9 Reasons Pluto is a Planet: Is Pluto's Dismissal as a Planet More Political than Factual?

[Shemjaza]

I love Pluto, but it's still a dwarf planet.

No one talks about calling Ceres a planet, and it's easy more likely to be relevant in human space exploration.

 

[Warden_of_the_Storm]

I honestly have to ask: who would seriously benefit politically from Pluto being called a dwarf planet instead of a regular planet?

 

[Aureus]

We kicked Pluto out because it wasn't fair to Ceres to let Pluto in the first place. Poor Ceres already got kicked out a century before Pluto was discovered.

 

[Radrook]

Why not simply call those additional planets Transplutonian planets?
That leaves the classification of Pluto as a planet intact.
After all, it is perceived as a planet regardless of how many times we are told that it isn't.
It even has moons orbiting it. One, Charon, is so large that the Charon/Pluto relationship was until very recently considered a dual planet system.

 

[Gxg (G²)]

False. Pluto's orbit is not on the same plane as the other planets, which indicates that it is a Kuiper belt object and not a planet that formed from the accretion disk that existed during the formation of the Solar system and where the true planets formed.

May wish to pay more attention next time, as I did not say Pluto's orbit was the same as the other planets and neither did the OP at any point (assuming you actually read it or watched the documentary on it). We already know it's a part of the Kuiper Belt but that is not the discussion....nor was it ever the case that true planets were solely those formed from the accretion disk since that was never the only definition.

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If it was a planet, then its orbit would be on the same plane as the other planets. It isn't.

That was never the definition of a planet, dude. Having a unique orbit does not mean you're not a planet when not on the same plane since there are other planets beyond Pluto that have been deemed to be planets since they're far larger and yet have a unique orbit.

As another noted best on the issue when seeing Pluto misunderstood:

The ability to clear an orbit depends on the star, not just the planet
It has been argued that the notion of a planet “clearing out its orbit” is too vague, so it has been made more rigorous by defining the “scattering parameter”. This parameter quantifies the ability of a planet to scatter all the smaller bodies out of its neighborhood. It is calculated as the planet’s mass squared divided by its orbital period, multiplied by another factor that describes the swarm of smaller bodies in its path (which we can consider a constant factor). In our solar system, all the planets with a scattering parameter larger than unity have mostly cleared out their orbits, whereas those with scattering parameters smaller than unity share their neighborhood with a swarm of other bodies that have not been cleared away. So it seems that this scattering parameter really works! Pluto is one of the largest bodies that has a scattering parameter less than unity, and there is a gap of over 4 orders of magnitude between scattering parameter of the weakest major planet (Mars) and the strongest dwarf planet (Pluto). This is shown in the figure, below. Doesn’t this tell us without any doubt that Pluto is not a true planet? Some people certainly think so.

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Plot of Scattering Parameter for bodies >300 km diameter in our solar system. Moons are included just for general interest, since they aren’t in orbit directly about the sun so the scattering parameter does not really apply to them. Reference Stern and Levison, “Regarding the Criteria for Planethood and Proposed Planetary Classification Schemes,” Highlights of Astronomy 12, 205—213 (2012).

While it seems simple enough, it gets fuzzy as soon as we consider other solar systems. For one thing, the scattering parameter depends not only on the planet itself, but also on the size of the star it orbits. That’s because the planet’s orbital period is shorter when the star is more massive and vice versa. Let’s consider a couple of examples. One day we will find a body the same size and mass as Saturn’s little moon Tethys but orbiting in the habitable zone of a red dwarf star. That’s not an unlikely scenario: there are billions and billions of red dwarf stars in our galaxy. It turns out that this Tethys twin will have a scattering parameter greater than unity everywhere in that star’s habitable zone. That means Tethys is a major planet when it’s at a red dwarf star.

Some day we will also find a twin of the Earth in the habitable zone of a large, O-type star. Everywhere within that zone, the Earth twin will have a scattering parameter that is two orders of magnitude too small to be a major planet. Sadly, the Earth twin will be classified as only a dwarf planet at that star.

So when we find these worlds, we will make this poster:

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According to the scattering parameter, Tethys would be a major planet in the habitable zone of a red dwarf star, while Earth would be a dwarf planet in the habitable zone of an O-type star.



