You've underscored my problem with this new approach right here: I'm in full support of reclassifying the planets/asteroids/whatever as we discover more stuff, but the "dwarf planet" category seems like it's designed to single out Pluto and similar bodies without upsetting the status quo everywhere else.
Only under the IAU definition that defines dwarf planets as somehow being not planets. The person who actually coined the term several years earlier intended it simply to mean a planet that's comparatively small -- a logical counterpart to the use of "giant planet" for Jupiter, Saturn, etc. If "dwarf planet" were defined that way, as its name literally suggests -- a small planet -- then the status quo does need adjustment and nothing is singled out/marginalized.
If the terrestrial planets and the gas giants can both be considered "true" planets despite the huge variation in their sizes, compositions and other properties, why do small icy and/or rocky bodies get singled out? Either create a definition of planet that's as inclusive as possible, or start getting more specific across the board and create new sub-classes for the current planets to reflect their different properties.
We need to do both. As we discover more exoplanets and trans-Neptunian planetary bodies, we need to develop our concept of a planet into something that's at once more inclusive and more systematically categorized. No need to say that they aren't all planets, any more than you need to say that bears, cats, and lemurs aren't all mammals. They're just different subcategories of mammals, but the term "mammal" has to be inclusive enough to incorporate everything that fits the same fundamental parameters.
I haven't seen that much of Dr. Tyson, with the exception of a few appearances on the Colbert Report. But--at least based on those few impressions--I have to agree with Trekker. When Colbert interviewed him about the upcoming IAU ruling on the status of Pluto-like bodies, he seemed very self-righteous. One thing that stuck in my memory was his reaction to the idea that Pluto could be a "true" planet--because then, the solar system would have upwards of TWENTY-SEVEN PLANETS!!! *gasp*, a statement accompanied by no small amount of sarcastic eyerolling. I mean, it's so obvious that twenty-seven planets is absurd, right? So we must reclassify Pluto to avoid this horrible, ridiculous fate.
Yeah -- what's wrong with having a large number of planets? Heck, for decades back in the 19th century, when asteroids were thought to be planets, the system was believed to have dozens of planets, although there were decades' worth of debate about whether to keep the "planet" label for asteroids or not.
Back during the running commentary thread, just after IAU's ruling, I recall you were in support of the new classification system. What changed your mind?
I wasn't strongly partisan on the issue; actually, before the decision was made, I was one of the people saying there was nothing wrong with the possibility of having dozens of planets. I actually wanted the IAU's decision to go the other way, but I'm not passionate about it and so I tried to look on the bright side and be open-minded and fair. And the basis of the IAU's definition seemed reasonable at first, if a little awkward and in need of refinement. But as I've read more criticisms of the ruling and the process underlying it, I've become more aware of its flaws.
And it's not really a question of what I believe; I'm just saying that based on what I'm reading, the community of planetary scientists isn't embracing the definition and it's likely to be changed in the future. And the reasons for this sound good and valid to me.
And really, I'm pushing for a middle ground here: keep the dwarf planet category, but as a subset of planets rather than something that isn't a planet. I think the category is useful, but its segregation from other planets is not.
That's because it deserves some. Most of the same people who resist the reclassification of Pluto would likely resist the addition of 18 "new" planets to the solar system.
Just because people would be resistant to an idea doesn't mean it deserves ridicule. The general public is always resistant to accepting new understandings of the universe -- just ask Charles Darwin. That doesn't make them right to be resistant. Science is not about clinging to comfortable traditions, it's about discovering new truths.
Besides, it wasn't wrong to add Uranus or Neptune to the roster of planets, or to add dozens more to the roster of known moons of Jupiter, Saturn, Uranus, and Neptune. I remember back in third grade or something, I was rather upset when a filmstrip presentation claimed that Jupiter had 13 moons when I
knew it had only 12. We now know it has at least 63. It's silly to resist new knowledge. And ultimately pointless, because the knowledge isn't going to go away.
