Dwarf Planets Accumulate In Outer Solar System

An anonymous reader tips a piece in Australian Geographic indicating that Pluto may be in for another demotion, as researchers work to define dwarf planets more exactly. "[Australian researchers] now argue that the radius which defines a dwarf planet should instead be from 200–300 km, depending on whether the object is made of ice or rock. They base their smaller radius on the limit at which objects naturally form a spherical rather than potato-like shape because of 'self-gravity.' Icy objects less than 200 km (or rocky objects less than 300 km) across are likely to be potato shapes, while objects larger than this are spherical. ... They call this limit the 'potato radius' ... [One researcher is quoted] 'I have no problem with there being hundreds of dwarf planets eventually.'"

Comments

Proper nomenclature

[BadAnalogyGuy]

The preferred term is size-challenged planets.

Re:Proper nomenclature

[Waffle Iron]

The preferred term is size-challenged planets.

That projects too much negativity. The new recommended term which looks to a more positive future is: "Presently Accreting Planets".

Re:Proper nomenclature

[John Hasler]

> The new recommended term which looks to a more positive future is:
> "Presently Accreting Planets".

That's not so positive if you live on one.

Re:Proper nomenclature

[saider]

All planets are presently accreting. Go outside tonight and look for accretion events (AKA shooting stars).

Re:Proper nomenclature

[Heed00]

Radially challenged.

Re:Proper nomenclature

[blackbear]

Pluto may be a dwarf planet, but it has a LONG orbit.

Re:Proper nomenclature

[jonadab]

Why not just officially label them "developing planets" and have done? Then people who don't care about being politically correct can go right on calling them "third world planets".

Let it go

[sznupi]

Planets were in the past, for example, emissaries of the gods. The Moon was considered a quite distinct body. Epicycles and heavenly sphere have also went away.

Re:Let it go

[john83]

Actually, our own moon is a planet according to their definition - it's over 3000 km across. As I understand it, it's not currently classified as one because the earth-moon system's centre of gravity is inside the earth.

Re:Let it go

[Theuberelite]

A dwarf planet must "not be a satellite of another planet" so our moon does not count as a dwarf planet according to the IAU.

Re:Let it go

[budgenator]

The IAU’s definition decrees that a dwarf planet must orbit the Sun, must not be a satellite of another planet, must not have cleared its orbit of debris (like larger planets do) and it must be of sufficient mass to assume a nearly round shape. They also state that dwarf planets should be of a certain brightness, which is only possible with objects with a radius of more than 420 km.

The moon orbits the Earth, not the sun.

Re:Let it go

[nottheusualsuspect]

As I understand it, it's not currently classified as one because the earth-moon system's centre of gravity is inside the earth.

The moon orbits the Earth, not the sun.

Isn't that what he said?

No, seriously, doesn't

"the earth-moon system's centre of gravity is inside the earth"

mean the same as

"The moon orbits the Earth"

?

Or do I just read it like that as I already knew it, therefore I automatically draw a conclusion?

Re:Let it go

[Nadaka]

The moon does not orbit the earth, nor the sun. The earth doesn't orbit the sun for that matter.

The earth/moon system orbits its barycenter and that barycenter, the sun and barycenter of the other planet systems orbit the combined barycenter of the solar system.

I am technically correct, and that is the best type of correct.

Re:Let it go

[Red Flayer]

I am technically correct, and that is the best type of correct.

No, that is the worst kind of correct. It does not allow one to make up different interpretations to confirm our pre-existing beliefs. The best kind of correct is when the correctness leads to the maximum of amount of feel-good among believers.

*This post is ambiguously serious. This is the best kind of serious.

Re:Let it go

[GigaplexNZ]

must not be a satellite of another planet

This point emphasizes it better.

Re:Let it go

[Teancum]

So what is the point of the heliocentric definition of a planet?

I don't mind if Io, Europa, Ganymede, Callisto, Titan, and other similar bodies are all classified as "dwarf planets". It would be an excellent definition.

Heck, I think Mercury ought to be classified as perhaps a "dwarf planet" and that a line be drawn between planets that are merely large enough to become spherical and those which can hold an atmosphere. Another category ought to also include those planets for which a majority of the mass is gaseous (aka the "gas giants"). BTW, I would put Titan in the same classification as the Earth, Venus, and Mars under this non-heliocentric classification.