What if biologists did classification like this? We could ask them, is a dog a mammal? They might say, well, it depends upon the island where the dog lives. If it’s a big island that supports a population of tigers, then the dog is just a dwarf mammal, and dwarf mammals are not mammals. But if it swims to a smaller island that doesn’t support tigers, then it can dominate the island and then it’s not a dwarf mammal any more. Then it is a true mammal. (Hmmm…)

Pluto, being a medium-sized planet, would clear its neighborhood around our sun if it were at the orbit of Venus or Mercury. It would almost do so if it were at the orbit of the Earth. It would definitely do so if it were in the habitable zone of any star slightly less luminous than our own, which includes the vast majority of stars in the universe. Therefore, if there is life beyond the Earth, one could argue that it is likely to be found on a medium-sized planet like Pluto. (And by the way: in terms of percentile, Pluto has one of the highest scattering parameters of all the planet-sized bodies in our solar system.)

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The 169 planet-sized bodies (probably planet sized) discovered beyond Neptune so far.



The scattering parameter gives us great insight into the dynamics of planets in a given solar system at a snapshot of time, and therefore it allows us to create subcategories of planets based upon how they have behaved in their current or (presumed) past environments. This is how the scattering parameter was intended to be used. However, most of us will agree that it doesn’t capture the essence of planethood, itself. We feel something is wrong when we look at the poster with the twins of Tethys and Earth. The problem is that dynamical requirements (doing) are relative whereas physical requirements (being) are universal.

..... We can’t create good dynamical definitions from a sample of one
Clearing out an orbit isn’t something that a planet does all by itself, unaffected by neighboring planets. It is all interconnected. It takes a solar system village to raise a planetary child.

A lot has been made about the gap of four or five orders of magnitude between the scattering parameters of Mars and Pluto. Where are the planets with scattering parameters closer to unity? According to theory, they could have existed, but maybe the giant planet migrations eliminated them. That might not have happened the same way in other solar systems. As far as we can tell, our solar system is the oddball. Whatever dynamics happened elsewhere did not happen here and vice versa.

So far we have studied only one solar system in great depth, so our dynamical definitions are based on a sample of one. It’s unlikely that a sample of one oddball solar system is giving us an adequate picture.

...Pluto’s planethood indicates the reality of nature
As I discussed in a prior post, the old concept of the planets was that they are gods ruling over the Earth, each reigning in its own orbit. After we realized the planets are worlds like the Earth going around the sun, we continued to believe they are an orderly, clock-like system with no sharing of orbits. Only in the most recent century have we realized that countless smaller bodies share and cross the orbits of all the planets, and an entirely new zone of the solar system exists beyond Neptune (the Kuiper Belt) where sharing orbits is normal, not an

exception.

Furthermore, we started realizing that even the inner planets have been moving around and messing with each other’s orbits. No part of the solar system is clock-like when you consider how it behaves over long enough timescales. When the IAU changed the definition of a planet to exclude all the worlds of the third zone, it was an attempt to keep the old clock-like view intact with planets that reign like gods within their own orbits. But that’s not the reality of nature. To include Pluto as a planet is to embrace the new view of the solar system, the reality that it is a wonderfully messy, ever-changing place.................

Pluto’s planethood enhances education
Right now, most elementary and secondary education focuses on the eight largest planets, creating the outdated impression of a static solar system where everything important is closer to the sun than Neptune. That’s a bias of perspective just like the geocentric bias of earlier generations. Including Pluto and the hundreds of worlds in the Kuiper Belt as planets will change our perspective and change the language of the classroom. Imagine when teachers say, “The planets are in four zones in our solar system: an inner zone with rocky planets; a middle zone with giant planets; an outer zone with hundreds of newly discovered, medium- and small-sized ice planets; and a far outer zone (the Oort Cloud) with planets we have not even begun to discover or explore.”

That will create an entirely new understanding of reality. It is a reality freed from the artificial boundaries of anthropocentrism. It is a reality full of unsolved mysteries about planetary science and our place in the cosmos. It is a reality where students have a lot to contribute by becoming scientists themselves, because we are communicating the fact that there is more we don’t know about our solar system than what we do know.

Students today aren’t getting that impression when we tell them there are exactly eight planets, all of them were discovered long ago, they orbit in these eight regular paths, and that’s that.​

 

[Gxg (G²)]

no, i haven't really been into the study of Pluto....i just think it's wrong to call Pluto a planet and then to snatch away the status of Pluto as a planet....

why Pluto gotta be picked on like that? What's Pluto ever done to anyone?

So you're not really interested in the science aspect and more so on the status aspect

 

[Gxg (G²)]

Why not simply call those additional planets Transplutonian planets?
That leaves the classification of Pluto as a planet intact.
After all, it is perceived as a planet regardless of how many times we are told that it isn't.
It even has moons orbiting it. One, Charon, is so large that the Charon/Pluto relationship was until very recently considered a dual planet system.

