Pluto is no more

[Las Naranjas]

Textbooks are generally a mangled version of what was cutting edge 3 decades ago anyway.

[deadsuperhero]

On the bright side, if we take all the "incorrect" textbooks, we'd each get an instant collectable. Then we could wait some 50 odd years and pass it along as a family heirloom, until some stupid descendant would sell it at an auction and make thousands of dollars off of it.

[EagerMind]

Previously to this, there was no scientific definition of the word "planet". That was the whole point of the exercise. According to the one they decided on, it's not one in the scientific sense, since they couldn't come up with a sane (i.e. not ridiculously gerrymandered) one that limited our solar system to the nine classical planets.

It sounds like they could have used the assistance of some of those people that negotiate athletes' salaries. I'm sure they could have come up with some sort of "Pluto clause" or something .... 

The article linked in the first post originally had a comment about "erasing" Pluto out of the textbooks. Makes you kind of wonder how this is done: will they in fact "erase" Pluto, or just add a blurb about it being "reclassified?" Makes you kind of wonder what's been "erased" in our textbooks before now ....

[Shane 'ProgZmax' Stevens]

NEWZ UPDATE!!!11
http://uncyclopedia.org/wiki/UnNews:Vogons:_Earth_no_longer_a_planet

Also, if you actually go to the source rather than rely on watered down soundbites on news pages you can read about the details of their silly re-classification scheme.

http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html

It was a rather amusing read.  Scientists/astronomists every so often get an urge to be recognized and remembered.  This is such a case.  We've went with a simple but effective definition of a planet for many years but now it's extremely necessary to create an overcomplicated one?  And why does it have to include 'clearing the neighborhood around its orbit'?  It seems rather arbitrary that planets can no longer be planets if they move in an elliptical--and bear in mind that Pluto's orbit is somewhat elliptical--orbit--yet that's one of the definitions of a dwarf planet as well!  The 'very nearly' clause in their definition is absolutely arbitrary as well.  What classifies very nearly spherical?  They haven't bothered to release evidence of what 'very nearly means' aside from our existing 8 planets and Pluto, so I really can't respect this kind of arbitrary science.  I'd still like to know why they feel that 'clearing the neighborhood' around its orbit is tantamount to a planet's behavior.  Does this mean if Venus' orbit shifts slightly and its path overlaps with Saturn it will become a dwarf planet as well?  According to the new definitions the answer is yes.  Bollocks.

[Erenan]

That had me worrying there for a moment, anxiously searching for traces of sarcasm.

Its... not a difficult word... or are you refering to the obvious mispelling "Dr. Pepper"?

EDIT:

Because, as we all know, there hasn't been a period in the name "Dr Pepper" since the 1950's.

No sarcasm. I was serious. I was referring to the fact that there's no period.

Sarcasm: I think we should destroy Pluto with explosives.

[Cosmo]

If Pluto isn't a proper planet anymore, where does that leave Charon? It can't be a dwarf planet itself, as it doesn't orbit the sun.

does it get down-graded from a moon to "one large lump of rock"?

How sad. :'(

[Las Naranjas]

Since the centre of gravity of the Pluto-Charon system isn't within either body, it's actually a binary dwarf planet, both are going around the sun together, as opposed to one doing so and the other around it.

At least, as far as following the straightforward definitions we have now, which are far far better than the old definition which....didn't exist and therefore had little practical value in such a state of being.

[Traveler]

We've went with a simple but effective definition of a planet for many years but now it's extremely necessary to create an overcomplicated one?

As far as I know, there was no clear definition for planets, but I might be wrong. The definition that I knew was that a planet is a celestial body that orbits a star and does not shine with its own light. A comet can easily make this definition, while we clearly don't intend to classify it as a planet.

The 'very nearly' clause in their definition is absolutely arbitrary as well.  What classifies very nearly spherical?  They haven't bothered to release evidence of what 'very nearly means' aside from our existing 8 planets and Pluto, so I really can't respect this kind of arbitrary science.

"Very nearly" is actually a fairly close definition. It means, that the planet has large enough mass so that gravity can distort it's shape into a perfect sphere.

