Slashdot Mirror
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?'"
Although you have to admit that we NEED a planet named after the god of the dead. Perhaps we can put some trash out there and christen it.
What we call folk wisdom is often no more than a kind of expedient stupidity.-Edward Abbey
Err, by 'FP', I am of course refering to 'Final Planet'.
Of course. What did you think I meant?
Huxley
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.
The two-legged things in my office have names?! Not just email addresses?
".. pours on some gasoline with this article..."
You haven't seen squat until you've seen astronomers argue.
Planet or not, it's out there and it's circling the sun. It's large enough to attract space dust and rocks in its vicinity. It will eventually grow larger and then there will be no doubt that it is a planet.
But really, who cares? Is this a big deal?
I have been pwned because my
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.
---
Never criticize religion on Slashdot. You will be modded down for "Troll" no matter how factual it is.
...a pluto. And Sedna a sedna. The solar system would have 8 planets, a pluto and a sedna, then. :)
by growl you must mean an ear-drum piercing yip ;)
The Atlantic Monthly had an article about the Pluto situation years ago. The problem, though, is that "kids love Pluto." Scientists have tried to change names before (such as the dinosaur example). It'll be interesting to see what the public says about Pluto's demotion (if it occurs).
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.
0
Nope. Guess not.
"You wouldn't deny a chihauhau a place among dogs because it is too small."
Dog? I always that chihauhaus were large rats.
Alan Stern said, "wouldn't deny a chihauhau a place among dogs because it is too small"
yeah, no kidding. But if the definition of a dog included, "must not weigh less than 30lbs" then yes, a chihauhau would most certainly not be dog.
I know there is no such definitive critereon for planets, but jeeze...a simple webster's definition includes the phrase "...large heavenly bodies..." (emph mine). Any reasonable defintion of large would probably exclude pluto, just as any reasonable definition of "large dog" would most certainly exclude the lowly chihauhau
My charts are going to have to all be recalculated if Sedna is a planet. What a PITA if there ends up being 900 planets! How will I ever be able to calculate this week's horoscope before the week is up?
We should have stuck to the original five. Mercury, Mars, Venus, Jupiter and Saturn. Earth doesn't count, since all these revolve around it.
Let's not mess with our destinies. Don't upset the natural systems any more.
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.
But how irregularly shaped does it have to be to qualify for non-planethood? Even the gas giants aren't perfect spheres.
Have you been touched by his noodly appendage?
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.
Saskboy's blog is good. 9 out of 10 dentists agree.
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!
Lucky for us we didn't discover a 'planet' with a completely liquid surface yet, to limit the 'round' criterion even more. This way astronauts making water bubbles on ISS can claim they're planet-makers ;-)
(hint: surface tension also makes things round)
He wants a Boolean criterion (yes/no) for planethood, but then criticizes a 'minimum mass' limit as being arbitrary. It is not possible to impose a Boolean criterion onto parameters that vary continuously without there being an arbitrary boundary somewhere.
Other than that, a pretty good discussion. His suggestion will still require an arbitrary boundary (how round is round?) but it is not totally arbitrary.
His rule has a problem that it turns into planets objects that we had previously decided were not planets. It has the advantage of being less arbitrary than the alternatives. Whether the advantage outweighs the disadvantage is a matter of taste.
Quattuor res in hoc mundo sanctae sunt: libri, liberi, libertas et liberalitas.
I think part of the problem is the fact that memorizing the 9 planets are all most people really know about our solar system, and so they tend to be fairly sentimental about it. I think a much more accurate and interesting approach to teaching kids would be to start of by brainstorming all the different types of objects in space - galaxies, solar systems, stars, moons, astroids, comets, nebulas. Then instead of memorizing just the planets memorize all the different regions of our solar system and what makes them special. Start with the sun, then you get to the inner planets, then astroid belt made mostly of rock, then giant gass planets, then the Kupier Belt full of icy objects and finally the Oort cloud. Then lastly you describe the interesting features of each area, including the planets and what makes them unique.
This journey approach would be far more interesting to the kids and by the time you got to the point of describing pluto and charon, they would have an understanding of how diverse (for lack of non PC word) matter in space is and would be less concerned about sticking a specific catagory on it, and just be excited that it was yet another unique and interesting thing.
It's the difference between decribing the cool terrain, people and features in country as opposed to just memorizing the state capitals. The former is far more interesting, and informative, and kids will eat it up.
The whole thing is SOOO silly. Who cares if its called a planet or not. The universe doesn't differenciate between the two. Theres just stuff floating around in space...without a name. The human race has become far to obsessed with naming things. Why can't we just experience the universe directly without the interference of sybols and concepts.
