Experts meet to decide Pluto fate, Finally we'll know what a 'planet' is... Options

[MizarKey]

One of many articles regarding the upcoming conference...

Experts meet to decide Pluto fate

[volcanopele]

I must say that I disagree. As reductionists, it is our job to categorize. Finding a workable definition
for a planet has only become necessary, and painful, because we have made so many fundamental
discoveries in our solar system and others since 1992 (the year the first KBO and the
first pulsar planets were detected). It's not about culture. It's about good science.

But we also shouldn't present the solar system as a neat and tidy place when it isn't. The discoveries since 1992 have allowed us to appreciate the complexity of not just our solar system, but other solar systems as well. From other solar systems, we have found large planets that don't following neat and tidy orbits, some have high eccentricities for example. We have found stars with two accretion disks at different inclinations. In our own solar system, we have found icy dwarf bodies that follow a miriad of orbits and have various shapes, and there maybe some the approach the size of the terrestrial planets.

The solar system (and other systems) are not neat and tidy places and we shouldn't pretend that it is. Listen, I understand we need a system for categorization. It allows us to more easily make sense of our world or the worlds around us. I understand that. But the amount of press this has gotten and the amount of breath and time spent on this is not worth it. Pluto is still Pluto whether it is a planet or a TNO, or any icy dwarf, or a dog.

Setting arbitrary definitions also makes the word less useful for scientific purposes. A TNO at 4000 km probably didn't form fundimentally any different from a 2000 km wide body (or a 1900 km wide body). As long as we make it clear to the public what the words value is (for classification purposes and for nomenclature purposes), I think we can come to an understanding. But if we treat it as if objects that are planets are some exclusive group or club and those that are just moons or minor planets are inferior and aren't worth our time in terms of exploration purposes (just because they are not planets), then we have a problem.

Okay, I'm sorry about the rant...

[Guest_AlexBlackwell_*]

But we also shouldn't present the solar system as a neat and tidy place when it isn't. The discoveries since 1992 have allowed us to appreciate the complexity of not just our solar system, but other solar systems as well.

Jason, I think you and everyone else are missing Alan's point. No one is trying to obscure the fact that our "solar system [isn't] a neat and tidy place." Quite the contrary. Taxonomies and classification systems are very useful in science, especially in astronomy. Discerning hierarchical relationships, ipso facto, can lead to scientific discoveries.

[David]

No one is trying to obscure the fact that our "solar system [isn't] a neat and tidy place." Quite the contrary. Taxonomies and classification systems are very useful in science, especially in astronomy. Discerning hierarchical relationships, ipso facto, can lead to scientific discoveries.

I don't disagree; but if it were the business of the IAU to try to make its nomenclatorial system conform to any one of several possible planetary taxonomies, surely the first order of that business would be to find a way of pointing out that Jupiter and Mercury are not the same kind of object?

[Alan Stern]

I don't disagree; but if it were the business of the IAU to try to make its nomenclatorial system conform to any one of several possible planetary taxonomies, surely the first order of that business would be to find a way of pointing out that Jupiter and Mercury are not the same kind of object?

Perhaps these will help some who have not seen them; sorry for spamming those who did-- the
links will save me from typing my views:

Gravity Rules: http://www.spacedaily.com/news/outerplanets-04b.html

Copernicus Smiled: http://www.thespacereview.com/article/450/1

[David]

Perhaps these will help some who have not seen them; sorry for spamming those who did-- the
links will save me from typing my views:

Gravity Rules:

I'm quite fond of the "rounded by gravity" criterion myself; but the presence of objects like 2003 EL61 and Iapetus makes it rather difficult to apply. Objects with diameters between 400km and 1600km exhibit a wide variety of shapes: spheres, near-spheres, flattened spheroids, spindly spheroids, nicely rounded ellipsoids, bumpy, lumpy, and partially concave ellipsoids, and plain old irregulars. If there's a direct correlation between shape and size or mass, it's not an obvious one.

Why wouldn't a cutoff above 1600km diameter be just as defensible a gravity-based division as one below 400km?

[Alan Stern]

I'm quite fond of the "rounded by gravity" criterion myself; but the presence of objects like 2003 EL61 and Iapetus makes it rather difficult to apply. Objects with diameters between 400km and 1600km exhibit a wide variety of shapes: spheres, near-spheres, flattened spheroids, spindly spheroids, nicely rounded ellipsoids, bumpy, lumpy, and partially concave ellipsoids, and plain old irregulars. If there's a direct correlation between shape and size or mass, it's not an obvious one.

Why wouldn't a cutoff above 1600km diameter be just as defensible a gravity-based division as one below 400km?

Careful, careful, careful! The roundness argument is not about whether an object is round or not-- because it could for axample be tidally bulgded or rotationally distorted. I's about whether its massive enough **Tto be rounded by gravity** in the absence of
the other effects. You will see this speccifically noted in the IAU language tomorrow.

-Alan

ps. EL61 is probably not a big egg: it's most likely a huge contact binary. At least that's where my money is.

[vexgizmo]

Alan Stern discussed this above (post #28 above, with my reply following).

I's about whether its massive enough **Tto be rounded by gravity** in the absence of
the other effects.

-Alan

But Vesta likely was once "rounded" by gravity, and then later smashed. In fact, its post-impact shape still may prove to be well-approximated by a hydrostatic figure. Again, I like this definition, but I suspect we will be arguing Vesta and others, and arguably Xena doesn't seem to make the cut until we measure its triaxial shape.

-Bob P.

[David]

But Vesta likely was once "rounded" by gravity, and then later smashed. In fact, its post-impact shape still may prove to be well-approximated by a hydrostatic figure. Again, I like this definition, but I suspect we will be arguing Vesta and others, and Xena doesn't seem to make the cut until we measure its triaxial shape.

I have some thoughts on Vesta's shape, but having gotten myself unnecessarily exercised on this point in the past, I'm inclined to wait until either Dawn or the next generation of telescopes provides a much better image of Vesta's shape than we currently have. I don't mind waiting several years to have an argument.