So whoever would have thunk it. The IAU duly appoints a small committee to settle the definition of a planet. Said committee duly arrives back with a unanimous verdict, then heads off for fun and frolic. Upon receiving definition-by-committee-with-no-dissent, the general membership of the IAU, by all appearances entirely on its own, starts a major dust up over the matter. Something seems rather unseemly here.

Not only that, but there's been some hand wringing among a few at the IAU over the fact that the moon might transition to a planet, in such a way that you would think it's going to happen this century rather than billions of years down the road. Less than two actual centuries from now, asteroid 3753 Cruithne will go from being an Aten class to an Apollo class. No big deal, it'll just be deleted from one list and added to the other.



A true story here. I myself once asked Professor Tombaugh, in person, about the mass limit of a planet.

It was at the University of Kansas, where I was a student, shortly after the discovery of Charon. He gave a lecture to a packed audience at the student union, during the course of which he stated that Charon had shown Pluto's mass to be "shockingly low".

After the lecture, I got in the long line to meet him. I remember the fellow in front of me (with trembling hands) asked for four special envelops to be autographed, which Tombaugh graciously did. Then I stepped up, nervously cleared my dry throat, and dared to ask where between Ceres and Pluto he would draw the line on planets.

His answer? He didn't have one! He was settled on the idea that, no matter what, Pluto was a planet, but he frankly admitted he didn't know where to draw a line. He mostly pointed out the large gap between the relative sizes of Pluto and Ceres. True enough then, but now we have gap fillers.

So that is that.

One last thing, about short lists for the benefit of kids. When I was growing up, Jupiter had twelve moons (five named), Saturn nine, Uranus five, and Neptune two. Fairly easy to keep track of. Now we have substantially more inner moons, which by themselves would still be concise enough. Then add on the mass of outer moons multiplying like rabbits, and now the situation is impossible for any student who is not obsessive over them (and I'll admit to being a borderline case).

Then end result? Nobody is asking for fewer moons. Kids simply study and remember what seems important. What's important is, first, what has been photographed up close, and second, what stands out.

Phobos and Deimos because they're martian. Io with the volcanoes. Europa cause it might have life. Titan absolutely. Enceladus maybe. Of course, there is also that one with the footprints on it.

My prediction is that the most important planets will be the classical ones, the ones that can be seen.
Mercury, Venus, Mars, Jupiter, and Saturn. Uranus and Neptune will continue to be remembered by every kid serious about astronomy, especially if they have a pair of binoculars. Paradoxically, the fact that Vesta can be seen, barely (and I have seen it naked eye, barely), may insure that it stays an asteroid. That way those blessed with good vision have the hope of seeing, naked eye, an asteroid. This rather than just another faint planet, like Uranus.

It's possible that Ceres may take Pluto's place in peoples affections as a "pet" planet. Who can resist a tiny little giant? After DAWN we will have side by side scale pictures of Vesta and Ceres. I'm expecting that Vesta will look like a very big asteroid, and Ceres will look like a very small planet. And then when New Horizons reaches Pluto, Pluto is going to look an awful lot like a planet.

Pluto will retain it's position in the cultural consciousness both from history, and by the fact that it's currently the only one among the outermost swarm that can be directly viewed with a modest sized telescope.

As for the rest of the planets lurking out there, well... The average student of astronomy will consign them to where they are, relegated to obscurity in the anonymous outer darkness, where there is weeping, wailing, and gnashing of teeth.

Unless we find a really big one. Or a really strange one. Or a really big, strange one.

Lastly, I have no idea what to think of Charon as a planet, although at first I was vaguely against it. Maybe I still am. Not sure.

Well that's my say. Now I'll retire to await the final verdict with the rest of you, and read others opinions here in the short meantime.

Dunno if someone already highlighted here, but David posted another possible definition of double planet that seems to better match the unicity of Pluto-Charon and (eventually) similar double asteroids/KBO objects.

Debate goes from bad to worse:
http://www.newscientistspace.com/article/d...s-heats-up.html

Planetesimals To Brown Dwarfs: What is a Planet?

Authors: Gibor Basri, Michael E. Brown (Univ. of California, Berkeley and California Inst. of Technology)

Journal-ref: Annual Reviews of Earth and Planetary Science, 2006, v. 34, pp. 193-216

http://arxiv.org/abs/astro-ph/0608417

Looks like the anti-Pluto bullies might have got their way after all...

Astronomers lean towards eight planets

Michael Brown: discoverer of the 10th planet sounds much more significant than Michael Brown: discoverer of 20 out of 80 plutons.

[...]

I still don't see the rationale for demoting Pluto, which has been a planet since the 1930s. Deciding Pluto is too small is just arbitrary. How does one justify keeping Mercury and rejecting Pluto based only on that criterion?

However, it seems this question has come up because of the discovery of KBOs. So now there is a sense that there will be "too many planets". Again, that is arbitrary. What if there are many Mars-sized objects out there? Will we go through through this whole silly exercise again, with dozens of ad hoc definitions of "planet" being proposed?

The IAU doesn't seem to have been any more scientific about this than the hundreds of opinions I've read on multiple forums. It's become a big joke in the media now, so from a public-relations standpoint it was handled very badly by the astronomy community.

