That would do it, if there were a fundamental difference
Not true.
Mercury is not a planet? That'll never fly.

Yup, the last one (big3.jpg). I didn't know how accurate it was supposed to be, but figured it would show if it was a gross overlap. Maybe that orbit will change with more observations.

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The Planet Lila page that 01101001 linked to says that its orbit is from 97AU to 38AU. Neptune's orbit doesn't go much beyond 30AU.

And the chart just below that is interesting. The chart assumes that if the object is Pluto like, it will be 125% of the diameter of Pluto--but if it is really Pluto like, it will have a large moon. That's one of the reasons that Pluto used to be thought bigger--the light reflected from Charon was taken into account. Just imagine the wild-eyed theories if that turns out to be the case.

Very good points so far.

I still do not understand why everyone sees size as the most important factor, I believe orbital characteristics and composition are the most important. Mercury is a planet, Pluto might be a planet, Sedna is not a planet- why because of orbit. Do not be fooled by flashy numbers they are essientially the same mass more or less.

Besides pluto was defined as a planet long before we knew anything about its neighborhood or that it had a moon, seems like a premature decision. And for that matter "planets" were defined by ancient astronomers thats determined a planet's status based solely on its relation to earth, and its movement in the night sky.

Still these are very good arguments, and I never thought of it this way.

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Daryl Zero

Yes, I would agree with you, except for composition. Only orbital characteristics would suffice. As I said earlier, we could have thousands or millions of planets, each one in its own track, in a stable orbit (by stable I mean something that´s not gravitationally perturbed by other bodies; this is not the case of comets and asteroids*). The solar system would be comprised mostly of planets. According to this criterion, a peeble neatly circling the Sun somewhere beyond Pluto would be a planet. Big Titan would still be a satellite (see Argos´ rules for defining a satellite, above in the thread). Size, shape and composition wouldn´t matter.

By the way, when observed from Earth, Ceres (and many other "asteroids") displays the same trajectory pattern of a planet. Additionally, it is located on the general plane of the ecliptic. It´s a planet to me. Why creating unnecessary categories?

(*)

My definition of asteroid: a body with irregular, prone to disturbances, orbit, whatever its size, shape and composition.

My definition of a comet: an active asteroid.

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Yeah I purposely left the gas giants out because the scale would become so large it'd be hard to discern the gaps I noticed between the much smaller bodies.

Also, I'm not convinced orbital parameters are that crucial. Correct me if I'm wrong, but aren't there many Jupiter-size giants around other stars that have very elliptical orbits? Also, temporary interactions could lead to a gas giant in an inclined orbit as well (if there is a crowd issue, this is feasible). I certainly wouldn't want to exlude a large gas giant from planet status if it's elliptical or inclined.


The only problem I have w/ this idea is classifying lopsided objects as planets. When you think of planets, do you think of a 100km potato? I don't...rather, that's what comes to my mind for an asteroid. I think your definition could work, except I still think a planet has to be spherical. If it's lumpy, it's an asteroid or KBO.

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In my scheme, there would be no problem with elliptical or inclined orbits. The only requirement would be stability.


In my scheme I´m interested in getting rid of too many assumptions (like spheroidicity) and arbitrary limits (like 1,000 km diameter). I have no problem with a potato-sized, or potato-shaped planet, as long as it displays the typical planetary orbital pattern across the sky.

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That means that Moon (Earth's moon) is a planet.

(I actually agree with this classification, but I can see how others would have issues)

Robbo

I see where you're coming from, but what you're saying is there's basically no limit at all on size. So by that scheme, a pebble-size rock orbiting the sun in a stable fashion would be a planet. What about grain or beed size...those would be planets too? If you don't draw the line between spherical and lopsided, it might as well be between microsopic and macrosopic (otherwise it'd be arbitrary). Just don't see that working out...we'd have trillions of billions of planets, 99% being no bigger than a house.

Also, having planets and asteroids cross in size seems a bit messy. A 1000km object could be an asteroid because of an instable orbit, but a grain is a planet because it's stable. I'd rather put more weight on size.

What Shaggy is suggesting w/ the 2000km+ limit is right where I found a good place to draw the line in the original post...and I'm still not opposed to that idea at all. The other option that's growing on me is anything spherical is a planet...

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We´ve grown up with the notion that a planet = a world; a place we could call home. It doesn´t have to be so, rigorously. Even if you had thousands or millions of planets, as in my scheme, you could divide them into major, minor and micro planets. That would make more sense than to consider Titan a planet because of its size. I find that scheme quite simple. I recognize that it defies certain preconceived notions, though.

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I think the best approach is to use multiple categories with differing levels of elimination.

First, use size: 1-anything smaller than a gravisphere is not a planet; 2-anything big enough to start fusion is not a planet.

Second, use orbital characteristics: 3-anything off the ecliptic by more than 5 degrees is not a planet; 4-anything with an eccentricity higher than .2 is not a planet; 5-anything that does not orbit the sun is not a planet, 6-anything that orbits a mass that is not a star (or former star) is not a planet.

Third, use composition characteristics: 7-anything with a density lower than water is not a planet; 8-anything that does not have a density based stratified differentiation is not a planet.

Fourth, use population characteristics; 9-anything that exists within a narrow co-orbital and co-planar population where no object contains more than 50% of the total population mass is not a planet, 10-anything that is co-orbital with a mass that contains 90% of the mass or greater in that orbit is not a planet.