"It is not how things are in the world that is mystical, but that it exists." -Ludwig Wittgenstein
Tomato is, and always has been, a fruit.
The supreme court rulled that it was a vegitibale so schools could meet nutrienal guide lines.
Rule of thumb:
Does it have seeds? then its a fruit.
The Kruger Dunning explains most post on
chihauhaus are clearly rodents, not dogs. Therefor, Sedna is not a planet, but a rodent.
One future, two choices. Oppose them or let them destroy us.
planot
How about the simple argument that planets are gravitationally strong enough to pull themselves into nearly spherical objects, whereas asteroids are not.
:)
:) But clearly a planet-sized object orbiting another planet is a moon. Again, this definition makes perfect sense.
:)
I like this definition a lot. While it does leave some wiggle room as to what exactly constitutes "spherical", it is still based on a physical property of the object related to its mass. This makes it better than any arbitrary size/mass requirement (e.g. "Anything as big or bigger than Pluto").
Pluto, BTW, Sedna, and many of the largest moons can all do this.
I'm going to be extremely unhappy with any definition that demotes Pluto. Also, anything that makes Pluto not a planet is going to be close to making Mercury not a planet, and that's just not acceptable.
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.
Titan is bigger than Mercury, so a Sun-orbiting Titan not being considered a planet is unacceptable.
I'm not an astronomer (but I play one on occasional weekends), but of all the definitions I've heard, "big enough to be spherical and orbiting a star" is the simplest and most logical.
And for the record -- if there was some comet out in the Ort cloud with an incredibly eccentric orbit around the sun that was the size of Titan, that'd be a planet too. IMHO.
The enemies of Democracy are
Anyone think to ask Disney?
I wrote a cool program in Matlab for a graduate astrodynamics class I took that would plot the planets and their orbits at any time. One thing immediately jumps out at you.... Pluto is not a freakin planet! Any good diagram of the solar system shows to screwed up Pluto is.
For those who hate pictures, here are the orbit elements of the planets in tabular form
First off, note that Pluto has an eccentricity of almost 0.25, that is WAY oblate. Now, someone will probably point out that Mercury is nearly that oblate and we can argue whether Mercury is really a planet also. It probably is, however, it is soooo close to the Sun that it has comparatively zero angular momentum - and remember, that is the job of the planets, to store the bulk of the angular momentum of the solar system as it was formed (you do remember that right?) Anyway, Mercury is so close to the Sun, that its orbit is much more easily perturbed by higher J2 and J3 harmonics of the Sun and you would expect it to have be a little out of plane and eccentric due to multibody effects as well.
Moving on, how about that inclination... 17 degrees. Again, excluding Mercury, the next closest is 3.4 deg and the next closest outer planet is 2.5 deg.
And how bout these data. Check out the rotational period... 153 hrs.. the next closest outer planet is 17 hrs.
Sorry folks, it is a captured Kupiter belt object... move along.
"It takes considerable knowledge just to realize the extent of your own ignorance." - Thomas Sowell
Perhaps he didn't mention it because all objects meeting his "gravity rules" requirement happen to have stable orbits.
It isn't Sedna, get it right, it's Rupport. They practice astrology, and we really do need some sort of planetary shielding right about now!
Dogs, coyotes, and wolves can interbreed and make perfectly fertile offspring -- that's a real problem for the preservation of wolves and coyotes. There's a quite a bit of tradition involved in deciding what is a species and what isn't. Greeks and Romans saw wolves as something other than wild dogs, and thus we do too. And of course, the vast majority of organisms on Earth are asexual, making the whole issue of "fertile offspring" moot. Logically, all decisions should simply be based on percent identity of DNA, but then the question becomes what percent should be the cutoff.
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.
Astronomer: "oh oh oh, yeah, well, you have your head up Uranus"
"There is no teacher but the enemy."-Mazer Rackham
My
Very
Evil
Mother
Just
Sent
Us
Nothing
I think there needs to be the addition of an atmosphere to be considered a planet. Really it's just a round rock without one. It pretty much classifies moons as planets without that qualifier.
First -- how much atmosphere? Every sizable rock will be able to hold onto at least a few gas molecules.
Second -- the Sun has stripped Mercury of its atmosphere. However, if Mercury were orbiting at the distance of Mars, it would have been able to retain quite a bit more air. Your definition is biased against close planets.
Third -- our atmosphere came largely from outgassing. A planet with a different composition (say, similar to the moon), or less active tectonics, might have dramatically less of an atmosphere.
