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Harvard astrophysicists favor revival of Pluto the planet

Off-topic, everyday life: I returned from an "extreme reunion" with 1st and 2nd grade classmates whom I haven't seen (in almost all cases) for over 32 years because I moved to another school (moreover, the class I left got slightly expanded afterwards). Pretty cool to meet someone who was arguably one's best friend at age of 7, and so on. How many of you have ever attended such a reunion after a longer break than me? ;-)
Eight years ago, 9,000 astronomers gathered in Prague, the Czech capital. A somewhat important vote took place on the last day when most people had left. Only 400+ folks voted that Pluto would no longer be a planet.

Eris vs Pluto. Eris is a bit larger but otherwise they're almost "twins". It's hard to claim that Pluto has a "virtue" that Eris doesn't.

This 2006 vote is known as the Fourth Defenestration of Prague. The word "defenestration" comes from Latin and it means "throwing something or someone out of the window" in Prague. ;-) The first one took place in 1419 (Hussites threw 7 members of the city council), the second one in 1618 (regents were thrown by some wealthy protestants for sectarian reasons), and the third one took place in March 1948 when communists who took complete power 2 weeks earlier apparently threw out Jan Masaryk, the foreign minister and son of the founder of Czechoslovakia TGM.

I have been trained and hardwired to love Pluto as the ninth planet, much like most of you. It's a matter of terminology and people may distort the meaning of the planet so that the list is whatever we like. Nevertheless, at the end, I would prefer some meaningful scientific basis for such terminology.

The post-2006 definition of a planet says that it is an object

* orbiting a star, or a star remnant, and directly (not like a moon of something else)
* massive enough to be rounded due to its gravity
* light enough to avoid fusion
* massive and old enough to have cleaned its vicinity of junk (planetesimals)

It sort of makes sense. Pluto, now a dwarf planet, failed to fulfill the last condition. There's just too much junk on its orbit around the Sun.

Whether one decides that the definition of a planet should be more inclusive or less inclusive is arguably debatable. But what seems independent of these sensible choices is that Pluto simply doesn't make it among the 9 most important bodies orbiting the Sun – whatever criterion you pick as long as it is simple enough.

In particular, Eris, another dwarf planet, is round and 27% heavier than Pluto. It even has its own moon, Dysnomia. Like Pluto, it has a frozen surface with nitrogen ice and methane. Eris' potential "defect" is its too high relative eccentricity 0.437. The orbit is very far from a circular one. You could use it in a new definition of a planet, to eliminate Eris. But that would not be helpful to eliminate Ceres, another dwarf planet, whose orbit between Mars and Jupiter is nearly circular. Just for the completeness, Haumea's orbit is very similar to Pluto's (both the semi-axes and eccentricity) but its mass is 1/3 of Pluto's.

Note that these small objects' mass is estimated once you find some satellites orbiting them. Two satellites are known around Haumea. However, Makemake, the last dwarf planet among the five, has no known satellites which also means that we don't know Makemake's mass. There are complications like that.

Given the annoyingly messy classifications schemes for the pieces of rock in the Solar System etc., I would think that a step towards some "logic" and "rules" is a step in the right direction. We've had some recent exchanges with Lisa Randall about the words "meteoroid" and "bolide" (not to mention "meteor" and "meteorite"). There exist ambiguities in the meaning of these words and I think it is unfortunate. The rocks in the Solar System (and other systems) come in very many sizes but there are just several conditions discriminating these objects truly "qualitatively" and it is unfortunate to divide the rocks into finer groups than what is defensible by these "qualitative" differences. Boundaries that cut through the "bulk" and divide objects according to some "quantitative" properties just look awkward, especially because in the case of the marginal cases, one needs to measure an object accurately to decide what "kind of an object" it is.

But to return to the present, Metro, WaPo, Tech Times, and many colleagues told us that there was an event at Harvard-Smithsonian some two weeks ago. A vote over there made it clear that the folks in that institution – and participants of conferences that would be organized by someone at Harvard – would endorse the revival of Pluto as a planet.

