Pluto Might Be Bigger Than Eris

astroengine writes "Look out, the battle of the dwarf planets is about to re-ignite! During last weekend's rare occultation of a star by Eris, astronomers managed to gain one of the most accurate measurements of Eris' physical size. When three Chilean telescopes watched the star blink out of sight, astronomers were shocked to find that Eris is actually a lot smaller than originally thought. So small that it might be smaller than Pluto. On speaking with Discovery News, Eris' discoverer Mike Brown said, 'While everyone is more interested in the "mine is bigger than yours" aspect, the real science is the shockingly large density of Eris.' The mass of Eris is well known, so this means the object is more dense than Pluto. Does this mean the two mini-worlds have different compositions? Did they evolve differently? In light of this finding, is the underlying argument for Pluto being demoted from the planetary club on wobbly ground?"

Comments

Pluto controversy

[falldeaf]

I remember being so confused about the Pluto controversy. Maybe it's just because I'm not an astronomy nerd but I don't understand the uproar about correcting a miss-classification of a heavenly body... I remember Neil Desgrasse Tyson on the Colbert Report chiming in that it was just a simple fact. Any of you astronomy nerds reading that could explain the emotional reaction? (Not to assume, was it astronomy nerds that were upset? Maybe it was Astrology people that were upset.)

Re:Pluto controversy

[just_another_sean]

I also seem to remember Neil blaming most of the uproar on Disney. Paraphrasing - "if they hadn't named that darned dog Pluto nobody would have cared".

It's hard to tell with Neil how serious he was on that one. :-)

Re:Pluto controversy

[fuzzyfuzzyfungus]

I think that it has a lot to do with the fact that a shockingly large number of people confuse nomenclature with knowledge. Because of that, a fairly fiddly technical discussion over how best to handle astronomical nomenclature hit the popular press as "zOMG pointy-headed scientists don't even know if Pluto is a planet!!!!!"

Naming is not a trivial thing, good nomenclature makes the world a much easier place, crap nomenclature makes it a mess wholly without reason; but either way it seduces people into forgetting that names are simply constructs, assigned for our convenience to bundles of real things. Sometimes, you have to revise the constructs to make the nomenclature better, simpler, more expressive, whatever; but that is very different from changing the bundle of real things and attributes.

Re:Pluto controversy

[sznupi]

At least we can be certain it will pass - I don't see any people lamenting that Ceres, Pallas, Juno and Vesta have lost their planetary status.

Status which they had, for half a century after their discovery. Similar to Pluto.

(for that matter, the same applies to the Sun - it was also classified as a planet at some point)

still not a planet per the IAU

[heptapod]

Size does not matter. Clearing its path matters. Per the IAU Pluto has not cleared its orbital path and can not be considered a planet by the current definition.

Re:still not a planet per the IAU

[molo]

Please compare the total mass of all Neptune-crossing bodies to those gravitationally bound to Neptune. You will clearly find that Neptune has cleared the neighborhood. Neptune has a planetary discriminant of 2.4 x 10^4. A body with discriminant >= 1 is considered a planet.

-molo

Re:still not a planet per the IAU

[Tumbleweed]

Yes, let's come up with a definition that excludes Pluto - that way we can exclude Pluto. Makes sense.

Re:still not a planet per the IAU

[MozeeToby]

Don't forget the real reason that they wanted to change the definition in the first place: current theory predicts that there are probably hundreds, if not thousands of bodies in the outer solar system with basically the same composition and orbit as pluto, and only slightly smaller. There would be no logical reason to exclude those hundreds of bodies from the list of planets without also excluding Pluto, since there is little qualitative difference between them.

Re:still not a planet per the IAU

[Anonymous Coward]

actually, that was the reason they wanted a clear definition of 'Planet'. Pluto not being a planet is the results of those discussion, not the cause.

There would be nothing wrong if the definition included Pluto and all those other similar objects.

That may be true but it's naive to think that the motivation for defining "Planet" didn't color the discussion. The original parent is correct in that this was motivated by the perceived inconvenience of having a large number of bodies that would meet the definition.

The whole vote was a sham in my opinion, because there were other reasonable definitions that got suppressed in discussion because of the issue the parent is raising. The current definition is ridiculous and completely context-dependent, fuzzy, and completely colored by some emotional concern about how many planets there "should" be and how convenient it is for people to ennumerate them. I'd also point out that the hundreds of bodies that caused so much anxiety for so many people haven't become an issue.

Re:No.

[Sique]

The argument was as arbitrary as any others. It was basicly "which property is common to both Pluto and Eris, but not found in the other objects traditionally considered planets?".

Pluto always was a weird object to be called a planet, with his density somewhere in the nowhere between the earthlike planets and the gas giants, and being pretty similar to the large moons of the gas giants.

But only when Eris was found, there was a second objekt thought to be similar enough to Pluto to define a new class of "plutolike objects", which allowed Pluto to be demoted from planet status.

So yes, the classification of Pluto in the class of "plutolike objects" (pardon, "Dwarf planets") seems to be on pretty firm ground, considering there are now more objects known in that class (Makemake for instance), though Eris now seems to be a weirdo within this class.

