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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?'"
Does that mean every comet/asteroid that orbits the sun is technically a planet? If you throw out the size requirement, what other criteria remain for designating something a planet?
The Doormat
If you're not outraged, then you're not paying attention.
How about the simple argument that planets are gravitationally strong enough to pull themselves into nearly spherical objects, whereas asteroids are not. Pluto, BTW, Sedna, and many of the largest moons can all do this.
I also think, for the record, that if something as large as Luna, or Titan, or Europa were out floating in space orbiting the sun and not another planet, they would be considered planets too.
A problem with this is that there really is no clear-cut differentiation between "planet" and "planetoid". There's no qualifying size-- it's more subjective than anything. Almost like different species: we all differ genetically, yet a species is a generally-recognized "set".
One agreed-upon qualification is being formed round by its own gravity. I'm not sure if that applies to Sedna.
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Never criticize religion on Slashdot. You will be modded down for "Troll" no matter how factual it is.
And just what makes me an authority on this? I've taken more Astronomy classes in University than the average bear.
So here is my reasoning from an old assinment of mine:
A) Is Pluto a planet? Many measurable criteria signify that Pluto is a planet, but it is not a major planet. It is too small to be a major planet, so it is a minor planet or a giant comet. The only reason some astronomers still accept Pluto as one of the major planets, is because an American astronomer discovered Pluto in 1930, and they feel that changing its status to "minor" will minimize Pluto's significance in the solar system. Obviously books will need to be changed to reflect its new status, and many feel it would just be simpler to let it continue to be seen as a major planet, despite the facts saying otherwise.
It might make sense to consider placing Pluto into different categories, such as minor planet and comet. "Dual status already exists for some comets and minor planets, which are given formal numbers and names in both kinds of catalogues." [Green] The various categories we have for collections of matter in our solar system are many. The main categories are star, giant gaseous planets, smaller rocky planets orbiting the sun inside the "asteroid belt", satellites orbiting both major and minor planets, trojans, comets, trans-Neptunian objects (TNOs) and Kuiper belt objects. Meteoroids, and bits of dust, gas, and sub-atomic particles round out the other matter in our solar system.
A large asteroid named Ceres was first discovered in 1801, and was first presumed to be a comet. Then it was classified as a planet. That means astronomers, hypothesized that Ceres was a planet, they tested their hypothesis, and upon inspection of the available data years later they concluded that Ceres part of a new family of minor planets that was just being discovered. We now know of other TNOs, and Pluto doesn't look all too different from them, so we could adjust our view to place it as one of those other 100+ objects.
We can teach school children valuable lessons about science and astronomy if we teach them the history of the classification of Pluto, but stop calling it the ninth major planet. Pluto would not be called a major planet if it were discovered today, so it is a bad lesson in science to ignore data in favour of political concerns. People who say Pluto should remain a planet because for 70 years we have called it so, do not know the history of astronomy. They either don't know or don't care that many celestial bodies have been reclassified as new scientific data is gathered. Outdated models are thrown away in favour of newer, and more accurate models. Pluto no longer fits the major planet model that we use for the 8 major planets, so with our new data we should find Pluto a new category.
B)
Pluto was classified as a planet, when the data available to astronomers indicated it was one. Now the technology has allowed us to gather more accurate data about Pluto's characteristics, we should re-evaluate it's current categorization. People have had to re-evaluate "scientific facts" for millennia. Classifying the Earth as the center of the universe made sense several hundred years ago, but now we know more data that shows it cannot be the center.
From what we know about the physical characteristics about Pluto, I say it is a special minor planet. It seems odd to classify it as a kind of a comet, since I've seen no evidence that it leaves a trail of debris, and we don't know if the core is rocky, or ice like.
Saskboy's blog is good. 9 out of 10 dentists agree.
So what do we settle upon for criteria? Size is actually rather an arbitrary and vague boundary at both ends.
The fact that it orbits the Sun, specifically? The Sun's nothing special, we've found plenty of other stars that have planets. And if the Sun snuffed it tomorrow, would the Earth cease to be a planet? Would Ganymede be a planet if it were let loose for a stroll on its own away from Jupiter?
What about moons? Venus and Mercury don't have them, and those two rocks around Mars don't count.
It can't be geological activity, because Mercury is dormant and Io, a moon, beats everything we've yet seen for volcanic eruptions.
I think that having a discernable stata and a core of different composition than the crust sounds like a good rule of thumb, because then you're not just talking about a lump of rock that happens to be round, like Ceres. Now we just need to see what Pluto, Quaoar, Sedna have got in that department.
