Is Pluto A Planet Now? (Synopsis)

“Words are the source of misunderstandings.” -Antoine de Saint-Exupéry

Back in 2006, the International Astronomical Union officially defined the word "planet" for the first time, claiming that as long as something met all three of the following criteria:

  1. It was in hydrostatic equilibrium (pulled itself into a spherical/spheroidal shape under its own gravity),
  2. It didn't orbit any other body larger than itself (i.e., wasn't a moon), and
  3. Cleared its orbital path of all other major bodies,

then it got to be a planet.

Image credit: © 2015 The Johns Hopkins University Applied Physics Laboratory LLC, via Image credit: © 2015 The Johns Hopkins University Applied Physics Laboratory LLC, via….

By that definition, Pluto was out. And yet, Pluto and all the other Pluto-like objects out there in the Kuiper belt are not only paramount to the formation of our own Solar System, those objects vastly outnumber the planets. Not only our Solar System's planets, but these icy, lonesome objects outnumber the planets galaxy-wide. With everything we're learning about it, is it time to reinstate Pluto's planethood?

Image credit: Facebook user Pedro P. Coniconde, via Image credit: Facebook user Pedro P. Coniconde, via

A full look at the question, and a proposed solution by me over at Starts With A Bang on Medium!

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Since ' rogue planets ' are considered as planets, maybe Pluto can be accepted as a ' captured ( rogue ) planet ' .

Aren't you confusing Alan Stern (the principle investigator) and Mike Brown (a.k.a. @plutokiller)?

"So why do we stop calling it a planet because it’s in our Solar System? "
Because of planetcism.
It's not PC to be that way though because today, if Pluto "Feels" like it's a planet well, then it's a planet.

Your right though, "World" suffices.
I am a tiny bit disappointed in the opening poetic sconce (that is if anyone cares what I think).
I would have thought Shakespeare's "What’s in a name? that which we call a rose
By any other name would smell as sweet"
Would have been more fitting.

Congrats, Ethan and too all. Ethan you seem as excited as a kid in a candy store.
Good for you, I am happy for you.
I appreciate this stuff but I am most impressed with the " spacecraft has spent that last 9 1/2 years in the frigid conditions of outer space and still managed to operate all of its instruments flawlessly"

By Ragtag Media (not verified) on 14 Jul 2015 #permalink

Thankyou for being willing to reconsider and make what I think is very much the right choice here.

Yes it is time, overdue even to reinstate Pluto's planetary status and adopt a far better broader definition of planet.

I certainly agree with your conclusion on the other version :

"In a very meaningful way, including Pluto back in the picture of our Solar System’s planets may be more honest — in terms of understanding what’s really important when it comes to a planetary system — than leaving it out. "

I think thedefinitionof planet taht may work best is that a planet is an object that :

1) Doesn't and never has been self-luminous through internal nuclear fusion thus not a star or brown dwarf.

2) Rounded by its own gravity (hydrostatic equilibrium) thus not a comet or asteroid.

3) Not directly orbiting another planet thus not a moon.

I reckon this is a pretty simple, easy to determine and straightforward definition that really works.

The IAU's third orbital clearance criterion strikes me as raising far more problems than it solves and being illogical in a number of ways - starting with the little known fact that it rules out planets round other stars at all as well as those that orbit without stars and lets not forget that Earth at Pluto's distance probably couldn't clear its orbit either.

I think its also really wrong and obviously inconsistent to say that a dwarf star is always a star - and most stars are dwarfs - but a dwarf planet is somehow not a proper planet.

Seems to me that by the IAU "logic" grass wouldn't count as a plant because its small and grows differently and there's a lot of it!

I really hope the IAU do fix the horrible mistake they made in Prague and accept the ice dwarfs are indeed legitimate planets.

Today though most of all, I'd like to say thankyou to 'New Horizons' and all those who've made her fly. Seeing Pluto for the first time! Wonderful! Been looking forward to this for so long and so good to see.

The people from one dwarf binary planet around a dwarf star have sent a dwarf spacecraft (piano-sized) to fly past another dwarf binary planet. One of these dwarf worlds is bigger than the other, the other is more a binary than the other. But both are remarkable and marvellous worlds and both count as planets and deserve respect and awe and discovery.

By Astrostevo (not verified) on 14 Jul 2015 #permalink

After manoeuvers are completed, both antennae are going to be used to almost double the data stream (1.9 x). Transmission will be shut down for further navigation, then the next data stream will be sent, & so on. Clever.
Congrats to all the staff & backroomers involved - job well done.

