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  Is Pluto a planet or not?

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Author Topic:   Is Pluto a planet or not?
Astro Bill
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posted 02-17-2005 11:54 PM     Click Here to See the Profile for Astro Bill   Click Here to Email Astro Bill     Edit/Delete Message   Reply w/Quote
Today is the 75th anniversary of the discovery of Pluto by Clyde Tombaugh in 1930. The question remains - Is Pluto a planet or what? I attempted to answer this question in an article in the "Astrophile" of the Space Unit (July/Aug 2002, pp. 148-152 and Nov/Dec 2002, p. 241).

There is no definition of a "planet." Pluto (1/5th the size of Earth) orbits the Sun and is spherical like the other planets, but it has a highly eccentric elliptical orbit inclined 17 degrees to the plain of the ecliptic, it rotates on its side (like Uranus) and in the opposite direction of most planets (clockwise). Pluto is smaller than 7 moons in our solar system: the Moon, Callisto, Europa, Ganymede, Io, Titan and Triton. Pluto orbits the Sun every 248 days and has a thin atmosphere and a rather large moon - Charon. So the question remains - is Pluto a planet?

If Pluto is a planet, is Quaoar a planet? Quaoar is 1/2 the size of Pluto and 1-billion km beyond Pluto, but it is spherical and it orbits the Sun, our Sun. Suddenly size and distance from the Sun are important in deciding if an object is a planet. Then, what about Sedna (discovered in 2004)? There is a need for a definition of what a planet is. What about Ceres (discovered in 1801), a large spherical asteroid in the Asteroid Belt. Ceres was thought to be a planet, until it was discovered that it was part of the Asteroid Belt between Mars and Jupiter. What are your thoughts and your definition of a PLANET?

Bill York, Director, Space Unit

[This message has been edited by Astro Bill (edited February 17, 2005).]

DavidH
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posted 02-18-2005 08:58 AM     Click Here to See the Profile for DavidH   Click Here to Email DavidH     Edit/Delete Message   Reply w/Quote
Pluto probably has zero right to be a planet, but in my opinion is grandfathered in, and will remain one.
However, after Pluto, the gates to planetary status are probably now locked, and nothing else in our solar sytem will receive the designation, barring the surprise discovery of something much larger out there.

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Philip
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posted 02-18-2005 12:22 PM     Click Here to See the Profile for Philip   Click Here to Email Philip     Edit/Delete Message   Reply w/Quote
Well, I guess we could translate the old Greek word " PLANET " into the English word " WANDERER " ... referring to the movement ( during several months ) of the planet in the nightsky as seen on the background of stars.
PLUTO is a very special case, isn't its moon CHARON bigger than the planet itself ?

DavidH
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posted 02-18-2005 12:25 PM     Click Here to See the Profile for DavidH   Click Here to Email DavidH     Edit/Delete Message   Reply w/Quote
Nope. The two are closer in size that most planets and moons, but Pluto is larger, with an equatorial radius of 1,151 km, versus Charon's 593 km.
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Pluto

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"America's challenge of today has forged man's destiny of tomorrow." - Commander Eugene Cernan, Apollo 17 Mission, 11 December 1972

BLACKARROW
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posted 02-18-2005 01:21 PM           Edit/Delete Message   Reply w/Quote
I define a planet as a body orbiting the sun (but not in orbit around another planet) which is the diameter of Pluto, or larger. You have to set a size limit somewhere, and I'm happy to draw that arbitrary line at the diameter of Pluto. The fact that Pluto has a moon of its own simply emphasises its right to "planethood."

dss65
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posted 02-18-2005 08:44 PM     Click Here to See the Profile for dss65   Click Here to Email dss65     Edit/Delete Message   Reply w/Quote
I agree completely with David H. Pluto is grandfathered in and we can't take that status away. Not very scientific, I suppose, but rather human. And I guess we have assumed the right to make the designations, haven't we?

