Ok… Let’s try this again shall we?
It’s official! There are eight planets in our solar system. The vote has taken place. Last week’s proposals have been rejected. Pluto has been demoted, apparently.
They were voting on the following (I got this here):
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”.
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.
That was resolution 5A, and then there is 5B:
Insert the word “classical” before the word “planet” in Resolution 5A, Section (1), and footnote 1. Thus reading:
(1) A classical planet1 is a celestial body . . .
And then the Pluto-specific resolutions (actually, 5B seems to lay the groundwork of the attempt to save 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.
RESOLUTION 6B
The following sentence is added to Resolution 6A:This category is to be called “plutonian objects.”
Anyway, from what I heard, looks like these attempts to keep it have been rejected – those last few resolutions were voted down – and Pluto has been given the chop.
More later…
[Update: BBC story on it here.]
-cvj
i think that CERES IS NOT A PLANET AND SHOULD NOT BE ON THE DIAGRAM!!!!!!!!!!!!!!!!!!!! SO KICK IT OFF
i think that Pluto exist!
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Neptune today is not at the orbit where it was formed. It formed at about 18 AU and migrated outwards to 30 AU. Neptune actually might have moved further to 50 AU and then migrated inward to 30 AU (bouncing!). The other giant planets: Uranus and Saturn migrated outwards, and Jupiter migrated inwards.
Along the way, the giant planets scattered the planetesimals (asteroids, etc.) into the asteroid belt, the Oort cloud, and into the ‘scattered disk’ part of the Kuiper Belt. Neptune had a large role in shaping the Kuiper Belt that we see today, according to Hal Levison, Alessandro Morbidelli, and their colleagues. For Papers, Morbidelli’s web site is a good start.
Go to Morby and his colleagues’ journal papers of which this 3 Nature papers combined with supplementary material might be most relevant. And here is his and his colleagues’ review papers. This “Planet migration in planetesimal disks” review paper by Levison H.F., Morbidelli A., Gomes R., Backman D. (2006) in the Protostars and Planets V conference proceedings could be best to cover the topic of the “why” and “how” of planetary migration.
You can see other (recent) papers by his colleagues on Google Scholar too: (not sure that this
link will turn out; so if not, then search on Levison, Morbidelli, Neptune)
http://scholar.google.com/scholar?q=Neptune+Levison+Morbidelli&ie=UTF-8&oe=UTF-8&hl=en&btnG=Search
Keithe:
Well that sounds nice and rigorous, but the astronomers not adopt Steven Soter’s definition? They adopted some vague orbital-clearing definintion.
The IAU’s new definition of planet says a planet is a body that “has cleared the neighbourhood around its orbit”. Many people do not yet understand why this type of orbital-clearing criterion is so scientifically significant, nor do they understand how this criterion could be clearly quantified. I recommend the excellent and readable paper by Steven Soter named “What is a Planet” (http://arxiv.org/abs/astro-ph/0608359). Soter explains why mass dominance is significant. The IAU’s definition is not exactly the same as Soter’s, but it is the same idea, and they both conclude that there are eight planets. Soter defines a planet as “the end product of secondary accretion from a disk around a primary body”. He has a couple of parameters that can measure this. Lambda quantifies the extent to which a body scatters smaller masses out of its orbital zone. Lambda = kM^2/P, where k is a constant, M is the object’s mass, and P is the orbital period. Mass and orbital period are pretty easy to measure even for extra-solar planets. A second parameter is mu = M/m where M is the object’s mass and m is the mass of everything else in its orbital zone. In Soter’s paper, if mu is greater than 100, then the object is a planet. The remarkable thing is that both these measures (Lambda and mu) show an immense gap of five orders of magnitude (!) between the eight main planets in our solar system and all the other debris, like Pluto and Ceres. The log-log plots at the end of Soter’s paper show this gap very starkly. There is no gray area between planet and non-planet. Soter convinced me that nature has clearly sorted solar system bodies into two very distinct classes, the dominant (eight) objects and the non-dominant objects.
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Amara: That sounds very interesting. Could you direct us more precisely please?
k95: If you read recent papers by Morbidelli, Levison, et al, you will find that Neptune’s influence on the bodies in its vicinity are important indeed. The present orbital structure of the outer solar system could be due to Neptune.
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ksh95. Yes indeed, almost.
The first set of proposed rules allowed a planet in another solar system which had an eccentric orbit around another planet to be a moon then a planet then a moon, as it orbited, if you see what I mean.
Now we have planets which have to have cleared their neighbourhood, sort of. Since Neptune has Pluto in the way it has not cleared the local space of junk therefore Neptune is not a planet. haha.
How rude, calling Pluto junk. Apologies.
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Pluto and Neptune have intersecting orbits, therefore Pluto has not yet cleared it’s orbit and is a dwarf.
Doesn’t the reciprocal argument make Neptune also a dwarf????
I guess the astronomers are not explaining themselves clearly, or perhaps I didn’t pay sufficient attention.
Must say it all strikes me as somewhat simpleminded. A bit like botanists declaring that everything with roots over 6 feet high is a tree, everything less than 2 feet high is a plant and all the rest are bushes.
Still, it’s better than what we had before, and the unseemly bunfight has caught the publics imagination in a scientific way.
Incidentally, 81 percent of 13,000 AOL members think that Pluto should still be a planet. How many of that number are astrologers is not divulged 🙂
Back to mneumonic production…
Hooray !!!!
Makes mnemonic writing even easier now 🙂
There’s a conceptual clarity in these resolutions that I appreciate, and that I prefer over the habits of speech and of feeling that incline some of us oldies to say “I want to keep calling Pluto a planet.” It seems to me one could teach a machine, or a Martian, the meaning of these definitions, which is another way of saying we can use them like a razor to decide which object goes into which pile. (I admit I haven’t considered the implications fully; maybe there will be difficulties.) Since science is a human activity, there may be more to consider than just what we could rigorously formalize and teach to a machine, but at least that approach is apt to be self-consistent, isn’t it?
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