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Why Planets Will Never Be Defined
Ad Astra, via Space dot com ^ | November 21 2006 | Robert Roy Britt

Posted on 11/21/2006 8:42:28 PM PST by SunkenCiv

Before the dust even settled after the Great Pluto War at the International Astronomical Union (IAU)'s General Assembly in Prague, one thing became clear: There will never be an accepted scientific definition for the term "planet." Rather than crafting an acceptable definition, the IAU alienated members, put the group's authority in jeopardy and fueled schisms among astronomers on theoretical grounds and even nationality. And the whole affair was scientifically pointless, many astronomers say..."It is a little-known fact that nearly 25 percent of the known extrasolar planets are in binary- or multiple-star systems," said Stephen Kortenkamp , a research associate at the University of Maryland. "That further complicates the notion of creating a universal definition of planet." ...The known setups are a tiny sample of what's out there. There are perhaps 250 billion planets in our galaxy, says Gregory Laughlin, an exoplanet hunter and planetary system theorist at the University of California, Santa Cruz. Eventually, astronomers could find two Earth-size objects orbiting each other around a center of gravity in the space between them, Laughlin said. Other worlds might be accompanied by planet-size "Trojans" that move with them in a horseshoe-shaped pattern. The present IAU definition, requiring a planet to clear out the path of its orbit, is not set up to handle such offbeat configurations. It's also possible two planet-mass objects could be found orbiting each other with no star involved.

(Excerpt) Read more at space.com ...


TOPICS: Astronomy; Science
KEYWORDS: astronomy; pluto; science; space; xplanets
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Why Planets Will Never Be Defined

1 posted on 11/21/2006 8:42:29 PM PST by SunkenCiv
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To: annie laurie; garbageseeker; Knitting A Conundrum; Viking2002; Ernest_at_the_Beach; mikrofon; ...
The MSNBC version of this story (same author etc) had the very fine headline, "How the Great Pluto War became a quagmire".

2 posted on 11/21/2006 8:43:43 PM PST by SunkenCiv (I last updated my profile on Thursday, November 16, 2006 https://secure.freerepublic.com/donate/)
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related topics:

The Fight for Pluto Rages On
Skytonight | September 1, 2006 | Editors of Sky & Telescope
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Interview with the IAU President on Pluto's Demotion
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Posted on 09/11/2006 7:09:36 PM EDT by KevinDavis
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Astronomical debate
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Posted on 09/23/2006 9:38:54 PM EDT by KevinDavis
http://www.freerepublic.com/focus/f-chat/1707188/posts


3 posted on 11/21/2006 8:45:47 PM PST by SunkenCiv (I last updated my profile on Thursday, November 16, 2006 https://secure.freerepublic.com/donate/)
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To Pluto -- And Far Beyond "To Pluto And Far Beyond" By David H. Levy, Parade, January 15, 2006 -- We don't have a dictionary definition yet that includes all the contingencies. In the wake of the new discovery, however, the International Astronomical Union has set up a group to develop a workable definition of planet. For our part, in consultation with several experienced planetary astronomers, Parade offers this definition: A planet is a body large enough that, when it formed, it condensed under its own gravity to be shaped like a sphere. It orbits a star directly and is not a moon of another planet.

4 posted on 11/21/2006 8:46:26 PM PST by SunkenCiv (I last updated my profile on Thursday, November 16, 2006 https://secure.freerepublic.com/donate/)
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To: SunkenCiv
How does the mass of Pluto compare with the mass of the largest closer thing that isn't a planet, and how do their orbital radii compare?

I wonder if it would be meaningful to classify objects as planets based upon how strong their gravity field is at the object they're orbiting? I would think such a definition might be especially useful with extrasolar planets, given that AFAIK the only way we even discover their existence is by their gravitational influence on the stars they orbit.

Obviously Pluto is much further away than the asteroid belt, and this greater distance would make it hard to compete by that measure, but it would still come out well ahead of the objects that are even further out.

What do other people think of that idea for a metric?