Indeed - and when comparing Earth to the other large planets, we're the equivalent of Pluto with Jupiter or Saturn. But of course, we say it's relative...

 

[Murby]

One of the biggest reasons Pluto is no longer considered a planet is because it has not cleared it's own orbit of debris.

To consider a planet for "planet-hood", one of the criteria is that the object in question must have a gravity-well sufficient enough to have cleared it's own orbital neighborhood of left over debris such as small rocks, dust, and other objects.

If you were to take a planet such as Mars or Earth and place it in orbit like Pluto, the Kuiper belt would not exist as it does today.

When a planet encounters a smaller object in its orbit, it can do three basic things.. Suck it in, orbital capture, or gravitationally sling shot it away. Pluto is not large enough to do much of any of these so its orbit remains littered with solar system left-overs. Hence, not a planet.

 

[Gxg (G²)]

I think it is a genuinely difficult question. Recall that Pluto was discovered by Clyde Tombaugh as part of a deliberate search for the planet thought to be responsible for irregularities in Neptune's orbit. (We now know these irregularities were computational errors.)

So, when Pluto was spotted in the right place for where we expected to find a planet it was natural to call it a planet. Had we stumbled on it by chance and recognised from the outset the peculiarities of its orbit and its low mass, it is doubtful it would have been classified as a planet. Had we simultaneously been aware of Sedna and Eris and Makemake and the rest of the Trans-Neptunian bodies it certainly would not have been called a planet. A new category would have been created, just as had been done a century earlier for the asteroids of the main belt.

Although, for sentimental reasons, it would be nice if it were still a planet, the reclassification is logical. What I do object to is the rather sloppy rules that have been established by which to distinguish between planets and dwarf planets - but that is another matter.

A final observation: one thing that pleases me is that the downgrading occurred after Clyde Tombaugh had died. He did not have to witness the downgrading of the hallmark achievement of his life.

I'm not so certain it'd be an issue of irregularities making all the difference alone when it comes to what Clyde stumbled upon. Even without them, the aspects of what make a planet would still be present even if Pluto was not the planet they were looking for. But dwarf planet as a designation is fair for now....

With Clyde, what's interesting is that he really was not the person who discovered Pluto....and as said best elsewhere:

Just weeks before New Horizon's highly anticipated flyby of the dwarf planet, Pluto, researchers at the Carnegie Institution for Science found a 1925 astronomy image with notes pointing to what appeared to be Pluto.

Cynthia Hunt, who found the evidence in the institution's historical collection of astronomical photos, cross-referenced the location of Pluto with software and confirmed that the scientists who had made the notes had, in fact, identified Pluto correctly.

With a little more digging, Hunt found that the men behind the photos were astronomers Gustav Stromberg and Nicholas Mayall.

After Clyde Tombaugh discovered Pluto in 1930, it was not uncommon for astronomers to go back and reanalyze older images they had taken of the sky in search for the new then-planet, Natasha T. Metzler of Carnegie told Business Insider in an email.

And it appears that's exactly what Stromberg and Mayall did. They reported their findings a year after Tombaugh announced his discovery in a 1931 paper.

Here's, Pluto, circled in blue, in one of the 1925 photos:

CarnegieInstitution on YouTube​

It's likely Stromberg and Mayall didn't know what they were seeing; if they had known, they might have published their findings much sooner.
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And as it concerns the way that Pluto is still uniquely amazing after being deemed a dwarf planet:

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Stop hoping that Pluto will regain its former designation as a planet. It isn’t going to happen. But the good news is, Pluto is something much cooler than a mere planet. It’s the largest dwarf planet we know, and one half of the first binary planet system. Pluto didn’t get demoted, it got promoted.

We’re all feeling the Pluto-love, with all the new data flowing in from the New Horizons probe. I keep getting asked, “When will we get the ‘Pluto is a planet again!’ article? Or is it silly and nostalgic to hope?” That article isn’t coming—but instead of sorrow over Pluto losing its old title, I’m celebrating all the awesome titles getting piled on to this little world after its reclassification.

I am in love with Pluto and all its moons. I love the weirdly young surface. I love its strange thermal-driven geomorphology. I love that I’m constantly surprised by all its mysteries. But Pluto isn’t a planet, and I’m happy for it. Ever since Pluto’s reclassification as a dwarf planet, it’s been piling on far cooler titles. See for yourself!


