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Is Pluto A Planet? (Merged Thread)


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#1    Waspie_Dwarf

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Posted 23 June 2006 - 12:59 AM

Pluto - is it a planet?

The British National Space Centre (BNSC) press release is reproduced below:

At its conference this August, the International Astronomical Union (IAU) will make a decision that could see Pluto lose its status as a planet.

For the first time, the organisation will be officially defining the word "planet", and it is causing much debate in the world of astronomy.

There is only one thing that everyone seems to agree on: there are no longer nine planets in the Solar System.

The debate has been brought to a head by the discovery of a potential 10th planet, temporarily named 2003 UB313 in January 2005. This new candidate planet is bigger than Pluto.

The question now facing the IAU is whether to make this new discovery a planet.

Pluto is an unusual planet as it is made predominantly of ice and is smaller even than the Earth's Moon.

There is a group of astronomers that are arguing for an eight-planet SolarSystem, with neither Pluto or 2003 UB313 making the grade as a planet; but a number of astronomers are arguing for a more specific definition of a planet.

One of these; Kuiper Belt researcher Dr Marc Buie, of the Lowell Observatory in Arizona, has come up with a clear planetary definition he would like to see the IAU adopt.

I believe the definition of planet should be as simple as possible, so I've come up with two criteria," he said.

"One is that it can't be big enough to burn its own matter - that's what a star does. On the small end, I think the boundary between a planet and not a planet should be, is the gravity of the object stronger than the strength of the material of the object? That's a fancy way of saying is it round?"

This definition could lead to our Solar System having as many as 20 planets, including Pluto, 2003 UB313, and many objects that were previously classified as moons or asteroids.

One possible resolution to the debate is for new categories of planet to be introduced. Mercury, Venus, the Earth and Mars would be "rocky planets". The gas-giants Jupiter, Saturn, Uranus and Neptune would be a second category.

Whatever the outcome of this debate there is only one thing that we can be certain of; by September 2006 there will no longer be just nine planets in our Solar System.


Source: BNSC press release

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#2    OrthoChristian

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Posted 26 June 2006 - 02:58 AM

I think that Pluto should remain designated a planet. It has been so since its discovery in 1930, so for 76 years we have thought that Pluto is a planet, so I think it should remain so. And in 2005 we have discovered an additional 2 moons circling Pluto for a total of 3. yes.gif


#3    SAMURAI-X

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Posted 26 June 2006 - 03:11 AM

Why must they make things so difficult

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#4    Waspie_Dwarf

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Posted 26 June 2006 - 07:50 AM

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Why must they make things so difficult


Unfortunately nature has made it difficult. It was the fact that astronomers tried to keep it simple that got us into this mess in the first place.

Quote


I think that Pluto should remain designated a planet. It has been so since its discovery in 1930, so for 76 years we have thought that Pluto is a planet, so I think it should remain so. And in 2005 we have discovered an additional 2 moons circling Pluto for a total of 3. yes.gif


For 76 years Pluto was thought to be a unique body in a unique orbit. It isn't. It's just one of thousands of Kuiper Belt Objects, and not even the largest.

If Pluto is a planet then other larger KBOs will also be planets. So what becomes the lower limit in diameter for a planet? If some arbitary diameter is decided for the lower limit then this problem may just return. What if a body is found which has a diameter about that of the limit? There will be debates about whether it is or isn't a planet just as there is with Pluto.

What is needed is a scientific definition which settles the question once and for all. I hap the IAU have the courage to do the right thing and tell the world that Pluto isn't a planet.

The number of moons it has got has no bearing on whether it's a planet. Many small irregular shapped asteroids have moons.

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#5    FrankBlunt

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Posted 26 June 2006 - 09:08 AM

In 1991 when I began college, one of my general ed courses happened to be Astronomy.  The professor included an exam question in reference to Pluto, asking what the implications would be if Pluto became declassified.  In my infinite wisdom, I responded, "They'll have to modify current charts of the solar system [and create new editions of textbooks at a whopping cost to students]."

