The Great Planet Debate conference, August 2008 - Washington DC Options

[Ken McLean]

Should we be waiting on further evidence to support planetary accretion such as the Modern Laplacian Theory before deciding what constitutes a planet, ie. born out of solar/stellar system formation? And if so, does that mean we need to class satellites like Titan - which has been postured by some to have formed independently of Saturn's orbit - as a planet? If the orbit of a planet changes does it cease to be a planet, despite being compositionally very similar?

[Greg Hullender]

Don't forget that planetary scientists also study moons, asteroids, comets, dwarf planets, Trans-Neptunian Objects, etc. We don't just study planets

You're undermining Alan's argument, you realize. I don't think we can allow you to study comets, unless they're REALLY big comets. :-)

--Greg

[Juramike]

Objects smaller than the Sun occupy a multidimensional continuum of values. Level of differentiation, atmosphere, size, distance from parent star, crustal materials, past/current geologic processes, etc. Choose the right dimensions, and there will be a unique place for everything: Io, Pluto, Eris - everything could get a coordinate.

All of the objects could be plotted in multidimensional space and subjected to cladistic analysis to draw up clusters of objects. But when you define up the clusters, you may end up drawing up borders that may not make much sense in the big scheme of things. And some of these might end up being arbitrary.

For instance, my own personal level of "interesting objects" [crustally differentiated objects with a solid surface]: Mercury, Venus, Earth, Moon, Mars, Ceres, Io, Europa, Ganymede, Titan, Triton, Pluto.

My list of "preferred interesting objects" [crustally differentiated objects which may have had "recent" geological activity] includes: Venus, Earth, Mars, Io, Europa, Ganymede, Triton, Pluto

And my arbitrary list of "most preferred interesting objects" [crustally differentiated objects which may have had recent geological activity and have an atmosphere and that we've taken pictures of the surface]: Venus, Earth, Mars, Titan

All this demonstrates that the groupings and definitions we give objects are arbitrary in the eye of the definer. (The data itself is not, but how the data is grouped is.)



Pluto will still be Pluto whether we call it a "planet" or not.

Whether Pluto is a "planet" or not, for me, is an uninteresting discussion.

A much more intereseting discussion would try to answer:
How is Pluto similar/different from Earth?
How is Pluto similar/different from Titan?

Those are the questions I get excited about.

-Mike

[djellison]

FYI - the actual pluto debate itself has been had at UMSF before. It ended with raised tempers, arguments, attacks, people running to teacher to claim they were getting bullied. See the several threads in this sub-forum for further examples.

We're not going down that road again.

Whos job it is to decide, however, is a pertinent and interesting debate. Keep it nice.

[Alan Stern]

Objects smaller than the Sun occupy a multidimensional continuum of values. Level of differentiation, atmosphere, size, distance from parent star, crustal materials, past/current geologic processes, etc. Choose the right dimensions, and there will be a unique place for everything: Io, Pluto, Eris - everything could get a coordinate.

All of the objects could be plotted in multidimensional space and subjected to cladistic analysis to draw up clusters of objects. But when you define up the clusters, you may end up drawing up borders that may not make much sense in the big scheme of things. And some of these might end up being arbitrary.

For instance, my own personal level of "interesting objects" [crustally differentiated objects with a solid surface]: Mercury, Venus, Earth, Moon, Mars, Ceres, Io, Europa, Ganymede, Titan, Triton, Pluto.

My list of "preferred interesting objects" [crustally differentiated objects which may have had "recent" geological activity] includes: Venus, Earth, Mars, Io, Europa, Ganymede, Triton, Pluto

And my arbitrary list of "most preferred interesting objects" [crustally differentiated objects which may have had recent geological activity and have an atmosphere and that we've taken pictures of the surface]: Venus, Earth, Mars, Titan

All this demonstrates that the groupings and definitions we give objects are arbitrary in the eye of the definer. (The data itself is not, but how the data is grouped is.)



Pluto will still be Pluto whether we call it a "planet" or not.

Whether Pluto is a "planet" or not, for me, is an uninteresting discussion.

A much more intereseting discussion would try to answer:
How is Pluto similar/different from Earth?
How is Pluto similar/different from Titan?

Those are the questions I get excited about.

-Mike

Mike-

I agree with the high "interest factor" in your two questions just above, start a thread! But regarding planet definition, I hope the the topic (for everyone, not just this forum) needs to move from a "contest" over Pluto (let it fall where it may) to a rational one about planet categorization in general. Putting Pluto in the middle of it clouds the arguments, with people claiming there are issues of sentimentality, American pride, etc.; it distracts attention from the important issue of getting a workable definition and categorization of planets. Sykes and others (not sure about Tyson) have accepted this point and will be echoing it at GPD later this week.

Are you going to be at GDP?

-Alan

[JRehling]

A title I always find funny is "Lunar and Planetary Science", as though the Moon is the sole non-planet to be discussed in this context. Either it should be called "Lunar and Ganymedian and Mirandan and ... [...] ... and Cometary and Planetary Science" or the lunacy of listing just ONE exception should be discarded immediately. Why would the term be broad enough to include Saturn's rings, but not the Moon?

I think the simple category error here is the presumption that the question "Who decides" is going to end up having an answer. Who decided how Pittsburgh would be spelled? Who decided that the English definite article was "the"? Who decided that when a group is asked to identify itself, the answer that sounds best is "It's us" rather than "It's we"?

The IAU *happened* to come up with a torturous and counterintuitive definition and they *happened* to make a decision that impacts the textbooks and they *happened*, as Alan notes, to consist more of people who study stars rather than solar system objects. However, the idea went sour at the point that the implication was made that a smoke-filled room owned the term "planet", which provides no service to science for objects in our solar system.

