IAU Demotes Pluto to 'Dwarf Planet' Status

davidwr writes "It's official. Pluto's been demoted. It's now one of several 'dwarf planets.' I guess we can drop the 'Period' from 'Mary's violet eyes make John stay up nights.'" (Of course, no one says you have to privately agree with the International Astronomical Union.) Several readers have contributed links to the BBC's coverage of the downgrade, as well as the usefully illustrated story at MSNBC.

Comments

my take on it:

[SomeGuyFromCA]

many very educated men just screwed up nine planets...

Re:my take on it:

[Average_Joe_Sixpack]

I agree! Not since Brontosaurus was renamed to Apatosaurus have I been this upset.

Re:my take on it:

[Adhemar]

I wouldn't call it a screw-up.

The draft proposal was:

A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

Pluto would continue to be a planet, and Ceres, Charon and 2003 UB313 would become planets. However, this criterium is reached by hundreds, even thousands of other celestial bodies in our solar system. Under that proposal, all could gain planet status.

The final text is:

A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.
All other objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

This definition does not define the terms "nearly round", nor "neighbourhood". But having a definition, rather than just an enumeration, is in my opinion a big leap forward. Demoting Pluto is a small price to pay.

I quite like the additional criterion of dominance of a body in its neighbourhood. It's not as arbitrary as simply requiring a minimum mass or size.

On the other hand, I do not like the fact that a planet should orbit to Sun to be called a planet. On this point, I preferred the original proposal in orbit around a star. I don't see why our solar system should be any different, why planet-like celestial bodies orbitting other stars are not called planets.

Re:my take on it:

[Amazing Quantum Man]

Reminds me of a guy from JPL I used to know. His plates read "MVEM JSU" with a license plate holder that read "This License is NP-Complete".

What's he going to do now?

Re:my take on it:

[Rei]

...Walt Disney may be rolling over in his grave, but Pluto discoverer Clyde Tombaugh is surely spinning!

At a rate of once every six days, nine hours, seventeen minutes, and thirty six seconds.

Re:my take on it:

[Rei]

One thing that annoys me is that they added "is not a satellite" to specifically exclude Charon.

Pluto orbits the sun, but it also orbits a point in space above its surface. Charon doesn't orbit Pluto, but orbits a point in space above the surface of Pluto, while it too orbits the sun. Can someone explain to me why this shouldn't be called a double?

Re:my take on it:

[uniqueSnowflake2]

My comments are mostly gas...I don't want to speak for anyone else here.

Re:my take on it:

[2short]

Not so. The original proposal (which had the "is not a satelite" clause) made Charon a planet.

"is not a satellite" does not exclude Charon, because they picked a somewhat peculiar definition of "satelite" (barycenter of gravity inside the primary), which excludes almost everything we typically think of as a moon, but not Charon. This definition makes the Moon a satelite, but if the Earth had a slightly smaller radius but the same mass, the Moon would follow exactly the same orbital track, but suddenly be a planet.

I beleive they picked this definition of "satellite" specifically to exclude Earths Moon. If you actually plot the orbital tracks of the Moon, Charon, and any other moons you like, one stands out like a sore thumb as the one that should obviously be said to be orbiting the Sun. It's not Charon.

Re:my take on it:

[Chysn]

> On the other hand, I do not like the fact that a planet should orbit to
> Sun to be called a planet. On this point, I preferred the original
> proposal in orbit around a star. I don't see why our solar system
> should be any different, why planet-like celestial bodies orbitting
> other stars are not called planets.

Because they're only defining what a Solar planet is, not the general meaning of the word "planet":

        The IAU therefore resolves that planets
        and other bodies in our Solar System be
        defined into three distinct categories
        in the following way...

So the new definition doesn't apply to extrasolar planets. Why didn't they broaden their scope? Maybe the whole point of the exercise was just to deal "once and for all" with the Pluto problem. It's not going to affect the current work of people looking for extrasolar planets.

Re:my take on it:

[SEE]

The thing that bothers me is that if Pluto hasn't cleared Neptune out of it's orbit, then neither has Neptune cleared Pluto

I'm assuming for the moment that it was misreported. The real problem with Pluto is the thirteen other known Plutinos -- objects not gravitationally related to Pluto, but also in highly elliptical Neptune-cossing orbits with a 3:2 resonance to the Neptunian orbit.

Now, there are a bunch of objects which have stable solar orbits with a period the same as Mars. But in the case of Mars, they all either orbit Mars itself, Sol-Mars L4, or Sol-Mars L5. Same can be said for Jupiter, Saturn, Uranus, and Neptune (and Earth, except the "bunch", since Earth only really has the Moon).

Instead, Pluto is like Ceres, which has a number of objects in the same orbit which are all more-or-less doing their own thing. Demoting Pluto now that we know about the Plutinos is like the demotion of Ceres after the discovery of a bunch of other asteroids in the same orbit.

Re:my take on it:

[Cro Magnon]

I don't care what those new-fangled books say! I called them "brontosaurus" when I used to ride them to school, and I still call them that.

Mnemonic device update

[jameseyjamesey]

My Very Eager Mother Just Served Us Nachos

Now every geek's question is...

[Lothar 0]

How will this affect Sailor Pluto?

Re:Now every geek's question is...

[zyl0x]

Um, how about Sailor Moon? It's never been said that the Sailor Scouts have to be representative of the nine plan-- Oh my god. It just occurred to me that I'm a nerd.

Astrologers panic!

[grub]

So will this render all astrological predictions which took Pluto into account as invalid? I'm sure the kooks will come up with some excuse to explain how their previous charts were accurate at seeing the future as if they ~knew~ this all along.

Re:Astrologers panic!