The articles don't quite explain whether they want to change the definition of a planet, or define planets just by the full list – which may be hard, given the fact that lots of experimenters love to study exoplanets these days. It's clear that the "clean orbit" condition would have to be suspended in some way for Pluto to "come back" but we were not told whether they actually want the tons of larger or similar objects to make it to the list of planets as well, or whether they want to add some special new conditions that would reduce this competition.

I would like to know whether this opposition to the "demotion of Pluto" exists among the young people who have never been taught that Pluto has to be a planet. If people like to defend Pluto as a planet just because it's a habit they have been programmed to worship, perhaps because that keeps some of their childhood alive, I think it may be a good idea to jettison that excess baggage in order to make progress.

No imminent gathering that could change the status of Pluto is being prepared. On the other hand, it can't be excluded that the status of Pluto will change again as soon as 2015.

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reader Mark Berton said...

I never liked the demotion of Pluto simply because it didn't make sense to me as a layperson. Having read this, I am even more against it. If it is true that 400 out of a conference of 9,000 held a subversive vote when many were leaving for home, that means just 4% of the group was represented. I was under the assumption that the demotion of Pluto was more of a broad consensus of the scientific community as a whole. I am all for reinstating Pluto. The last criteria for defining a planet regarding clearing junk is a pretty arbitrary one. I would support its demotion if there were a broad vote among the scientific community that represented a true consensus and not a seemly sophomoric prank among a group of rogues hanging out after all the cool kids went home ;-p

reader Rehbock said...

The argument in favor of reinstating Pluto comes from Harvard. Its alumni are correlated to success and power.
The planet Pluto is identified by as the planet of power and success. perhaps Harvard should adopt Pluto as its new mascot.

reader Luboš Motl said...

It's all great but do you understand that if you simply drop the "clean orbit" condition, there will be many more planets obeying the definition and children will become effectively unable to memorize all the planets in the Solar System?

Our observational tools have become much better so we see lots of "historically unimportant junk". So there is an inevitable tension between the "simple ancient history and legends" and the "scientific picture".

The old planets were simple. Each of them was different. Venus is like an extra star people could observe for quite some time in the morning and evening etc., and an evil sister of Earth. They found another one, the male Mars, on the other side which is still visible almost without anything. Mercury is very close to the Sun, some dirt on the Sun sometimes. Jupiter is visible because it's so large and Saturn even has a ring which is cool. With easy telescopes, one sees a blue Uranus, and Neptune was predicted because of some slightly mismatched predictions of the trajectories, and it was indeed found.

Pluto gets us to the 20th century and the reasons for its discovery are already a bit bizarre. The search for Planet X and they found some random rock Pluto instead and gave it a very catchy ancient name. The name makes it look like it's a part of some harmonic legend or story but this is already a fabrication. When it was being discovered, there was no reason to think it was too special, too similar to the previous planets, unique, predicted, or something like that.

reader Gene Day said...

When I was a small child my mother taught me the mnemonic, MVEMJSUN, in order to memorize the planets. It is pronounced "em-vem-jay-sun” but it became "em-vem-jay-sun-pee”. I’m happy that the pee has been eliminated.

reader strictly speaking... said...

Imho, I think the terminology would be clearer if "planet" became the more general term and planet was the special case.

It would certainly be more convenient for casual speech if "planet" was a general term for any body(including moons and rogue planets) large enough to be in hydrostatic equilibrium/round, and the current "planets" were termed "major planets" or something like that to denote that they orbit the sun and have cleared their neighborhood. Or maybe let "planet" denote orbiting around a star, and "planetary body" for the more general class.

Either way, the existence of a good term for more general classes of objects seems quite necessary to me, and ideally it should follow proper grammar unlike the current terminology.

reader Mark Berton said...

I understand that, but I would counter that life is full of things that deviate from the rules because of history. In my forte, the word "ain't" is considered flawed English; a verb of the ignorant. But if you were to recreate a natural-sounding dialogue for a work of literature, usage of "ain't" might be completely apropos. In fact, look at how often it's used in drama and marketing despite the fact that it's not proper English. Time and usage has accepted this "mistake" into common parlance. In the case of Pluto, I agree with all of your history and your rationale behind the clean orbit rule. It's just my opinion that if that mistake happened today, by all means reverse it and correct the record. Changing the makeup of the solar system after several generations have known it as being so? It just doesn't sit right with me.

reader planet mickey mouse said...