These four categories organize ten points. It is a negative assessment, but could be rewritten to be affirmatives for planethood, but the idea is a process of elimination. An object can have two elimination points but still be a planet. Anything with 3 or more elimination points is not a planet.

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Your categories sound good, but what names do we use for bodies that don't meet the criteria.


Here I am thinking of some of the extra-solar planets already announced. Many of these have elliptical orbits, possibly being the only planet that orbits that star etc.



1-anything smaller than a gravisphere is not a planet

-- fair enough, you have to be "this big" to use the name Planet

2-anything big enough to start fusion is not a planet.

-- else you're a star

3-anything off the ecliptic by more than 5 degrees is not a planet;

-- bye bye Pluto, bye bye Mercury

4-anything with an eccentricity higher than .2 is not a planet;

-- bye bye Pluto, bye bye Mercury
-- but also kiss off many of the extra-solar planets as mentioned above


5-anything that does not orbit the sun is not a planet,

-- the Sun, or a Star. Do you mean to limit the designation "Planet" only to our solar system?

-- Note, the Moon orbits the Sun in an interacting orbit with the Earth.


6-anything that orbits a mass that is not a star (or former star) is not a planet.

-- ok, but see hypthetical case below


7-anything with a density lower than water is not a planet;


-- Saturn would fail this categor, and otherwise this knowck off a few smaller icy bodies.


8-anything that does not have a density based stratified differentiation is not a planet.

-- this one might be a bit hard to determine without actually visiting the "alleged planet"'s location


9-anything that exists within a narrow co-orbital and co-planar population where no object contains more than 50% of the total population mass is not a planet

-- define narrow. The TNO and KBO areas are hard to describe as "narrow", even the Solar system's asteroid belt is hardly narrow.
-- Is this intended to eliminate Pluto, UB 313 etc. or just the asteroids?

10-anything that is co-orbital with a mass that contains 90% of the mass or greater in that orbit is not a planet.

-- OK so this lets off the Moon, and all the gas giant satellites, but not Charon.

Sad really since Triton, Ganymede etc are likely to be quite nice worlds, that our descendents might conceivably choose to live on. Would they be happy living on a mere moon?




Here's a hypothetical... we have a brown dwarf that orbits a red dwarf, and a rocky body that orbits the brown dwarf.

By the above definition list, the rocky body is a planet if the brown dwarf is fusing, but is a moon if it is not. Too bad if it starts fusing then runs out.. or if it stutter starts.. or if it gets enough mass added that fusion kicks in, then we promote the "moon" into a "planet" :-)


It's all a bit iffy... we could adjust the rules to include or exclude specific bodies based on the solar system, but we're begging to be hit by exceptions as more stars are examined.

I'd prefer a simpler set of rules, and let the chips fall. We can demote Pluto, or add UB313, but I'd rather not lose Mercury :-)



cheers,
Robbo

Jupiter?? how long ago did you grow up?

Here´s a good set of rules:

Definitions:

Planet: any body that orbits a star on a stable orbit.
Double Planet: any two bodies orbiting a common center of gravity around a star on a stable orbit
Satellite: any body that orbits another body (except a star) which is ten times more massive.
Asteroid: any body that orbits a star on an unstable or hyperbolic orbit
Comet: any icy body with meaningful surface activity, which orbits a star on an unstable or hyperbolic orbit.

Classification:

Planets

Major planet : >1,000 km diameter (ex: all solar system planets, including Xena)
Minor planet: between 100 m and 1,000 km diameter (ex: Ceres)
Micro planet: < 100 m "diameter" (ex: anything)

Satellites

Major satellite : >1,000 km diameter (ex: the Moon, Titan)
Minor satellite: between 100 m and 1,000 km diameter (ex: Deimos)
Micro satellite: < 100 m "diameter (ex: space junk)

Asteroids

Major asteroid : >1,000 km diameter (ex: none, so far)
Minor asteroid: between 100 m and 1,000 km diameter (ex: a great deal of them)
Micro asteroid: <100 m "diameter (ex: most of them)

Comets
Major comet: > 10 km
Minor comet: < 10 km

All of them (except the comets) can be classified into rocky, icy, and gaseous. Asteroids can retain their classification.

Few assumptions, few arbitrary boundaries. Ain´t it really simple?

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You just got rid of most of the asteroids, didn't you?

Isn't that true for every planet-moon combination? Or do you mean that the common center of gravity is outside of the confines of the larger body?

If the common center is inside the larger body, perhaps you have a combo of a satellite and a planet.

The location of the CCG wouldn´t be an issue. All that matters is the mass ratio between the two bodies. In my scheme, the pair Earth-Moon is comprised of a planet and a satellite (mass ratio ~ 0.01). The pair Pluto-Charon is a double planet (mass ratio ~ 0.15), because Charon is more than 10% (one of the few arbitrary limits of the present scheme) as massive as Pluto.

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I was thinking of changing the inclination query from 5 to 10 degrees, but Mercury only had two negatives, so it is still a planet either way. Perhaps it should be 5 or 10 degrees from a plane that is the median angle of the 8 classic planets (somewhat circular reasoning, I admit). These ideas are not set in stone, but are just a framework.

The issue with rule #5 (solar orbit) should be