I asked this group some time ago if anyone knew of a consensus definition for "planet" and one of you responded with what I believe was the IAU's working definition for a range of objects. Forgive me, but I don't now remember who that person was. As I recall, the definition went something like this: If an object is massive enough to be spherical and is in orbit about a star, it's a planet. If an object is massive enough to sustain the fusion of hydrogen to helium, it's a star, and so forth. Does anyone recall and can cite the source for these definitions? I've always thought that the simplest definition is the best, and I liked this definition for planets. Therefore, both Pluto and "Xena" are planets, as may also be the larger Kuiper Belt objects. All the rest of the arguing by the "professionals" about what constitutes a planet seems somewhat petty and fruitless. After all, we still have to fix those damn tiles... Kim
Very good point, Kim. When I was talking with my son, Sky, the other day he suggested the same definition for a planet, large enough that its gravity forms it into a sphere and in orbit around a star. The last part rules out moons that are big enough but are not planets. I think it's a fine definition. The question is, what is the lower limit for enough mass to create a sphere? And how perfect does the sphere have to be? (In a sense, Jupiter isn't a very good sphere -- it's oblate because it is so large and spins so fast and is so gaseous that it bulges at the belt).) -- Joe
That definition is a bit too broad. Water in zero-G will form spheres of microscopic size. I'm sure there are other materials that can form spheres in a vacuum and at very low temperatures. We must take it as given that those favoring that particular definition are talking about "threshold mass" rockpiles and extremely large volumes of gas and liquids, in a DIRECT orbit around a star. Otherwise astronauts in space would create planets when they sneeze. ;) --- Joe Bauman <bau@desnews.com> wrote:
Very good point, Kim. When I was talking with my son, Sky, the other day he suggested the same definition for a planet, large enough that its gravity forms it into a sphere and in orbit around a star.
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Ah, yes, Chuck is right. However, the astronaut would have to sneeze with enough energy for the ejecta to escape earth orbit and become gravitationally bound to the sun. In that instance, perhaps other parts of the astronaut would also attain escape velocity. I'm sure that the definitions quoted to me were more specific - I just can't find the reference. ----- Original Message ----- From: "Chuck Hards" <chuckhards@yahoo.com> To: "Utah Astronomy" <utah-astronomy@mailman.xmission.com> Sent: Tuesday, August 02, 2005 2:44 PM Subject: Re: [Utah-astronomy] planets - or not | That definition is a bit too broad. Water in zero-G | will form spheres of microscopic size. I'm sure there | are other materials that can form spheres in a vacuum | and at very low temperatures. | We must take it as given that those favoring that | particular definition are talking about "threshold | mass" rockpiles and extremely large volumes of gas and | liquids, in a DIRECT orbit around a star. Otherwise | astronauts in space would create planets when they | sneeze. ;) | | --- Joe Bauman <bau@desnews.com> wrote: | | > Very good point, Kim. When I was talking with my | > son, Sky, the other | > day he suggested the same definition for a planet, | > large enough that | > its gravity forms it into a sphere and in orbit | > around a star. | | __________________________________________________ | Do You Yahoo!? | Tired of spam? Yahoo! Mail has the best spam protection around | http://mail.yahoo.com | | _______________________________________________ | Utah-Astronomy mailing list | Utah-Astronomy@mailman.xmission.com | http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy | Visit the Photo Gallery: http://www.utahastronomy.com | | ______________________________________________________________________ | This e-mail has been scanned by Cut.Net Managed Email Content Service, using Skeptic(tm) technology powered by MessageLabs. For more information on Cut.Nets Content Service, visit http://www.cut.net | ______________________________________________________________________ | |
I was just playing wit' cha, folks. But remember that anything in earth orbit is already gravitationally bound to the sun, which is why I said in a "direct" solar orbit. FWIW, I agree with Joe's boy and that was the gist of the chauvenism post. A planet, by any other name, would still smell as sweet- or something like that...I wonder if anyone has ever given much thought to planetary smells, come to think of it? Uranus, being composed largely of methane, would smell like a swamp or cattle ranch or- an outhouse. I find it strangely apropos that Uranus would smell like that...but I digress. Where one happens to come from should have no bearing on the true nature of one's character. And if a planet did happen to come from a stellar accretion disk other than old Sol's, that just makes it more interesting. --- Kim Hyatt <kimharch@cut.net> wrote:
Ah, yes, Chuck is right. However, the astronaut would have to sneeze with enough energy for the ejecta to escape earth orbit and become gravitationally bound to the sun. In that instance, perhaps other parts of the astronaut would also attain escape velocity.