The Largest of the Dwarf Planets, and the King of the Kuiper Belt
Instead of being the very smallest planet, Pluto is confirmed as the largest dwarf planet. We had trouble measuring the world’s size from afar due to the haze of its thin atmosphere creating a fuzzy mirage. The New Horizons probe finally pinned down its size: 2,370 kilometers (1,473 miles) in diameter with a potential error of ±20 kilometers (12 miles). This means that Pluto is the undisputed largest dwarf planet. The next in line is Eris, with a diameter of 2,336 kilometers (1,451 miles) with an error of ±12 kilometers (7 miles). Even if Pluto is at the very smallest end of its potential size range and Eris at the very top of its range, Pluto still comes out bigger as the largest dwarf planet.

It’s close, but Pluto narrowly edges out Eris for title of the largest dwarf planet. It also gets a more interesting moon system, even if it is less dense. Image credit: NASA​

This also means Pluto is slightly larger than our earlier estimates—which means that it is a bit less dense, with a larger proportion of ice layered onto the active world. A deeper layer of ice means, in turn, that the troposphere is lower than our previous atmospheric models. The weirdest bit is that this also means that Pluto is less dense than Eris, which is smaller yet heavier. For all the strange wonders we’re finding at Pluto, whatever is going on at the next-largest dwarf planet Eris is going to be entirely different.

The dwarf planets in our solar system are located in two places: in the main asteroid belt between Mars and Jupiter, and in the Kuiper Belt beyond Neptune. Like the main asteroid belt, the Kuiper Belt is full of dwarf planets, jagged asteroids, and shards of ice. Being largest of the dwarf planets also makes Pluto the de facto King of the Kuiper Belt.

Classifying objects serves a function by organizing similar objects together to make them easier to analyze. Despite our nostalgic affection for classifying Pluto a planet, its dramatically eccentric, inclined orbit makes it stand out as the outlier in a game of, “Which of these is not like the others?” Instead, the dwarf planet is the namesake for an entirely new class of solar system objects: the plutoids.

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Pluto’s orbit is far too eccentric and inclined to match the planets in the rest of the solar system. Image credit: NASA

Plutoids are dwarf planets outside of Neptune’s orbit. These trans-Neptonian objects have highly elliptical orbits that send them on journeys outside the plane defined by the rest of the planets in our solar system. As dwarf planets, these bodies are massive enough to be near-spheres, butnot so massive that they clear out their orbit. In a more technical definition, every plutoid is massive enough that their self-gravity overcomes rigid body forces so they’re in hydrostatic equilibrium (aka, spherical), but not massive enough to gravitationally dominate their orbit.

While some moons are big enough to border on dwarf planets, any satellite of a plutoid is a moon, not a plutoid.

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Pluto is the namesake of the plutoids, dwarf planets in trans-Neptunian orbits. Image credit: NASA

Pluto and Eris are both plutoids with orbits farther out than Neptune. ButCeres isn’t a plutoid because it’s located in the main asteroid belt between Mars and Jupiter. Haumea and Makemake are also plutoids, and Quaoarmight be one. Sedna may be a plutoid, but it doesn’t have a moon so we won’t know its mass (and thus if it’s massive enough to be a sphere) unless we send a probe out to investigate. It’s also possible that Neptune’s largest moon Triton is actually a former plutoid that was captured into orbit around the gas giant.

Pluto is also the namesake of plutinos, which are any trans-Neptonian object of any size that are in orbital resonance with Neptune. Most plutinos have an orbital period about 1.5 times that of Neptune, clustering with an average of 247 years to make a complete trip around the sun. Along with Pluto, other plutinos include Orcus, Ixion, Huya, and an entire scattering of objects that still wear more dull monikers like 2002 VR128, 2004 UX10, and 1999 TC36.

Our First Binary Planetary System
By far the coolest title Pluto has earned isn’t a solo title, but a joint honor shared with Charon. Charon is Pluto’s largest moon: at half the diameter of Pluto and 10% of its mass, Charon is the largest moon in relation to its primary world in our entire solar system. Charon is big enough that if it weren’t orbiting Pluto, it’d be a dwarf planet in its own right.

The mass ratio between parent and moon for Pluto and Charon is 10:1, far closer to parity than the usual hundreds to one of terrestrial planet-moon systems, or the thousands or even tens of thousands to one of the mass of gas giants to their diminutive moons.