Can't believe money and work/leisure hours are spent on this topic.
Hey you know roman gods don't exist either we should change the planets' names too. I mean are we doing science or religion here.
In other news, is there a precise line where hills turn into mountains?

reader Eric Simpson said...

Pluto should be reinstated as a planet.

It's not simply a matter of whether it meets certain celestial conditions. Pluto had been an icon as a planet for 80+ years. It's long history as a designated planet should have been taken into account, and it should have been grandfathered in and allowed to remain a planet.

reader Luboš Motl said...

Dear Richard, the word "planet" - in "asters planetai" - was first used by ancient Greek astronomers and it means a wondering star.

The word "planet" itself is linked to "wandering" but be sure that people knew that it had to be a round body. We know that these objects are round due to self-gravity today but even if this weren't added, it won't really change anything about the definition.

The word "gravity" in the form "gravitas" in Latin has meant the same force attracting objects in the downward direction for millennia.

I don't miss cultural implications. I just find it essential that they shouldn't play any important role for decisions in science. Your suggestion of distort the scientific terminology in order to help or hurt a particular person - for political or personal reason - is a nice example how toxic it becomes if such political things start to influence science, although it's "just" the scientific terminology now.

Also, I assure you that if one would revert the decision that Pluto isn't a planet, Tyson would become even more powerful. The real reason is that despite all his emptiness, bias, and limitations, he is defending the scientific attitude to these things while people like you defend the medieval superstitions. The more you show this basic conflict, the more people will see Tyson as a savior of the modern world against medieval stupidity (which he is not).

reader Luboš Motl said...

The word "planet" is just a word so discussions about it are just "scientific terminology", not "science" itself.

But be sure that it is scientific and not religious terminology. The word, used from the ancient Greek "asters planetai", meant a wandering star, particular real objects that may be observed in Nature.

The words "Jupiter" etc. are used for real objects, too. Every word that scientists use - and as I repeat, "planet" is primary a scientists' word - is meant to denote something that exists.

Analogously, "Zeus" is a word that also has a meaning, namely a chap who doesn't exist (apologies to this über-guru). It's a piece of literature, traditions, fairy-tales, humanities, and in humanities, people use lots of words for things that don't exist. That's an aspect by which humanities differ from natural science.

The demarcation between "hills" and "mountains" is vague and subjective but this doesn't matter at all because unlike "mountains", "hills" isn't a scientific term at all. Check a list of scientific papers and you will only find "hills" as parts of names of (usually lower, less steep) mountain ranges which is done because people called them "XY Hills". But science-wise, it isn't supposed to mean anything specific.

On the other hand, "planet" is one of the central words in astronomy and it is meant to denote real objects and rather specific objects.

reader Rambo Cnc said...

reader David Nataf said...

A surprising fact, the moon experiences more gravity from the Sun than it does from the Earth, as the Sun is ~300,000 times more massive but only ~400 times further away. Technically, the moon orbits the Sun and not the Earth, so it might actually be a planet by the above definition.

reader Luboš Motl said...

The gravitational acceleration goes like GM/R^2 and the Sun's contribution is indeed larger than the Earth's at the Moon. But the tidal forces scaling like GM/R^3 are already greater from Earth's, and they're really more representative of "who matters for the motion".

A natural satellite or a moon in general

is defined by its orbiting around a larger body. Well, it always orbits around the common center-of-mass, and the latter is also moving, but in the terminology, it's just done in a particular way. Take the largest thing in the system and look what blocks of matter orbit it, and what is the largest object in each group.

Be sure that the definitions of the planet and the moon are indeed such that the usual planets and the moon obey these definitions, and it's your fantasies what the definitions should look like that probably lead you to a different, wrong conclusion.

