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:

[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:

[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:

[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

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: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.

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?

[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).

Holst was right.

[telchine]

Gustav Holst was right all along!

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.

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.