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Another good point. It has to be big enough to form a sphere through its gravity (rather than surface tension, which I think is what would form a droplet into a sphere). Other thoughts? -- Joe
As I recall, the definitions extended to planets' spherical moons, irregular moonlets or satellites (I don't recall the terms used), brown dwarves, etc. I also don't remember if the definitions included a class for large, non-stellar interstellar interlopers. I don't know the mass/gravitational threshold for spherical objects, nor do I possess the math and physics skills to calculate that. Maybe someone else knows? One could calculate the theoretical limit for becoming spherical, then define any object larger than that limit as a planet, provided of course it fits other criteria, such as orbiting a star or star-like object. Imperfections such as Earth's slightly pear-shaped "sphere" and the oblateness of Jupiter would then not matter. ----- Original Message ----- From: "Joe Bauman" <bau@desnews.com> To: "Utah Astronomy" <utah-astronomy@mailman.xmission.com> Sent: Tuesday, August 02, 2005 1:52 PM Subject: Re: [Utah-astronomy] planets - or not | Very good point, Kim. When I was talking with my son, Sky, the other | day he suggested the same definition for a planet, large enough that | its gravity forms it into a sphere and in orbit around a star. The | last part rules out moons that are big enough but are not planets. I | think it's a fine definition. The question is, what is the lower | limit for enough mass to create a sphere? And how perfect does the | sphere have to be? (In a sense, Jupiter isn't a very good sphere -- | it's oblate because it is so large and spins so fast and is so | gaseous that it bulges at the belt).) -- Joe | | _______________________________________________ | Utah-Astronomy mailing list | Utah-Astronomy@mailman.xmission.com | http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy | Visit the Photo Gallery: http://www.utahastronomy.com | | ______________________________________________________________________ | This e-mail has been scanned by Cut.Net Managed Email Content Service, using Skeptic(tm) technology powered by MessageLabs. For more information on Cut.Nets Content Service, visit http://www.cut.net | ______________________________________________________________________ | |
--- Kim Hyatt <kimharch@cut.net> wrote:
One could calculate the theoretical limit for becoming spherical, then define any object larger than that limit as a planet, . . .
My two cents - A planet is (1) a non-fusor that orbits a fusor and (2) that has accreted more than 50% of the mass in its orbit. A moon is (1) a non-fusor that orbits a non-fusor planet and (2) that has accreted more than 50% of the mass orbiting a planet and is more than 1% of the total mass of all other moons orbiting the non-fusor planet. A planetiod is (1) a non-fusor that orbits a fusor and (2) that has accreted less than 50% of the mass in its orbit. An asteriod is (1) a (non-fusor that orbits a fusor and (2) that has accreted less than 50% of the mass in its orbit and (3) that is of a mass less than "X", "X" being culturally defined. See also Mike Brown's 2004 discussion of "Is Sedna a planet?" Brown is part of the Kupier Belt hunting team that discovered Sedna and "Xena". His Sedna discussion also reviews using gravitational rounding as a possible criteria. - Canopus56 (Kurt) ____________________________________________________ Start your day with Yahoo! - make it your home page http://www.yahoo.com/r/hs
Kurt, I'm assuming the accreted material referred to is the material gravitationally available to the enlarging protoplanetary mass, not 50% of the mass of the object itself, right? What is the justification for the 50% accretion cut-off? Assuming the in-situ accretion percentage makes or breaks a planet, what do you call those objects that look like planets, walk like planets, & quack like planets, but didn't form in the system you find them in or don't orbit a fusor at all? And what do you call a planet if it's star has evolved into a non-fusor over time, as opposed to a br And what nags at me is, WHY do scientists even WANT to make the "home-grown" distinction at all? It seems totally unnecessary and pathologically compulsive. Could there be a real, scientific reason for it? Seems easy enough to distinguish between natives and non-natives with a few words if and when required, without resorting to an entirely new object classification. --- Canopus56 <canopus56@yahoo.com> wrote:
A planet is (1) a non-fusor that orbits a fusor and (2) that has accreted more than 50% of the mass in its orbit.
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I've been enjoying this discussion. It says everything about humans and nothing about chunks of rock going 'round in the sky. it's funny how we've just got to put a label on things. I'm not innocent myself.
--- Chuck Hards <chuckhards@yahoo.com> wrote:
And what nags at me is, WHY do scientists even WANT to make the "home-grown" distinction at all? It seems totally unnecessary and pathologically compulsive.