Pluto's moon Charon has radius over 50% of Pluto's, so it's really "comparable". You may get even closer to parity but you will never find any biplanet for which the mass equality will be "exact", so only "minor improvements" of the Pluto-Charon situation may occur.

reader David Nataf said...

1) Can you explain why you think tidal force is the more natural delimiter?

2) What's sensible for this solar system may not be the best set of definitions, as the definition of a planet should apply to other solar systems too. I think it's critical that the definitions be generalisable.

What would happen if a Jupiter-like planet had a Neptune-sized planet in its L4 or L5 Lagrangian point? Would that be two planets or just one? Because that's a stable configuration.

3) There will never be absolute parity, but if there were a double planet involving say, Earth and Venus, that would be close to enough to parity to consider it a double planet and not one planet and a moon, in my opinion.

We have not seen these kinds of systems yet but there's a case to be made that we don't yet have the sensitivity.

reader David Nataf said...

A lot of things have been icons for 80+ years.

reader David Nataf said...

Related, but not quite the same:

Carter et al.

The ratio of masses is a factor of two, and they heavily influence each other's orbits due to being related. Neither really dominates its own orbit. Thus my biggest complaint is with the "cleared its neighbourhood" aspect, as sometimes (though rarely) neighbourhoods will be shared.


Returning to the example of the moon, in a billion years or so the Moon will have continued to drift outward and the Earth and Moon will then be a double planet. That's not relevant for this system, but given that it will happen here it has probably happened for another system by now.

Admittedly, all of my criticisms can be taken into account if a definition of double planet is appended.

reader Giulio said...

Could it be that Pluto's orbit is not clean yet because it takes more time to clean a longer orbit? Is it going to be clean eventually? Could this be a better criteria? (I'm not an expert, so this could just be nonsense)

reader Luboš Motl said...

Dear Giulio, first, yes, absolutely, a more distant orbit is harder to be cleaned, the discriminant "how much able" is proportional to mass^2 / radius^1.5, see

and No, it is a long-term problem, so Pluto will probably never clean it. It is a necessary condition for the quantity mass^2/radius^1.5 to exceed a certain bound for it to clean, after a sufficient time (another condition), the orbit.

reader HelianUnbound said...

At the very least, Pluto should be promoted back to planet status before the New Horizons flyby next year. Then, after a decent interval has been allowed to elapse it could be quietly demoted to a dwarf again.

reader Luboš Motl said...

1. It depends on the definition of "natural" which can't be settled objectively. But because the Earth's tidal forces exceed the Sun's on the moon, it is a hint that the Earth is a nearby important object that can't be neglected for the gravitational forces on the Moon, so the relationship between the Moon and the Sun isn't direct.

2. All star systems are qualitatively the same. There is no known reason why something should be "totally different" around other stars.

In the L4,L5 Lagrange points of Sun and Jupiter, there are small objects known as Jupiter Trojans. But these simple properties of the Lagrange points only hold because there are just 2 important sources of gravity, Jupiter and the Sun. If there were 3 or more, the problem would be much more complicated, much like the usual 3-body problem, and there wouldn't be stable points like that.

3. If the Moon were as large as the Venus, it would still be a Moon according to a very sensible definition, and your "opinion" would be demonstrably wrong.

reader Swine flu said...

When one says that "Harvard astrophysicists" want Pluto back as a planet, was it a vote in the whole Astronomy Dept and/or by the Smithsonian-affiliated astronomers there, or was it just a small subset? I somehow had the impression that it all took place at a public night of some sort at their observatory, with just a few scientists present.

reader Curious said...

Any thoughts on Sean Carroll's blog promoting philosophy in cosmology?

reader Fer137 said...

"...a planet is and has been for some thousands of years defined as a celestial body that can be perceived from the Earth as moving relative..."

You have removed Uranus and Neptune. At this rate we will run out planets.

reader Swine flu said...

That mission has already been funded, so there is no need for a temporary status upgrade.

reader papertiger0 said...

Neptune isn't easy.

reader David Nataf said...

