That was what I thought at the time.
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You might already know this. But the reason is, that the way they hunt for exo-planets is to trace their transit in front of their home star. We can’t use this method to find planets in our own system, because they’d have to pass the sun in front of us. It’s like could detect a moth flying around a lamp on the other side of the road, but not a moth flying behind you. Yeah, right behind your head, it almost touches your ear.
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Yes, Mars did. If it hadn’t the asteroid belt would have continued there. There is a genuine difference between how the eight major bodies orbiting our sun formed, by accretion of most of the material in their orbit and then ejecting the rest, and the belt objects that coalesced from the remainders.
That’s one way. Another way is to look at the wobbles they cause in their star’s motion…so far I believe most not found by Kepler are still in that category. Which, incidentally, is a technique that counts large objects that have cleared their orbits but not smaller objects that are part of belts. So far extrasolar belts are detected by infrared excess from the dust they create instead, and we are not anywhere near seeing individual objects in them.
I can see why people might think so but it’s a really awkward definition for three reasons:
- Round isn’t defined and is actually kind of difficult to do so…for instance, it’s tough to say is Vesta is round with a dent or not. You basically have to pick a cut-off and unlike orbit-clearing there is no natural gap there.
- In focusing only on the object’s shape and not size or gravitational influence, it’s hard to justify why it wouldn’t include moons.
- Like you said, it includes a lot of objects and doesn’t match up with the traditional use of planet, where for instance asteroids stopped being considered as examples once it became clear there was a whole belt of them.
It’s true that Mars and Jupiter aren’t very similar…but then that’s actually part of the point. There is nothing quite like Jupiter, only a few things kind of like Mars, and a ton of things a lot like Pluto. Because Pluto is part of a belt and Mars and Jupiter aren’t, which is at this point what planet literally means.
And by the way, it’s important to have a distinction somewhere because there are so many non-planet objects that they need numbers to have any chance of keeping track of them, like 1 Ceres and 624 Hector and 99942 Apophis. Planets never have and that’s ok because there are less than a dozen of them…it won’t if you include little things like 136199 Eris and 136472 Makemake and 134340 Pluto.
It’s so deeply frustrating to me that all these years later people still say that’s arbitrary. They put the line in the least arbitrary spot possible, one where there is a genuine change in how the objects in question fit into the solar system.
You know, it’s funny. I was literally just reading about the fish family Cyprinidae and how it was recently split into about a dozen smaller families, because what they had was too large and not actually a helpful framework for organizing the diversity of different fish being classified under that banner. Bunch of rubes, changing taxonomy just because it wasn’t useful.
This is really what it’s come down to, huh? We’re just going outright slander the interests of scientists because we don’t like the taxonomy they use? Because astronomers definitely aren’t interested in the barely-explored category of Kuiper belt objects. After all, it’s not like they’ve been sending robot probes out to any asteroids and comets. 
God, this topic gets stupider and more frustrating every year that passes. I know it’s traditional to assume that scientists must not actually have any idea what they are doing when it interferes with nostalgia, but come on.
I used to love astronomy and was so interested in all of this. I still am on my own. It was wonderful seeing what Pluto was actually like, it will be wonderful learning more about all its kindred, and it will be fascinating to see if Brown’s hypothetical ninth planet can be found. But I hate that every single report on it is an excuse for people to go on about how they don’t understand what Brown and the IAU were thinking as if that’s something wrong with astronomers and not themselves. 
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So what is your opinion on Mercury also not being a planet, according to the data from the MESSENGER probe? I don’t think the current solution of “don’t talk about it” is a good one.
And I will be the first to admit that it wouldn’t be a planet under the definition I gave, which gets us back to the “definition of what a planet is being arbitrary” problem.
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I’m not really sure what you’re referring to. Mercury is definitely round by almost any cut off, and it definitely goes around the sun. Is there some 1023 kg of material crossing its orbit that we didn’t know about? 
Apparently not.
Pages 72-73 say that it isn’t.
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That’s interesting, and thank you.
The thing is that like being round in shape, hydrostatic equilibrium isn’t really an absolute but a question of degrees. Even Mars has an odd thing where one hemisphere is about 5 km higher than the other. Note this says that Mercury is farther out of hydrostatic equilibrium than the Moon…but it would still be much closer to it than something like Vesta would be.
So I would say the problem is they never really formalized what they meant by that. And it doesn’t really matter for the definition of planet, because there are no objects that have cleared their orbit that aren’t large enough to be “nearly round” in an intuitive sense. But I think at some point they really will need to come up with some kind of sharper criterion for dwarf planets.
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Which means if you’re going to start splitting into different categories it makes logical sense to start by splitting the things with the biggest differences. For example, gas planet vs. rocky planet.
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Pluto should stay a planet. And Charon and Ceres and Eris and Haumea and Makemake and all the rest should be planets too.
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“My Very Educated Mother Just Served Us Nine Pies, And There Are So Many Of These Planets I Can Make This Mnemonic Say Whatever I Want It To”
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Mike Brown did NOT lead the demotion of Pluto in 2006. He actually had nothing to do with it. It was done by 4% of the International Astronomical Union (IAU), most of whom are not planetary scientists, and was opposed by an equal number of planetary scientists in a formal petition. The debate over Pluto’s status and planet definition remains ongoing. Most planetary scientists prefer the geophysical definition over the IAU one. The geophysical definition does not require orbit clearing as a condition for planet status and includes dwarf planets as a subclass of planets. According to this definition, our solar system has a minimum of 13 planets and counting.
