Why Pluto Is a Planet, and Eris Is Too

[One Bad Pig]

Source: Tim DeBenedictis

Let's dissect resolution 5A "Definition of 'planet'", one of six IAU Resolutions that were passed at the Closing Ceremony of the General Assembly in 2006.

The IAU therefore resolves that planets and other bodies in our solar system, except satellites, be defined into three distinct categories in the following way:

(1) 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 neighborhood around its orbit.

(2) 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 [2], (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

(3) All other objects, except satellites, orbiting the sun shall be referred to collectively as small solar-system bodies.

*Nearly round shape. There is an element of something good here. We all intuitively feel that a planet should be round, or nearly so. But what is "nearly"? How lumpy and bumpy must an object be to no longer qualify as a planet or a dwarf planet? How smooth must the "ball" be? The Earth, which we all agree is a planet, is nearly round on some scales, but on others, it's not. If you're standing in the bottom of the Grand Canyon, the Earth isn't even close to nearly round.

*Cleared the neighborhood. I've tried to wrap my head around this phrase for years, tried to convince myself that it makes sense — but I just can't swallow it. The IAU is trying to express that, in addition to being round, a planet should be the dominant gravitational force in its local region of the solar system. That's not an unreasonable position. Certainly the Earth and Jupiter are the dominant objects in their local regions. Neptune surely is, too. Even though Pluto's orbit crosses Neptune's, Neptune forces Pluto into something called a 3:2 resonance (for every three times Neptune goes around the sun, Pluto goes around twice), preventing collision. But have any of these planets actually "cleared the neighborhood" around their orbits? No. Pluto is still clearly in Neptune's "neighborhood." For that matter, Jupiter has two well-known groups of asteroids, the Trojans, which lead and follow Jupiter along in its orbit. For that matter, the Earth hasn't quite "cleared the neighborhood" around its orbit, either, to which anyone who saw the near-Earth asteroids that entered Earth's atmosphere near Chelyabinsk, Russia, on Feb. 15, 2013, or Tunguska, Siberia, on June 30, 1908, can attest. So are Earth, Jupiter and Neptune the dominant gravitational objects in their local neighborhoods? Yes, clearly. Have they cleared their neighborhoods? No. Not by a long shot.

Other scientists have weighed in on the matter. Alan Stern, principal investigator for the New Horizons mission to Pluto, made it clear he disagrees with the IAU resolution. "Any definition that allows a planet in one location but not another is unworkable. Take Earth. Move it to Pluto's orbit, and it will be instantly disqualified as a planet," Stern said.

The biggest problem with the IAU's planet definition is that it replaces an already-ambiguous concept ("What is a planet?") with three more ambiguous concepts, ("nearly round," "cleared" and "neighborhood"). Indeed, the only definitive part of the IAU resolution on which everyone can agree is the first part: (1) A planet is in orbit around the sun. It's why the moon is not a planet. My 6-year-old niece intuitively understands this. It's the only part of the IAU definition I would keep.

...

So, what would be a better definition for the objects in the solar system?

(1) A "planet" [1] is a celestial body that (a) is in orbit around the sun, and (b) has a maximum surface radius greater than 1,000 km (620 miles).

(2) All other objects, except satellites, orbiting the sun shall be referred to collectively as small solar-system bodies.

"But that's completely unscientific," you may say. "Why 1,000 km? Why not 1,200, or 750?"

I submit that the precise definition of a planet as an object with a radius of at least 1,000 km is no less scientific than the definition of a kilometer as being a unit of distance equal to 1,000 m, or a degree being 1/360 of a circle.

And there are other reasons why the 1,000-km definition is more scientific than it might seem at first. But let's put that aside momentarily. Instead, let's see what would have happened if the IAU had adopted this definition.

Here is a list of the largest known objects orbiting the sun, and their radii in kilometers:
Object Radii (km)

Jupiter: 69,911 km (43,441 miles)
Saturn: 58,232 km (36,184 miles)
Uranus: 25,362 (15,759 miles)
Neptune: 24,622 (15,299 miles)
Earth: 6,378 (3,963 miles)
Venus: 6,052 (3,761 miles)
Mars: 3,390 (2,106 miles)
Mercury: 2,440 (1,516 miles)
Pluto: 1,184 (736 miles)
Eris: 1,163 (723 miles)
Makemake: 715 (444 miles)
Haumea: 620 (385 miles)
Quaoar: 555 (345 miles)
Sedna: 498 (309 miles)
Ceres: 475 (295 miles)
Orcus: 458 (285 miles)

By the 1,000-km definition, all eight classical planets would remain planets. So would Pluto. And we'd add Eris. The solar system would have exactly 10 planets, a number that is deeply satisfying to two-handed, five-fingered humans who've been practicing base-10 mathematics for thousands of years. The "Plutophile" camp, fond of keeping Pluto's planetary status for historical reasons, would retain its dignity. And elevating Eris to a first-class planet would be an honorable nod to the cutting-edge astronomers whose work led to a need for this new definition in the first place.

And finally, the 1,000-km rule, like any good arbitrary rule, actually does a pretty good job of respecting the underlying physical phenomena that it purports to define. Planets are made of physical materials like rock, metal, gas and ice. They may come in different proportions, but those materials all respect the same physical laws. When you put together a lump of rock, metal or ice, in any proportion, certain things start to happen as that lump approaches 1,000 km in radius. The materials will pull together under the force of their own gravity. Solid rock will start to deform. Ice, even frozen hard as granite at the edge of the solar system, will slowly flow. There is no known substance that can resist the force of its own gravity when made into a lump with a 1,000-km radius. Any object, made of any substance, of approximately that size, will eventually flow under action of its own gravity into a shape that is "nearly round" when viewed from far away. The 1,000-km radius just happens to describe something that naturally takes place for objects of a certain size and results in what we all intuitively want a planet to look like.

