What "Planet clears its neighborhood" really means.

[Hornblower]

While Earth was the center of the known universe, it wasn't some sort of apex. It was more the basement, the one step above Hell. An analogy is the drain in your shower: all the water swirls around it, but it's just a stop on the way to the sewers.

That is a set of mythological and/or religious tenets, with no basis that I can see in mechanical terms. I was thinking in terms of position, motion and the center of the apparent motions of the celestial objects.

[George]

I made this figure for a class on exoplanets last year:

planet_counting_label.png

Nice! The spike is what got people's attention, especially Herschel's. The link I gave has some interesting history on this which involved Herschel turning to the Royal Society head (big in taxonomy) delegating to another who proposed "asteroids", but a few months after Herschel had already suggested this term.

Any proposal that adds 500 to your graph should consider the prior, now tiny, spike, IMO.

[George]

It's the penguin's name, not the penguin's classification. Organisms can be named after locations where the type specimen was found, colors, legendary beings, physical characteristics, funding agencies, or celebrities. True, there are no natural penguin populations north of the equator, but that's not because of how they classify birds, it's because the birds that fit the penguin taxonomy don't live north of the equator, not that the system of taxonomy is defined so penguins only live south of the equator and pseudo-penguins north of it.

Those are fair points. I see their classification, in English, is "wedge like". What you state makes sense for penguins because that is what they are. Planets, like penguins, also have a location-like existence -- orbits. Like Nascar drivers who may, perhaps, be classified similar to planets (both only turn left *wink*), we can consider what takes place in their respective orbits that distinguishes them from other objects that... turn left or live in Antarctica.

But, according to the graphic provided by Margot (http://mel.ess.ucla.edu/jlm/epo/planet/planet.html), neither Mars nor Mercury would be planets much beyond 40 AU. Saying the Earth would still be a planet out there is fine and dandy, but doesn't address the main issue: that an identical object would be classified differently depending on location.

True, but at least you aren't making gross errors about it.

If we place a population of penguins in the Sahara desert what differences would we observe? If we place Mercury way out there, would differences would we observe? Each would be different due to location. Mercury would no longer be capable of clearing its orbit and the penguins would suffer worse.

Similarly, Pluto would be a planet if its semi-major axis was less than about 1 AU. I think one can accept the IAU both as a fait accompli and as intrinsically flawed.

No doubt true, but Pluto would come alive (I do like penguins) and would finally start behaving by clearing its orbit like it's supposed to. [It would be interesting to see how many people would pay to move Pluto in to a "planetary" position. Ug.]

One example of a flaw is that Ceres and Pluto are radically different objects, but have the same classification.

Yes, I would guess this will become increasingly useful in time. This is not unlike the development of stellar classifications. We now have, after a few bends in the road, the suffix of Roman numerals to better define the stars.

[swampyankee]

That is a set of mythological and/or religious tenets, with no basis that I can see in mechanical terms. I was thinking in terms of position, motion and the center of the apparent motions of the celestial objects.

I'm just trying to put the "center of the universe" thing in context. Very often, the logic seems to be that the opposition to the heliocentric theory was motivated by worries about humans being knocked out of their supremely important position in the center of the universe. Most (all?) of the opposition to the heliocentric theory was political, possibly more than religious. Of course, until the discovery of the aberration of light and of stellar parallax, there was really no direct evidence for the Earth's motion.

[George]

I'm just trying to put the "center of the universe" thing in context. Very often, the logic seems to be that the opposition to the heliocentric theory was motivated by worries about humans being knocked out of their supremely important position in the center of the universe. Most (all?) of the opposition to the heliocentric theory was political, possibly more than religious. Of course, until the discovery of the aberration of light and of stellar parallax, there was really no direct evidence for the Earth's motion.

Earth becoming a planet in the modern sense was a big step. It took Aquinas about 3 hard years, IIRC, to get Aristotle integrated into the theology, and it was tough to dethrone it. [I won't risk going too far off topic. This would make for an interesting thread if you should elect to start one. I would like to see more on the political vs. theology arguments.]

[swampyankee]

Earth becoming a planet in the modern sense was a big step. It took Aquinas about 3 hard years, IIRC, to get Aristotle integrated into the theology, and it was tough to dethrone it. [I won't risk going too far off topic. This would make for an interesting thread if you should elect to start one. I would like to see more on the political vs. theology arguments.]

I fear such a thread would immediately fall afoul of the no religion rule.

