[X] My Assistant

[dilo]

In a previous thread, few months ago, I proposed an arbitrary mass lower limit of 0.001 Earth masses for a planet definition and this translates in a 1500-2000 Km minimum diameter, depending on density. Even though mass is a more objective parameter than diameter (especially for non perfectly spherical objects), I must admit that can be very difficult or even impossible to measure it for so distant objects, if they do not have small satellites like Pluto or 2003EL61...
So a lower limit of 2000Km seems the best thing also to me, but only for now! Probably they will discover other far object larger than this in the next few years; in fact, as reported in the Space.com article, "Alan Stern, of the Southwest Research Institute and leader of NASA's New Horizons mission to Pluto, predicted in the early 1990s that there would be 1,000 Plutos out there. He has also contended, based on computer modeling, that there should be Mars-sized worlds hidden in the far corners of our solar system and even possibly other worlds as large as Earth."
So it seems we need a different approach, otherwise we would fall in the same ridiculous situation of natural satellites count: most of the ever increasing objects around giant planets are in fact boulders, very few kilometers across and otherwise classified as captured asteroids or planetesimals. If we really want to call them "moons", why do not include in this cathegory also rings particles ?!?

[tedstryk]

Bruce. Just wait until they have found a thousand objects with a diameter between that of Pluto and Mars's (and perhaps a few Earth sized ones also) , I dont think the schoolchildren of the future will have to memorize the names of a solar system of more of a thousand planets!

If we do indeed make such discoveries, it will sure make exploration difficult. It might be the time to develope a large space-based array of telescopes to map them.

[Alan Stern]

Bruce. Just wait until they have found a thousand objects with a diameter between that of Pluto and Mars's (and perhaps a few Earth sized ones also) , I dont think the schoolchildren of the future will have to memorize the names of a solar system of more of a thousand planets!

That day will come, in this century. And so what of it? School children aren't expected
to know the names of all the stars, and no one claims we should arrange the definition
of stars (or galaxies, etc.) just to keep the number of them small.

-Alan

[Guest_Sunspot_*]

Any idea what the dates would have been for the last perhilion passage and the next - also the expected magnitude?

[alan]

about 1700, roughly magnitude 14.5
next about 2250

This post has been edited by alan: Jul 30 2005, 03:14 PM

[Rob Pinnegar]

Personally, I still favor
setting a maximum diameter of 2000 km as the arbitrary planet/nonplanet
dividing line, which will allow Pluto to retain its longtime seat in the
Planet Club without allowing too many newcomers to crash the party.

Yeah. A radius of 1000 km is a good dividing line. If we're going to be arbitrary (which is inevitable) we might as well be arbitrary in a way that's easy to remember and hawks the Metric System. Might as well throw in a mass of 10^22 kilograms while we're at it; Pluto barely passes that test too.

This analogy has been raised before, but the argument about whether Pluto and this new object ought to be called planets or asteroids is reminiscent of the old question about whether Australia should be called an island or a continent. (It might be more appropriate to substitute Greenland for Australia, in this case, though.)

I can't speak for anybody else, but to me the most interesting about this new trans-Neptunian thingamabob is its 44-degree orbital inclination. Could it have accreted in an orbit like that? My semi-educated guess is that it couldn't, which implies it's been tossed there by something more massive. Does that mean that the Big Prize is still out there? Maybe the same thing responsible for the 50-AU cutoff of the Kuiper Belt? Could something that big and that close in have remained undiscovered until now?

[Decepticon]

With all these discovers which web page has a list of these objects?

[Guest_Myran_*]

Alan Stern said: That day will come, in this century. And so what of it? School children aren't expected
to know the names of all the stars, and no one claims we should arrange the definition of stars (or galaxies, etc.) just to keep the number of them small.

You took me a bit to literally there.
My point is that I dont really think that the larger KBO's eventually wont be seen as anything else but just objects of the 'Kupier belt' even if we happen upon even larger ones. This is just a repetition of what happened when the asteroids were discovered, the first ones were viewed as planets then also. With more knowledge the perception of things will change.

[Mongo]

Personally, I still favor
setting a maximum diameter of 2000 km as the arbitrary planet/nonplanet
dividing line, which will allow Pluto to retain its longtime seat in the
Planet Club without allowing too many newcomers to crash the party.

It now looks like 2003 FY9 is bigger than originally reported. The original MPEC circular listed H = 0.1, but its absolute brightness was increased today to H = -0.4 (compared to Pluto at H = -0.5).