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Pluto and Charon mutually orbit their barycenter in these photographs taken b the New Horizons spacecraft. Image credit: NASA/JHUAPL/SwRI

That’s where things get interesting: Charon orbits Pluto, and Pluto orbits Charon. The center of mass between the two worlds — the barycenter — is in empty space, not nestled within Pluto’s rocky core. This goes beyond the distinct wobble that our moon induces in Earth’s orbit—instead, this creates a distinct point that both Pluto and Charon orbit around.

When it comes to stars, any time the barycenter of two stars’ orbit is beyond the surface of the primary object, and is instead out in space somewhere, that’s enough to declare them a binary star system. The same is true for asteroids — we’ve found asteroid pairs with barycenters outside both rocks, and declared them binary asteroid systems. Since the barycenter of Pluto and Charon is an empty point in space, surely that means that Pluto-Charon a binary planetary system. This would make Pluto and Charon not only the first binary planet system in our solar system, but the first one we’ve found among the literally hundreds of Kepler exoplanet worlds.

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.........Pluto doesn’t need to be a planet to be cool. We already have eight planets. Instead of being the smallest of the four rocky, terrestrial worlds, Pluto gets to be something truly special. It’s a geological mystery, the first Kuiper Belt object we’ve ever studied, and a constant source of scientific surprises. With all these awesome titles — the most massive dwarf planet, the namesake of an entire classification of solar system objects, and the primary body in the first binary planetary system of any variety — Pluto was never downgraded. Instead, it was upgraded to become something far more interesting.


'
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Definitely something to keep in mind with how unique Dwarf Planets are...

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[Gxg (G²)]

Why not simply call those additional planets Transplutonian planets?
That leaves the classification of Pluto as a planet intact.

After all, it is perceived as a planet regardless of how many times we are told that it isn't.It even has moons orbiting it. One, Charon, is so large that the Charon/Pluto relationship was until very recently considered a dual planet system.

There's actually an excellent article on what you noted that I think you'd appreciate it.

• Pluto - When Is A Dwarf Planet Not A Planet? - Science 2.0

As said there:

Let's declare Pluto as a dwarf or sub dwarf planet AND a planet! This is about the vexed question of whether Pluto is a planet or not. Yes the International Astronomical Union "settled the question" but I'm not sure their decision is one that will work all the way into the future, for instance if we find Earth or Neptune sized objects beyond Pluto. And it also stretches language in an awkward way to say that Pluto is a dwarf planet but not a planet - and the English word "dwarf" seems to have little to do with the concept of clearing your neighbourhood.

Their decision doesn't have unanimous support amongst astronomers. Alan Stern of the New Horizons mission is one of those who argues strongly that Pluto is a planet. Note that he is one of the two authors of the paper that introduced the idea of "Clearing the neighbourhood" which the IAU used to motivate their decision that Pluto is not a planet. Indeed the ones who are most against their decision are planetary scientists, and the decision, it seems, was made by astronomers most of whom study stars and galaxies and such like rather than planets, so don't have to live with this awkwardness in their daily life so much.

There are plenty of people to explain why it is a dwarf planet and not a planet. This originated as my answer to the Quora question "How can Pluto be called a "dwarf planet" if it's not a planet?" - so see the other answers to that question for more about this.

I think for balance, it may help to present the other POV (Point Of View), that perhaps we should re-classify it back as a planet again.

At the moment there's a nice clear division between planets that clear their neighbourhood according to their definition, which all happen to be big, with Mercury the smallest. Then the ones that don't, like Pluto, Eris, Ceres, etc, are all rather small.

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But in the future we will surely find more objects larger than Pluto.

The WISE spacecraft has has ruled out the possibility of a Saturn sized object out to 10,000 times the Earth - Sun distance, and a Jupiter size or larger object out to 26,000 times that distance. (I.e. 26,000 AU.)

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Artist's impression of the NASA Wide-field Infrared Survey Explorer. Researchers proved that Nibiru and Tyche can't exist in results published in 2014, using data that it collected in 2010 to 2011.But we might still find Earth or Mars sized objects out there. Some think that's the explanation of the Kuiper cliff.

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Has a dip at 50 au. Models showed the number of objects should double at 50 au. So some suggest there must be a planet, as large as Mars or Earth to "shepherd" the kuiper belt like Saturn's shepherding moons for its rings. What if we find an object large enough to do that, also as large as Mars or Earth, but which can't "clear its orbit" according to the IAU definition.
If we find a Mars sized object, or even an Earth sized object, a bit beyond 50 au, which helps to shepherd the comets in the Kuiper belt - but is not able to clear its orbit according to the IAU definition - is that a planet?