So, you have now tied the definition of what is a planet to a complicated interplay between its size, composition, geology, distance from the sun, and who-knows-what-else factors.
Can't we just say "You need to be this big to be called a planet" and leave it at that?
ZFS: because love is never having to say fsck
This is exactly the same as the continent-versus-island debate in geography. Why isn't the United Kingdom considered a continent by anyone except the Brits? Why is one single monolithic land mass with a dotted line down the middle considered two continents, Europe and Asia? How is Australia not a continent?
Yes, that would be why he stated that GRAVITY would be making the object round, not some other force. So, no, by that definition water bubbles would not be planets.
common sense: noun
What those who are ignorant of the subject matter think; usually wrong.
Everything you said was right,
Though the earths "moon" should be considered a second plant in our bi-planetary system. It is large enough that the center of gravity for our orbit is well off from the center of the earth.
(just another pet peeve of mine when it comes to astronomy)
I live in a giant bucket.
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'
...kinda threw me for a loop. Look, chihuahuas are classified as dogs due to their genetic composition. They're sorted down to a certain genus and species and are most certainly close enough (in their genetic makeup) to other animals we call dogs to be considered one. Size does not matter.
:P
If we classifed planets using the closest possible method, say material composition ("This rock is 70% nickel, 30% iron... whatever, you get what I mean?"), it would be too damn hard to classify an object as a planet properly. Compare Jupiter's chemical composition to Earth's. Doesn't work. So instead, I believe planets should be classified by relative size, orbital pattern, and possibly the rate at which they orbit.
It's easy to say that in order to be a planet, the object must be determined to be orbiting a solar system's sun. You could also possibly define a certain speed of orbit to further clarify things, though it would be harder to settle on. The first (relative size) isn't as easy to define as the others though, for example say a rock out there orbiting the sun is, oh, 5km in diameter. We decide to call it a planet. It's fairly big, it's orbiting the major heavenly body (sun) in the solar system in question. Then suddenly from around the planet comes another rock with a slightly larger orbit and different orbital trajectory. It's big too, but only 4.9km in diameter. Or 4km. Or 3km. Size is hard to include in the definition of a planet. But it does matter.
But no matter what you decide, with the knowledge we have of the universe today, it's going to be hard to settle these little conflicts over what's a planet, and what's just an asteroid, and what's just a little rock that happens to be orbiting the sun. Maybe a committee should come up with a set of standards for defining and classifying heavenly bodies?
I dunno. I just kinda jumped on the chihuahua thing and wanted to contest that argument.
Is Ceres the fifth planet from the sun, then? It is shaped into a (rough?) sphere by gravity.
Ewige Blumenkraft.
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?'"
Mmmmmm... Somebody who likes the sound of his own voice way too much.
As far as planet vs planetoid goes, I'd think the difference relies on how much influence is imparted on what body of mass. For instance, the majority of influence imparted on asteroids comes from the planets and stars they revolve around, whereas the planets principle influence is the sun.
So which influnces these celetial bodies more? The sun or other planets around it? Does the body influence other celetial bodies a great deal? Does it have it's own bodies trapped in orbit around it? If this body careened through the solar system close to a planet (say, earth), how much influence would it impart on us??
I'm leaning more toward planet, especially in the case of Pluto. Sedna, I'm not so sure about given the lack of hard data, but I'm pretty sure a near pass from Pluto would seriously screw things up here. Besides, all this crap is relative anyway. I'm sure if you had a huge enough planet, Earth could be considered a moon or something.
You need a FREE iPod Nano
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.
Ceres is round. Vesta is nearly so. Do they get promoted to planet status?
"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.
There's a growing sense that even if The Future comes,
most of us won't be able to afford it.
-- Lemmy
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.
Pluto by 2 Skinnee J's
With depravity I break laws of gravity
Blast past the atmosphere to the last frontier
I go boldly through space and time
The skies the limit but they're limiting the sky
I break orbit by habit, ignite satellites and leave rings round the planet
A flying ace like that beagle
Nevertheless this alien remains illegal
'cause their discovery don't cover me
the immigrant's been left in the cold to grow old and disintegrate
discriminate against the distant and disclaim this
cause small minds can't see past Uranus
But I shun their rays, 'cause stuns just a phase
And my odyssey runs in two thousand and one ways
And I can see clearly now like Hubble,
Shoved off the shuttle, here's my rebuttal
It's a planet
Who you represent? I represent the smallest planet
Attorney in this tourney versus those who've tried to ban it
If you don't agree go see Interplanet Janet
Cause sun is star, like Pluto is planet
Lend me all your ears and let me state my case
About all the types of satellites you must embrace
Cause like my parents, great grandparents
This planet was an immigrant
To deport it makes no sense
It's an upstanding member of the solar system
Apply the laws of earth and make it a victim
Of Proposition 187
When Pluto spawns a moon it will apply to the heavens
I will damn thee like Judas of Iscariot
If you demote this mote remote to affiliate
It's like taking ET's custody from Elliot
Support your Lilliput, cause simply put
Pluto is a planet
Do it for the children
Lyrics - MP3
-prator
Ida and Dactl
If we call Jupiter a gas giant, can we call Pluto a ice midgit?