© Copyright Original Source

[fm93]

I'd prefer that My Very Excellent Mother Just Serve Us Nine Pizzas rather than nachos anyway.

[Truthseeker]

The list of the largest known objects of the solar system left out two objects.

[Roy]

Source: Tim DeBenedictis

Let's dissect resolution 5A "Definition of 'planet'", one of six IAU Resolutions that were passed at the Closing Ceremony of the General Assembly in 2006.

© Copyright Original Source

I'm not convinced by some-one who doesn't seem to know why the Trojan asteroids haven't been cleared out of Jupiter's orbit, or that roundness is a relative measure and thus independent of scale.

And this: "If you're standing in the bottom of the Grand Canyon, the Earth isn't even close to nearly round" is ridiculous.

Edited to add: Also, he doesn't seem to know that Ceres has also pulled itself into a spherical shape, despite being smaller than his 1000km radius criterion, nor that the size at which gravity will pull an object spherical is dependent on the composition of the object, since gravitational pull will vary by density.

Roy

[Duragizer]

This is why we need to blow Pluto up. Once that ball of ice is reduced to snowflakes, this whole pointless discussion of whether its a planet or not will finally be put to rest.

[Jedidiah]

Pluto is a planet because it was a planet when I learned the names of all the planets. Eris was not on that list so it is not a planet.

[rogue06]

This is why we need to blow Pluto up.

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[Paprika]

Indeed, the only definitive part of the IAU resolution on which everyone can agree is the first part: (1) A planet is in orbit around the sun. It's why the moon is not a planet. My 6-year-old niece intuitively understands this.

Nah, the moon orbits around the sun too.

[lilpixieofterror]

This is why we need to blow Pluto up. Once that ball of ice is reduced to snowflakes, this whole pointless discussion of whether its a planet or not will finally be put to rest.

You shouldn't be putting these ideas into people's heads. Some might actually try it (IE Rogue06).

[mossrose]

Please note my avatar.

[Catholicity]

Just wait till oxmix gets word of this.....

[rogue06]

Interesting bit about the erraticness in the orbit of some of Pluto's moons: Chaotic orbital interactions keep flipping Pluto’s moons

NASA describes them as " "wobbl[ing] unpredictably." The New Horizons spacecraft is due to fly by next month and should provide a lot of new inflrmation.

[One Bad Pig]

I'm not convinced by some-one who doesn't seem to know why the Trojan asteroids haven't been cleared out of Jupiter's orbit,

Take that up with those who proposed the definition - and I'll note that he says nothing about why.

or that roundness is a relative measure and thus independent of scale.

I'm not sure how you managed to read that purported ignorance into the text.

And this: "If you're standing in the bottom of the Grand Canyon, the Earth isn't even close to nearly round" is ridiculous.

I think he's exaggerating for effect here, but I could be wrong about that.

Edited to add: Also, he doesn't seem to know that Ceres has also pulled itself into a spherical shape, despite being smaller than his 1000km radius criterion, nor that the size at which gravity will pull an object spherical is dependent on the composition of the object, since gravitational pull will vary by density.

Roy

Please point out to me where he spoke in absolute terms that would exclude Ceres. He's speaking in generalities as far as I can tell.

[Paprika]

After more detailed reading:

*Nearly round shape.

The specific criterion is 'assumes a hydrostatic equilibrium', with 'nearly round' being the simplifying elaboration, so he's just being intellectually sloppy by criticising the simplified version.

The biggest problem with the IAU's planet definition is that it replaces an already-ambiguous concept ("What is a planet?") with three more ambiguous concepts, ("nearly round," "cleared" and "neighborhood").

The biggest advantage of the definition was that it uses astronomically significant features as criteria to try to bring order to a classification which had none.

I submit that the precise definition of a planet as an object with a radius of at least 1,000 km is no less scientific than the definition of a kilometer as being a unit of distance equal to 1,000 m, or a degree being 1/360 of a circle.

Defining a multiple of a unit of measurement is hardly the same as making a definition for a dividing criterion.

And there are other reasons why the 1,000-km definition is more scientific than it might seem at first....The 1,000-km radius just happens to describe something that naturally takes place for objects of a certain size and results in what we all intuitively want a planet to look like.

His definition is hardly workable when Ceres (for example) which has hydrostatic equilibrium and with respect to the definition only differs from Pluto in not falling under the arbitrary 1000 km radius limit is not considered a planet while fulfilling the logic that underlies (1b) - ie. a planet should possess hydrostatic equilibrium.

[Paprika]

Please point out to me where he spoke in absolute terms that would exclude Ceres. He's speaking in generalities as far as I can tell.

(1) A "planet" [1] is a celestial body that (a) is in orbit around the sun, and (b) has a maximum surface radius greater than 1,000 km (620 miles)...
Object Radii (km)
Ceres: 475 (295 miles)

By the 1,000-km definition, all eight classical planets would remain planets. So would Pluto. And we'd add Eris. The solar system would have exactly 10 planets

He is pretty clearly explicitly excluding Ceres.