I'm beginning to realize that there is a real cultural issue here -- but it's not about popular culture. It's about the professional culture of astronomers.

With a few exceptions, it seems that "planet" is a word that many astronomers want to own. After all, if "planet" belongs to some group of people, it ought to be the astronomers, right? So how do you show that you own a word? By putting your own stamp on it -- by giving it the definition that you want. And if it's a definition at variance with common perceptions, all the better -- because that way you show that you are not just following popular thought, but really lending it the benefit of your uniquely scientific thought. But when you invoke that kind of justification, the matter becomes personal. Hence the insistence that one definition is scientific and another is not.

It's been noted that debates become most heated when the least significant and least verifiable issues are on the line. I think that's because those issues allow more of the personal element to be injected into the fray. Perhaps they do need several years to hash this out, although that could also be several years in which opinions become hardened.

And yes, at present the IAU does not seem to be coming out of this smelling like a rose.

You make some very good points there David, food for thought indeed.

Personally I will be bitterly disappointed if Pluto loses its planetary status. Not only will the IAU be belittling and snubbing the achievements of Clyde Tombaugh, they'll be showing - to some degree - they can be cowed by or at least influenced by aggressive almost bullying tactics. That committee, in good faith and knowing they were on (as we put it here in the UK) a hiding to nothing, put out a proposal for considered debate and discussion, and then certain groups attacked them and their work in a needlessly angry and confrontational way. I think quite a few astronomers have tried to create influence and make names for themselves here with no real respect or passion for the actual issue itself.

If Pluto is demoted I predict two things. 1. There will be an even bigger "Save Pluto" campaign than before, and 2. astronomy, and astronomers, will come out with a very tarnished reputation that will take some time to repair.

...which, I fear, seems like it would do anything to stop young whippersnappers like Mike Brown, with their new-fangled sky surveying telescopes, get acknowledged as "planet discoverers". Professional science at its worst here, I fear, more about clashes of personalities and character and - perhaps professional jealousies - than doing what's good and best for science and The People.

Here's a thought that hasn't apparently yet been expressed: What happens to this whole house of cards if we find a Mercury-sized or better KBO? I personally think that the odds are way better than 50-50 that we'll find one (or ten, or a hundred) of such beasts within the next twenty years.

I have to admit that I'm a size chauvinist; the thought of all these small bodies classified as planets seems completely inappropriate. So, how about this: Any spherical body in an independent orbit around the Sun that is equal to or greater in diameter to the largest natural satellite (Ganymede) in the Solar System is by definition a planet?

To clarify: I'm not a Pluto-hater, but let's face it: Pluto's been considered an anomaly since its discovery, and nothing in this debate has really changed that. In this view, the Pluto system still gets to be the crown jewel of the KBO population (at least for now...)

Edit: Rats; forgot that Ganymede is bigger than Mercury, and Mercury is most definitely a planet. Guess that's why I never get invited to IAU meetings...

Seems to me there is another way of approaching the "planet definition" question.

Method of formation.

The inner planets and the gas giants / ice giants were formed by accretion via the dynamics of a rather tightly planar accretion disk. These planets were formed as part of the accretion dynamics of the Sun itself.

the Kuiper Belt and the Oort Cloud are both clouds of matter that seem to surround the Sun in a roughly spherical array. The material out in these clouds do not appear to have been formed within the main accretion disk that made up the original solar nebula.

Yes, I know this would reduce us to only nine planets if you count Ceres, and eight if you don't. But perhaps "planets" are objects formed from the dynamics of the original accretion disk, and everything else is debris that accreted outside of these dynamics? Therefore, a planet would have to occupy the plane of the ecliptic in order to have been born directly from the original accretion disk...?

-the other Doug

Mercury's smaller than Ganymede -- although due to its unusually high density, it is considerably more massive. If you made it "as massive or more than Ganymede" you'd include Mercury.

I say that "planet" is any natural object in a stellar system; that sub-groups among these planets are comets, asteroids, KBOs (although we should find a better name for them), SDOs, terrestrial planets, Jovian planets, and satellites, none of which are defined by size (though they may have typical sizes); that there is some special category, let's say "planete" for objects that are "big enough to be round"; and that multiple membership is possible.

So: Jupiter is a Jovian planet and a planete;
Earth is a Terrestrial planet and a planete
Ceres is an asteroid (minor) planet and a planete
Eros is an asteroid (minor) planet and not a planete
Pluto is a Kuiper belt planet and a planete
1992 QB1 is a Kuiper belt planet and not a planete
Sedna is a Scattered Disk planet and probably a planete
Ganymede is a satellite planet and not a planete
Phoebe is a satellite planet and not a planete

There, now everybody can be a planet! Are we all happy now?

I can't find the exact article, which I believe was on space dot com, but this was addressed once by a Pluto demoter. Since KBOs constitute a "swarm", like the asteroid belt, and there should be a gradation of sizes where nothing stands out by, say, orders of magnitude, then it was the opinion of the writer that none should be planets. Not Pluto. Not a Mercury size object. Not a Mercury mass object. Not the biggest, even if it's bigger than Earth.

If a Mars size object is found out there, there is a segment of astronomers that want it designated as 2009 XB14, for example. Then handed number 301576 when its orbit is established.

I hasten to point out that I don't share this opinion.