When looking at the vast differences in other planets, is there yet a significant scientific reason to classify Pluto as something different.
(That is the question.)
I won't join Slashcott. OTOH, If Beta goes live, I just won't be back until it's fixed. Sorry Dice.
There was a good presentation at today's blackboard lunch at the Institute for Theoretical Physics in Santa Barbara today. The first 15 minutes or so are a great summary of why Sedna is important for our understanding of the solar system.
Unless you want to say that a vegtable is any sort of large plant product that doesn't classify as a fruit, I'd say that the Tomatoe can reasonably classify as both.
Free Software: Like love, it grows best when given away.
exist to illustrate similarrities and differences. It is less useful to argue whether this definition of a planet or that definition of a planet should rule but rather we should be discussing what is a useful classification system.
Pluto has more in common with a whole class of objects which spend most of their time out past the orbit of Neptune. Sedna is another such large object but there are hundreds more identified.
That Neptune and Pluto's orbits cross is, I think, a major blot on our current classification.
I have discovered a truly marvelous sig, unfortunately the sig limit is too small to contain i
This excludes stars and any gas giants so massive that they could become stars at some point in their existence.
Ummmm... given that the only difference between a gas giant and a star is their mass, does this statement make any sense at all? If a planet has "enough mass to become a star at some point" then it will immediately ignite. If it doesn't, it won't.
It's not like planets get a choice in the matter. It's not like Jupiter might get ambitious one day and decide to get lit.
Clear, Dark Skies
Well, since we are throwing out just about every possible way to classify a planet, or in this case, a way to keep Pluto and exclude Sedna, why not use the old method that led to the discovery of Neptune and Pluto?
If the body in question exerts a measurable gravitational force on another planet, then it's a planet. As I understand, Sedna is much too distant to exert any kind of force on the orbit of Pluto. This also can discount Ceres and other large planetoid objects in the asteroid belt, since most of their gravitational exertion can only really be measured on other objects in the belt.
If we keep promoting solar objects with a mass greater than a Volkswagon to planetary status, then I guess all those numbers in Drake's equation just skyrocketed...
"Luck is what others call skill when they have none." --Phelan Kell
Let's be honest. The New Horizons mission is being launched for completeness. Once it completes its flyby, all of the nine planets will have been explored by a visiting spacecraft.
Classifying Sedna and Quaoar and all that other stuff out there as planets will require more missions. Demoting Pluto would elimiate the need for New Horizons. So lets make sure New Horizons hits space before doing anything else.
The first asteroid, Ceres, was predicted before it was discovered. According to the Titus-Bode series, there is a gap between Mars and Jupiter where there must be a planet. So, astronomers looked and surprise surprise, there's Ceres. But it didn't take too long to discover Vesta and Juno and Pallas and all of Ceres' other friends. Its those friends, immediately discovered, which caused Ceres to be demoted.
Pluto was predicted to explain otherwise unexplained perturbations in the orbits of Neptune and Uranus. So, astronomers looked and looked and looked and eventually found Pluto in the predicted position. Then they stopped looking. If they had kept it up, they would have discovered the rest of the Kuiper belt, and Pluto would have suffered the same fate as Ceres.
The proposed Gravity Rule would cause the answer to the question "How many planets are there in the solar system?" to change from "9" to "we don't know." We can be reasonably confident based on the tracks of the Pioneers and Voyagers that there aren't any other large masses out in the outer solar system. We will never be sure we have discovered all the round things out there.
Besides, there are objects which should be round, but aren't, like Hyperion, and things which have no business being round, but are, like Comet Wild/2. How would the Gravity Rule treat those?
Lots of technical and environmental problems are solved by the application of vast amounts of nuclear power
So if we accept that a planet is anything that has been rounded by its own gravity, we have to make assumptions about the density of the matter that it's made of. Stars that went through a supernova and no longer have enough mass to undergo fusion may be round and of comparable size to known planets, but many times denser. They clearly are not in fact planets but the author's criterion would have it that way. Of course we can say, "They aren't planets because they used to be stars" but he threw out the entire argument-from-origins criterion as being too difficult to establish.
