[X] My Assistant

[dvandorn]

OK -- fair enough. Though that *is* allowing a gravitational definition of real estate to affect the terminology...

Isn't there some theory out there that Triton was once a KBO that was later captured by Neptune? If that's the case, then you would say that Triton is a planetary body that was once a planet but is now a satellite?

I think the reason people are so energized over the classification issue is that most people have a need for a world around them that they can hold in their heads at one time. When we think of a city, for example, we think in sequential sequences of roughly ten-square-block areas that we can characterize in our minds "at once." We even give names to these small, handle-able chunks, calling them districts or neighborhoods. But we chop things into segments that we feel comfortable dealing with as single entities.

If you want people to feel like they have a place in a larger Universe, you need to give them relatively "small" chunks to deal with. Giving school kids ever-widening chunks (Earth as a planet, Earth/Moon System, Inner Solar System, Solar System) you keep the number of elements of each "chunk" under 10. That seems to be a good limit, since anything that gets a lot more complex is difficult for an average person to handle in one "chunk."

So, I think it is important to give people some kind of system that identifies a Solar System consisting of a number of objects (in the vicinty of 10 or so objects) that they can grasp in one "chunk." When you add thousands of objects, you need to add them "en masse," is with the Asteroid Belt, the Kuiper Belt, the Oort Cloud -- note that those are all singular, not plural, nounds. Otherwise, people begin to feel lost and overwhelmed by the sheer number of "things" they need to consider or account for within a given system.

(Note -- I'm not talking about the scientists and engineers, here, who are trained, or have an affinity, to think in different scales. I'm talking about your normal, salt-of-the-Earth kinds of people who make up 95% of the world's population...)

-the other Doug

[DEChengst]

Just ask a kid to draw
a planet; they always draw a circle, never an egg or a box or a pyramid or
even a hamburger.

I bet Borg children would draw a box

[Alan Stern]

Other Doug-- I respectfully disagree; people I speak to in public talks find the i
dea of there being a lot of planets new and captivating, many even find it really
enchanting.

I don't think they have any problem at all getting used to lots of planets as long
as they know that, like stars, they won't have ot be able to name them all.

-Alan

ps. Yes, Triton's retrograde orbit means it is a capture from heliocentric orbit. It
formed as a planet and was captured to become a satellite. Because it orbits
a planet now, rather than old Sol, it is best to refer to this kind of object as
a planetary body or a planetary-scale satellite. If you do look at the Icarus
paper reference I sent a yesterday, you will see that Triton is hard to capture,
indicating that indicates there were a lot of them once around in the U-N zone,
in order to get one into orbit around Neptune.

[dilo]

Alan, you observed that my scheme wouldn't correctly classify eventual Mars-sized objects in the Oort Cloud or ejected Earth-sized bodies...
Maybe, but the constraint I made in terms of orbit and population sizes goes in the same direction of your criteria: "If is large enough the remove or control other objects in its vicinity it is a major planet", otherwise is a minor planet. I must admit that latter criteria is simpler and less arbitrary, but mass treshold would be harder to calculate, because depends on distance from the Sun...
If joined to the spherical shape constraint, this criteria appear acceptable (I wouldn't classify as a planet an isolated chunck of rock orbiting the Sun!)...
Based on this, I'm happy for the satellite statement too: "if an object fitting the planetary definition orbits another planet then it is called a satellite"; now, if "planetary definition" includes also constraint on size/shape, IAU shold update the list of planets orbiting giant planets or, at least, divide them in the two cathegories of major and minor/dwarf satellites!

[Mongo]

Just to let everybody know ... there is a new Yahoo Group called 'majororminor' located here, concerned with discussing the definition of a planet, and related topics. I have already posted some of my earlier posts from this forum there, but thought it would be good to have multiple viewpoints from outside the mpml community.

Bill

p.s. Is it okay for flog this new site here?

[Guest_BruceMoomaw_*]

Alan Stern: "The one comment I'll make here is that we have been
careful not to insist that an object BE spherical, only that it be massive enough
to become spherical. This avoids issues like the one you brought up (who
spherical does it have to be?), as well as issues related to rotational
flattening (ala Jupiter and Earth)."

Ah, but whether an object is "massive enough to become spherical" depends on what kind of substance it's made of, and thus how rigid it is. And determining that is a lot more difficult than determining how spherical it actually is.

Moreover, we once again have the problem of fuzzy borders. Pallas (which Mongo accidentally left off his list of Class 3 planets) and Vesta are mildly but significantly non-spherical, and NOT because of rotational flattening. Among the moons, so are Iapetus and Mimas -- to say nothing of Neptune's second-biggest moon Proteus, which looks like nothing so much as a giant marshmallow. And given the number of KBOs, there are bound to be a lot of them that fall into this "Is it spherical or not?" category.

Parenthetically, has anyone come up with a convincing explanation for why Pallas -- alone of all significantly large asteroids -- has that incredible 30-degree orbital tilt? Either something really big ran into it (in which case one would tend to think its parent body would have shattered into multiple objects), or something even bigger brushed past it and tidally yanked it into that orbit.

[Guest_BruceMoomaw_*]

More news from Mike Brown via Ron Baalke in the "Planetary Sciences" webgroup: the Spitzer Telescope's IR observations of 2005 FY9 show it to be only 50-75% of Pluto's diameter -- which takes it off the "planet" list even if one defines a planet as something more than 2000 km wide. So we're back to 10.