Adjacent to the X, granting me zero points for such a submission, the professor noted the triviality of my input.  But I ask, of the implications for a planet so distant and unreachable, what implications could we possibly mitigate or prevent?  Pluto will eventually collide with Neptune, but that occurs regardless of Pluto's planetary status.  

When volcanoes erupt on Jupiter's moon, Io, lava is ejected.  That's the consquence.  Does it affect me?  No.  Can I do anything about it?  No.  Am I cynical about the Plutonic test question after 15 years?  Indubitably.  grin2.gif

Does anyone else have an idea as to what these grand implications may be in the face of declassification?

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#6    ROGER

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Posted 27 June 2006 - 09:06 PM

tongue.gif And if in 200 years a valuable mineral is found on Pluto , the same debate will start again . Asteroid miners verses Governments selling planetary property.

I smell a SCI-FI story in the works.  thumbsup.gif

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#7    Trinitrotoluene

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Posted 16 August 2006 - 08:35 AM

Source : http://www.cosmosmagazine.com/node/563

Quote

SYDNEY, 16 August 2006: The Solar System may gain an extra three planets after today’s meeting of the International Astronomical Union (IAU) in Prague, upsetting text-books and science-classroom mnemonics world-wide.

With the advent of ever-more powerful telescopes, objects orbiting our Sun out beyond Pluto have been discovered. Some of these, Xena being the most famous example, are actually larger than Pluto and, its discoverers declare, should be considered as much of as planet as the others.

Other astronomers contest that these far distant objects are merely bits of rubble in an area of space filled with other bits of rubble, the Kuiper Belt.

The discoveries and the associated debate has thrown into question the definition of a ‘planet’ (see Cosmos, Issue 9 p 74) and at its general meeting this week, the governing body of such matters, the International Astronomical Union has sat down to nut out the problem.

The solution they propose is to describe all the objects as planets. Eight of them will be the ‘classical’ planets we have come to know and love. Ceres, an asteroid floating in the asteroid belt between Mars and Jupiter will become a planet. And a kind of sub-planet category known as 'plutons' will be invented for the remaining three planets. They will be Pluto, it's moon Charon and 2003-UB313, nick-named Xena. Xena's real name will be determined at a future date.

The part of "IAU Resolution 5 for GA-XXVI" that describes the planet definition, states: "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (cool.gif is in orbit around a star, and is neither a star nor a satellite of a planet."

A recent poll of Cosmos Online readers found that a slim majority (28 per cent) preferred Pluto remaining a planet. Twenty per cent of respondents thought Pluto should be a Kuiper Belt Object, and 26 per cent wanted a new category for Pluto and the Kuiper Belt Objects. Some 25 per cent of readers playfully suggested that Pluto was a Disney dog.

Plutons are to be distinguished from classical planets in that their orbits around the Sun that take longer than 200 years to complete and their orbits are highly tilted with respect to the classical planets. Plutons also typically have orbits that are far from being perfectly circular.

The IAU said that all of these distinguishing characteristics for plutons are scientifically interesting in that they suggest a different origin from the classical planets.

Member of the Planet Definition Committee, Richard Binzel said, "Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet."

The draft “Planet Definition” resolution will be discussed and refined during the General Assembly and then it will be presented for voting on 24 August 2006.


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Posted 17 August 2006 - 08:38 AM

I saw this in the paper today.  Most interesting.  Though since our solar system is still the same size, it actually feels a little smaller.

As Douglas Adams might say - Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.  The addition of three planets newly categorized in our solar system has made space look tiny, but scientists assure us that space is still big.  Tiny would be microscopic.  Bacteria, viruses, nano-technology, that is tiny.  So have no fear folks, space is still big.

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#9    Startraveler

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Posted 18 August 2006 - 03:10 AM

As has been reported in at least two threads the International Astronomical Union has at long last offered up a definition of the word "planet":

"A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and ( b ) is in orbit around a star, and is neither a star nor a satellite of a planet."

In other words, if it's round(ish) and circles a star, it's a planet.