[pumpkinpie]

I've registered to watch the debate but I never got any email confirmation. Am I all set? What do I have to do to watch it?

[Juramike]

I think the Great Planetary Debate will work itself out over the next several years to decades. And it won't be from a small group in a smoke-filled room, it will be from scientists looking at a much larger dataset (of as-yet undetected and unanalyzed objects) and grouping things according to trends.

A likely example is the historical development of the Periodic Table. From the initial five elements: earth, wind, fire, water, and lifeforce (see avatar), we now have a clear grouping into 118 (and counting) atomic elements. What is very interesting is that early attempts tried to pigeonhole the elements strictly according to atomic mass (law of octaves). It wasn't until Mendeleev ordered elements according to chemical reactivity with guidance from atomic mass that a clear pattern emerged that allowed elements to be systematically grouped.

[Now, what would've happened if the "inert gas" compound XeF2 [CAS# 13709-36-9] had been known back then? Again, there are always exceptions to definitions.]

Years from now, we'll all look back on this and laugh....then get back to work colonizing Titan.

-Mike

[Alan Stern]

I've registered to watch the debate but I never got any email confirmation. Am I all set? What do I have to do to watch it?

Pumpkinpie- I just inquired on your behalf with meeting organizer Hal Weaver. He says you and about 500 of your closest friends who have also registered for the web participation will be getting email info later today answering your particular questions and more.

-Alan

[laurele]

We may need to come to the realization that no one permanently "decides" such things. Students actually learn more from finding out there are multiple schools of thought driven by different interpretations of the same facts. Maybe some discussions need to be left open for now with caveats that things may change as we learn more. No one is teaching there are X number of exoplanets because it is obvious that we are discovering new ones all the time and that the number and types of exoplanets discovered will inevitably keep changing. It's just a suggestion, but maybe the best way for textbooks and educators to approach the planet question is to point to the Dawn and New Horizons missions, which in only seven years will provide us with a whole new set of data on which to base these decisions. Every time we send a mission to another planet, whether a flyby, orbiter, rover, or lander, we discover new things that no one could have anticipated, such as the water vapor in Mercury's very thin atmosphere.

It's just my opinion, but a reversal of the IAU statement that dwarf planets are not planets at all would go a long way in resolving this issue regarding Ceres, Pluto, Eris, MakeMake, and other round KBOs.

I'm going to be attending the GPD in person, and I look forward to a wonderful, tremendous learning opportunity.

[tedstryk]

I think a lot of this trouble stems from the questionable process by which the IAU adopted the resolution. Waiting until after most of the delegates had left to bring it up created a lot of bad feelings. Not to mention that the definition passed doesn't really make sense. My fear is that this mess might weaken the IAU's role, something that would create chaos.

[Greg Hullender]

But regarding planet definition, I hope the the topic (for everyone, not just this forum) needs to move from a "contest" over Pluto (let it fall where it may) to a rational one about planet categorization in general. Putting Pluto in the middle of it . . . distracts attention from the important issue of getting a workable definition and categorization of planets.

It does seem to me that Planetary Scientists are the only ones in a position to do such a thing. If there were a consensus around a really solid definition, I'd expect the other IAU members would defer to it.

Maybe it would help to give a special name to the Magic Eight; call them "Classical Planets" for example. Round, isolated, in circular orbits in the ecliptic -- they certainly SEEM special enough to merit special attention.

Things like the moon or Ganymede could be "Secondary Planets". Again, from a Planetary Science point of view, I don't see how any rational definition can leave them out.

Planets that are part of a belt (like Ceres or Pluto) could stay "Dwarf Planets" but those would now be "planets," not minor planets.

Stuff above the meteorite level could be "subplanets" and we'd just retire the term "minor planet".

Probable planets (e.g. fairly bright KBOs) could be called "planetoids" until we had enough data to class them as dwarf planets or subplanets.

I'm still not ready to consider comets to be any kind of subcategory of planet, but I suppose you could "define" comet as an icy subplanet in a highly elliptical orbit etc.

Kids in school would learn the classical planets. Enterprising ones would learn the secondary and dwarf planets on their own.

--Greg

[surreyguy]

Maybe I'm just belabouring the obvious here, but while some categorisations (e.g., all bodies whose name begins with 'E') are arbitrary, the good categorisatons are those which reflect theory in some way. That's what distinguishes categories such as '>2000km diameter' from 'roundness'. The thing which I guess I find odd is when people find it hard to switch viewpoint, and thus what categorisation is relevant for the discussion at hand. If you want to discuss tectonic features then the roundness criterion is relevant; if you are talking about the way the architecture of the Solar System has been sculpted over time, then orbital dominance is what you want.

Of course, some definitions may be more useful than others for the purpose of knowing which committee gets to name a body, but you'd think that issue could be settled without people crying 'but what about the children'...

I guess I'm saying that I don't think that there's any such thing as an inherent quality of 'planetness' which we can know - that seems a no brainer, put like that, but a lot of the arguments one sees seem to be predicated on something like it. For example, the use of 'what if we discovered...' scenarios.

[Alan Stern]

Pumpkinpie- I just inquired on your behalf with meeting organizer Hal Weaver. He says you and about 500 of your closest friends who have also registered for the web participation will be getting email info later today answering your particular questions and more.

-Alan

Now I am told they are getting flooded with extra log in requests for web participation and won't cut off the list until morning. Only then will they send out the instructions, answers to questions, etc.

-Alan

[Juramike]

Are you going to be at GDP?

I guess I was more interested than I thought. I signed up to get the webcast.
(Hopefully it'll get through my work's firewall.)