Mike Brown is not and never has been a member of the IAU. He did not take part in the vote. However, he is obsessed with the idea that he “killed Pluto” and uses that as a brand to pursue money and fame. He was one of a team of three who discovered the planet Eris in 2005. The other two co-discoverers, David Rabinowitz and Chad Trujillo, reject the IAU planet definition and demotion of Pluto. Rabinowitz even signed the 2006 petition of planetary scientists who opposed it.
Brown deliberately refers to this hypothetical but undiscovered planet as “Planet Nine” to promote himself and his own agenda and pretend the debate over planet definition is over when that is far from the case. It is very sad to see a scientist mislead the public for his own personal gain. The appropriate term for a hypothetical but as yet undiscovered planet is “Planet X,” with “X” referring to the unknown, not the number 10. In August 2018, scientists at the Planetary Science Institute published a letter requesting this undiscovered planet be referred to by the neutral term “Planet X.” I urge the media to do just that when covering this issue.
The media also needs to stop enabling Brown in this delusion and stop being the PR agent that promotes his book, which contains many errors.
There are no “the eight planets.” Dwarf planets are a subclass of planets. Our solar system has a minimum of 13 planets and counting.
By the IAU treatment that’s simply incorrect. Dwarf planets are no more a subclass of planets than minor planets (asteroids) were. Citation very much needed that most planetary scientists prefer the “geophysical definition”…descriptions of Ceres as a planet are scarce.
I literally just explained why that would make things difficult, but sure, who cares about whether it works for astronomers when sentiment is involved?
Both of those are thought to develop from the same processes, and then it’s a question of if they get massive enough to retain a thick atmosphere. Still that’s always how they had further divided the planets: bigger gas giants (and ice giants), smaller terrestrial planets, and much smaller Pluto. The first spaced in relatively coplanar circular orbits outside the ice line, the second spaced in relatively coplanar circular orbits inside the ice line, and Pluto.
The difference between them may seem more important to you. If so, good news, we have words for you to use too…gas giants (and ice giants) and terrestrial planets. But it still doesn’t in any way make it arbitrary that Pluto doesn’t belong with any of them the way you accused.
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You know, it’s funny. I was literally just reading about the fish family Cyprinidae and how it was recently split into about a dozen smaller families, because what they had was too large and not actually a helpful framework for organizing the diversity of different fish being classified under that banner. Bunch of rubes, changing taxonomy just because it wasn’t useful.
While there are indeed “lumpers” (taxonomists that combine existing groups into larger supergroups) and “splitters” (taxonomists that split existing groups into smaller ones), it is important to realize this isn’t due to personal preference or because it wasn’t “useful” (whatever that means). Instead, changes in taxonomy reflect changes in what we believe the evolutionary history of these organisms to have been. In many cases, there is conflicting evidence as to their histories and that’s why different scientists support different taxonomies. Ideally, with more data there eventually comes a general consensus among taxonomists as to what the true history was.
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Exactly how does splitting up a monophyletic group like Cyprinidae possibly conform to this description instead of what I said?
If it was believed to be a monophyletic group, there is no justification to split it up regardless of how diverse it is. If people split it up, they are implying that (at least in their opinion) it is not monophyletic.
What about the category of ocean worlds-- Europa, Triton, Pluto, Enceladus? We know that Arrokoth (a Kuiper belt object along with Pluto) doesn’t have one.
From an article on the 2066 decision.
The Harvard professor emeritus blamed the outcome in large part on a “revolt” by dynamicists - astronomers who study the motion and gravitational effects of celestial objects.
“In our initial proposal we took the definition of a planet that the planetary geologists would like. The dynamicists felt terribly insulted that we had not consulted with them to get their views. Somehow, there were enough of them to raise a big hue and cry,” Professor Gingerich said.
Planet nine falls nicely into that category-- it’s known solely through its dynamic effects.
What about them? All I have said about those kinds of ice worlds is that they don’t resemble the eight larger masses, so I’m not even sure what you are asking here.
But Dr Stern pointed out that Earth, Mars, Jupiter and Neptune have also not fully cleared their orbital zones. Earth orbits with 10,000 near-Earth asteroids. Jupiter, meanwhile, is accompanied by 100,000 Trojan asteroids on its orbital path.
I have a tough time taking an article seriously when the main source apparently didn’t even make it through what’s on the wikipedia article on the topic (now at least…that’s a really old source). The definition is not inconsistent. The Trojan asteroids do not have a mass anywhere near 1027 kg.
As I have pointed out repeatedly, the criterion used may be a dynamical one, but it’s not coincidental it also reflects a significant change in physical properties. Nearly round doesn’t. As far as I can tell nobody’s even defined it yet, 15 years after that article is written. Where has the “revolt” been since?
Oh, straight up no then. Cyprinidae, which like I said I was literally just reading about, was a case of splitters. The family was considered inconveniently diverse compared to other fish families to justify lumping. But nobody was actually saying they don’t still form a monophyletic group, as trees in the same papers make very clear.
Taxonomy does have a real use, you know, to organize the living world around us in a way that allows us to discuss it. That’s why you get lumpers versus splitters in the first place. I get the cladist ideal of labeling points on a tree without worrying about what they’re like…that’s still not the universal approach, especially where ranked taxa are still used.
I wish people would consider that on occasion, I might pick my examples from things where I have some idea what I’m talking about.
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I think I found the proper forum for this discussion. (lower left)
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