New Planets!

[cheesehead]

jasong,

I've noticed that Mally recently seems to have developed a habit of criticizing other posters without first getting the facts straight, perhaps without awareness of that omission, and apparently without noticing that sometimes his own statements contain the types of error he attributes to others. He seems to misinterpret what he reads more often recently than he did earlier. I hope this is temporary.

Your 22 Aug posting was correct, and Mally's subsequent condescension was unjustified.

[jinydu]

http://www.iau2006.org/mirror/www.ia...603/index.html

Originally Posted by International Astronomical Union website

RESOLUTIONS
Resolution 5A is the principal definition for the IAU usage of "planet" and related terms.

Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the prototype. The IAU will set up a process to name these objects.

IAU Resolution: Definition of a Planet in the Solar System
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation "planets". The word "planet" originally described "wanderers" that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

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

(1) A "planet"1 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 except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".




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



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IAU Resolution: Pluto

RESOLUTION 6A
The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

Still rather vague and imprecise for a scientific definition, in my opinion.

[jinydu]

Some of the public reaction to the news has been less than informed:

http://news.yahoo.com/s/usatoday/200...tusgoeslongway

Originally Posted by Editorial

Our traditional values in this country are taking a big enough beating, and now the IAU wants to mess around with the traditional organization of the solar system. When is this going to stop?


This Pluto decision must be reversed. Tradition must prevail.

Originally Posted by Another Editorial

When I was a kid in school studying the solar system was one of my favorite parts of science class. Now I’m being told we were wrong. Oh man, now I’m worried I really never will need the algebra, trigonometry and physics classes I haven’t used since then either! Were all my years in school a lie? Why are we here? The ponderous weight of these thoughts are clouding me over. Help me Obi-Wan Kenobi. You’re my only hope.

[cheesehead]

Originally Posted by jinydu

Still rather vague and imprecise for a scientific definition, in my opinion.

How would you modify or replace it to be less vague and more precise?

Keep in mind that the IAU's definitions of "planet", et al., are intended to be category labels for classifying objects differentially for various purposes of ongoing scientific work. The IAU doesn't mean that there's any inherent scientific difference between objects just above and just below the line between "planet" and "dwarf planet".

The IAU definition of "planet" is not intended to be the same type of scientific definition as those of "mass" or "light-year".

[jinydu]

Originally Posted by cheesehead

How would you modify or replace it to be less vague and more precise?

Admittedly, I don't know. I just noticed that the definition is nowhere near as wonderfully precise as the definitions I learn in physics classes.

[roy1942]

Originally Posted by xilman

The killer criterion, in my opinion, is that the moon's orbit is everywhere concave to the Sun. That's why I account it double-planet status and not merely a satellite of planetary mass.

Paul

Actually the "concave orbit" criterion depends on the relative sizes of the earth-sun and earth-satellite orbits, and the relative masses of the earth and the sun, and (to first order) is independent of the satellite mass. The orbit is concave if at all points in the satellite's orbit, the sun is attracting it more than the earth is. A dust mote at the moon's orbital distance would also have an orbit always concave to the sun, but you wouldn't want to call that a double planet with the earth!

[mfgoode]

Originally Posted by roy1942

Actually the "concave orbit" criterion depends on the relative sizes of the earth-sun and earth-satellite orbits, and the relative masses of the earth and the sun, and (to first order) is independent of the satellite mass. The orbit is concave if at all points in the satellite's orbit, the sun is attracting it more than the earth is. A dust mote at the moon's orbital distance would also have an orbit always concave to the sun, but you wouldn't want to call that a double planet with the earth!

Roy: You have a very valid point and the mass counts a lot.
Could you please explain what you and Xilman mean by 'concave Orbit'?
Asfaik, the satellite orbits the earth say, and the earth orbits the sun. Thus there is more than one orbit of the satellite and the final orbit is much more complicated than 'concave'? IINM this is the case with Pluto
Thank you,
Mally

[xilman]

Originally Posted by roy1942

A dust mote at the moon's orbital distance would also have an orbit always concave to the sun, but you wouldn't want to call that a double planet with the earth!

Correct.