I place the whole "it's not a planet" controversey in the same place I store Saint Thomas Aquinas' philosphical transactions on whether several angels could be in the same place at once - the source of the "how many angels can fit on the head of a pin" popular witticism. I'm perfectly happy with the informal cultural definition of a planet as something (1) alot bigger than me, (2) somewhat rounded by gravitational attraction and (3) orbits a sun. I loose no sleep wondering if Sedna, Xena or Pluto are planets. Personally, I feel that none of them are planets; they are all big tail end of the normal distribution of Kupier Belt objects. My intent was to show that IAU could reduce the amount of energy that people put into this nonsensical question by adopting _any_ formal scientific criteria for the definition of a planet.
<snip> What is the justification for the 50% accretion cut-off?
It is arbitrary but seems to work as a rule-of-thumb from what little I know of moons in the Solar System. I can't think of any moons in an orbital track that haven't sucked up at least 1/2 the matter in its orbit. There's either a Moon in an orbital track or an orbital track that is a highway of rubble. Know of any exceptions?
Assuming the in-situ accretion percentage makes or breaks a planet, what do you call those objects that look like planets, walk like planets, & quack like planets, but didn't form in the system you find them in or don't orbit a fusor at all? <snip>
Well, when professional astronomers _find_ any physical evidence of such an object - call me and I'll revise my cosmology. -:)
Could there be a real, scientific reason for it?
Good clear criteria in definitions that are easy to apply advance knowledge in other more potentially fruitful areas of inquiry by reducing the energy that people spend on non-sensical controver ---- just loop back to the top of this post. -:) Best wishes - Canopus56(Kurt) __________________________________ Yahoo! Mail for Mobile Take Yahoo! Mail with you! Check email on your mobile phone. http://mobile.yahoo.com/learn/mail
Canopus56 wrote:
I loose no sleep wondering if Sedna, Xena or Pluto are planets.
Just a reminder that the discovery team has stressed that Xena is only the working name used amongst team members and is not the name officially submitted to the IAU. Even so I must admit that Xena has started growing on me (although on the tv series I always preferred Xena's side kick Gabrielle). As an aside, remember that when Herschel discovered what we now call Uranus, he named it George (for King George III).
My intent was to show that IAU could reduce the amount of energy that people put into this nonsensical question by adopting _any_ formal scientific criteria for the definition of a planet. Apologies to those who get News as this was in this morning's edition, but for non-News subscribers:
+++++ Astronomers To Decide What Makes A Planet The discovery of a new addition to our Solar System has prompted astronomers to fast-track plans to decide what is and is not a planet. http://www.nature.com/news/2005/050801/full/050801-2.html +++++ Nothing to do with planets but this next gave me a chuckle when it showed up in my in box. And, no, Guy is not the author, or at least I don't think he is. :-) A couple had been debating the purchase of a new vehicle for weeks. He wanted a truck. She wanted a sports car. He would probably have settled on any beat up old truck, but everything she seemed to like was way out of their price range. "Look!" she said. "I want something that goes from 0 to 200 in 4 seconds or less. And my birthday is coming up. Surprise me." So he bought her a brand new bathroom scale. Services will be at Downing funeral home on Monday.
Kurt, no need to revise your cosmology. I was thinking of planets orbiting "former fusors", such as neutron stars and black holes, or dead, black lumps of carbon that lacked sufficient mass to evolve into either. Seems I've read about an exoplanet detected orbiting a neutron star, perhaps I'm wrong. My memory isn't among my strong points- probably the biggest reason I never became a scientist. No recall. Admittedly I've been playing word games here, but my intent is to show the ridiculousness of requiring a home-grown pedigree or tidy orbit to qualify a body as a planet. It infers some kind of special status that isn't deserved. My original term was planetary chauvenism but perhaps a better term would be "solar chauvenism". Taken verbatim, some of these scientist's definitions would mean that only the sun has true planets. What they mean, and fail to articulate, is that they want to differentiate between the sun's ORIGINAL planets and late-comers. Defining a body as a planet (or not) doesn't decide where it formed- and I'm still in the dark as to why it gets so many scientists shorts in a knot... --- Canopus56 <canopus56@yahoo.com> wrote:
Assuming the in-situ accretion percentage makes or breaks a planet, what do you call those objects that look like planets, walk like planets, & quack like planets, but didn't form in the system you find them in or don't orbit a fusor at all? <snip>
Well, when professional astronomers _find_ any physical evidence of such an object - call me and I'll revise my cosmology. -:)
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participants (6)
-
Canopus56 -
Chuck Hards -
Joe Bauman -
Kim Hyatt -
Michael Carnes -
Patrick Wiggins