[Ara Pacis]

I just saw Neil deGrasse Tyson on Colbert tonight. They talked about planets. He says that Pluto crosses Neptune's orbit, which is something a planet doesn't do. Um, if we use that criterion, then do we need to demote Jupiter and Saturn, since some models suggest they crossed paths and switched orbits at one point. Without a time machine, we may never know, so we'd just have to list them with an asterisk. Also, some suggest that the pair migrated inward and then outward due to a resonance between them, so was it just one planet that cleared the neighborhood or a group effort, and does that matter?

I don't like the use of the word "world" in place of "things that could be called planets with a sensical definition." The word has other meanings and astronomers have already stolen too many useful words and then played technogrammarnazis against people. Let them come up with a neologism. Maybe call it an Ohse for "Object with hydro-static equilibrium.

[swampyankee]

I wonder if you took Neil deGrasse Tyson aside and fed him a few drinks whether he would be as defensive of the IAU definition as he is in public.

[George]

I fear such a thread would immediately fall afoul of the no religion rule.

A lot has to do with the posters. There was a "belief's" thread a while back that went for many pages because it stayed on the scientific aspects of faith, beliefs, and science. How science impacts religion or philosophy is a scientific topic. More threads than not, admittedly, have been closed for ignoring the rules.

[Hornblower]

I just saw Neil deGrasse Tyson on Colbert tonight. They talked about planets. He says that Pluto crosses Neptune's orbit, which is something a planet doesn't do. Um, if we use that criterion, then do we need to demote Jupiter and Saturn, since some models suggest they crossed paths and switched orbits at one point. Without a time machine, we may never know, so we'd just have to list them with an asterisk. Also, some suggest that the pair migrated inward and then outward due to a resonance between them, so was it just one planet that cleared the neighborhood or a group effort, and does that matter?

I don't like the use of the word "world" in place of "things that could be called planets with a sensical definition." The word has other meanings and astronomers have already stolen too many useful words and then played technogrammarnazis against people. Let them come up with a neologism. Maybe call it an Ohse for "Object with hydro-static equilibrium.

My educated guess is that if we substituted another large planet for Pluto in that orbit, it would catastrophically perturb Neptune and create an unstable combination. If I am not mistaken a Trojan or 3:2 Pluto-type orbit is stable only if the large planet's mass is small compared with the Sun and the third body's mass is very small compared with the planet. In the model in which Jupiter and Saturn crossed paths, that condition would only have been temporary, lasting a very short time on the cosmic time scale, and should not disqualify them from being considered planets.

[Ara Pacis]

My educated guess is that if we substituted another large planet for Pluto in that orbit, it would catastrophically perturb Neptune and create an unstable combination. If I am not mistaken a Trojan or 3:2 Pluto-type orbit is stable only if the large planet's mass is small compared with the Sun and the third body's mass is very small compared with the planet. In the model in which Jupiter and Saturn crossed paths, that condition would only have been temporary, lasting a very short time on the cosmic time scale, and should not disqualify them from being considered planets.

Why would they not be disqualified? Or would Jupiter be a planet, but its dominance over Saturn makes the latter a non-planet?

[Hornblower]

Why would they not be disqualified? Or would Jupiter be a planet, but its dominance over Saturn makes the latter a non-planet?

Because they were the same high-mass objects then as now, with the same dominance capabilities once they got through that brief chaotic crossover period. Your post reinforces my opinion that the framers of the IAU resolutions did a poor job of choosing their words, apparently in haste under the pressure of unscientific feedback from the general public.

[swampyankee]

Because they were the same high-mass objects then as now, with the same dominance capabilities once they got through that brief chaotic crossover period. Your post reinforces my opinion that the framers of the IAU resolutions did a poor job of choosing their words, apparently in haste under the pressure of unscientific feedback from the general public.

I don't know what kind of "unscientific feedback" was coming from the general public, mostly because the general public didn't particularly care until after the definition was released. I think the reasons were very much politics within the IAU: astronomers are academics, and academia in the U.S. has a reputation for the sort of political infighting that would fit well in Renaissance Italy. I don't know anybody from non-US academe, but I suspect it's not much different there.

[Hornblower]

I don't know what kind of "unscientific feedback" was coming from the general public, mostly because the general public didn't particularly care until after the definition was released. I think the reasons were very much politics within the IAU: astronomers are academics, and academia in the U.S. has a reputation for the sort of political infighting that would fit well in Renaissance Italy. I don't know anybody from non-US academe, but I suspect it's not much different there.