This is only going to become much more of an issue when the exo-planets are studied in detail, and we start to discover smaller non-stellar bodies that don't even orbit stars of any kind except the center of the galaxy. I'd even be willing to wager that the number of these objects that don't even orbit stars that are "planetary sized" outnumber those objects which orbit stars under a reasonable definition of what constitutes a planet.

The current IAU definition really is a poor solution, particularly if they keep tweaking the definition in all sorts of weird ways.

Re:Let it go

[MartinSchou]

The moon does not orbit the earth [...] The earth/moon system orbits its barycenter

And where, pray tell, is the Earth/Moon barycenter?

Why, Nadaka, it's 1,710 km below the surface of the Earth. I'm pretty sure that qualifies as "The Earth".

The Earth/Sun barycenter is below the surface of the Sun (449 km from the center), and I'm pretty sure that qualifies as "The Sun".

So you're not technically correct in saying that the Moon doesn't orbit the Earth or the Earth doesn't orbit the Sun. But you are, technically, an idiot.

Re:Let it go

[Tablizer]

I bought a Russian space trip to this "Bary's Center" place that's allegedly so important, and it turned out there was nothing there. No refund allowed either. Thus, as far as I'm concerned, it doesn't exist! Bary moved, folks.

Re:Let it go

[interiot]

What happens in Titan's neighborhood is determined mainly by the gravity of Saturn. Everything that happens now (its orbit, what impacts it) is dominated by Saturn's gravity, and everything about Titan's past (formation out of the dust disc that circled the early Saturn) was dominated by Saturn's gravity.

It's not heliocentrism, it's a recognition that gravitational relationships are hierarchical. That's true from moons to plants to stars to galaxies to clusters.

Going in the other direction, there's debate about whether rogue planets (planet-like bodies that orbit a galaxy but aren't attached to any star) can be properly called a "planet".

Re:Let it go

[Red Flayer]

Shame on you for not recognizing a Futurama reference. Please surrender your geek card immediately

Aw, crap. Please, can I get an exception this one time? Please?

I've been a little busy building and programming my fleshy robot companion, so I'm behind a couple seasons on watching Futurama and other nerd canon.

I promise I'll get caught up as soon as I get the hip actuators working properly.

Re:Let it go

[Teancum]

Although there certainly deserves to be a classification note of some kind that identifies those objects which are dominated gravitationally by some other body, I don't think it really is well deserving to be so hard-nosed about dismissing Titan as a planet here either. With the sole exception that it happens to be dominated by Saturn rather than the Sun, it really does fit every other conceivable definition of a planet, and certainly would be called one if it were merely orbiting the Sun.

I do envision that a problem is soon going to develop going the other direction too, in terms of the very small bodies of the Solar System. We have asteroids as a very loose definition of being something roughly inside the orbit of Jupiter that is not really planet sized. A slightly better definition is something smaller than a dwarf planet. Just how small of an object can something be to still be classified as an asteroid? Something the size of a city, a house, a sofa, or a fist (to give some rough scale equivalents)? Does a grain of sand count as something which should be named and given a classification number in the IAU minor planet catalog? We are very nearly at that point of absurdity right now.

I could argue that perhaps a legal definition might even be eventually adopted, where something formally classified as an asteroid might have some sort of "protected" status but smaller object can be manipulated and refined into manufactured good. When people start to get serious about mining asteroids, it will become a big deal. The date that will start to happen is sooner than you think.

BTW, stars are classified based strictly on its spectral type and raw physical characteristics, not by its particular relationship with other objects in the universe. Why should planets be different?

Pluto is pissed

[tpstigers]

And is forming a gang. We could be in big trouble here.

Re:Pluto is pissed

[thijsh]

So that's why mother earth never goes out anymore, she's afraid of the gang of dwarf planets hanging around the neighborhood.

Re:Pluto is pissed

[GodfatherofSoul]

A cold-blooded gang of hoodlums cruising around the neighborhood waiting to drop in unannounced.

Re:Pluto is pissed

[jimbolauski]

We represent the dwarf planet guild
the dwarf planet guild the dwarf planet guild
I just hope Pluto doesn't hang itself.

The gang shall be called

[spun]

Snowy cold and the several dwarfs.

Demotion?

[K-Mile]

Why is Pluto in for another demotion? It still fits the proposed Dwarf Planet description, right?

Re:Demotion?