At the moment we have two discrete populations, planets that clear their orbit and small "dwarf planets" that don't. But there may be many things in between that are large and don't clear, or only partly clear.

I think the IAU definition will begin to fall apart if we find planets like that as it will seem odd to many to say that a Mars or Earth sized dwarf planet is not a planet.

ORIGINAL DECISION WAS MADE BY ONLY 1% OF WORKING ASTRONOMERS
The IAU made the decision but immediately afterwards there was a fair bit of criticism of how the decision was made by other planetologists.

For instance the decision was made in a meeting that might not have been very representative of astronomers generally. Only 5% of the members of the IAU were present to vote - and perhaps 1% of all working astronomers had the opportunity to vote. Were they representative enough of the general astronomical community to make this decision? Should non attending members been involved as well, and astronomers not in the IAU?

Inside the planet definition process

WE HAVE FREE SPEECH - YOU CAN CALL IT A PLANET IF YOU WANT TO

Anyway - there's nothing to stop anyone calling Pluto a planet if they want to . We have freedom of speech, and it's not harming anyone to do that.

Sometimes people do call Pluto a planet, especially in conversation, sometimes in articles also, if they are sympathetic to this idea that it should be classified as a planet.

PETITION IN WAKE OF DISCOVERIES BY NEW HORIZONS

Actually there's a petition you can sign here if you want to support the view that Pluto should be reclassified back as a planet again, in view of the new discoveries, showing a surprisingly complex world with geological processes, nitrogen glaciers etc.

International Astronomical Union: Declare Pluto a Planet #PlutoFlyby

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I signed the petition myself, and this is what I said there:

"The rule about not clearing neighbourhood doesn't make too much sense to me what about the Jupiter trojans (about as numerous as the asteroids of the asteroid belt) and Jupiter family comets and even Neos?

And for that matter - has Neptune cleared Pluto out of its orbit?

And anyway - could anything as far out as Pluto clear its neighbourhood given the wide variety of inclinations and eccentricities of KBOs?

- and it's a bit of a strange definition if you don't know if it is a planet until you have done a census of all the other objects in its neighbourhood - and if applied to exoplanets - would mean that you just don't know if they are planets or not until you know a whole lot more than we can expect to know by remote observation in early stages.

And does that mean that in the early solar system - e.g. in late heavy bombardment when a whole lot of objects came into the inner solar system - possibly from the outer solar system, or wherever they came from, that for a while Earth ceased to be a planet until it had cleared them all out of its orbit?

Or if you make it a kind of theoretical thing "capable of clearing its orbit, but right now it is a bit overwhelmed by new material so hasn't actually done it yet" then it just seems a rather quixotic way to define a planet.

I just don't find the idea very convincing myself, from what I've seen of it so far.

So, I'm inclined to go with Alan Stern there, it doesn't make much sense to me to not call Pluto a planet for this reason.

Though calling it the largest of the dwarf planets is also good, but a dwarf planet surely is a planet? Again it seems kind of logically odd to not call them planets. I'd call them all planets, and qualify them as "dwarf planets" to say they are small - and add in Ceres as a planet, but a dwarf planet. And leave "dwarf" as vague so that compared to the gas giants, then Earth and Venus are also dwarf planets, but in ordinary use a dwarf planet would be a planet that is smaller than Mars, around the size of the Moon, not much larger than it.​

VALUE OF "CLEARING ITS NEIGHBOURHOOD" AS A SCIENTIFIC CONCEPT

There's no doubt that this idea of "clearing the neighbourhood" is a useful concept in study of planetary systems. Where there's a parameter you can calculate to tell you how effective, e.g. the Earth is at clearing its neighbourhood. The answer is, very effective, any NEAs will be cleared out within about 20 million years unless in a resonant orbit - either to hit the Earth, Moon, Sun, Jupiter, Mars etc, but they can't stay in the vicinity of the Earth in an independent non resonant orbit.

It's a continuous parameter so even Pluto does clear its orbit to some degree, and so you have to choose a "cut off" point at which the planet clears its orbit sufficiently for the definition. But Pluto is many orders of magnitude less effective at "clearing its orbit" than Earth. So if that's your criterion, to use that parameter, it is reasonable to exclude Pluto.

The only question is a terminological one - is this how we want to understand the word "planet"? Is it a convenient and good and useful way of using the word? I.e. more a question of language than science.

After a fair bit of thought, then I'm inclined to side with Alan Stern on this, for the reasons given.