I have no doubt that the definition will be upgraded as free-floating planets and double planets are eventually discovered in great numbers.You can call that a fantasy, but Iit's inevitable as long as science keeps moving forward. Free-floating planets in particular should be abundantly catalogued by 2018.

This definition was and is great and adequate for our solar system, which is fine for 2006. It doesn't generalize well to other solar systems which show greater diversity (obviously), but with this conversation I'm not concerned at all, having given it some thought, I'm now sure they'll ammend the definitions eventually.

reader papertiger0 said...

Can you define "clean orbit". I'm fairly sure there are fewer bits of debris per cubic kilometer in Pluto's orbit than in Ceres' orbit.

reader Tibor Arouet said...

Offtopic: Salam, Sciama, Witten and Budinich having a chat

reader Smoking Frog said...

Lubos - Re your "reunion" question: Yes. Two years ago I attended the 50th reunion of my high school class, the first I had ever attended. At first It was weird, almost as if my classmates were wearing masks that looked like them, only uglier. In some very few cases the person didn't look at all like what I had expected, not even like the same "type" of person.

Some of my classmates had followed surprising paths in life, and some very few had gone further than I would have expected; e.g. of the latter, a friend of Latvian background (his father had escaped) had achieved great things in the Air Force and retired as a brigadier general. I can't say I met anyone who had achieved less than I would have expected, but I didn't speak with everyone.

reader Luboš Motl said...


reader Luboš Motl said...

Dear papertiger, I can, simply by copying and pasting the 2000 paper which is used for the official definition.

By focusing on a "clean orbit" even though the official condition talks about the verb "cleaning" or "clearing", you are actually missing the whole point!

The initial value of the concentration of debris is completely irrelevant for the condition because, and that's the point, a heavy enough object will reduce this concentration around a short enough orbit "exponentially", so whatever the initial concentration was, it will win in a very short (logarithmic) time.

As a result, after a very short time, the concentration of debris around the (dwarf) planet is actually almost directly dictated by the mass and the radius of the (dwarf) planet, and not by the initial concentration.

For this reason, Ceres has actually cleaned the neighborhood of its orbit about "4 times" more clean than Pluto, see the table

and your expectation is wrong.

reader Luboš Motl said...

Dear David, what you write is complete bullšit. None of the "great discoveries" you are foreseeing - which are not great or new at all - will locate any flaw in the current definitions of those celestial bodies.

reader Luboš Motl said...

Dear David, could you please explain why you expect "great numbers", what it means, and why it matters?

The number of exoplanets that will be known will surely be increasing as we observe more. Agreed?

The percentage of planets that will be seen as parts of "double planets" of almost identically heavy planets will stay extremely small, won't it?

And none of the "double planets" differs from Pluto-Charon too much, anyway, so it's not qualitatively new, is it?

reader David Nataf said...

1) Double planets are probably rare, so when and if they're found, they can either be ignored or included as a small amendment to the definition.

The Kepler mission has yielded a very robust catalogue of planets closer to their star than Mercury's orbit around the Sun (~2,000 planets) and within that sample double planets are rare.

Pluto/Charon I have not thought too much about. If an Earth-Venus pair were found there would be a challenge as to how to form this pair, probably some sort of three-body scattering. It would be a different class of object due to the large difference in mass, and also because they would have had to form in a different place, where silicates are common rather than further out in the accretion disk where ices are common. It might benefit the habitability of those planets by stabilising the angular momentum vector, as the moon does for the Earth.

2) Free-floating planets were thought rare/irrelevant until recently. It turns out they're likely common, and the ability to detect them is about to come on line due to various instruments, like the Korean Microlensing Telescope Network.

The nature paper I linked counted them statistically, soon we'll be able to count them individually.

Thus, the fraction of those planets among total planets will rise from ~0% to ... ~50%? Or maybe 25%, or 75%, we don't know the precise number actually, but recent research suggests they're common, and precision is about to improve by more than an order of magnitude. This is a qualitative change in our understanding as previously the instrumentation to detect these kinds of planets was not available.

Once that number is added to the integral it will change how people model young stars, in terms of their disk mass and initial angular momentum, et cetera.