However, all planets rotate around their axis (it's impossible to not to*), and the forces that awake because of the rotation slightly distort the shape to "very nearly spherical": the planet is squeezed along the rotation axis and bulges out a bit along its equator perpendicular to the rotation axis.

So, if a celestial bode doesn't have a large enough mass, gravity will not be strong enough to distort its shape.

* A planet *may* lose its angular momentum over time, due to forces acting on it and stop rotating. Earth, for example, will stop rotating in the distant future, becase tidal friction is slowing down its rotation.

I'd still like to know why they feel that 'clearing the neighborhood' around its orbit is tantamount to a planet's behavior.  Does this mean if Venus' orbit shifts slightly and its path overlaps with Saturn it will become a dwarf planet as well?  According to the new definitions the answer is yes.

All planets orbit their star in an elliptical orbit. Even a theoretical perfect circle orbit is elliptical, as a circle is just an extreme case for an ellipse. Anyway, Pluto's orbit is highly elliptical and tilted, which suggests that it didn't form as planets of the solar system, but in some other way.


I actually find it somewhat amusing to see what people think about Pluto. I just came back from Washington DC and I saw that inside the space museum, the part regarding Pluto was framed black and someone put a flower under it.

Pluto is not going away, it's going to be a named part of the solar system, it just won't be called a "planet" anymore.

[Shane 'ProgZmax' Stevens]

As far as I know, there was no clear definition for planets, but I might be wrong. The definition that I knew was that a planet is a celestial body that orbits a star and does not shine with its own light. A comet can easily make this definition, while we clearly don't intend to classify it as a planet.

1.  A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

It's a simple definition agreed, but it has worked for years.

"Very nearly" is actually a fairly close definition.

Hogwash.  Quantify very nearly for me.  All of the planets swing in elliptical orbits, whether extreme or not.  Nevertheless, Pluto's orbital shape isn't the issue here, it's the overlap with Neptune.

Observe:





You will note that the only anomaly Pluto seems to suffer from is sharing an overlapping path with Neptune.  This seems like a rather poor reason to discount Pluto as a planet and makes the new definition seem arbitrary to me.  Why?  A meteorite impact of sizeable mass could push any of the planets in our system off orbit just enough to create a similar situation, and then by the new definition they would no longer be planets.  Based on articles I have read, it seems more likely to me that the definition was made to quell the Xena (UB313) and Ceres controversy that has sprung up in the last few years.

[MrColossal]

http://www.nasm.si.edu/ceps/etp/pluto/pluto_orbit.html

maybe this will help understand pluto's orbit better than your images.

http://www.space.com/scienceastronomy/solarsystem/pluto-ez.html

some pluto facts!

[Traveler]

1.  A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

It's a simple definition agreed, but it has worked for years.

You must have used a different source than I did for the planet definition. This itself shows that there was a problem with that definition, since we came up with two different (albeit very close) definitions. So having a unified definition is actually good, because there is only one way to define a planet.

Anyway, there is no clear-cut definition for asteroids, either.

Hogwash.  Quantify very nearly for me.  All of the planets swing in elliptical orbits, whether extreme or not.  Nevertheless, Pluto's orbital shape isn't the issue here, it's the overlap with Neptune.

Of course it is not the *shape* that is the issue (I didn't imply it was. Pluto actually has a fairly spherical shape, but that partly comes from the fact that it's a binary system with Charon, so tidal forces help to achieve the sperical shape.)

The issue is the *mass* and the *force of gravity* that awakens from that mass. A planet - according to the new definition - should have enough mass to generate sufficient gravity to form a sphere on its own, without outside help.

This is also the reason for the clause to "clear the neighborhood": if a planet is massive enough, it'll have enough gravity to attract smaller bodies from a distance, thus sweeping an elliptical path in the accretion disk around a newly formed star. A small body would have a lot less gravity (gravity weakens according to the inverse square law), so it would have a lot less chance to do the same.

That's why there are no significant bodies around the orbit of Mercury, Venus, Earth, Mars, etc. - they swept a significant path out by accreting matter from the dust when they formed.

-------
Note: I think you have a misunderstanding here. I just realized, that when you talk about shape, you talk about the shape of the orbit, right? Ignore this if not.