[reverseengineer]

It wouldn't be demoted in terms of moving out of its present category. I think the sense is more that the class of dwarf planets, which now comprise only five known objects (Ceres, Pluto, Haumea, Makemake, and Eris) would admit many more members if the minimum radius necessary for the category were revised far downward. The argument then is that the category would be somehow less "special" if there were hundreds or thousands of dwarf planets instead of a handful.

At least from an aesthetic viewpoint, I actually like this proposed new definition though- the size at which an object forms a spherical shape under its own gravity seems like a significant transition. I feel that if an icy sphere the size of Enceladus were discovered out in the Kuiper belt, an assignation of "dwarf planet" would be logical, but such an object would be considered too small under the current IAU definition.

Re:Demotion?

[GigaplexNZ]

It's not demotion if you are purely going via the name of the title, but it is demotion if you are going by what the title actually means.

Re:Demotion?

[jellomizer]

I was thinking the same thing too. A demotion in the sense that you got other people promoted to your level and you got nothing.

I had no idea.

[Dr Caleb]

I knew where the Planet of the Apes was, but I had no idea where the Planet of the Dwarfs was.

Thanks!

nobody tosses a dwarf planet off the list

[jollyreaper]

Plutoids ain't got no reason to live.

This

[Hognoxious]

This is just totally fucking unfair. Leave Pluto alone!!!!!

Better name

[jlebrech]

Just name them potato planets.

Re:Better name

[Kaki Nix Sain]

"Potanets"

Bad Astronomy, Bad Taxonomy

[syousef]

I for one couldn't care less what category Pluto falls under. Planet, Dwarf Planet, Pototoid, Potato Chip. Who cares. I have no emotional attachment.

What I do care about is bad science and bad classification. The current definition stinks. The problems I have

1. A 'dwarf planet' is not a subclass of 'planet' as one would expect from the name. It should have been named something different.
2. The definitions refers to our the sun. Not the star which the planet orbits but 'the sun'. That makes it sound like extrasolar planets are not planets either.
3. The definition of planet requires that the body has cleared it's orbit. So while it is forming early in the solar system it is not a planet then one day "poof" by magic we have a planet.
4. The draft proposal was nothing like the final proposal. The definition was passed on the last day of that IAU conference when lots of scientists had already gone. That suggests a political pissing match rather than well thought out science.
5. The definition is not consistent with what had been taught for decades, and there was no good reason for that.

I have an Astronomy degree that I did for fun and that I have never used professionally. I lost all respect for the IAU on the day they released their crappy definition.

Re:Bad Astronomy, Bad Taxonomy

[dkleinsc]

I'm fairly certain the 'dwarf planet' classification was so-named because it was a foolish compromise with those who wanted to believe that Pluto was still a planet, because they'd been taught for generations that there were 9 planets. If I remember correctly, changing Ceres from being a planet to an asteroid to a dwarf planet wasn't anywhere near as controversial.

Re:Bad Astronomy, Bad Taxonomy

[GigaplexNZ]

The definition of planet requires that the body has cleared it's orbit. So while it is forming early in the solar system it is not a planet then one day "poof" by magic we have a planet.

The moon still keeps getting in the way of the Earth. Perhaps we should demote ourselves too.

Re:Bad Astronomy, Bad Taxonomy

[unixan]

2. The definitions refers to our the sun. Not the star which the planet orbits but 'the sun'. That makes it sound like extrasolar planets are not planets either.

Given that we don't have much information about extrasolar planets yet, making up such definitions is bad science in general, not just bad astronomy.

We aught to survey another system by probe before determining whether our local definitions apply to other systems. Especially, for example, rules for small objects determined by politics.

Re:Bad Astronomy, Bad Taxonomy

[Jiro]

Specifically referring to our sun and not to any primary star was done on purpose. After all, there's really no way you can tell whether something in another solar system has cleared its orbit. If you applied the definition to other systems you'd never be able to know if anything there is a planet.

Re:Bad Astronomy, Bad Taxonomy

[Sparklepony]

An object can be "cleared" from another object's orbit without actually leaving the physical proximity of the other object. It suffices that the dynamics of the situation are such that there's no chance that the two objects will ever collide. This means that both moons and objects in resonant orbits (eg Jupiter's Trojan asteroids, which are in a 1:1 resonant orbit, and Pluto, which is in a 3:2 resonance with Neptune) are considered "cleared" from a planet's orbit.