Not surprisingly, many astronomers think this definition sucks. The very first asteroid ever discovered--the first of its class--would become a planet under this new definition. In fact, if the IAU votes in this new definition the solar system will have at least 12 planets; more likely, the number of planets will be ballooning into the hundreds. That impressive list of exoplanets might start to look a bit sparse when compared with our own solar system. Indeed space.com has an entire article about just how divided the astronomical community is becoming over this one:

Quote

An informal SPACE.com survey of astronomers who study planets in and out of our solar system found six in favor of the resolution and seven against. A separate private straw poll being conducted by the National Academies of Sciences has so far yielded an overwhelming "No" response, a source told SPACE.com.

'Terrible definition'

Clearly no consensus has emerged, however.

"I think it's a terrible definition," said David Charbonneau, a researcher at the Harvard-Smithsonian Center for Astrophysics who searches for and studies planets around other stars. Charbonneau joins two other astronomers close to the issue who sharply criticized the plan [see yesterday's story].

Charbonneau said the definition was motivated by a desire to determine whether Pluto and another object, 2003 UB313, are planets. But the IAU now says there are a dozen other objects that might be planets but need further study.

"It is ironic that we are left with more, not fewer objects for which we are uncertain of their 'planetary' status," Charbonneau told SPACE.com. "Perhaps astronomy will undergo a schism, with sects of astronomers proclaiming different numbers of planets."

"As representatives of an international community of planetary scientists, we urge that the resolution be approved," said the DPS statement, signed by chairman Richard French of Wellesley College.

In an email interview, French said he supports the defintion but realizes its shortcomings.

"My own personal definition would have been different from the final IAU resolution, but scientists have been stalemated for years by defending their own pet definitions," French said. "I understand the appeal of a simple declaration that Pluto is no longer a planet and that the solar system has only eight, but I also think there is value in the present definition that has applicability to planets around other stars as well."

The DPS has about 1,300 members, at least one-quarter of which are outside the United States. The statement does not represent the views of all members, said DPS Press Officer Sanjay Limaye. "There has been some feedback saying, 'I don't like it,'" he said.

'Worst' decision

The definition would make a planet of the asteroid Ceres and also reclassify Pluto's moon Charon as a planet, on the logic that the center of gravity around which Charon and Pluto orbit is not inside Pluto but rather in the space between them. (Earth's Moon orbits our planet around a center of gravity that is inside Earth.)

Pluto and Charon would be called a double planet, and they'd also be termed "plutons" to distinguish them from the eight "classical" planets. Ceres would be termed a dwarf planet.

The definition entirely misses the key element of a solar system object, namely its role in the formation of the solar system," Charbonneau said. "There are eight fully formed planets. The other objects—Ceres, Pluto, Charon, [2003 UB313], and hundreds of thousands of others, are the fascinating byproducts of the formation of these eight planets."

David Jewitt, an astronomer at the University of Hawaii who searches for objects in the outer solar system, told SPACE.com that the proposal is "the worst kind of compromised committee report."

Jewitt has long avoided the whole debate over whether Pluto is a planet "because I think it is essentially bogus and scientifically it is a non-issue." He waded in reluctantly this week.

"Scientifically, whether Pluto is also a planet is a non-issue," Jewitt writes on his web site. "No scientific definition of planet-hood exists or is needed. Is that a boat or a ship? It doesn't matter if you are using it to float across the ocean. Scientists are interested in learning about the origin of the solar system, and setting up arbitrary definitions of planet-hood is of no help here."

Geoff Marcy, who has led the discovery of more planets around other stars than anyone, called the definition arbitrary.

"Pluto, its moon, and large asteroids cannot suddenly be deemed planets," Marcy said in an email interview. "How would we explain to students that one large asteroid is a planet but the next biggest one isn't?"

Astronomers made a mistake when they deemed Pluto a planet in the 1930's, Marcy and many other astronmoers say. "Scientists should show that they can admit mistakes and rectify them," he said.

'Just might work'

However, one mild endorsement came today from Brian Marsden, who heads the Minor Planet Center where asteroids, comets and other newfound solar system objects are catalogued.

