posted 02-17-2005 11:54 PM               

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

posted 02-19-2005 08:35 AM               

Sorry if its stupid qustion.

posted 02-19-2005 11:04 AM               

quote:

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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

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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." []

posted 08-02-2005 10:00 AM               

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

posted 08-02-2005 10:06 AM               

quote:

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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?

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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?

posted 08-02-2005 11:46 AM               

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Brian Peter