Pluto no longer a Planet

[MKruer]

I only post this in the hope that someone from the IAU may read it and inspire them to have a clean cut standard of a planet.

Mercury 3.3E+23
Venus 4.87E+24
Earth 5.97E+24
Moon 7.35E+22
Mars 6.42E+23
Ceres 9.50E+20
Jupiter 1.9E+27
Io 8.90E+22
Europa 4.80E+22
Gaymede 1.48E+23
Callisto 1.08E+23
Saturn 5.68E+26
Titan 1.35E+23
Uranus 8.68E+25
Neptune 1.02E+26
Triton 2.15E+22
Pluto 1.27E+22
2003 UB313 (Xena) >1.27E+22 / < 3.00E+22 kg

My Definition of a planet:
1. Must Orbit a star
2. Must have a mass greater then or equal too 1.0E+23 kg

Optional: Planetary Moon
1. Must orbit a planet
2. Must have a mass greater then or equal too 1.0E+23 kg

Both these are straight forward to the point, radius and composition irrelevant, and one of the easier metrics to obtain.

The 1.0E+23 kg is arbitrary and can be redefined, but the metric or mass is what i would hope they would use to classify planet status

[chris]

I think that the new IAU resolution is a very sensible one. Mike Brown has written a well thought out statement in support of the decision:

http://www.gps.caltech.edu/~mbrown/eightplanets/

He addresses a few of the common (and I think rather silly) objections to the new definition of planet, including the ones involving the Trojan asteroids, the near-Earth asteroids, and Pluto's and Neptune's orbits crossing.

--Chris

[Malenfant]

I think that the new IAU resolution is a very sensible one. Mike Brown has written a well thought out statement in support of the decision:

http://www.gps.caltech.edu/~mbrown/eightplanets/

He addresses a few of the common (and I think rather silly) objections to the new definition of planet, including the ones involving the Trojan asteroids, the near-Earth asteroids, and Pluto's and Neptune's orbits crossing.

--Chris

He's right in saying that the problem is the vaguely worded nature of the definition. I think the concept's reasonably sound but we've got to have something that is unambiguous and exactly phrased pretty soon to limit these arguments.

[MKruer]

That?€™s a good argument, better then most I have hear, but there is still too much wiggle room in my opinion.

Let me see if I can explain.

What about multiple planets in roughly the same orbit with the same mass, both of them would be dominate, but none of them would dominate each other.

What about the possibility of rouge planets or extremely distant objects. If Pluto was outside the system 100?€™s of AU?€™s and was the only object, then by that definition it would be a planet. It would have either directly or by other external actions not related to the planet, have its own path that was cleared or would clear its own path be considered the dominate object in that part of space would it not?

If there were multiple mercury?€™s around close into the sun?

I think this is a good step in the right direction, but I think they are still missing the point. Any object of sufficient mass should be a planet regardless of how it got there, regardless of how many there are.

[chris]

That?€™s a good argument, better then most I have hear, but there is still too much wiggle room in my opinion.

Let me see if I can explain.

What about multiple planets in roughly the same orbit with the same mass, both of them would be dominate, but none of them would dominate each other.

Then under the current definition the objects would not be planets. However, I have doubts that several planet-sized objects in similar orbits would stable over long time scales. One planet could occupy another's L4 or L5 Lagrangian point if the mass ratio between the two was 25:1 or greater, e.g. an Earth sized object sharing an orbit with a Jupiter sized planet. It's not clear if both objects would be planets under the IAU's definition, but I think the language could be clarified to allow such cases. *If* they even exist.

What about the possibility of rouge planets or extremely distant objects. If Pluto was outside the system 100?€™s of AU?€™s and was the only object, then by that definition it would be a planet. It would have either directly or by other external actions not related to the planet, have its own path that was cleared or would clear its own path be considered the dominate object in that part of space would it not?

I don't think such an object would exist in isolation, at least not for long. It would be in a cloud of other ejected objects.

If there were multiple mercury?€™s around close into the sun?

Again, I have doubts that such a system would be stable. I'd like to see the 'clearing the neighborhood' requirement more clearly stated, but I suspect that may not actually be that many troublesome intermediate cases in nature. Dynamics seem to demand that an orbital zone is dominated by a single object or host to thousands of similar objects and not, say, four.