About that graphic of the nine "planets"...that poor user needs to start over, the first picture is of Ganymede, not Mercury.

By Grebuloner (not verified) on 14 Jul 2015 #permalink

All heavenly objects are "worlds" in their own rights, even comets like the one that Philae landed on. I think it's designation as a trans-neptunian object (TNO) is good enough for it. Anything else is just disguised sentimentality. Yes, even I do not think dwarf planets is a good term for these tiny worlds. It felt like a backhanded way to still define Pluto as some kind of "planet".

I was on the edge of my seat during the flyby. I watched the countdown live on NASA's ustream channel, and then watched other live events like the "Breakfast with Pluto" event with Neil Tyson. I think it's spectacular that we were able to flyby Pluto. I ended up watching a few documentaries about Pluto (and Clyde Tombaough) on youtube to satisfy my thirst for all things Pluto.

Yet I still would never support re-categorizing Pluto as a planet. It's a TNO, a KBO, a failed comet (just like Eris in the Scattered Disc). When it was measured as being slightly bigger than once thought, it meant that there is more ice, and less rock, in Pluto than what was once supposed. It is an ice world with a small rocky core.

BTW, you mixed up Alan Stern (the project lead for New Horizons) with Mike Brown (the Pluto killer and the authority on TNOs) in your article. I have listened to the audiobook of Mike Brown's memoir "How I Killed Pluto" and am thoroughly persuaded with his position.

Yes it is time, overdue even to reinstate Pluto’s planetary status and adopt a far better broader definition of planet.

No it's too late and a broader definition doesn't work.

Get over it.

"Is Pluto A Planet Now?"

Pluto is a planet since 1930. Next question please.

Pluto isn't a planet now.

So your answer is incorrect. Why do you want another question? Get one right first, then we can talk about trying another.

Seems to me that by the IAU “logic” grass wouldn’t count as a plant because its small and grows differently and there’s a lot of it!

Well, you're right, according to the IAU grass isn't a planet.

Hell,even by the geophysical definition,it isn't a planet.

Oh. did you say "plant"? Biology isn't astronomy. I thought you should know that.

What is the emotional attachment so many people have with Pluto? Is the objection to the change in status simply due to the cute name or that so many grew up with it counted as a planet? It seems (to me) to make no more sense than saying "We need it considered a planet so that the number of planets is the largest perfect square less than 10."

If Pluto is once again classified as a planet, what would Charon be classified as? Keep in mind that Charon does not orbit Pluto but rather a barycenter located between Pluto and Charon.

"What is the emotional attachment so many people have with Pluto?"

It was an american who found it. No disrespect since the ones earlier in were all much more obvious, so they had already been discovered.

They "grew up" with it and just do not like change. That earlier Ceres was likewise "downgraded" (which isn't a damn downgrade at all, FFS) because they had no involvement in that one.

"what would Charon be classified as"

Yup. This is why the IAU took such a long time working out a definition that could work. It's *easy* to fit the definition you want if you don't have to work out how it fits to EVERYTHING ELSE. The ones complaining are only matching those that their argument works with. No wonder it matches...

You know, 'Dwarf Planet' does include the word "Planet". The problem is that we have no special term for bodies that match requirement 3, so what we should really do is come up with a separate name for orbitally-dominant bodies, and leave Pluto in the category of Dwarf Planet.

I'm with Wow and Dean - the fuss bemuses me. Who cares?

Pluto is Pluto, and it's not a planet.

In any case, in the long run, the most interesting bodies in our solar system - by far - are going to be non-planets: Europa, Enceladus, Titan and maybe Ganymede and Callisto.
Not planets, but far more interesting than any of the planets. But I don't hear people whining about them, probably because it really isn't important how you classify it - it's what you find there that is important.
Have they found anything important on Pluto?

Another thing that I found kind of ridiculous and 1950s-ish was some footage of Ground Control (somewhere) getting back these pictures of Pluto - everybody was waving little US flags about.
You couldn't satirise this frame of mind if you tried.

By Craig Thomas (not verified) on 15 Jul 2015 #permalink

Consider this, Craig. It took 9 1/2 years to get there, we now know what it looks like in part. The next phase is the coming data sets which will satisfy many answers of Plutos 'worth'. The thing is, without going there, nothing major would be learned.
The people celebrating their achievement can celebrate however they wish - their choice - not yours.