[Decepticon]

Boy would I love to see one of these up close.

Makes you wonder if all stars in the Milky Way have KBO.

[Rob Pinnegar]

Forgetting about trans-Neptunian objects for the moment: I'm going to play Devil's advocate here...

If I recall correctly, the millenia-old original list of "planets" included seven objects: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn.

Since that list was defined, we have been finding ways of updating it by redefining the word "planet" in ways that incorporate new discoveries while conveniently keeping most of the original members on the list. But even if we only consider the "Mercury-through-Neptune" list, we hit some problems.

Does it really make sense to put Saturn and Mercury in the same taxonomic classification, when one of them is two thousand times more massive than the other, and their compositions are completely different?

On the same note, is Jupiter a planet in the same way that Mars is a planet?

Some people call the Earth-Moon system a "double planet" -- so why don't we cut Neptune and Triton the same slack? Or Saturn and Titan? And why isn't the Jupiter system called a "quintuple planet"?

If we are going to consider demoting Pluto from planetary status, then maybe it's time to throw out the baby with the bathwater, and demote _everything_ from planetary status, because "planetary status" has become a quaintly outmoded concept. We need to divorce ourselves from our sentimental attachment to the word "planet". Five thousand years ago, it meant something, but not any more. It is obsolete.

Pluto is what it is, no matter what we call it.

Cheers
Rob, Canada

[abalone]

Alan Stern: "The one comment I'll make here is that we have been
careful not to insist that an object BE spherical, only that it be massive enough
to become spherical. This avoids issues like the one you brought up (who
spherical does it have to be?), as well as issues related to rotational
flattening (ala Jupiter and Earth)."

The chondrules that make up carbonaceous chondrites what about them?

"A roughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at about 1400°C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5 to 2 mm in diameter and are usually composed of iron, aluminum, or magnesium silicates in the form of the minerals olivine and pyroxene, with smaller amounts of glass and iron-nickel. Together with calcium aluminium inclusions, which predate them by a couple of million years, they are among the oldest objects in the Solar System with an age of about 4.57 billion years. They formed when dusty regions of the solar nebula were heated to very high temperatures, became molten, and then resolidified as tiny droplets. "
From http://www.daviddarling.info/index.html

Were they all once planets and are there enough combinations of letters to name them all if they were?

This shows the pointlessness of trying to invent rules where none are needed.

[Alan Stern]

Chonrules are spherical due to surface tension, not gravity. It's not that
an object *is* spherical, the thing that counts, that reveals it acts like a planet in
a fundamental way, is that it is massive enough to have its shape controlled
by self-gravity.

[abalone]

Chonrules are spherical due to surface tension, not gravity.

I realise this Alan, I was just taking the rule to its ultimate extreme. Exactly how big would it have to be before gravity is stronger than surface tension, 10m, 50m certainly by 100m?
If there was a material that had no surface tension then gravity would cicularise it irrespective of mass and if a 100km lump of rock had enough internal heat generated by short lived radionucleides it would melt and form a sphere due to gravitational forces in the brief time before it resolidified, but now I am getting ridiculous again

It is like making up a rule for what constitutes a tall or a rich person

[dvandorn]

I've been enjoying the discussion, actually. It's obvious that human beings need to classify things into systems -- it's how we look at things. And whenever you get into defining rules for systems, you get into such wonderful discussions as this...

Alan, believe me when I say that I respect your opinion and your accomplishments. Quite a bit. I've also enjoyed how you have stuck to a single set of referents within the argument, no matter how many people have tried to place their own rules onto the system.

I still think there need to be more useful and handle-able subclassifications within the system for "popular consumption." But other than that, your logic holds sway, Alan.

-the other Doug

[Guest_BruceMoomaw_*]

Today's MPEC ( http://cfa-www.harvard.edu/mpec/K05/K05P01.html ) knocks the absolute magnitude of 2005 FY9 down to 0.3 (as against 0.4 for 2003 EL61 and minus 1.2 for 2003 UB313).

[edstrick]

I haven't read every work exhaustively in the "planet definition" discussion, but there's one idea I've had for some time that seems to cut to the core of the distinction between planet and "minor body".

As planetismals form in scenarious of planet, asteroid and comet formation, they grow by hierarchial capture of smaller objects, so that at any time there's a few relative big ones, lot's of relative small ones, and Sagans (billions and billions) of tiny ones. But initially, they the big fish all eat the little fish by simply running into them. Their cross-section for capture is simply their projected surface area.

As the biggest planetismals grow bigger, they start to get significant gravity. At whatever the average local encounter velocity is for that part of the nebula at that time, eventually their gravity bends approaching objects in toward collision and accretion. As that threshold is passed, the cross-sectional capture area becomes larger, then much larger than the purely geometric area.

At that point, a vaccuum-cleaner effect starts to become significant, and the biggest fish start eating faster and faster, as you start to get almost a run-away accretion. (True runaway accretion is what's proposed to happen around a 25 or so earth-mass core when a proto-gas-giant starts sucking hydrogen and helium in a purely hydronamic infall)

My proposal is a planetismal becomes a planet when it's capture area is some somewhat arbitrary times larger than it's cross-sectional area. The asteroid belt and Kuiper belt seem have to have the largest objects get up to or slightly above this threshold.

I don't know whether models show a distinct break in the size-frequency distribution of objects on a log-log plot that's related to incresed gravitational cross section. That break in the curve would be the division point for any segment of the solar system to decide between planet and planetisimal.