There actually are more rigorous ways of defining and measuring the degree to which a planet has cleared its orbit, or the degree to which a planet is capable of clearing its orbit. You can see a good summary of them here: http://en.wikipedia.org/wiki/Clearing_the_neighbourhood. I expect the Stern-Levison method is the main one the IAU had in mind since the paper describing it was presented to the IAU back in 2000. It's a mathematical formula that's applicable to any planet that you know the mass and orbital period of and gives an objective value for how capable it is of clearing its orbital neighborhood. Plotting the Stern-Levison parameters for the major bodies of the Solar System shows an orders-of-magnitude gap between the planets and the dwarf planets.

The IAU already has a separate working definition for what constitutes an "extrasolar planet", BTW: http://www.dtm.ciw.edu/boss/definition.html. It says that "The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System." So probably the 2006 IAU definition of Sun-orbiting planets already applies to extrasolar ones even though the 2006 definition explicitly required planets to orbit the Sun. It just doesn't matter much yet because we generally can't detect anything small enough around other stars to be considered borderline.

Re:Bad Astronomy, Bad Taxonomy

[GigaplexNZ]

Wow, that was a rather informative way to destroy an attempt of a joke.

Re:Bad Astronomy, Bad Taxonomy

[Teancum]

Which is precisely why the definition including a reference to Sol is a bad definition. Taxonomy of astronomical bodies should be made based on the physical characteristics of that body, not the evolutionary state of whatever place it happens to be located near.

I still don't get why Mercury is a planet, yet Titan isn't. Or better yet, why Mercury isn't a dwarf planet either? This was an arbitrary decision of an arbitrary definition that in the long run is going to need an adjustment when other things show up. Indeed I would argue that the definition of clearing its orbit really only applies to Mercury, as the other "major planets" all have atmospheres that could fit a better definition of a planet. Oh wait.... Titan has a substantial atmosphere too, and they didn't want to give that body a "promotion"?

As for how you can tell whether something in another stellar system has cleared its orbit, I would presume that would imply sending a probe to that star system and checking out the planetary bodies when that probe arrives. That kind of astronomical observation is merely going to take a few centuries before NASA or some other similar agency decides to make such an effort.... the definition itself doesn't necessarily have the teeth to cope when new discoveries are found (and have been found!)

I say we stick with the good an ancient definition of planets, which includes the Moon and the Sun, but not Uranus or Neptune. Something like "Visible objects that 'wander' in the sky". Then again the Space Shuttle and the ISS would fit that definition of a planet. See what a reference frame can do to really muck things up?

Re:Bad Astronomy, Bad Taxonomy

[Tablizer]

Classifications of such are mostly to make conversation and communication simpler. Technically, they are probably imprecise, but it only matters if used in the wrong context. Language is a useful lie. Trying to force language to be thorough and concise at the same time is often an impossible goal. If you want to know the size, weight, and shape of a specific "thing" in space, then ask.

That being said, I'd personally vote to call them "Kuiper belt objects" (KBO). That way one doesn't have to worry about classification conflicts with the "asteroids" between Mars and Jupiter; and size doesn't matter[1]. If they discover an Earth-sized KBO, so be it. It's just a big-ass KBO.

[1] No, I'm not subconsciously projecting personal insecurities into this.

Re:Bad Astronomy, Bad Taxonomy

[syousef]

Yes classification is hard, human language is imprecise, but there's no need to throw the baby out with the bath water.

Re:Bad Astronomy, Bad Taxonomy

[syousef]

I think you'll find that what's actually occurred here is that just as a "dwarf planet" is technically not a planet, an "extra-solar planet" also doesn't fit the definition and therefore also isn't technically a planet.

A good definition makes things clearer. This definition clearly fails by that criteria.

Also my point about clearing the orbit was more about the formation of the planet. Even with a solid body formed and most of it's mass present, if it's still part of the solar disk and still subject to bombardment it is not a planet. What the hell it is during the early part of solar disk evolution isn't clear.

Re:Bad Astronomy, Bad Taxonomy

[Sparklepony]

Extrasolar planets have a whole other definition that was established just for them already. If you've got a problem with that one, arguing about the 2006 definition of solar planets instead is kind of pointless. The extrasolar one is labeled as a "working" definition anyway so I doubt there's much controversy over the fact that it's not very good yet.

The Stern-Levison parameter of an object is unaffected by whether it's being bombarded, it's only meant to show the object's orbit-clearing capability. Unsurprisingly, now that billions of years have passed since our solar system formed, objects with high orbit-clearing capability happen to have orbits that are clear of other debris. But Earth would have had the same Stern-Levison parameter four and a half billion years ago as it does now, so by that measure it's quite clear what it would have been.