FOUR GAS GIANT PLANETS, FOUR MEDIUM SIZED ROCKY PLANETS AND NUMEROUS SMALLER DWARF SIZED PLANETS - WHY NOT?

So, the solar system, I'd say, has four gas giant planets: Jupiter, Saturn, Uranus and Neptune, four medium sized rocky planets Mercury, Venus, Earth and Mars, and numerous dwarf sized planets Ceres, Pluto, Eris, Haumea, ... And some of them are better at clearing out their neighbourhood than others.

But anything large enough to reach a gravitational equilibrium shape is a planet, if it orbits a star and is not a moon of another planet and if it isn't a star itself or a brown dwarf "almost star". If two such objects orbit an axis not within either of them it's a double planet, making Pluto and Charon the only known double planet so far.

Any gravitational equilibrium shape would count including rapidly spinning three axis ellipsoids like Haumea, and roche-world type double planets, or planets with three or four Roche type lobes, or even "donut shaped planets" if such exist (So You Thought You Knew What Planets Look Like? ... Shapes Of Rapidly Spinning Planets) - I'd call them all planets so long as they are big enough to self gravitate and reach gravitational equilibrium - unless, of course, they are moons, brown dwarfs or stars.


I'd call Haumea a planet also.

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The IAU definition also says a planet has to be a "hydrostatic equilibrium (nearly round)" shape - which seems like a definition not written by experts on planet formation, because there are other shapes planets can take and still be in gravitational (hydrostatic) equilibrium, including an "over contact binary" and for that matter, for rugby ball shaped planets, I think it's a stretch to call them "nearly round". You can also have a contact ternary or quarternary planet in theory, and in theory at least, though if it is possible at all, this might be a short lived phase, a donut shaped planet. '

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Whether the more complex shapes here actually exist is unsure but the contact binaries seem quite likely as many asteroids are contact binaries though not quite in gravitational equilibrium.

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Though we don't know any planets like this yet, many scientists think it's only a matter of time before we find a contact binary planet, either in our solar system or as an exoplanet.

For more on all this: So You Thought You Knew What Planets Look Like? ... Shapes Of Rapidly Spinning Planets

AND USE THE WORD "DWARF" TO REFER TO SMALL PLANETS
And I'd use the word "dwarf" to refer to the size of the planet, not to whether or not it clears its neighbourhood. As it happens none of the dwarf planets in our solar system clear their neighbourhood, if you use the word to refer to e.g. planets of around the size of the Earth's Moon or smaller.

But in other exoplanet solar systems around dwarf stars, perhaps some tiny planets would "clear their neighbourhood", if so I'd still call them dwarf planets if they are small. But as a flexible word so in some contexts, so long as you make it clear what you mean, you could call the Earth a dwarf planet.

If you want a more precise term, Alan Stern suggested calling Earth, Mercury, Venus and Mars dwarf planets, and Pluto, Ceres, Eris etc "sub dwarf" planets. See Page on swri.edu

​

Alan Stern's suggestion is to call Earth and Venus, Mars, Mercury - the planets to the left of the picture, dwarfs.

Then he would call the really tiny Pluto, Charon, KBO objects etc, to the right of the picture, sub dwarfs.

But they are all planets. Image from: Illustrations - Roberto Ziche​

MOONS LARGER THAN PLANETS
If we go with this definition we end up with moons that are larger than planets. But that's true already with the IAU definition, as Ganymede and Titan are larger than Mercury.

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It it is a moon if it orbits a planet and the barycenter is inside the planet it orbits (as it is with the Moon and the Earth)

​

and otherwise, it's a planet which could be part of a system of two or more planets orbiting their common barycenter.

In the case of Pluto and Charon, the barycenter - the point they both rotate around - is outside of Pluto, so although Charon is lighter than Pluto, it's not so clear whether Charon is orbiting Pluto or they both orbit the barycenter. So this is a double planet.

Like this: animation created before the New Horizons flyby of course:

​

See Can we call Pluto and Charon a "binary planet" yet?
DWARF PLANETS OF INTERMEDIATE STATUS
With brown dwarfs there's a gray area between the smallest brown dwarf and the largest Jupiter sized object.

In the same way there are asteroids that are not quite in hydrostatic equilibrium and have many of the features of dwarf planets such as differentiated interior.

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Again, this is the same for the IAU definition. It's then a borderline dwarf planet. This doesn't change anything.