[Rob T Firefly]

Astrologers will just keep on as before. The Astrological usage of the word "planet" includes the traditional planets as well as the Sun, the Moon, the planetoid 2060 Chiron, and really whatever else one desires to keep track of in their system of astrology.

A new one

[suso]

Much vodka easily makes John seek urination naturally

Why is this "breaking news"

[HikingStick]

I just can't understand why this story of Pluto's reclassification is deemed "breaking news" on the major news websites. It's not as if it just changed orbit and was streaking straight for New Jersey...

Now that would be breaking news!

Re:Why is this "breaking news"

[Skye16]

And incredibly freaking awesome, even if it would be an extinction level event. I don't want to die or anything, but if I'm going to, a: I want it to be in a really huge explosion, and b: I want all of New Jersey to go first.

Pluto demoted?

[jo42]

Walt Disney is turning over in his grave...

That changes everything

[Frequency Domain]

I learned "Mary Virginia eats many jam sandwiches under Ned's porch." Now it will have to be "...under Ned."

Re:That changes everything

[MBGMorden]

Is it just me or does anyone else find it harder to remember these damn phrases than it would be to just remember the planets and their order?

So why does Neptune qualify?

[s-gen]

Much-maligned Pluto doesn't make the grade under the new rules for a planet: "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit." Pluto is automatically disqualified because its oblong orbit overlaps with Neptune's.

So how does Neptune qualify? Seems to me it too has failed to clear its orbit... of Pluto!

Re:So why does Neptune qualify?

[unjedai]

If you consider that comets cross the orbits of all the planets, then none of the planets qualify.

Re:So why does Neptune qualify?

[shma]

Actually, while Pluto comes closer to the Sun than Neptune, they are never that close. Pluto's erratic orbit ensures that it is well above the solar system equator when it does cross. The chart here shows how far it really is at the cross (chart is in AU =~ 149 billion meters).

Re:So why does Neptune qualify?

[phlegmofdiscontent]

"Clearing" basically means that all other bodies within an orbital range are much smaller. There are many asteroids that cross Earth's orbit, but none are larger than a few tens of km. All 8 planets have cleared their orbital zones. The remainder of objects in those oribtal zones are assorted junk (comets, Trojans, NEOs, Centaurs, Atens, etc). Pluto and Ceres do not qualify because there are objects of comparable size in their respective orbital zones.

Son of a B*****!!!

[Anonymous Coward]

I remember failing a second grade test because I missed pluto! Time I march down to the nursing home and give Mrs Johnson a piece of my mind!

Napoleon...

[turthalion]

I guess we can drop the 'Period' from 'Mary's violet eyes make John stay up nights.'

I don't like this at all.

You IAU bastards! Now, My Very Educated Mother no longer Just Sat Under Napoleon's Picture. Now, My Very Educated Mother Just Sat Under Napoleon.

You guys are sick. Leave my mother out of this.

Holst was right.

[telchine]

Gustav Holst was right all along!

Anyone? Anyone?

[darkitecture]

Quick, someone who actually knows what they're doing, please give me a rough answer/calculation to the following queries:

Pluto is automatically disqualified because its oblong orbit overlaps with Neptune's.

1) - Is it possible for Pluto and Neptune to one day (like within the next couple billion years) collide? Or are their respective orbits degrading to the point where by the time they'd be near each other orbit-wise, their orbits would no longer overlap significantly? Or by 'overlap' do they mean "diagrammatically speaking, on a two-dimensional representation they overlap but even at their closest possible point they're still a squillion miles away from each other"?

2) - If so, how cool would that be? Would it be funny enough to make it onto an America's Funniest Home Videos video montage? Would it need special clown-horn-honking sound effects?

3) - Considering their distance from Earth and their relatively small size, would a collision of the two have any noticeable effect here on Earth?

4) - Seriously, how cool would worlds colliding be?! Costanza jokes aside, I think it'd be awesome to the max.

Re:Anyone? Anyone?

[Nemosoft Unv.]

Is it possible for Pluto and Neptune to one day (like within the next couple billion years) collide?

Nope. Their orbits are in 3:2 orbital resonance; basicly this means they constantly miss each other (a bit like your average commuter bus and train schedule :P). Also, due to the declination of the Pluto orbit it doesn't even touch the Neptune orbit. When seen straight from above, the orbits overlap, but if you go off-angle to just the right spot the Pluto orbit can be seen to be completely separated from Neptune.

Re:Stupid

[Reality Master 101]

Given the level of scientific illiteracy, what the hell is the point of taking something that everyone does know and declaring it to be wrong?

"Everyone" knew there were eight planets prior to 1930. Did the world end when it was changed to nine, especially with something that wasn't even obviously a planet?

Guess what? A whole generation of children will grow up with the new, consistent rules and won't know any different. What's unarguable is that the new rules are better. I'm all in favor of fixing things that are broken, even if certain curmudgeons are too mentally inflexible to make the adjustment. See also: the metric system in the US, which is kept down by the same curmudgeons.

Pluto and Neptune

[thundergeek]

FTA - "Pluto is automatically disqualified because its oblong orbit overlaps with Neptune's." and from the definition, "and has cleared the neighborhood around its rbit."

Doesn't that mean that Neptune also hasn't cleared it's neighborhood? It's orbit overlaps that of Pluto. So why is IT a planet?

Re:Ok, so no we have...

[plague3106]

Excuse me, that's Differently Proportioned Planets!!!

Re:.. but given enough time...

[shellbeach]

Yes, in a few billion years. If you're still around then, you get to say "nyah nyah told you so".

Actually (if I did the math correctly) in about 3,529,037,195 years. That's still within the projected lifetime of the solar system, so yes ... at that stage the IAU might need to come up with a new definition of what forms a planet, or accept that we're living on a dwarf planet. At least they've got a while to think about it ...