Marsden was on an IAU committee of planetary scientists that tried for a year but failed to come up with a definition for the word "planet," which was never needed until recent discoveries of Pluto-sized worlds out beyond Neptune. The newly proposed defintion was crafted by a second IAU committee of seven astronomers and historians.

Marsden is a firm believer that there are eight planets, but the new proposal has him sounding more flexible than in the past.

In an email message from Prague, Marsden said the new definition is "intended to satisfy the eight-planet traditionalists (such as myself) and the 'plutocrats.'" He added that he's "not against" the idea of using roundness as a determining factor.

The IAU proposal will be voted on by IAU members Aug. 24.

"It all just might work," Marsden said.

In email interviews, several experts in planetary science share their views:

"The definition itself is not that important.  There are lots of interesting bodies out there for us to study.  We need to have a definition, though, because it makes it easier for people to understand what we mean."
—Amy Simon-Miller, NASA scientists and member of the DPS Committee that endorsed the IAU resolution

"I think most astronomers agree that there are eight planets, and (like myself) are not particularly passionate about either Pluto's status or the outcome of the 'debate.' It's clear, however, that removing Pluto from the list rouses strong emotions within the public (who ultimately pay the bills). So I would just retain the eight planets plus Pluto."
—Gregory Laughlin, University of California, Santa Cruz extrasolar planet researcher

"It [the definition] makes a lot of sense. There has to be a physically meaningful definition for a planet since we are finding lots of KBOs and planets around other stars.  If you had an arbitrary cutoff at say Pluto or even Mercury, how would you justify it when looking for other bodies in the solar system or in other stellar systems?"
— Larry Lebofsky, senior research scientist at the Lunar and Planetary Laboratory

"My prejudice is to restrict the definition of planet and put Pluto and its large Kuiper Belt cousins in a different class...with a name to be determined (planetoid seems to serve well)."
—Jonathan Lunine, professor of planetary science and of physics at the Lunar and Planetary Lab

"The whole debate, with Pluto as the pivot point, seems a bit silly to me, to make such a big deal of it. If planets are round, then there are a whole lot more than 12 of them."
—Laurance Doyle, SETI Institute extrasolar planet researcher

"The astronomers who oppose the resolution on pure or ostensibly pure science grounds find the criterion that makes Charon a planet —the center of mass is outside the body of the more massive partner—most objectionable.  I also think that this criterion is new to them and they might fine it less objectionable after it gets to be a familiar rule."
—Stephen Maran, retired NASA astronomer and author of " Astronomy for Dummies"


There are, of course, lots of suggestions for just what a planet is. Mike Brown, the astronomer perhaps most responsible for the current predicament besides Clyde Tombaugh, has a webpage that discusses possible definitions one could consider. Brown is the discover of objects like 2003 UB313 and Sedna and if you read that first space.com (along with the link to his page just provided) you'll see he's no fan of this new proposed definition. On that page Brown considers scenarios like the Purely Historical (the 9 planets we all know and love), a Historical Plus classification system (the 9 planets plus anything larger than Pluto), and the sort of Gravitational Rounding definition being considered by the IAU right now. He ultimately dismisses this last one, by the way, writing "Roundness is an important physical property, and gravity is the dominant force in the solar system, so perhaps it  is  important to have a special word which describes the class of objects in the solar system which are round. But simply because all historical planets are round does not at all mean that it is good science to define all round objects to be planets."

The definition Brown likes the best hinges on how social the object in question happens to be:

Quote

Population classification.  This definition requires a little more explanation and a little more understanding of the solar system, but, in the end, leads to the most satisfactory definition of "planet". Just like the solar system very naturally divides itself between round objects and non-round objects, it also very naturally divides itself between solitary individuals and members of large populations. The best known example of a large population is the asteroid belt. We call it a population because one region of space contains objects with a continuous range of sizes from one moderately large object (Ceres) to a handful of slightly smaller objects (Vesta, Pallas, Hermione) to a huge number of extremely small objects (rocks, dust particles). The solitary individuals are much different. In their region of space there is only them (Earth, say) and then a collection of much much smaller objects (the near-earth asteroids), with no continuous population in between. A single example helps to dramatize the difference between a continuous population and a solitary individual. Ceres, the largest asteroid, has a diameter of 900 km. The next largest asteroid, Pallas, has a diameter of 520 km. After that is Vesta at 500 km, and Hygiea at 430 km, and the list continues on down. The jump in size between asteroids is never more than a factor of two. In contrast, the earth has a diameter of about 12,000 km, while the largest other object in the earth's vicinity, the asteroid Ganymed, has a diameter of about 41 km, a factor of 300!

