Is {pluto|sedna} A Planet?

Dr. Zowie writes "NASA's announcement last week of Sedna's discovery reignited the debate over whether Pluto is a planet. Dr. Alan Stern a noted planetary scientist and leader of the New Horizons mission to Pluto, pours on some gasoline with this article in which he skewers the various arguments against Pluto-as-planet. Choice quotes include 'You wouldn't deny a chihauhau a place among dogs because it is too small,' and 'if your brain was so completely full of names of people that it just couldn't take any more, would anyone new who you met after that, therefore not be a person?'"

Comments

Re:I love this stuff

[Anonymous Coward]

Well Pluto's moon is called Charon. The ferryman of the dead. Is that good enough for you.

Re:I say it isn't a planet, Harvard

[saskboy]

Harvard has a nice page with lots of links an references for people looking to dig deeper into the Minor Planet definition under which asteroids like Ceres and Sedna fall under.

Re:Well..

[loyalsonofrutgers]

Well the concept of a species might not be the best example. Two individuals are generally recognized to be of the same species if they can mate and produce fertile offspring. So, for the most part it's not simply arbitrary set determination.

See: Mule

Re:Let the Astrologers decide.

[Imperator]

We should have stuck to the original five. Mercury, Mars, Venus, Jupiter and Saturn. Earth doesn't count, since all these revolve around it.

The original seven, actually. The sun and the moon were planets. And yes, they all revolved around Earth. Church "scientists" were the first to add to this system; they put Heaven above the planets.

Re:I love this stuff

[Unknown Kadath]

I cite Cecil Adams of The Straight Dope, font of all knowledge, to say with authority:

The tomato is botanically a fruit.

Brontosaurus never existed.

And you can blame the Greeks for the continent thing.

That Cecil! Is there anything he doesn't know?

-Carolyn

Re:I love this stuff

[spun]

Potatoes are vegetables. Specifically, they are tubers, parts of the root system that enlarge and store energy. They are also in the same family as peppers, eggplants, and tomatoes, all of which actually are fruits.

However, peppers, eggplants, and tomatoes are also all vegetables. The two are not exclusive, as fruit is a technical term with a specific definition. A vegetable is any plant grown to be eaten, or the part of the plant that is eaten. Fruits are vegetables. So are nuts and grains.

Everyone knows that a tomato is a fruit. Most wrongly assume that means it isn't also a vegetable. The lesson here is, check your facts before you go spouting off in a supercilious manner.

The problem is:

[Bendebecker]

"One doesn't deny a Chihuahua a place among dogs because it is too small."

First: The designation 'planet' should mean something. Sure we can group small dogs under the category of dogs but that doesn't mean we can go around calling pomeranians' greyhounds. The same with planets. We can group pluto and sedna under the category of masses but we shouldn't call them planets. Planets should be its own category of the junk floating around in the universe, just as asteroids and comets are categories. When someone says this object is a planet we should thus be able to make some assumptions about that object. Otherwise we have to break that category up even more. If we have to have sedna added and a couple hundred other, the category of planets becomes so vague that it becomes meaningless. Thus we will have to break the category of planets up into sub categories in order to get any meaning out of it: gas gaints, rocky planets, etc.. Think of it like the dogs again. If we call every dog a pomeranian then the label 'pomeranian' loses its meaning.

Now the problems with his gravity rules. The first problem is moons. No one wants to call luna a planet. If we go around saying a planet in the solar system (Jupiter) has 32 other planets orbiting it, things will get very confusing awfully quick. So we would want to declare that for it to be a planet it has to orbit the sun. But then their is the problem of 'planets' that orbit each other. For example, we see this in some asteroids - two asteroids that orbit each other while traveling in a circular path around the sun - similiar to binary star systems where two stars orbit each other and tavel in a circular path around the galaxy. They can't both be moons. They can't both be planets. And what about rogue planets that no longer orbit a star but have been orphaned and are currently floating in interstellar space.

The second problem is comet-like bodies. What if you have a planet that as it orbits its sun sheds its atmosphere and mass to the point that it loses the gravity necessary to keep it circular. Likewise, what if you run into an asteroid that through a series of collisions gains enough mass to become a planet. This is fine but what happens when you have a whole belt of such objects. When you classify something, its best it stays in that classification for awhile or else the act of classification becomes somewhat meaningless. For example, you don't classify water by its mass in a rain storm cause that mass is constantly changing. Rather you state the rate of that change. If you didn't, you'd be forced to constantly reclassify it every observation.

