluto has an active geology, Ceres does not.

Will this suffice or should I add more?