Okay, both are spheroids. Both have atmospheres. Both orbit our sun. Both even have satellites of their own. The only reasons one COULD say that they're not are that they're small, and they're way far out there. Both of those 'arguments' are pretty pathetic, IMO.
In short, there are more reasons for them to BE classes as planets than for them NOT to be.
On a related note, 'Sedna' is a really good name for an HMO, but a really _horrible_ name for a planet! *booo* Hell, even 'Planet 10' is a better name than Sedna!
There may be another Earth-sized planet that was ejected by Neptune and that in turn shifted the orbit of Sedna. Why don't we see that planet? Because it may be in the aphelion, perhaps light week away. Not only it is far away from us, but it's also in the darkness, being far away from the Sun.
Or maybe the Sun approached another star in the past, which changed the orbits of the outermost Kuiper Belt Objects. Finally, maybe it was our Sun that snatched Sedna from another star.
Second, the object must be round. This criteria excludes most asteroids.
Third, the body must be large enough that its own gravitational forces can account for it shape. This criteria excludes any objects which might happen to be round but can't really be called planets, such as small round rocks or asteroids.
Fourth, there must not be any similarly sized objects in the same orbit unless the gravity of one significantly affects the orbit of the others. This requirement excludes comets and all remaining asteroids.
Fifth, the object cannot be in orbit around another object that otherwise qualifies as a planet. Objects which orbit eachother may qualify as a double planet if all other criteria are met.
File under 'M' for 'Manic ranting'
The definition that makes Pluto not a planet is not based completely on arbitary size and so would probably not demote Mercury as well. The most prevalent definition of a planet (which was not stated in the article as far as I could tell, at least not completely) is any gravitationaly round object that is more massive than the rest of the mass in a similar orbit COMBINED. Mercury would be safe with this definition, while Pluto would be quickly tossed out. I actually like this better as the term "planetoid" now means something different than either "planet", "asteroid" or "satelite." (it would become a synonym of one of these otherwise) I propose the following definitions:
Planetoid: Any object that becomes round by its own gravity but does not sustain fussion.
Moon: Any planetoid that orbits another planetoid (let's face it, it's a generic term and nothing will ever change that). BTW: This would demote a lot of "moons" to mere satelites.
Planet: Any planetoid that is more massive than the the rest of the matter in its orbit combined.
common sense: noun
What those who are ignorant of the subject matter think; usually wrong.
I bet the folks who live on Jupiter think our solar system only has 4 planets. After all, Earth and Venus aren't so much bigger or different than Pluto or Sedna. Certainly, Earth is closer in size to Pluto than to Jupiter.
People argue so much over where to draw the line between Planet and non-Planet, but everyone seems to take for granted that Earth is a member of the former class.
Bigots.
Paranoid
Bwaahahahahaa.
Asteroids are also called planetoids, which just flips the above comparison on its head -- they're like planets, but they're not exactly like planets. The really amusing thing about this double terminology is the way it confuses Star Trek writers.
Then there's the word planet, from a Greek word that translates literally as "wanderer". All the objects in the sky that move with respect to the stars were originally considered planets. Not including the asteroids, because you can't see an asteroid without a telescope which hadn't been invented yet. But what about the Sun and Moon? These were considered planets too. But not the Earth, because everybody knew that the Earth didn't move. Hey, motion is define in reference to the Earth, how could the Earth move? What is that Copernicus dude taking, anyway?
Incidentally, that's why there are seven days to the week. Each planet that you can see without a telescope (and thus that is actually considered to exist) is dominated by a deity, and each deity has their own special day: Saturn Day, Sun Day, Moon Day, Mars Day, Mercury Day, Jupiter Day, and Venus Day. Most of the names we use in English come from Norse gods that medieval scholars thought were cognate with familiar Roman gods; their logic was a little stretched, but nobody cared, since the Norse religion was already dead, and hadn't involved planet worship anyway.
But I digress. The important point it that all these names are historical relics -- there's no way to be really precise with them. The cover issues we no longer care about, and don't cover issues we do. If you want to be more precise than anybody is in real life, you refer to rocky body, gaseous bodies, and Kuiper objects. But in real life you use familiar terms, because they're, well, familiar. If there are confusions and ambiguities, you take a moment to clear them up ("for the purposes of this discussion, any large body that orbits the sun is a planet; also Greenland is an island, not a continent"), and then you move on to stuff that really matters.