My criterion is not the sole requirement for planethood. I also require self-gravity sufficient to overcome rigidity of the solid component of the body. The latter rules out dust motes, comets, most asteroids, Phobos, and so forth. The former criterion rules out the Galilean satelites, Titan, Triton, etc.

Under my definition, the four terrestial planets and four gas giants would be accompanied by Luna, Ceres, Charon and at least two more KBOs.

Here's a question that does not seem to have been satifactorily answered yet. Is a black hole of 0.001 solar masses and orbiting a star in its own right a "planet". It would appear to be under the present and proposed IAU definitions. It's not big enough to sustain fusion, it's spherical under its own self-gravity, it's not orbiting another planet and it's easily large enough to have swept its orbit clear of other material.

BTW, I find it immensely reassuring that the Earth is a planet according to the IAU. By definition, it has swept its orbit clear of other material and so we never again need worry about an asteroid impact causing major damage of the kind we last experienced 65My ago.


Paul

[Uncwilly]

Originally Posted by xilman

Here's a question that does not seem to have been satifactorily answered yet. Is a black hole of 0.001 solar masses and orbiting a star in its own right a "planet". It would appear to be under the present and proposed IAU definitions. It's not big enough to sustain fusion, it's spherical under its own self-gravity, it's not orbiting another planet and it's easily large enough to have swept its orbit clear of other material.

BTW, I find it immensely reassuring that the Earth is a planet according to the IAU. By definition, it has swept its orbit clear of other material and so we never again need worry about an asteroid impact causing major damage of the kind we last experienced 65My ago.

How did the BH form? If it was a stellar class object that has lost mass through Hawking radiation, I would think that it would not be considered a star. If it was a primordial BH, then I would think that it still would not be, because it would not have formed with the star, but on it's own and been captured. I think that the IAU recognizes that BH are special objects. Wait 100 years and there should be a definition of a stellar object, etc. Each of these should include references to other classes (as does the planet def.).

The orbit is clear. Your message falsely implies that the SS is not a dynamic and chaotic place.

[ewmayer]

Originally Posted by xilman

I find it immensely reassuring that the Earth is a planet according to the IAU. By definition, it has swept its orbit clear of other material and so we never again need worry about an asteroid impact causing major damage of the kind we last experienced 65My ago.

Yes, I too am sleeping much better, knowing that this kind of dysfunctional anti-planetary behavior will no longer be allowed.

(Actually, technically the IAU is OK here, since AFAIK the major impactors beginning with the late heavy bombardment around all were the result of stuff that was perturbed into an earth-crossing orbit by the other planets or by collisions elsewhere in the solar system, not stuff that had eluded clearing of its own orbit by the earth.)

[xilman]

Originally Posted by Uncwilly

How did the BH form? If it was a stellar class object that has lost mass through Hawking radiation, I would think that it would not be considered a star.

Think about what you just wrote.

While cogitating, you may wish to consider the temperature of a stellar mass BH. Compare it with the temperature of the CBR during the history of the universe so far. If it helps, you could make the (rather implausible IMO) assumption that the CBR was at zero K until we started measuring it. That assumption doesn't change the conclusion you should reach.

Originally Posted by Uncwilly

If it was a primordial BH, then I would think that it still would not be, because it would not have formed with the star, but on it's own and been captured. I think that the IAU recognizes that BH are special objects.

Perhap you are right. I don't know and should find out. I am entirely happy to accept your opinion on this particular situation. However ...

What if the BH was a technological construct? For instance, the inhabitants of an honest-to-$Deity planet may have had an unfortunate accident when developing their gravitational engineering technology.

Originally Posted by Uncwilly

The orbit is clear.

The orbit is not clear, in at least two important respects. For a start, there are substantial numbers of very significant sized rocks crossing the Earth's orbit. If you are going to object to my (admittedly tongue in cheek) claim, do as Ernst did and point out that those rocks are (very likely) perturbed into their present orbits by other bodies such as Jupiter or Neptune.

The other counter-example is the Moon. As I pointed out earlier, it is co-orbiting the Sun with the Earth. The claim that it's a satellite is arguably shaky.


Paul