The shape of the orbit was not even a concern for Pluto - it was the *mass* of the planet that was a issue. The shape of the orbit didn't matter at all - it is not a requirement for a planet (even according to new rules) to have a nearly circular orbit. The requirement is to have a nearly spherical shape for the *planet* itself.
-------

And I think it is wrong to ask for an actual, quantified rule. Astronomers are well aware that the classification is artificial - it's really there to help sorting different bodies in the sky better. If we started quantifying, we could have 9 categories for planets right away.

So "very nearly spherical" tells us a good rule of thumb to make a decision, especially because we use the mass and the force of gravity as the basis, which are well-known quantities for known celestial bodies.

Mr. Colossal's links are right: there is no overlap between the orbit of Pluto and Neptune. The problem is that Pluto has a tilted orbit, so it implies that it didn't form as part of the planet formation process in the solar system.

When a star forms, a significant amount of matter orbits it, shaped like a wide, narrow disk, in the plane of the star's equator. This accretion disk is the source for the planets' material: there are different chunks of matter in the disk. Larger chunks have slightly greater gravity, so they have a better chance of attracting other (smaller) chunks and become even larger. But this results in planets in the plane of the accretion disk. Pluto could not have formed this way because of its tilted orbit - it must have joined the inner solar system in a different way and got hooked by the Sun's gravity.

[Nacho]

The diagrams showing the orbit of Pluto are generally wrong, they're just simplifications to make the representation easier. Actually, when the distance between Pluto and the Sun is higer, the distance between Pluto and Neptune is almost two times the distance between Pluto and the Sun.

Neptune is 30 AU from the Sun and Pluto reaches 50.

[EagerMind]

... if you actually go to the source rather than rely on watered down soundbites on news pages ...

Yeah, it does seem like a strange exercise. One thing that caught my attention is that this definition only applies to stuff in our solar system (since it explicity requires orbit around our Sun). This would imply that "planet"-like objects orbiting other stars have a different name, and are presumably described by a different definition. Indeed, Wikipedia describes them as "extrasolar planets" and their definition seems to depend on whether they're big enough to cause fusion (i.e. "they're not a star"). Talk about arbitrary.

It also seems strange that they're trying to classify these objects based on how they orbit the sun. It seems akin to classifing birds based on how they fly. I would think a more "scientific" method would be some physical critera that distinguishes planets from other objects. A quick search of Wikipedia would seem to indicate that all planets have a distinguishable core, as opposed to comets and asteriods which are basically rocks in space. Of course, this definition would seem to still leave out Pluto, but it seems like more solid reasoning to me than the fact that Pluto has a different orbit from the other planets.

[Traveler]

Yeah, it does seem like a strange exercise. One thing that caught my attention is that this definition only applies to stuff in our solar system (since it explicity requires orbit around our Sun). This would imply that "planet"-like objects orbiting other stars have a different name, and are presumably described by a different definition. Indeed, Wikipedia describes them as "extrasolar planets" and their definition seems to depend on whether they're big enough to cause fusion (i.e. "they're not a star"). Talk about arbitrary.

Nothing is arbitrary here. The limit that a planet doesn't cause fusion is the same as saying it doesn't have its own light, so it's really the same definition. You can replace Sun with the name of the other star and you get the same definition of a planet. The mass limit that causes nuclear fusion is very well known, and nothing is arbitrary about that.

We only know of *huge* planets in other star systems, simply because planets do not have their own light and it's very complicated to detect even a really big one. For big bodies, this definition is fine and when we discover smaller planets, we can see if the entire definition is good enough to cover all things that we would "instinctively" call a planet, but not others.

It also seems strange that they're trying to classify these objects based on how they orbit the sun. It seems akin to classifing birds based on how they fly.

I think you misunderstand it - no one is classifying celestial bodies based on their orbits. They're classified based on their mass - read my previous entry. The shape of the orbit is irrelevant.

The problem with Pluto's orbit is that it crosses the ecliptic plane of the solar system - this plane is well established by the orbits of other planets. So it tells us that 4.5 billion or so years back there was a disk of material around the Sun and the planets formed from this. The 8 planets' orbits fall very nearly into the same plane, with Mercury having the greates deviation, 7 degrees. Pluto has a much greater deviation, 17 degrees, which tells us that Pluto didn't form the same way.