So simply stating that gravity rule as the only criteria doesn't work. We'd have to make it more complex. Moons aren't planets (assuming you still want the word moon and planet to mean anything - and yes I know some moons could have their own moons). Belts like the asteroid belt and the kuiper belt where objects could conceivably change in every observation from planet to non planet and back would create a nightmare for astronomers using such a system. And remember these are only problems we face with a small data set like our solar system. Add in problems like the Super jupiters, some of which are undoubtfully brown stars or close to becoming them, and other as of yet unknowns and one could only imagine even more problems would arise in the gravity rule system. Now if these means adding addition requirements or not, or perhaps just abandoning the whole system is anybodies guess. He's write in stating you can't just use the old size requirement - but that was and is why we called pluto a planet and ceres an asteroid. Becuase someone said theres a size difference - there is really no other reason. Some asteroids have atmospheres. Some have moons. Some planets don't have moons. Some planets have moons larger than other planets. Perhaps the best bet is to just throw all the labels out and start over.

Photos of Minor planet with a moon

[erice]

Ida and Dactl

Re:Well..

[PatientZero]

It's a body of matter vastly more massive than other matter in the vicinity.

From the article:

Location Rules. "Let's use an object's location as the criterion to establish or reject it from planethood."

The most common form of this idea is to classify an object as a planet if it is the largest thing in its region. By this criterion, objects like Ceres and Sedna are planets, for they are the largest known things in their regions of the solar system.

The main problem is that as we discover new objects, some planets may cease to become planets. And what happens if a planet shifts its orbit closer to a bigger planet? Does it stop being a planet until it moves far enough away?

Having read the article, I like his criterion: massive anough for gravity to form it into a spherical object. This doesn't change over time; it's based on physics; and it's very similar to the criterion for whether or not an object is a star (massive enough for fusion to provide the majority of its energy).

Re:I love this stuff

[catbutt]

But we are talking about what you mean when you say a word. The only evidence that matters is usage. If you say "fruits" when actually what you are talking about is tomatoes and cucumers and squash and pumpkins, people are going to be misled.

In real world usage, having high levels of sugar is a requirement for fruit, being from an herbacious plant is a requirement for vegetables. Tomatoes don't have as much sugar as most things considered fruit, and they certainly are from herbacious plants.

Re:I love this stuff

[BrianGa]

http://caselaw.lp.findlaw.com/scripts/getcase.pl?n avby=search&court=US&case=/us/149/304.html

Continuous sets, discrete sets

[Anonymous Coward]

Things that we used to categorize, but no longer bother with:

• The ancients individually named the stars. Now that we can see zillions of them, they just get catalog numbers. You can argue about the exact division between brown dwarf and star, between main sequence and red giant, etc., but it's recognizeably not relevant.
  
• 'Race' used to be considered very clear-cut; you were African, European, Native American, unambiguously. Nowadays it's something closer to a continuum; rather than argue about who falls into what category, we're (perhaps) beginning to recognize the continuum.
  
• Early particle physicists classified particles by their masses: light (leptons), middleweights (mesons), heavyweights (baryons). Later they discovered that a more useful classification scheme, by quark content and quantum numbers, only sometimes coincided with the old one. Knowing about quarks, we now understand the naive mass categorization to have been arbitrary.
  
• Mendel determined that genes can be either dominant or recessive. In modern biology, we know that genes are extremely complicated, and the simple labels are only occasionally useful.
  
• New moons around the outer planets used to make the news. Nowadays, the half-dozen Volkswagens or whatever that turn up bimonthly around Neptune don't even merit names. 'Are they really moons?' we wail, 'aren't they just captured asteroids?'.
  

Anyway. The more phenomena we discover, and the faster we discover them, the less interesting each individual one becomes. The more diverse they are, the less likely it is that the 'labels' invented 3000 years ago will still make sense. We're lucky that the simple categorization 'planet/comet/asteroid' has held up as long as it has. We've patched it up with TNOs and KPOs and so on, but at some point it'll be a continuum. A sparsely sampled continuum, but a continuum nonetheless.

Re:Wrong

[Jeremy Erwin]

The words of the US Supreme Court:

Botanically speaking, tomatoes are the fruit of a vine, just as are cucumbers, squashes, beans, and peas. But in the common language of the people, whether sellers or consumers of provisions, all these are vegetables which are grown in kitchen gardens, and which, whether eaten cooked or raw, are, like potatoes, carrots, parsnips, turnips, beets, cauliflower, cabbage, celery, and lettuce, usually served at dinner in, with, or after the soup, fish, or meats which constitute the principal part of the repast, and not, like fruits generally, as dessert.

Nix v. Hedden, 149 U.S. 304 (1893)

Re:Wrong

[Tony Hoyle]

That is a legal fiction to satisfy the food regulations - not a reclassification, more a kind of dual-licensing :)

Carrot Jam is quite common - you can buy it in corner shops around here - however food regulations specify that jam must have a certain percentage of fruit to be called jam (which is good - I want to be sure what I'm buying is what it says on the pack). This is just a workaround for it.