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune all count as solitary individuals by this definition. Pluto and Quaoar do not. Pluto is clearly a member of the Kuiper belt population, as can be seen from the fact that there are objects in the same vicinity slightly smaller than Pluto (Quaoar, 2004 DW, Varuna), and then even a larger number slightly smaller than that, and then on down.

What about Sedna? Sedna is currently the only object known in its orbital vicinity, but we strongly suspect that there will be many others found out there with time. We thus feel it is more reasonable to classify Sedna as a member of a large population (the inner Oort cloud of objects) rather than a solitary object. This classification saves us from having to go back and reclassify Sedna in a decade when we find more objects!

Since there is a clear scientific distinction between solitary individuals and members of large populations it is instructive to come up with words to describe these objects. The large populations can each be described by the particular population (asteroid belt, Kuiper belt, inner Oort cloud, Oort cloud). What about the solitary individuals? Isn't the best word to describe them "planet"?

Let's examine this definition in more details. First, it is certainly scientifically motivated and well-founded. But so was the "gravisphere" definition above. Is there any historical basis for saying that a planet is a solitary individual that is not a member of a large population? Yes! As mentioned earlier, historically Ceres and the first few asteroids were initially classified as planets. Only when it became known that there were many many asteroids in similar orbits was it decided that they should no longer be classified as planets. Historically, there is a clear distinction between planets and populations. Any definition which fails to make this distrinction is in strong trouble on historical grounds. This simple look at history shows that Pluto is completely analogous to Ceres. Pluto was initially thought to be a solitary individual. Over time we found more objects in the vicinity and realized instead that it is a member of a large population. Historically, then, Pluto, too, should no longer be considered a planet.

We are thus left with a final concept of the word planet. Every object in the solar system quite naturally can be classified as either a solitary individual or a member of a large population. The individuals are planets. The populations are not. This definition fits the historical desire to distinguish between asteroids and planets, and this definition fits all of the requirements of scientific motivation.

Even this definition is not perfect. People will always be able to imagine (and perhaps even find) pathological scenarios in which the above classification scheme fails. In contrast, the first three definitions are much more rigorous and will never need refining. We don't find this aspect of the first definitions an advantage, however. As we learn more about our solar system our language -- both popular and scientific -- should change to fit our knowledge. We think that our proposed classification scheme will suffice for everything that is found in our solar system, but we would like nothing better than to find some object which defies everything that we currently think we know and forces us to completely rethink fundamental questions like "what is a planet."


All the rabble above aside, heading over to arxiv.org tonight to view tomorrow's preprints I saw that this brawl is spilling out of the pubs and IAU meeting halls and into the online preprint archive.

First up we have Steven Soter, an astronomer with the Hayden Planetarium in New York sharing a paper he's submitted to The Astronomical Journal. Soter offers a classification of planets that builds on Mike Brown's by linking in to the way planets form:

Quote

A modification of Brown’s definition can link it explicitly to the dynamics of planet formation: A planet is a body that has swept up or scattered most of the mass from its orbital zone in the accretion disk around a central star or substar. In this paper I propose an observational criterion to quantify this definition.

The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. Asteroids and comets, including KBOs [Kuiper Belt Objects], differ from planets in that they can collide with each other and with planets.


Soter goes on to define "orbital zones" around  stars--under his definition bodies share an orbital zone if their orbits cross somewhere at the same distance from their parent star and their ellipticities are remotely in the same ballpark. Using that notion of orbit zones Soter goes on to define a quantity μ, which is the ratio of a body's mass to the combined  mass of all the other stuff in the same orbital zone. If μ>100 (i.e. the body in question is at least 100 times bigger than the combined mass of everything else its sharing the neighborhood with) then the body is a planet.