I would think a more "scientific" method would be some physical critera that distinguishes planets from other objects.

There isn't any. The only really distinguishing property is mass. In our solar system, planets are made of different material, they have different sizes, different densities, different rotations, etc. Mass is pretty much the only thing that can be used as a general measure.

[EagerMind]

I'm starting to get the feeling that there's more to the story here than has been let on. Don't get me wrong, I'm not necessarily disagreeing with reclassifying Pluto, I'm just trying to understand the reasoning behind it. Traveler, you've provided the best reasons that I've seen so far.

But consider the news article that was originally linked to. It stated that the main impetus for the reclassification was to prevent having 50 planets in our solar system. But (as the article also states) if we're trying to describe the solar system "as it really is, not as we would like it to be," why would the number of planets in our solar system be any concern? Simply having too many planets seems like a bad reason, because than any distinction we make to keep the number down will be arbitrary. Discovering some new information or new understanding of our solar system that reveals some fundamental difference between Pluto and the rest of the planets seems like a good reason.

There also seems to be problem with the new definition. One of the requirements is for a planet to have "cleared its orbit of other objects," and Pluto is eliminated because it overlaps with Neptune. But doesn't the door swing both ways? Can't we say Neptune hasn't cleared Pluto from it's orbit, and therefore isn't a planet either? There must be a piece missing here.

Yeah, it does seem like a strange exercise .... Talk about arbitrary.

Nothing is arbitrary here.

I wasn't actually referring to the definitions themselves being arbitrary, but I may not have made that clear. My comment was referring to the fact that there are different definitions for planets and intersolar planets, with Wikipedia suggesting that there isn't even an agreed upon definition for intersolar planets. Functionally these definitions may be the same, but it implies to me that, realistically, we're approaching this from different angles.

Surely, it's reasonable to assume that whether we're talking about our own solar system or anyÃ, number of others, we're comparing apples with apples. Of all the uncountable number of solar systems in the galaxy, why should we assume ours is special, especially with how little we actually know? Let's get down to one definition and start trying to prove (or disprove) it.

I think you misunderstand it - no one is classifying celestial bodies based on their orbits. They're classified based on their mass ....

The problem with Pluto's orbit is that it crosses the ecliptic plane of the solar system ... which tells us that Pluto didn't form the same way.

This seems reasonable, and certainly points out something fundamentally different about Pluto from the rest of the planets. But I'm having a hard time seeing the connection to mass, or that this is the intention of the new definition. If this is the case, it would imply the following:
1. There's a relation between how big something is and its ability to wander through space (i.e. planets can't wander).
2. Pluto's orbit doesn't match the other planets', so it must have wandered in.

But if this is our intention - to define planets as something that was originally formed in the system in which it currently orbits - why not just make that the definition?

[Traveler]

...Traveler, you've provided the best reasons that I've seen so far.

Thank you, I'm glad to be of help.

Simply having too many planets seems like a bad reason, because than any distinction we make to keep the number down will be arbitrary.

Keep in mind, that the definition of a planet (or any other celestial body) will necessarily be arbitrary. Nature itself doesn't care about classifications: these bodies simply exist. The need to classify them is a human need, so we introduce categories.

The reason for the definition being arbitrary is simple: celestial objects come in all sizes (the range is continous, like the range of rational numbers), while we want just a few distinct categories. Pluto was not excluded because astronomers wanted to exclude it - it got excluded because the rule that we use to determine if an object is a planet doesn't apply to it.

One of the requirements is for a planet to have "cleared its orbit of other objects," and Pluto is eliminated because it overlaps with Neptune. But doesn't the door swing both ways? Can't we say Neptune hasn't cleared Pluto from it's orbit, and therefore isn't a planet either?

"Clearing its orbit" means that the planet must stand out. This is not a quantitative measure, but one can say by looking if an object stands out. Pluto is really part of a set of other objects (maybe part of the Kuiper belt), which orbit the Sun beyond the orbit of Neptune. Pluto (and the other objects in that area) are just too small to generate enough gravity to attract all other small bodies from the region - so they cannot stand out, they appear as part of a crowd.