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Don

Ashy
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posted 02-19-2005 08:35 AM     Click Here to See the Profile for Ashy   Click Here to Email Ashy     Edit/Delete Message   Reply w/Quote
Sorry for being a bit thick here, but if Pluto wasn't a planet what would it be defined as?

Sorry if its stupid qustion.

Si

Ashy
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posted 02-19-2005 08:37 AM     Click Here to See the Profile for Ashy   Click Here to Email Ashy     Edit/Delete Message   Reply w/Quote
It would also appear that I can't type today!

Astro Bill
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posted 02-19-2005 11:04 AM     Click Here to See the Profile for Astro Bill   Click Here to Email Astro Bill     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Ashy:
Sorry for being a bit thick here, but if Pluto wasn't a planet what would it be defined as?

Sorry if its stupid qustion.

Si


This is not a stupid question. It is an essential question for modern astronomers and space scientists, given the fact that large objects are continually being discovered in this area of space (e.g.: Quaoar - 2002, Sedna - 2004). If Pluto were discovered in modern times, instead of 75 years ago, it would be classified not as a planet but as a Kuiper Belt Object (KBO) or Trans Neptunian Object (TNO) or even as a "planetoid" or "minor planet".

In 1930 astronomers were looking for a "planet", which renowned astronomer Percivel Lowell predicted was out there. Since they were looking for a planet and since they did not know that what they were "seeing" was a small object AND a moon (2 objects), they declared that this object (Pluto) was the predicted missing planet.

Now, having discovered Quaoar and Sedna, we know that there are probably hundreds of large objects in that part of space, in the Kuiper Belt. The silence of modern scientists with regard to Quaoar and Sedna is deafening. These are major discoveries. Both "Popular Science" and "Discovery" magazine declared the discovery of Sedna as one of the major discoveries in 2004. If scientists were consistant, these two objects would also be labeled as Planets. But they are labeled as KBO's instead.

It cannot be denied that these new discoveries are new "worlds", which will eventually be investigated by spacecraft in the future. Pluto itself may have been a moon of Neptune which collided with another object millions of years ago, causing Pluto to break in half and altering its orbit to its present highly eccentric elliptical orbit around the Sun, inclined 17 degreet to the plane of the other other eight planets.

The IAU should issue an official definition of a PLANET. It should be spherical, in orbit around the Sun, not in orbit around a planet, not man made, not beyond 40 Astronomical Units (1 AU = distance between Earth and Sun), and no smaller than Pluto (1440 mi - 2320 km). This would eliminate Ceres (major asteroid in Asteroid Belt),Quaoar, Sedna and the many other KBO's recently discovered. Pluto would remain as a planet and everything discovered after Pluto would be labeled as a KBO.

I am sure that future astronomers will develop a better name for these distant "worlds" than KBO's. Then the only problem will be the possible discovery of a KBO that is LARGER than Pluto. What then? This could happen at any time. Modern telescopes are now exploring more of "near space" (up to 50 AU). Quaoar and Sedna will be explored by spacecraft in the far future. We cannot imagine what they will discover on these new "worlds." []

[This message has been edited by Astro Bill (edited February 19, 2005).]

Ashy
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posted 02-19-2005 11:27 AM     Click Here to See the Profile for Ashy   Click Here to Email Ashy     Edit/Delete Message   Reply w/Quote
Astro Bill thanks very much. I feel alot smarter than I did a couple of hours ago.

Si

BLACKARROW
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posted 02-19-2005 09:51 PM           Edit/Delete Message   Reply w/Quote
Astro Bill,
I think that's more or less what I said!

Moonpaws
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posted 02-19-2005 10:37 PM     Click Here to See the Profile for Moonpaws   Click Here to Email Moonpaws     Edit/Delete Message   Reply w/Quote
I think you guys have it all wrong. Sometimes kids come up with the best answers. You know the saying from the Bible:
"out of the mouths of babes...". Just ask my kid this question. His anwswer is: "Pluto isn't a planet dad, Pluto is a dog".