It's probably not a productive exercise to try to figure out the exact moment when a single grain of sand lands on a dwarf planet and makes it into a planet. The current definition has a clear bimodal distribution to it which is probably the best we can hope for for something like this.

Re:Bad Astronomy, Bad Taxonomy

[mdwh2]

But then there was the foolish compromise in order to not simply add more planets to the list, because they'd been taught for generations that there were only 9 planets.

Re:Bad Astronomy, Bad Taxonomy

[John Hasler]

What was foolish was attempting to come up with an algorithmic definition of planet instead of accepting that a planet (in our solar system) is any one of the nine objects on the list of planets.

Re:Bad Astronomy, Bad Taxonomy

[John Hasler]

> I say we stick with the good an ancient definition of planets, which
> includes the Moon and the Sun, but not Uranus or Neptune.

And not the Earth.

I say we stick with the traditional nine, and define "planet" as an object on the list of planets.

Re:Bad Astronomy, Bad Taxonomy

[LihTox]

They should just call Pluto an "honorary planet", give it a plaque and a gold watch, and be done with it.

Re:Bad Astronomy, Bad Taxonomy

[syousef]

So an extra-solar planet is not a planet. A dwarf planet is not a planet. Why the hell does the word planet appear in the definition? It's unnecessarily confusing and you are defending the indefensible. This is exactly the kind of stupidity that gets scientists labelled as socially incompetent propeller heads.

Another false & sensationalist headline

[SirGarlon]

TFA says nothing about re-classifying Pluto out of the "dwarf planet" category. In fact there is no FA, only a picture. And it's quite obvious from that picture that Pluto far exceeds the 300 km radius that is the proposed threshold.

Nice

[proxy318]

Dibs on the band name "Potato Radius".

Re:Nice

[blacksmith_tb]

Just what I was thinking! And now you've got dibs on it. Oh well. "Potato Radius UK" just doesn't have quite the same ring.

In space, no one can hear you sing.

[moviepig.com]

Dwarf Planets Accumulate In Outer Solar System

"Heigh-ho, heigh-ho..."

Re:In space, no one can hear you sing.

[gyrogeerloose]

"Heigh-ho, heigh-ho..."

And in this case, the vacuum is a merciful thing... ;-)

Re:In space, no one can hear you sing.

[517714]

Disney will get you for that. They already managed to keep Pluto from being referred to as a planet.

Re:In space, no one can hear you sing.

[mcgrew]

I heard a pimp singing that last week.

"High ho', High ho', it off to work she go..."

How the hell?

[Alsee]

Dwarf Planets Accumulate??

What, is our solar system some sort of drive-by dumping ground for other stars' litter?

-

Re:How the hell?

[eleuthero]

all current phase solar systems are... at least, if we get asteroids and other higher number elemental conglomerations from previous supernovas we do

Is a tomato a fruit or a vegetable ?

[Alain Williams]

Does it really matter ?

How science defines things is not always the same as how the ''common man'' calls things. For instance: botanically a tomato is a fruit, most of my friends think of it as a vegetable; also botanically rhubarb is a vegetable, most of my friends think of it as fruit.

To most people, myself included, pluto will remain a planet - I don't care what the astronomers think!

Re:Is a tomato a fruit or a vegetable ?

[NeoSkandranon]

People think of rhubarb as a fruit? It's like red celery...

Standardize Units!

[Sechr Nibw]

First, this isn't a call to standardize either the SI system or the metric system globally.

Anyone else notice that the image in TFA has everything in miles, but then the little information in the actual article was always in kilometers? Also, while the article mentions radius, they never specify that's what the image is showing, nor do they state the radius of any of the things in the image, so, without outside research, you couldn't tell where Pluto falls in their discussion for certain, even if you're adept at converting miles to kilometers!

I vote "Hemorrhoids"

[Tablizer]

after all, we've got "Uranus". Since the jokes aint gonna stop, let's just go with the theme and see where it takes us.

"I have no problem with there being hundreds...

[RichiH]

...of dwarf planets."

Well, neither do I. Noodles are fine for dinner, too.

That in and as of itself does not count very much unless said researcher is shown to be a leader of opinion and/or part of a larger consensus. I don't claim to know either way, simply wanted to point that out.