I think this is a good step in the right direction, but I think they are still missing the point. Any object of sufficient mass should be a planet regardless of how it got there, regardless of how many there are.

I disagree. I think it's right that the IAU is proposing a definition that tries to identify how planets are qualitatively different than non-planets rather than just setting some arbitrary cutoff. Mass is a continuum; 'orbital dominance' is likely not.

--Chris

[jebus]

In honor of Pluto's demotion, I reclassified it as an asteroid in Celestia!

ALso while there I chose to correctly name andromeda, Xena, and Gabrielle.

And in honor of Futurama, I renamed Uranus to end that stupid joke once and for all!

[Nick]

I think if you can't appreciate the sense in this new definition, you're not a very good scientist, cause it's pretty unquestionably reasonable.

[Malenfant]

I think if you can't appreciate the sense in this new definition, you're not a very good scientist, cause it's pretty unquestionably reasonable.

Just to re-iterate, this is what was decided on:

-----------------------------
*RESOLUTION 5A*
The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:

(1) A planet1 is a celestial body that (a) is in orbit around 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, and (c) has cleared the neighbourhood around its orbit.

(2) A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects3 orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

------------------------------------------------------------------------
^1 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
^2 An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
^3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
-----------------------------

The main issues are:
1) hydrostatic equilibrium is not easily measurable in an unambiguous way
2) "has cleared the neighbourhood around its orbit" is incredibly vague. Note that Mike Brown's interpretation of "gravitationally dominating its neighbourhood" is NOT what is stated in the definition.
3) The definitions work only around Sol, not any other star.

These are fairly critical issues, since there's no numerical, objective definitions involved here, it's all subjective.

So there's a lot of good scientists who don't appreciate the new definition precisely because (right now at least) it's so vague and ill-defined. One shouldn't have to "appreciate the sense" of a definition at all, one should just be able to apply it straight away with no ambiguity or interpretation.

[Johaen]

On a humorously related note:

A day after the planet Pluto was disowned by the world's astronomers, Texas Motor Speedway President Eddie Gossage is embracing the former baby in the solar system's nine-planet pantheon.

In honor of the planet formerly known as Pluto, Texas Motor Speedway will give residents of Pluto a free ticket to the Dickies 500. The ticket would be for the 2015 or 2016 Dickies 500 since it is a three billion-mile trip that takes roughly 9?? years in the New Horizons spacecraft. Pluto race fans must show a photo ID with a proof of residence at the TMS ticket office to be eligible for a Dickies 500 ticket.

"This is such a Mickey Mouse move by the astronomers, especially since my second-grade teacher told me it was a planet back then and still should be," Gossage said. "They may disregard Pluto, but Texas Motor Speedway won't. We already have all 50 states and several foreign countries with season tickets, but we'd love to be the first speedway to get a season ticket-holder from Pluto. We are always looking to expand our market."

Taken from: http://www.nascar.com/2006/news/opinion ... index.html (yes, I'm a big NASCAR fan )

lol

[hank]

"Pluto: Down But Maybe Not Out"

- Hank

[Scorpiove]

In honor of Pluto's demotion, I reclassified it as an asteroid in Celestia!

ALso while there I chose to correctly name andromeda, Xena, and Gabrielle.

And in honor of Futurama, I renamed Uranus to end that stupid joke once and for all!

Why would you do that? Even after being demoted, Pluto is still definately not an asteroid.

[PlutonianEmpire]

We didn't get "Plutonian Empire" comment yet ?

That's 'cuz i've been in Australia for 3 weeks.

I"m back now.

Oh, and my response:

NOOOOOOOOOOOOOOOOOOO!!!!!

[Planet X]

I definately agree with that! As it is, I've tried something new on my SSC files. Instead of using the "asteroid" classification, I decided to classify Mercury, Pluto, and Eris as "moon," followed by # Minor Planet. That way, their orbits uniquely stand out among the major planets and asteroids. Later!

J P

[Planet X]

Hey, that brings me to an important question. Has the term "Minor Planet" officially been ditched as a reference to small bodies? If so, I'd suggest that they revive the term to refer to bodies in the 2000-6000 km diameter range.

OK, 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. Later!

J P

[Malenfant]

I seem to recall that for some unknown reason they explicitly said that the term "minor planet" was to be ditched. I have no idea why, since it's a perfectly serviceable and useful term.