THE ENGLISH WORD "DWARF" DOESN'T MEAN SOMEONE OR SOMETHING WHO "CLEARS OUT YOUR NEIGHBOURHOOD", IT MEANS SOMETHING SMALL

It just seems rather odd to define "dwarf" as meaning "clears its neighbourhood" which has nothing at all to do with the way this word is normally understood in English.

With the IAU's definition, if we found a Neptune sized object somewhere far out in the Oort cloud - there is no way it could "clear its neighbourhood" so we'd have to call it a "dwarf planet" which seems a decidedly odd use of the word "dwarf", if you can in principle have a Neptune sized dwarf, and a Mercury sized non dwarf.

If you need a word for this concept, then coin a new word or just say "clears its neighbourhood".

VALUE OF SIMPLICITY IN LANGUAGE
I understand that this seems a minor matter. But I think clarity of basic concepts may have wider reaching effects. The simpler and easier to understand the basic concepts, then the easier it makes it for everyone all the way through the subject.

In maths when you work with axiom systems, you are constantly trying to find simpler definitions and easier to state and understand axioms. So I think that's why I, as a mathematician who worked on foundations of mathematics and axiom systems, find the simpler definition of a planet particularly appealing.

While, to call it a "dwarf planet" but not a planet, if it is the size of Neptune in the Oort cloud, and a planet, and not a dwarf if it is the size of Mercury and close to the sun - this is so far from natural ways of thinking about language - and based on rather recondite ideas - that it seems more likely to lead to confused ideas rather than to help with clarity of thinking in the future.

Same also for the other rather paradoxical conclusions you end up with if you use this definition, such as - that it might change from a dwarf planet (and not a planet) to a planet (and not a dwarf) and back again if some episode sends more than usual amounts of material into its neighbourhood.

Or for that matter, once we have megatechnology we could turn Pluto into a planet simply by artificially clearing out everything in its neighbourhood. Then there would be nothing for it to clear out and so by default it would then count as a planet by the definition, and would no longer be a dwarf. Again that just seems confusing, to use the words that way.

You can't say it is wrong, this is natural language and it is often paradoxical in how it works. But given the chance of a simpler way of using language I'm inclined to go with that where possible.

And because it is an observational science - simpler also would include easy to observe - seems to make sense to use concepts that are easy to observe as your basic concepts.​

 

[Gxg (G²)]

why Pluto gotta be picked on like that? What's Pluto ever done to anyone?

It's an underdog world for sure....

But being smaller in status does not mean smaller in significance..

 

[Warden_of_the_Storm]

Please, someone answer these questions:
Seriously, what changes in the world if Pluto is a planet or not?
What group benefits politically from Pluto being demoted from a planet?

 

[AV1611VET]

Wow 18 posts on Pluto and no AVVET.

You rang?

 

[AV1611VET]

Please, someone answer these questions:
Seriously, what changes in the world if Pluto is a planet or not?

The number of planets in our solar system.

What group benefits politically from Pluto being demoted from a planet?

The IAU?

And don't forget: Pluto was demoted by a rigged vote.

 

[Gxg (G²)]

The number of planets in our solar system.The IAU?

And don't forget: Pluto was demoted by a rigged vote.

Indeed - and this is something that has long been ignored. As it is, the director of NASA still calls Pluto a planet and he and other scientists realize the reality of political dynamic in the sense that size is really the leading factor above all.

 

[Gxg (G²)]

And don't forget: Pluto was demoted by a rigged vote.

I am surprised many do not know on the dynamics of that vote and the lack of integrity many saw in it...

 

[Radrook]

I am surprised many do not know on the dynamics of that vote and the lack of integrity many saw in it...

In the USA, many people have problems memorizing the locations of the nine planets.
So I imagine that school-teachers in the USA panicked when suddenly confronted with the task of requiring already-befuddled students with memorizing more. One additional planet beyond the traditional nine and the whole classroom might suddenly collapse from mental overexertion.

 

[Eudaimonist]

I honestly have to ask: who would seriously benefit politically from Pluto being called a dwarf planet instead of a regular planet?

Big Pluto. They have their fingers in everything.

 

[AV1611VET]

I am surprised many do not know on the dynamics of that vote and the lack of integrity many saw in it...

Have you seen this?

The Pluto Issue (again)

 

[AV1611VET]

In the USA, many people have problems memorizing the locations of the nine planets.
So I imagine that school-teachers in the USA panicked when suddenly confronted with the task of requiring already-befuddled students with memorizing more. One additional planet beyond the traditional nine and the whole classroom might suddenly collapse from mental overexertion.

Yet it ain't no biggie to require them to learn the elements in the Periodic table.