Conveniently, he did not make a biological analogy for the argument-from-origins -- that is because biologically, it is the most sensible argument.
What he really did was shift the question from "how big do you have to be to be a planet" to "how dense do you have to be to be a planet." Now we have to establish an upper limit to planetary density, which is hard since it's hard to see small, dense objects.
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.
We've been debating this here: vote totals so far:
Sedna is:
tenth planet 17 votes - 29 %
the eleventh planet 14 votes - 24 %
the 42nd planet 9 votes - 15 %
not a planet! 17 votes - 29 %
Energy: time to change the picture.
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.
Cool - another point to debate. What is the transition point from 'planet-moon' to 'bi-planetary' ?
Basing it on the center of gravity seems like a good idea, but 'well off from the center' is a little bit fuzzy. We could pick a number - say, 50% of the larger planet's radius - in which case the Earth-Moon system meets the criterion, since the center point is about 75% of the Earth's radius away from the Earth's center (some references).
But now we've done the same thing the original article was complaining about - we picked an arbitrary value, just, well, because.
It's seems like a physical point would work a bit better - say, the surface of the larger planet. Then the definition becomes a bit easier: if the center of gravity is in space, it is a dual-planet system. Otherwise, it's a planet-moon.
How you categorize a center of gravity within an atmosphere is left as an exercise ...
The IAU's current concern is to distinguish between extrasolar planets and dark stars. It takes about 13x the mass of Jupiter before an object generates the gravitational pressure needed to ignite the D-D reaction. So the IAU says that if it's smaller than 13x Jupiter, it's a planet. Bigger than that, it's a "brown dwarf" if not shining.
Tomatoes are planets.
Know your pads. One time pad: good for cryptography. Two timing pad: where to take your mistress.
When we see a white dwarf star orbiting around a red giant we don't quibble about whether the dwarf is a planet or star or star-oid or whatever. We classify it according to its intrinsic properties.
It would be useful to classify smaller bodies in the same way, regardless of their orbital situation:
1) Gas giants.
2) Bodies made of heavier elements large enough to have vulcanism and tectonic activity.
3) Smaller solid bodies large enough to be rounded by gravity.
4) Even smaller solid bodies, sub-classified into rocky and iron.
5)Dirty snowballs.
Of course there will not be an exact boundary between these classes. For example, vulcanisn in "planets" is fueled by long-lived radioactives but in Jupiter's "moons" by tidal energy.
Possibly there could be intermediate objects between 1) and 2) or between 1) and 5) or 3) and 5). Concievably there could be bodies made of water or solid methane or blue cheese but these would be unlikely to form naturally.
I believe the word "planet" has origins in Greek, and means "wanderer". Hence, the "planets" were called such because they appeared as wandering stars by human eyes on earth. Certainly anything in orbit around our Sun, out to about a light year, will have enough "wandering" associated with it (by parallax due to Earth's revolution about the Sun) to fit this classification. Unfortunately, that simply isn't a very useful way to classify things since we already have some sort of common notion about what "planet" means, even though it still a little vague.
Orbital inclination might be a good start to finding a not-so arbitrary way to classify things as "planets" or maybe at least as "primary planets". Maybe everything with an orbit inclined less than 10 degrees from the ecliptic can be a planet. Other than the specific degree-cuttoff, this criteria is not arbitrary, since the ecliptic plane exists due to the rotation of the Sun. Things orbiting far off of the ecliptic are more likely to be things which were thrown out of the system at the onset of fusion, so maybe they shouldn't be called "planets", or maybe they can be "secondary planets". (Pluto's orbit is inclined *17 degrees*, the next most inclined is Mercury at *7 degrees*)
Perhaps we can add a mass criterion based on the size of the Earth, which is admittedly totally arbitrary, but when coupled with orbital inclination, it's much less so. Maybe nothing with less than 1% the mass of the Earth should be classified as a planet. That excludes asteroids from accidentally being "planets" under the orbital inclination criterion, but we can still have Mercury, which has about 5.5% the mass of the Earth. (Pluto has about 0.2% the mass of the Earth)
Finally, to account for moons, we could add that for multiple bodies orbiting one another, and together orbiting the Sun, in the case that both are more than 1% Earth-mass, the one containing the center of mass is the planet and the other(s) the moon(s). If the center of mass is outside both or all (if there are many), then it's a double- or triple- planet system.
How does that sound?
-=[You cannot consistently judge this statement to be true.]=-
You might be disqualifying Mercury from planethood, which would be odd, considering it's one of the original 5.