OK, I may have overestimated the influence of public opinion and underestimated the interdisciplinary infighting. I was thinking in terms of remarks I saw in the video of the 2006 General Assembly. It appeared to me that the intent of saying "has cleared its orbital neighborhood" instead of "dominates its orbital neighborhood" was to try to make it easier for the public to understand.

[swampyankee]

OK, I may have overestimated the influence of public opinion and underestimated the interdisciplinary infighting. I was thinking in terms of remarks I saw in the video of the 2006 General Assembly. It appeared to me that the intent of saying "has cleared its orbital neighborhood" instead of "dominates its orbital neighborhood" was to try to make it easier for the public to understand.

If that was the intent, it failed miserably.

Neglecting what I think is an intrinsically flawed definition, one wonders why the IAU needed to go to such effort to categorize such a small number of Solar System objects with masses much greater than about 10^21 kg.

[George]

Neglecting what I think is an intrinsically flawed definition, one wonders why the IAU needed to go to such effort to categorize such a small number of Solar System objects...

Have you read Brown's book, "How I Killed Pluto and Why It Had It Coming"? He would have been the 2nd American astronomer to discover another planet, not to mention the loss of his gain in fame. He gave it up for reasons that make sense, and astronomy has already been down this road, giving us "asteroids" instead of thousands of little planets.

[Ara Pacis]

I imagine a sentient rogue planet roaming the galaxy looking at out solar system and saying, "Oy, look at those things, still living in their parent star's basement. When are they gonna grow up, leave the house and become real planets."

[swampyankee]

Have you read Brown's book, "How I Killed Pluto and Why It Had It Coming"? He would have been the 2nd American astronomer to discover another planet, not to mention the loss of his gain in fame. He gave it up for reasons that make sense, and astronomy has already been down this road, giving us "asteroids" instead of thousands of little planets.

Actually, I did. Note that redefining Pluto is not, per se, something I view as a problem; the definition is flawed by its dependence on the object's location: define "dwarf planet" vs "planet" by an intrinsic property, not a location-dependent one, on other words if it's a dwarf planet at 500 AU, it's a dwarf at .05 AU, not a dwarf at one and not the other.

I have wondered about the motivation of the redefinition of "planet," as there were some comments about there getting to be too many planets, but I'm not going to assign off-the-cuff comments any weight.

[George]

Per swampyankee: "Actually, I did. Note that redefining Pluto is not, per se, something I view as a problem; the definition is flawed by its dependence on the object's location: define "dwarf planet" vs "planet" by an intrinsic property, not a location-dependent one, on other words if it's a dwarf planet at 500 AU, it's a dwarf at .05 AU, not a dwarf at one and not the other."

As I've stated before, your view has merit, and it seems to be in-line with the article posted earlier from the Lunar and Planetary Science article. So allow me to address their points critically...

In the decade following the supposed “demotion” of Pluto by the International Astronomical Union (IAU) [1], many members of the public, in our experience, assume that alleged “non-planets” cease to be interesting enough to warrant scientific exploration, though the IAU did not intend this consequence.

It would not be the fault of astronomers if the public places less value on an object not called a “planet”. The idea that it would cause the public to actually “cease” exploration is absurd. Trips to asteroids and comets were hardly unwarranted in the eyes of the public, no doubt.

A rose by any other name is still….

To wit: a common question we receive is, “Why did you send New Horizons to Pluto if it’s not a planet anymore?” To mitigate this unfortunate perception, we propose a new definition of planet, which has historical precedence [e.g., 2,3].

Or, just show them one picture of Pluto, thanks to Horizon, and that alone should answer their question, and from now on. Did I mention comet and asteroid missions that I assume garnered public interest, if not before, then after, right?

In keeping with both sound scientific classification and peoples’ intuition, we propose a geophysically-based definition of “planet” that importantly emphasizes a body’s intrinsic physical properties over its extrinsic orbital properties….

A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.

I guess this is the intrinsic property that does the qualifying for planethood. Yet, how spheroidal? A “triaxial ellipsoid”? That hardly narrows it down any. How familiar is that term for the public? [I had to look it up – I admit that I’m quite average, which is one reason I like to comment on behalf of other “averagers”.] Each axis has a different length, so if we had two axes equal, it would not be a planet by definition, oddly enough, though now I'm being a bit too pedantic with it, no doubt. Regardless, to qualify an object to be spheroidal will also require a cut-off in the percent variance with each axis, right? I suppose an oblate spheroid would count, though even this term seems in question by some for Earth.

And, in attempting to eschew obfuscation, do we really need to drag the lack of fusion into the definition?