Now, the distance between Pluto and Neptune is really big. Pluto's orbit is tilted and the orbital cycles of Neptune and Pluto are such that when Pluto comes close to Neptune's orbit, Neptune is always far away from there.

Here comes the requirement of mass: Pluto couldn't possibly clear Neptune out of its orbit if they actually got close to each other (it's waaay too small), while Neptune could easily digest Pluto. And if you look at the general neighborhood of Neptune, you'll see that there is nothing in its path - it's really clear, just like the other planets.

Functionally these definitions may be the same, but it implies to me that, realistically, we're approaching this from different angles.

I believe the reason for this is that we know almost nothing about planets in general. We (humans) never left the Solar system and we detected only somewhat more than 100 planets in other solar systems. These are mainly gas giants, simply because a planet doesn't shine with its own light, so any smaller planets will be flooded out by their star's light. Huge planets can be detected by measuring the drop in brightness of their star when the planets periodically cross the line of sight between the telescope and the star.

On the other hand, we have a fairly good knowledge about the planets in our Solar system and the new planet definition is generic enough to use as a starting point to classify extrasolar planets.

But I'm having a hard time seeing the connection to mass, or that this is the intention of the new definition. If this is the case, it would imply the following:
1. There's a relation between how big something is and its ability to wander through space (i.e. planets can't wander).

There is no such relation. Any object in space can wander anywhere, as long as it's not under the influence of a greater mass that forces it to orbit (or fall in.) This is why I wrote that the orbit itself is irrelevant for the planet definition.

Greater mass results in greater gravity, which helps the object to attract smaller objects (thus to "clear the neighborhood" and stand out.) Attracting small objects increases the object's mass and its ability to attract other objects further.

2. Pluto's orbit doesn't match the other planets', so it must have wandered in.

But if this is our intention - to define planets as something that was originally formed in the system in which it currently orbits - why not just make that the definition?

Pluto's orbit is really just one of many similar objects at the edge of the inner Solar system. Don't forget that the Solar system doesn't end with Pluto's orbit: there are the Kuiper belt and the Oort cloud out there with tens of thousands of objects of different sizes and orbits. Most of these are very far away and very small, so we never saw them, but they're still there and they all orbit the Sun.

So Pluto didn't have to wander in from outside the Solar system - it comes from a region where there are many other objects with similar sizes and orbits. (Pluto's orbit is quite typical of Kuiper belt objects. This may have been one factor in not classifying it as a planet anymore. I think, though, that the linked article is in error stating that this was the sole reason.)

Edit: some typos fixed.

[Cosmo]

This thread made me dig out my old Astronomy text book i had for the semester i took of it.

"Discovering the Universe" by Neil F. Comins and William J. Kaufmann III



That last collumn is "Magnetic Field" and says Pluto's is unknown, yet it has been mentioned before in this thread...is my reletivly new text book out of date? (I mean discluding Pluto's status loss.

You know, I feel I could make an adventure out of this, "Pluto: The Little Planet That Couldn't."

[Nikolas]

Honestly...

Who cares?

Apart from the fact (I heard it, don't know if its true), that it's the only planet discovered by Americans I simply don't understand it.

Every forum I've been has a thread about poor pluto...

weird...

PS. Interesting though, as you get to leatn a little astronomy

[Nacho]

So... somebody cares?

[MrColossal]

You're right Nik, who cares... Let me just go into every thread on this forum talking about anything and write "Honestlly, Who cares!" in it.

Maybe if you phrased your question: "Does this class changing have any significance to the average person or is it mostly a classification thing that astronomers and scientists will use to further a general understanding of the universe?" your post wouldn't have seemed so rude.

To answer your hypothetical question I fear that the classification changing does have an impact on the average person and all it will do is help feed a distrust in science that I feel is held by many people. "Oh those scientists! This is what they're wasting our money on!" "I'm still going to say Pluto is a planet! These scientists have no idea what their talking about!" "If they were wrong about Pluto for so many years they must be wrong about other things therefore you can't trust them!