5 posted on 11/21/2006 9:14:16 PM PST by supercat (Sony delenda est.)
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To: SunkenCiv
Regulairity is what most ASStronomers and people want.
Pluto has given that; as in predictability....JJ61
6 posted on 11/21/2006 9:17:38 PM PST by JerseyJohn61 (Better Late Than Never.......sometimes over lapping is worth the effort....)
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To: supercat
I would think such a definition might be especially useful with extrasolar planets, given that AFAIK the only way we even discover their existence is by their gravitational influence on the stars they orbit.
Yeah, Geoff Marcy et al has co-discovered 121 of the known 200+ exoplanets, and sez:
At the telescope, we measure the change in the wavelength (color) of light coming from a star over the course of days, months, and years. This changing wavelength is the Doppler shift of the light, resulting from the star orbiting a common center of mass with a companion planet. For example, Jupiter's gravitational pull causes the Sun to wobble around in a circle with a velocity of 12 meters per second.
This is a very tiny shift. Another method is to watch for the slight dimming of a star's light as one of its planets transits the disk.
How does the mass of Pluto compare with the mass of the largest closer thing that isn't a planet, and how do their orbital radii compare?
The largest thing not classified as a planet is slightly larger than Pluto; Pluto's orbit is more out of the ecliptic (which is defined as the imaginary disk which transects the orbit of the Earth) than that of any other known planet, except possibly for 2003 UB313 Eris, because I haven't found that info. Discoverer Mike Brown sez:
The dwarf planet is the most distant object ever seen in orbit around the sun, even more distant than Sedna, the planetoid discovered almost 2 years ago. It is almost 10 billion miles from the sun and more than 3 times more distant than the next closest planet, Pluto and takes more than twice as long to orbit the sun as Pluto.
I think a simple definition is best, and that would be the one offered by David Levy (see above).
I wonder if it would be meaningful to classify objects as planets based upon how strong their gravity field is at the object they're orbiting? I would think such a definition might be especially useful with extrasolar planets, given that AFAIK the only way we even discover their existence is by their gravitational influence on the stars they orbit.
Yours sounds pretty good as well, since the shapes and masses of the extrasolar discoveries will be in question for a good while, in most cases. But again, where is the dividing line? Those in favor of dumping Pluto (which is a political stance, not a scientific one) would say that Pluto is out.
7 posted on 11/21/2006 9:40:22 PM PST by SunkenCiv (I last updated my profile on Thursday, November 16, 2006 https://secure.freerepublic.com/donate/)
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Artist's conception of how 2003 UB313 might look from space

8 posted on 11/21/2006 9:59:48 PM PST by SunkenCiv (I last updated my profile on Thursday, November 16, 2006 https://secure.freerepublic.com/donate/)
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To: supercat
Ah, here it is, on the page I just linked in the previous message:
"Approximate relative orbital sizes (top) and relative diameters (below). Note that it appears UB 313 passes inside Pluto's orbit, but this is due to the fact that UB 313's orbit is tilted significantly more than Pluto's orbit relative to the plane of the solar system (UB 313's orbit is tilted almost 45° and Pluto's is about 17°)."

9 posted on 11/21/2006 10:01:23 PM PST by SunkenCiv (I last updated my profile on Thursday, November 16, 2006 https://secure.freerepublic.com/donate/)
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To: SunkenCiv
But again, where is the dividing line? Those in favor of dumping Pluto (which is a political stance, not a scientific one) would say that Pluto is out.

Well, I just looked at a table of asteroids, and it appears a few of the bigger asteroids would probably beat Pluto in the gravitational-influence metric (assuming mass is proportional to R^3). If one defined "planetishness" to be the cube of the radius divided by the distance to the orbited star, Ceres would yield a value of 1.01E+8 km^3/AU^2; Pluto would yield only 8.75E+6 km^3/AU^2. Vesta yields 2.24E+7 km^3/AU^2 and Pallas 1.85E+7 km^3/AU^2. I would guess Pluto is third after those two. So we just need to blow up those who rocks and then Pluto can be a planet again. Seriously, though, I would think such a metric might be a good way of defining planets, since it represents something which can be measured for distant planets even if we can't see them. Just have to accept that Pluto's an oddball.