In peer-reviewed planetary science publications and talks, the word “planet” often substitutes for the given name of the world, even if the world is a moon or dwarf planet.

But why? Is there a problem for planetologists calling a moon a moon? Meteorologists are okay with meteors as a subset of atmospheric phenomena, aren’t they? [Ok, perhaps not. ] I don’t see how sticking with 17th century terms is necessary, and even then, they weren’t stuck on calling them planets. [Galileo named them the “Medici stars”, but he did call them the “Medici planets” at times, perhaps often.] It is hard to not associate the moons of Jupiter with the planet Jupiter, so saying “planets of Jupiter” would not make much sense, even for a respected planetologist, right?

Finally, and most severely, by requiring zone clearing the mathematics of the definition are distance-dependent, requiring progressively larger objects in each successive zone. For example, even an Earth sized object in the Kuiper Belt would not clear its zone.

Huh? What math are they using? I would be surprised if they, somehow, could discredit Margot’s work, especially given that it is based on the Hill’s Sphere work. The Earth’s clearing ability extends well passed the KB.

The eight planets recognized by the IAU [1] are often modified by the adjectives “terrestrial,” “giant,” and “ice giant,” yet no one would state that a giant planet is not a planet. Yet, the IAU does not consider dwarf planets to be planets.

That’s a fair point. The Sun is a "dwarf star", yet it's still a star, though it’s dwarfness is given to counter the idea that it’s not a “giant”. [It is also not yellow….I’m just sayin’. ] “Dwarf” would not be an adjective, however, at least I don’t think it would. So you could add these adjectives to either planets or dwarf planets; Pluto-- the icy dwarf planet.

So, per their statement, we seem to be back to the real estate importance of “location, location, location.” Adding location to the formal definition doesn’t seem that big a deal.

[swampyankee]

Roundness is, at least ideally, an intrinsic parameter (certainly stars can be pulled out of round; contact binaries are certainly far from spherical), and a roundness parameter can easily be set based solely on geometry, e.g.,by specifying limits on max vs min radius, or based on hydrostatic equilibrium, including rotation and tidal interactions.

As an aside, the IAU definition of planet is restricted to bodies in the Solar System.

[Hornblower]

Roundness is, at least ideally, an intrinsic parameter (certainly stars can be pulled out of round; contact binaries are certainly far from spherical), and a roundness parameter can easily be set based solely on geometry, e.g.,by specifying limits on max vs min radius, or based on hydrostatic equilibrium, including rotation and tidal interactions.

As an aside, the IAU definition of planet is restricted to bodies in the Solar System.

If I am not mistaken, they did not address the topic of exoplanets. At this time it is my educated guess that the framers of the final resolution would not quarrel if we call those objects planets.

[swampyankee]

If I am not mistaken, they did not address the topic of exoplanets. At this time it is my educated guess that the framers of the final resolution would not quarrel if we call those objects planets.

The IAU's definition (https://www.iau.org/public/themes/pluto/) specifies "orbiting the Sun," so they were either being very sloppy with their language or made an active decision to restrict their definition.

[Hornblower]

The IAU's definition (https://www.iau.org/public/themes/pluto/) specifies "orbiting the Sun," so they were either being very sloppy with their language or made an active decision to restrict their definition.

I cannot be sure without actually getting into their heads, but my educated guess is that they were sloppy from acting in haste, with some interdisciplinary rivalry and antipathy thrown in for good measure.

[George]

Roundness is, at least ideally, an intrinsic parameter (certainly stars can be pulled out of round; contact binaries are certainly far from spherical), and a roundness parameter can easily be set based solely on geometry, e.g.,by specifying limits on max vs min radius, or based on hydrostatic equilibrium, including rotation and tidal interactions.

Agreed, and roundness, as we know, is in the current definition, though the "clearing the neighborhood" requirement makes this rule somewhat superfluous and just appearance since.....hmmm.... wait a sec. Well, you'll probably like this... [Added: I was going to comment on how both are arbitrary, and that we need to allow such regardless of the definition selection. This appears ignored in the article I criticized.]

Uh oh....Trappist-1 presents the limit of the stellar mass spectrum (8% solar), and with exoplanets as close as about 2 Mkm. Under these extreme range conditions for the clearing rule, these small objects will not be round but would clear their orbits. [Or is my math off? I will check this weekend unless someone else can sooner.]

As an aside, the IAU definition of planet is restricted to bodies in the Solar System.

It has been commented that more of a "wait and see" (paraphrasing) approach for exoplanets is favored by some here. Enter Trappist-1 to justify that view.