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What is a planet? (discussion) (Read 16784 times)
Tony
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Re: What is a planet? (discussion)
Reply #15 - 08/25/06 at 14:08:02
 
You should post a few of your Neptune simulations.  They sound very interesting.
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thejames
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Re: What is a planet? (discussion)
Reply #16 - 08/29/06 at 09:09:46
 
perhaps I have missed this... but when I was looking at the definitions of a planet I thought it was odd that Uranus was still considered a planet.
 
If Pluto is not being considered a planet because it hasn't cleared its neighbourhood (I had read that it was because it's orbital crossed Uranus'), then wouldn't the same be said for Uranus? It hasn't cleared Pluto out of it's path yet, so why, is it being considered a planet?
 
Doesn't it have to meet all three criteria?
 
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Re: What is a planet? (discussion)
Reply #17 - 08/29/06 at 17:12:26
 
I personally think that as far as Prop V of the 2006 IAU GA, Section 1 Subsection c.  This should be defined as  
a toroid with,
major center at the solar system barycenter,
minor center at the center of the body in question,
major radius distance to aphelion from system barycenter,
minor radius of center of the body in question to maximum Hill sphere extent,
and the body in question must possess at least 50% of total mass that passes through this toroid.
 
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Tony
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Re: What is a planet? (discussion)
Reply #18 - 08/29/06 at 20:58:44
 
Quote from thejames   on 08/29/06 at 09:09:46:
perhaps I have missed this... but when I was looking at the definitions of a planet I thought it was odd that Uranus was still considered a planet.

If Pluto is not being considered a planet because it hasn't cleared its neighbourhood (I had read that it was because it's orbital crossed Uranus'), then wouldn't the same be said for Uranus? It hasn't cleared Pluto out of it's path yet, so why, is it being considered a planet?

Doesn't it have to meet all three criteria?


I think you mean that it crosses the orbit of Neptune.  In this case, Pluto is much smaller than Neptune, so Neptune is the dominant body in its orbit.
 
Most planets have smaller bodies known to cross their orbits.  But since the planets are much more massive than the combined masses of these objects they are not disqualified from being called planets.
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Planet_X
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Re: What is a planet? (discussion)
Reply #19 - 10/05/06 at 10:01:01
 
First off, I think the new IAU planet definition is poorly worded, with too many loop holes.
This brings me to an important concept. If the term "Minor Planet" has officially been ditched as a reference to small bodies, I'd suggest that they revive the term to refer to bodies in the 2000-6000 km diameter range.  
 
With that being said, I think it's time to spell out my own personal definitions of the various types of bodies in our solar system. Listed below are my classifications and what they mean, sort of based on the IAU definitions and sort of not.  
 
1. Major Planet - a celestial body that: (a) is above 6000 km in diameter, (b) directly orbits the Sun and not another body, (c) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (d) has sufficient mass to both have a differentiated interior and retain an atmosphere in a vacuum and finally, (e) has definitely cleared the neighborhood around its orbit (with the legitimate exception of Trojan Bodies).  
 
2. Minor Planet - a celestial body that: (a) is 2000-6000 km in diameter, (b) directly orbits the Sun and not another body, (c) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (d) has sufficient mass to both have a differentiated interior and retain an atmosphere in a vacuum, but (e) may or may not have cleared the neighborhood around its orbit (with the legitimate exception of Trojan Bodies).  
 
3. Planetoid - a celestial dwarf body that: (a) directly orbits the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has insufficient mass to have a differentiated interior or retain an atmosphere in a vacuum, (d) has not cleared the neighborhood around its orbit, and (e) is not a satellite of another body.  
 
(4) Planetesimal - a celestial dwarf body that: (a) directly orbits the sun, (b) has insufficient mass for its self-gravity to overcome rigid body forces so that it assumes a basically potato-like (irregular) shape, (c) has insufficient mass to have a differentiated interior or retain an atmosphere in a vacuum, (d) has not cleared the neighborhood around its orbit, and (e) is not a satellite of another body.  
 
That's my take on the matter. I think this would be a good starting point to fine tune the new IAU planet definition. In this scheme, we would have 10 principal bodies orbiting the sun: 7 major and 3 minor planets. I also think the public would accept a scheme like this, as it would allow Pluto and Eris to be planets, yet not force the classification of planet to a million round bodies below 2000 km in diameter. Plus, as Mike Brown even said after discovering Eris, it would give future generations the possibility of actually finding a new planet. Besides, it's highly unlikely the number of bodies above 2000 km in diameter will skyrocket to ridiculous numbers anytime soon, even out to several hundred AU distant.
 
I'm planning on e-mailing this concept of mine to Alan Stern, who is against the new IAU definition of planet and plans to come up with a counter definition in August 2007.  So, what are your thoughts on this matter?  Later!  
 
J P
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Tony
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Re: What is a planet? (discussion)
Reply #20 - 10/05/06 at 11:14:51
 
The word "celestial" needs to be replaced by "astronomical" in the IAU's wording.  Celestial refers to objects beyond Earth.  So interestingly, Earth is not a planet by the IAU's new definition.  It is a small solar system body.
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