I do feel like the discoveries are being oversold with breathless hyperbole, PJ. And it may be partly as a sop to American Exceptionalism being "hurt" by Pluto's "demotion" (remember, it isn't actually a demotion).

Jupiter and Saturn moons have more to say about themselves than Pluto will. The discoveries are amazing because we know so little about Pluto. And it doesn't look like there are any Palanians on it.

Things I think will be unique to Pluto we won't be able to do a good enough job on: its version of weather. We won't be there long enough to make any detailed analysis to learn about the phenomena.

What we could do is learn enough to make a package to GET that information.

Call me an inartistic nerd, but don't see why we can't just come up with a 1-N or A-X classification for the things that orbit stars. It strikes me that large gas giants, rocky worlds like ours, round belt-dwelling objects and the smaller stuff are different enough that we could just call them Type I planetoids, Type II planetoids, Type III, and Type IV (et cetera if needed), and be done with it. If many people dislike the current binary categorization because they disagree which of the two categories Pluto should be put into, maybe the solution is to get rid of the binary categorization altogether and go with a category scheme that has a bit more depth and nuance. I mean geez, stars have 7 major classes and we add on additional subclasses for temperature and luminosity. Why don't we do something similar (though I'm not suggesting we need that much depth) for planets?

A rose by any other name, eric.

Oh, we have gas giants, rocky planets, planetoids, planetesimals, ice giants and asteroids.

The definition of whatever "type III" (or whichever "planet" gets into) won't include pluto, whereas TNOs will exist in another type.

Precisely what would putting roman numerals mean for the difference?

@21: physically, nothing. I just find it to be a simpler and less confusing nomenclature. They're all stars and we may have names for some types (like brown dwarves), but calling M-types "suns", O-types "stars," and G-types "stellar objects" would be a needless confusion of fairly synonymous terms. Yet that's what we're doing with planet, planetoid, world, etc... Trying to use fairly synonymous terms as separate and distinct categories. Just admit the words have overlapping usage and call them type I planets, type II planets, etc. instead.

Of course YMMV. This is an issue of preference, not anything more, but my preference is for using a set of letter or number designations for different types of bodies rather than wrangling interminably over how the word 'planet' is to be used in the English language.

Yeah, but we call them stars (generic) with specific names of main sequence, Red/Blue/White giants, Red/White Dwarf.

The classification you're talking about is spectral classification and it isn't specifying a certain star.

Indeed, a metal rich or metal poor star gets a different classification despite being the exact same mass. So it doesn't describe the physical object's phenomena, only the visible light spectrum.

So not the same.

It really bothers me that Deimos and Titan are both known mainly as "moons" having nothing in common except orbiting something with the right mass for a limited time, while Triton, Charon and Pluto aren't even all "dwarf planets" (let alone planets) when they are clearly part of the same family of objects.

The orbital clearing requirement is hopelessly ambiguous, we already know that it will require changing for rogue planets, and it assumes and perpetuates among the public a static, clockwork-like conception of planetary systems, while with every new discovery we understand them as highly dynamic and ever changing. Astronomers are very good at counting and classifying stars, why exactly can't we have 200 planets in this solar system? Alan Stern came up with the lambda parameter used by IAU to clumsily justify the orbital clearing requirement, yet he is totally against the IAU definition, and favors calling them all planets, and subclassing them as uberplanets, unterplanets, satellite planets, etc... What's wrong with something like that?

Even according to Mike Brown calling "dwarf planet" something that is not a planet is a dumb idea: dwarf stars are still stars. It appears to me that you want to ignore that classification for the completely ad-hoc reason that it contradicts you: even keeping the mass constant a planet over time can become a satellite planet if captured, just like a star over time can become a giant star or a dwarf star. If stars can be subclassed by spectral analysis planets surely can be subclassed by orbital characteristics. Stars can still be co-orbiting in binary systems, nobody cares where they are or what they orbit, they are still stars, while there is no room for co-orbiting planets, only for co-orbiting dwarf planets. It is infuriatingly inconsistent.

More arguing on the definition of planet on this blog:

By Archibald Tuttle (not verified) on 16 Jul 2015 #permalink

Not only is Pluto small, it is getting smaller: icy objects on the inside edge of the Kuiper belt lose more mass by sublimation at perhelion that they are likely to capture by meteor impacts due to their low gravity field .