The thing I don't understand is, we get why planetary scientists wanted Pluto reclassified; the reasoning is all laid out. Yet those who want Pluto to keep its classification don't seem to have a real reason for it beyond preserving what they've been taught previously. This "I grew up being taught that it's a planet, so it's a planet, dammit!" mentality.
You've really misunderstood the debate. Many planetary scientists disagree profoundly with the reasoning of the IAU committee's definition for reasons that have nothing to do with tradition. They question it because it fails to apply to extrasolar planets, because it fails to categorize whether Charon is a dwarf planet or not, and because it includes a vague, poorly worded statement about "clearing the orbit" which, if taken literally, would exclude Earth from definition as a planet, and which is not universally applicable because the parameters of orbital clearance differ with distance from the Sun (i.e. a body that would clear its orbit at 1 AU would not be large enough to clear its orbit at 10 AU). A definition that would make Jupiter a dwarf planet if it were far enough from the Sun makes no sense at all.
And the scientists (as opposed to the laypeople who still make the mistake of thinking this is solely about Pluto) are not trying to preserve what we used to think -- just the opposite. Because if Pluto is a planet, then so are Ceres and Eris, so are most of the twenty-odd other known candidates, and so are the hundreds more yet to be discovered. Calling Pluto a planet
does not preserve the status quo; it upsets it hugely, because it requires the number of Solar planets to skyrocket with no discernible limit. It's the ones who said Pluto
isn't a planet who are trying to preserve the status quo, because it's the only way to keep the number of planets in the single digits.
I would like a definition of "planet" to reflect the internal structure of the body. For instance, the inner planets are all differentiated into metal cores and silicate mantles. The gas giants are all differentiated, too. Most of the asteroids probably aren't differentiated; Ceres, Pallas, and Vesta may be, so they would fall into planet status based on this criterion. Pluto may not be differentiated, we don't know. But if internal structure were added to the definition of planet, we could include some asteroids and Kuiper belt objects as planets while excluding others.
The issue with this idea is that it could lead to too many types of planets. The inner planets, Jupiter and Saturn, Uranus and Neptune, and Pluto and Eris all have largely different internal composition, even though they are differentiated. (I'm not sure that large KBOs are going to get warm enough to differentiate, even with a significant rocky content.) I've been dividing the planets this way for years, and this thread has made me decide to really emphasize this when I'm teaching in the fall.
You both have valid points. Again, I think the key is to have a broad, inclusive definition of what is a planet, but to divide that into a whole taxonomy of planetary types -- terrestrial planets, Jovian giants, ice giants, terrestrial dwarfs, ice dwarfs, ocean planets, carbon planets, iron planets, whatever other distinctive types we may find out there. It's silly to draw arbitrary lines saying "this is a planet and this isn't," because "planet" is too broad a category to be scientifically useful. What's valid is to say they're all planets but acknowledge and codify the enormous diversity of that phylum.
The asteroid Ceres is very likely differentiated, but it's too small to have had a significant amount of geological activity like the inner planets. Pluto, Eris, etc., are also small bodies, likely with little activity and development. Objects like Enceladus might indicate otherwise, but KBOs don't have the influence of a large body nearby to foster activity.
We don't know that Ceres lacks geological activity. We know it probably has an icy mantle like Europa and Enceladus, and may have had cryovolcanism in the past, at least. We'll know more once the Dawn probe reaches it, but it's premature to rule much out.
I'd like planets to be categorised based on the thickness of their atmospheres.
Either by the depth to/from the core or by percentage of radius that is their atmosphere.
I don't think that would be useful, since atmospheric thickness is a highly variable property. Take two geologically identical bodies and put them at different places in a star system and you'll get two radically different atmospheres. If, say, Uranus had migrated inward to Earth's distance, its hydrogen atmosphere would've blown away and it would've been left as an ocean planet. Also, a planet's atmosphere is not a constant; it evolves over time. It can grow thicker from vulcanism, it can be eroded by stellar radiation, it can be blasted away by comet impacts, it can freeze if the planet migrates outward from its star, etc.
Certainly an atmospheric categorization would be a useful thing to have, but as a secondary parameter, not as a primary basis for planet categorization.
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