Astro Bill
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posted 04-09-2005 04:43 PM     Click Here to See the Profile for Astro Bill   Click Here to Email Astro Bill     Edit/Delete Message   Reply w/Quote
Hopefully, the New Horizons spacecraft will be launched in mid January 2006 on its mission to Pluto & Charon. The spacecraft is now being tested but the launch has not yet been approved by the Bush Administration, which wishes to focus most of NASA's energy and budget on a return to the Moon (c2020)and a mission to Mars (c2038). But the question remains, is Pluto a planet? []

Astro Bill
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posted 08-01-2005 05:40 PM     Click Here to See the Profile for Astro Bill   Click Here to Email Astro Bill     Edit/Delete Message   Reply w/Quote
The recent discovery of an object in space that is bigger than Pluto has renewed the debate about the definition of a PLANET. What do you think are the determining factors. Is size the most important gactor?

Ben
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posted 08-01-2005 05:58 PM     Click Here to See the Profile for Ben   Click Here to Email Ben     Edit/Delete Message   Reply w/Quote
All these new objects, including Pluto, seem to be tiny, rock-ice balls that orbit in elliptical planes most of which are not in line with the first eight planets.

Those factors should be what defines them as different. I think we should say the planets stop at Neptune, and beyond it are numerous similar sized mini-planets (planetoids maybe?) orbiting in the "second" region out from the sun.

collshubby
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posted 08-02-2005 10:00 AM     Click Here to See the Profile for collshubby   Click Here to Email collshubby     Edit/Delete Message   Reply w/Quote
Most people would think that writing a definition for “planet” would be an easy task, but reading about this tenth planet has made me really think. I am starting to re-think my thoughts on Pluto, too. I guess I wanted to keep it as a planet for sentimental reasons, but I think it is more important to call it what it really is. So, I am just going to throw this out here:

I think Mercury is the smallest of the terrestrial planets, so maybe we should cut off the planet size as anything Mercury size or bigger. Set a degree limit…say 10 degrees either way on the ecliptic. Anything that meets the criteria is a “planet.”

Have anything between the size of Pluto and Mercury be a “minor planet.” Have anything smaller than Pluto as a Kupier Belt Obeject (KBO).

So, with that, the tenth planet should actually be classified as a minor planet. That means eight planets, two minor planets, and any number of KBO’s.

How does that sound?

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Brian Peter

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http://warhorse.omegappg.com

Astro Bill
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posted 08-02-2005 10:06 AM     Click Here to See the Profile for Astro Bill   Click Here to Email Astro Bill     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by collshubby:
Most people would think that writing a definition for “planet” would be an easy task, but reading about this tenth planet has made me really think. I am starting to re-think my thoughts on Pluto, too. I guess I wanted to keep it as a planet for sentimental reasons, but I think it is more important to call it what it really is. So, I am just going to throw this out here:

I think Mercury is the smallest of the terrestrial planets, so maybe we should cut off the planet size as anything Mercury size or bigger. Set a degree limit…say 10 degrees either way on the ecliptic. Anything that meets the criteria is a “planet.”

Have anything between the size of Pluto and Mercury be a “minor planet.” Have anything smaller than Pluto as a Kupier Belt Obeject (KBO).

So, with that, the tenth planet should actually be classified as a minor planet. That means eight planets, two minor planets, and any number of KBO’s.

How does that sound?



Brian:

So where does this leave Sedna and Quaoar? If Pluto is to be a "minor planet" with Xena, shouldn't Sedna & Quaoar be included with them because they are only slightly smaller than Pluto?

collshubby
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posted 08-02-2005 11:46 AM     Click Here to See the Profile for collshubby   Click Here to Email collshubby     Edit/Delete Message   Reply w/Quote
Well, this is where we run into trouble. You can fudge the rules and say just because the are only slightly smaller than Pluto, then we ought to call them minor planets. But if you stick to the rules, slightly smaller is still smaller, so they would be KBO's.

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Brian Peter

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B-17 Bomber "War Horse"
http://warhorse.omegappg.com

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