Kuiper bodies are further abstracted from planetary behavior by the wierd behavior of materials at the extreme cryogenic temperaures prevailing so many AU from the sun.

It's too cold for solid materials to experience the annealing phenomena and recystalization that mitigate radiation damage in Earthly rocks and ice, so surfaces are liable to be slowly stained and rendered amorphous by cosmic ray and solar wind particle tracks , While this phenomenon is only seen in naturally radioactive "metamict" minerals on Earth, such cold glass transitions affects practically everything at Kuiper belt temperatures , because in the cryogenic regime , the thrermal conductivity of rock froming matrials is also amplified by the low phonon density .

@18, WOW
totally agree with you on the ' package '. The thirst for information drives us on.

Pluto self-identifies as a planet, shouldn't we just all agree to call it a planet?

"Pluto self-identifies as a planet"

It doesn't

"shouldn’t we just all agree to call it a planet?"


Even according to Mike Brown calling “dwarf planet” something that is not a planet is a dumb idea:

Good. Did you have any reason whatsoever to let me know what Mike said?

dwarf stars are still stars.

They're still not dwarf planets.

even keeping the mass constant a planet over time can become a satellite planet if captured

In which case its name will be changed.

Just like the child will be called a different name when they grow up.

Where is your problem?

Hell, what the hell are you intending to solve here?

Oh well, we are commenting on a post arguing that not calling Pluto a planet might be a tad reductive, i pointed out that the IAU definition is lacking for several reasons that you simply ignored, i presented expert opinions that you simply disregarded, i linked external discussions that you simply didn't care about.

A reasonable person might understand that we are discussing the correctness and usefulness of the official definition of planet, decide to engage constructively in the conversation if interested or go away doing something more productive. Yet you continue to blindly apply the official definition as if it was an argument itself, completely oblivious to the fact that it obviously has problems. Are you a robot or something, or are you simply streetfighting against american exceptionalistm? (hint: i'm not american).

And what is wrong with you and analogies? An adopted child is still a child, an orphan child is still a child. Those are proper comparisons to a planet changing its position in space relative to other objects. Even by age alone a child is still a young person, like an adult or an elderly, but unlike an embryo: a planet can be born by accretion from an asteroid, and later become an even bigger planet of a different class. And i still don't understand how you can disregard the star analogy, bring up the child analogy and at the same time complain at #11 that comparisons to other scientific fields like biology are not relevant. Can't you at least try to be consistent?

By Archibald Tuttle (not verified) on 16 Jul 2015 #permalink

i pointed out that the IAU definition is lacking for several reasons that you simply ignored,

No, you gave reasons that were wrong.

There's a huge difference.

An adopted child is still a child, an orphan child is still a child.

A child who passes their 25th year is not a child, though.

I can play your silly game too: a commenter who refuses to engage the argument, rejects the opinions of experts and comes up with irrelevant analogies just to piss off his peers is most probably a troll, and yet he is still a commenter, therefore you are wrong.

By Archibald Tuttle (not verified) on 17 Jul 2015 #permalink

I've already looked at your arguments. They are not valid.

The IAU looked at the same arguments,. THEY decided they weren't valid.

If you don't want to accept a vote, start your own international astronomical society.

It was an analogy. You just don't like it therefore misunderstand and misrepresent it.

We change the names of things when the situation it is defined under changes which classification it resides in.

So when a wandering planet gets captured, it's no longer a wandering planet.

Where is the problem in changing the name where it changes its situation?


Yes Virgina there is a Santa Clause

By Peter Gutman (not verified) on 17 Jul 2015 #permalink

@32 Archibald Tuttle
...sadly too was an inability to discuss it with a certain levity as well.

Sorry, you seem to have missed the joke.

You are all overlooking a more significant question: Who really "killed" Pluto? Two people were named above, and I've heard Neil Degrasse Tyson called "the man who killed Pluto." In metaphorical terms, was it a contract killing or a mob "hit"?

By Cyndi Kurland (not verified) on 26 Aug 2015 #permalink

Too bad my comment was removed--not highbrow enough, as if the others added so much to the conversation--because I came back to add that Google gave me the answer. It was a "mob hit," Tyson admits he "drove the getaway car."

By Cyndi Kurland (not verified) on 26 Aug 2015 #permalink

"Who really “killed” Pluto?"

Nobody. Certainly not just "two people".

"Too bad my comment was removed–not highbrow enough"

Or maybe you're just talking rubbish.