Given any plausible albedo, this would also mean a diameter in excess of 2000 km.

So we now know of three TNOs with diameters greater than 2000 km:

2003 UB313 H = -1.1 D = 2600-3200 km
Pluto H = -0.5 D = 2390 km
2003 FY9 H = -0.4 D = 2200-2500 km (personal guestimate)

Plus five more TNOs (counting Pluto's moon Charon) in the 1200 to 1600 km range:

2003 EL61 H = 0.3 D = ~1600 km
Charon H = 1.2 D = 1270 km
Sedna H = 1.6 D = ~1600 km
90482 Orcus H = 2.3 D = ~1500 km
50000 Quaoar H = 2.6 D = ~1300 km

I have a feeling that we will be finding a great many objects with diameters greater than 2000 km in the coming years. My own opinion is that we need to either set the dividing line for 'major planet' status higher -- a minimum diameter of 4000 km would allow Mercury to remain a major planet, while keeping the number of trans-neptunian major planets to a minimum -- or to abolish the distinction altogether.

Bill

[gpurcell]

I wonder if a size+orbital mechanics might not be a way to proceed. It strikes me that the inclination/highly elliptical nature of the orbits of the TNO class will end up being one of the key elements of differentiation. Set Pluto as an lower limit for mass and size, but also require that they be within a certain number of degrees within the solar plane AND that their orbits be no more elliptical than Pluto's if they are to be considered planets.

[alan]

With all these discovers which web page has a list of these objects?

larger objects listed here, out of date now
http://www.ifa.hawaii.edu/faculty/jewitt/kb/big_kbo.html
all objects listed here
http://cfa-www.harvard.edu/iau/lists/TNOs.html
http://cfa-www.harvard.edu/iau/lists/Centaurs.html

[alan]

interesting note: 2003 EL61 has been found on plates from 1955, now part of Palomar digitized sky survey
I believe this is it at RA 10 09 55.6 Dec +24 51 40
http://www.ledas.ac.uk/staging/dss/dss0ef7_1.gif

[dvandorn]

Truthfully, I think that Pluto needs to be demoted to a KBO, and the rest of these objects need to be classified as KBOs. Regardless of size and mass. I think it's obvious that the KBOs, as a class, were formed rather differently from the rest of what we think of as planets -- most specifically, since they do not orbit in the plane of the ecliptic, they could not have been formed primarily from the solar nebula, since those nebulae always seem to array themselves in fairly thin disks.

I think that planets should be defined as objects that accreted directly from the solar nebular disk (defined by its orbital inclination) *and* that are larger than "X" in size and/or mass. Everything else, regardless of size and/or mass, should be defined as a KBO or an Oort cloud object (OCO).

So, simple rule -- formed from within the planetary disk = planet, formed outside of the disk = other.

-the other Doug

[Alan Stern]

I wonder if a size+orbital mechanics might not be a way to proceed.  It strikes me that the inclination/highly elliptical nature of the orbits of the TNO class will end up being one of the key elements of differentiation.  Set Pluto as an lower limit for mass and size, but also require that they be within a certain number of degrees within the solar plane AND that their orbits be no more elliptical than Pluto's if they are to be considered planets.

This idea is unworkable. If an Earth were discovered in a high inclination orbit, would it
not be a planet? Of course not. Location (orbital or otherwise) is for realtors. Planethood
is about what KIND of object it is. (By the way, I note that no one says a star should be
befined by its orbit, or even if it is in a galaxy or not-- it is defined by what KIND of
object it is. It is just such a definition that we seek for lanets.)

[Alan Stern]

I think that planets should be defined as objects that accreted directly from the solar nebular disk (defined by its orbital inclination) *and* that are larger than "X" in size and/or mass. Everything else, regardless of size and/or mass, should be defined as a KBO or an Oort cloud object (OCO).

So, simple rule -- formed from within the planetary disk = planet, formed outside of the disk = other.


I'm sorry to say it, but this won't work etiher. I am on the IAU committee
looking into planetary nomenclature and one thing we defintiely agree on is
that formation mode is not a good way to go. Why? Because we won't
be able to determine an object's oriigin from things we observe-- this
problem is particularly acute for extrasolar planets.

And as to Sedna and Pluto and UB313, as best we can tell, they were accreted just
like Earth, Mars, etc., but aborted early in their growth. They were then ejected
to these oddball orbits, so in fact it does seem that these dwarf planets were
made just like Earth, and Mars, and Venus, but they ran out of feedstock for
one reason or another and ended up stunted.



And