Re:Cue

[EyelessFade]

Professor: "I'm sorry, Fry, but astronomers renamed Uranus in 2620 to end that stupid joke once and for all."
Fry: "Oh. What's it called now?"
Professor: "Urectum. Here, let me locate it for you."

What does being old have to do with it?

[Valdrax]

Ask just about anyone geeky and my age,, and they'll telll you so: "yes, Pluto is a Fucking Planet, now stop trying to change things".

What does being geeky have to do with being old and too set in your ways to listen to reason?

Give us an argument why the IAU's definitions of a planet and of a dwarf planet are unreasonable. Please avoid any Appeal to Tradition. Also, can you craft a definition of a planet that covers Pluto but not Eris and Ceres other than "just what we used to arbitrarily call a planet?"

Re:What does being old have to do with it?

[GlassHeart]

I fear you fail to understand the reason behind the "demotion." What we call it has never made any real difference in what Pluto does or does not do. The only use of these names is to help us understand them better. As such, the terrestrial planets share much in common, the gas giants share much in common, and Pluto shares little with either group. Thus, if you're saying that all Pluto-like objects should be called "planets", it would make some sense except that there are lots of them. If you're saying that Pluto alone should be a planet while similar objects are not, then that's a far less defensible position.

Re:What does being old have to do with it?

[Beardo the Bearded]

NOT A FUCKING PLANET.

Re:No.

[MozeeToby]

It's perfectly simple: Humans are not animals, Humans are fucking special. Ask just about anyone religious and my age, they'll tell you so: "No, humans aren't animals, now stop trying to change things".

See how ridiculous your non-argument sounds?

Eris is more massive and may still be bigger

[mbone]

Don't give Eris out yet. There was a lot of discussion on the MPML about this.

First, Eris is definitely more massive, by about 28%. They both have satellites with good orbits, so their masses are pretty well determined.

Second, it is not really that clear that Pluto is really larger than Eris. There have been a number of estimates of Puto's size; by the most recent one presented by Angela Zalucha at the DPS meeting (a radius fit to occultation measurements with a new atmospheric model), Pluto and Eris have roughly the same radius within the respective error bars (1146 +-20 km in diameter for Pluto versus 1170 km for Eris).

What is more interesting to me is that Eris is dense and very bright - could something as rare as Deuterium snow be covering its surface ?

Re:No.

[Chris Burke]

The rest of the definition is noise because they failed to define "clearing the neighborhood".

Noise?!

There's FIVE ORDERS OF MAGNITUDE difference in planetary discriminant (mass ratio between the body and all other masses in that orbit) between the highest value for a dwarf planet (Ceres) and the lowest value for a planet (Neptune).

When the difference is a factor of 10,000, there's absolutely no need to create a precise definition, and it would be foolish to do so.

It's like you're arguing that the definition of what is or isn't ocean is useless because it doesn't precisely define at what point of the tide cycle, or how far up a river mouth, it transitions from ocean to not ocean. But the question we're asking is whether Topeka, Kansas is in the ocean or not and the answer is obviously NO!

You might as well argue that it's completely arbitrary to say that you are alive and Benjamin Franklin is dead, because science has not precisely defined the exact line between dead and not dead. Um, I think it's still pretty clear which is which in this case.

You see, if they did actually define "clearing the neighborhood" in a precise manner, that would be the truly arbitrary choice. But when you look at the bodies in our solar system, and you see that there's a small set of objects which outweigh everything else in their orbits by at least a thousand-to-1, and then a great many objects which weigh less than the rest of the objects in their orbit, then yes that actually makes a clear dividing line. You don't have to draw it with infinite precision to see that it's there.

The definiton of Pluto as a planet is far more "noise" than the definition that it isn't. We only called it a planet because we didn't know it was so different from the other ones. It's like when we first discovered Ceres. Only we changed that one pretty quick, even though it's more planet-like than Pluto is.

The goddess of discord

[Mspangler]

The goddess of discord is certainly living up to her reputation.

"So you think you have me all figured out, do you? Heh, heh, heh."

So how long will it take to get there, how big of a dish will it take to get a signal back, and how much plutonium to power the instrument package and radio to find out what is really going on out there?

IAU planet definition is pure junk

[syousef]

When the difference is a factor of 10,000, there's absolutely no need to create a precise definition, and it would be foolish to do so.

Well if the IAU was going for an IMPRECISE definition they couldn't have done better. Science is all about definitions. Precision is important. The current definition is an utter mess. I couldn't care less that Pluto was named after a Disney character. If we want to be precise it belongs in a different category BUT

1) They created pair of definitions where a "dwarf planet" is not a "planet. That is confusing and ridiculous.

2) They mention "the sun" and therefore the definition as written excludes extrasolar planets. So now we have "dwarf planets" that are not planets and "extrasolar planets" that techincally also are not planets.

3) The clearing the path part of the definition is an arbitrary requirement and a kludge. It is possible we will discover extrasolar planets that cross each other's orbits in a stable way. Fortunately extrasolar planets aren't planets anyway.

There are other things wrong with the definition, but lets just leave it at that shall we? The definition is beauracratic and in terms of science it is PURE JUNK. Science is about understanding things. We humans do this by classifying them, so definitions are important. However in this case everyone was more focused on whether or not Pluto is a planet and was bending the definition to fit their preference.