Scrolling a little further down the arXiv page you find another paper on the criteria for planethood. I'm not familiar with these authors but they've submitted this paper to The Journal of Planetary and Space Sciences. The definition they propose is sort of an interesting one:

Quote

As previously stated, the upper mass limit for a giant planet has to be below the limiting mass for thermonuclear fusion of deuterium (WGESP 2005). The lower mass limit for a giant planet is often assumed without much deliberation, but could be stated as: ‘An object is a giant planet if it is able to keep its atmosphere, composed mainly from the solar-composition gas of the parent body, against the surrounding vacuum.’

In the absence of the well-quantifiable physical property which would distinguish a rock (minor planet) from a (terrestrial) planet, we propose to apply the planethood criterion valid for giant planets also to the terrestrials. Previous similar concepts required a body to have an atmosphere. This was unsatisfactory because this requirement excluded atmosphereless planets (e.g. Mercury), and had a difficulty in setting the limit between an ‘atmosphere’ and a ‘vacuum’.

We argue that an object should not be called a planet if it is not capable to retain its envelope (volatiles) when connected to vacuum (i.e. to an empty space, as opposed to the
proto-planetary nebula gas cloud). We do not require an atmosphere, just the capability to retain it.


They take it somewhere a little more technical from there but that's the gist.

I imagine by this point pretty much no one's reading anymore so I'll wrap up. I don't think I'm liking the IAU definition but the Mike Brown definition (and perhaps the Soter definition, I need to chew on that one a bit) seems like a pretty decent one. Thoughts?


#10    RamboIII

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Posted 18 August 2006 - 03:35 AM

It is amazing we never had a definition for a planet in the first place...


#11    Waspie_Dwarf

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Posted 18 August 2006 - 03:54 AM

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It is amazing we never had a definition for a planet in the first place...


Until recently it didn't seem necessary, it was obvious what a planet was. Now the astronomers are discovering that things are not so neatly compartmentalised as they thought. Where do bigger planets end and brown dwarves start? Some asteroids are comet like so where do you draw the line there?

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#12    shun

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Posted 18 August 2006 - 07:55 AM

A planet is anything sufficiently round to gain the attention of an astronomer, and sufficiently small to gain the contempt of a star.  tongue.gif


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#13    jesspy

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Posted 18 August 2006 - 09:02 AM

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"A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and ( b ) is in orbit around a star, and is neither a star nor a satellite of a planet."

In other words, if it's round(ish) and circles a star, it's a planet.



im roundish and i circle a star am i a planet?  unsure.gif

Mind tricks don't work on me, only money.

#14    Startraveler

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Posted 18 August 2006 - 03:10 PM

No. At best you could be a moon of the earth.


#15    blucat

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Posted 18 August 2006 - 03:42 PM

In the absence of the well-quantifiable physical property which would distinguish a rock (minor planet) from a (terrestrial) planet, we propose to apply the planethood criterion valid for giant planets also to the terrestrials. Previous similar concepts required a body to have an atmosphere. This was unsatisfactory because this requirement excluded atmosphereless planets (e.g. Mercury), and had a difficulty in setting the limit between an ‘atmosphere’ and a ‘vacuum’.

We argue that an object should not be called a planet if it is not capable to retain its envelope (volatiles) when connected to vacuum (i.e. to an empty space, as opposed to the
proto-planetary nebula gas cloud). We do not require an atmosphere, just the capability to retain it.
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For what it's worth, I agree with this. A Planet is something which has enough gravity to retain an atmosphere in the 'empty' vacuum of space. If it happens to orbit really close to a sun which blows the atmosphere away, then it's an 'unlucky' planet. If Mercury was orbiting further out, it could retain an atmosphere.

Anyway, all these definitions are arbitrary and unimportant. Just look at the things without naming them. They are all amazing. It's like arguing if blue is blue or is it eally red? The words don't matter.

It's beautiful.

blucat@optusnet.com.au





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