10 posted on 11/21/2006 10:04:23 PM PST by supercat (Sony delenda est.)
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To: JerseyJohn61

:')


11 posted on 11/21/2006 10:05:51 PM PST by SunkenCiv (I last updated my profile on Thursday, November 16, 2006 https://secure.freerepublic.com/donate/)
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To: supercat
Seriously, though, I would think such a metric might be a good way of defining planets, since it represents something which can be measured for distant planets even if we can't see them. Just have to accept that Pluto's an oddball.
Thanks, great reply. Out of curiousity, what number does Mercury give you, compared with, say, Neptune? To do the whole broken record thing, the number needed to make a planet would have to be arbitrary; Ceres is known to be spherical (although that was doubted until recently); not sure about Pallas and Vesta, but I think that they qualify as dwarf planets under the IAU "standard".
12 posted on 11/21/2006 10:10:06 PM PST by SunkenCiv (I last updated my profile on Thursday, November 16, 2006 https://secure.freerepublic.com/donate/)
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To: SunkenCiv

Mercury and Neptune are apples and oranges. Mercury is a silicate world like the other inner planets (all the way to Mars) and Neptune belongs with the icy gas giants. Totally different worlds, totally different compositions.

I'd say that a planet is a typically spherical world orbiting a star with a gravity that allows one to walk around on it and that is not considered a moon. On Pluto a grown man is about 13 pounds, or half what he would weigh on the moon. And no, you could not launch yourself into space by jumping on Pluto any more than the astronauts on the moon could even reach 10 feet by jumping.

You can land a spaceship on Pluto, get out and walk around and it orbits the sun. Sure, it's in a dance with its moon but it's clearly bigger than it and it clearly has an atmosphere (one that freezes, but so what).

Also, there is Eris, a body which is even bigger than Pluto but much, much farther out. It has a moon (or moons, can't remember). Never made the planet list though.


13 posted on 11/21/2006 10:22:09 PM PST by spacecowboynj
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To: SunkenCiv

The pluto-planet-controversy illustrates a fundamental difference in scientific astronomy : the stay at home clerks that are only good for filing, sorting, etc(the military calls them clerk-typists)and the pioneers like Captain Kirk of Star Trek(cosmic warriors on a mission of discovery). Which are you, clerk-typist or warrior?


14 posted on 11/22/2006 1:19:25 AM PST by timer
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To: timer

Not sure, but I think Captain Kirk is fictional.


15 posted on 11/22/2006 7:27:08 AM PST by SunkenCiv (I last updated my profile on Thursday, November 16, 2006 https://secure.freerepublic.com/donate/)
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To: spacecowboynj
Mercury and Neptune are apples and oranges.
Not in the context of supercat's idea.
16 posted on 11/22/2006 7:27:46 AM PST by SunkenCiv (I last updated my profile on Thursday, November 16, 2006 https://secure.freerepublic.com/donate/)
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To: spacecowboynj
I'd say that a planet is a typically spherical world orbiting a star with a gravity that allows one to walk around on it and that is not considered a moon.
Yes, I agree, with the slight change (as seen in David Levy's suggestion) that the gravity was sufficient to turn it into a spheroid. :') That brings in Ceres, Pluto, Eris, and some other outer Solar System objects. I've been trying to avoid using the term Kuiper Belt Object (KBO) because I don't think it's relevant any longer.
17 posted on 11/22/2006 7:40:54 AM PST by SunkenCiv (I last updated my profile on Thursday, November 16, 2006 https://secure.freerepublic.com/donate/)
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To: SunkenCiv

I like Levy's gravity-turns-it-into-a-sphere definition.

They should go with that and then just sub-quantify like they do with stars. Ok, we know what a star is, but there are red ones, yellow ones, little ones, big ones, etc.

With planets we have "terrestrial" (or silicate) planets, gas giants, and icy dwarfs. Why do the icy dwarfs have to be out of the picture? Pluto has the land area of Africa and probably Europe combined. It's a planet.


18 posted on 11/22/2006 8:58:30 AM PST by spacecowboynj
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To: SunkenCiv

So is/was SHERLOCK HOLMES the detective/warrior, and his chronicler WATSON; and yet how many REAL people have lived and died in obscurity in the century since A.Conan Doyle created SH and JW, whereas fictional characters like these live on...and on...and on...through the ages...


19 posted on 11/22/2006 9:22:19 AM PST by timer
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To: spacecowboynj

Pluto is probably rocky, probably everything (even comets) are rocky, but have more or less gas either liquified or frozen; Jupiter, Saturn, Uranus, and Neptune are believed to have rocky cores, Earth-sized or better.


20 posted on 11/22/2006 10:13:15 AM PST by SunkenCiv (I last updated my profile on Thursday, November 16, 2006 https://secure.freerepublic.com/donate/)
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