[X] My Assistant

[tasp]

So, do we call Io a rock world, or a dehydrated ice world?

Just goes to show how rapidly even 'sensible' categories fray around the edges!

Bob Shaw

And it appears if we watch long enough, eventually Enceladus will run out of E-ring replenishment materials, and then we will have a rocky or metallic object orbiting Saturn.

{granted the time scale for this is rather on the longish side}

[SFJCody]

I've put together a plot showing all the bodies in the solar system more massive than asteroid (511) Davida (except for TNOs other than Pluto & Charon, will add those later):


The red line is the mass/density profile of a body composed of pure hydrogen at 5AU. No objects (aside from fluffy hot jovians) can exist above the red line.

Any catagorisation based on composition (icy, rocky, metallic etc) will have to have density boundaries that curve to the right with increasing mass, just like the profile for a pure hydrogen body. This is due to the effects of gravitational compression.

[SFJCody]

An interesting paper on 2005 FY9:

http://www.edpsciences.org/articles/aa/pdf...hj131_forth.pdf

[Bob Shaw]

I've put together a plot showing all the bodies in the solar system more massive than asteroid (10) Hygiea (except for TNOs other than Pluto & Charon, will add those later):
http://homepage.ntlworld.com/sfjcody2/planetary.png


The red line is the mass/density profile of a body composed of pure hydrogen at 5AU. No objects (aside from fluffy hot jovians) can exist above the red line.

Any catagorisation based on composition (icy, rocky, metallic etc) will have to have density boundaries that curve to the right with increasing mass, just like the profile for a pure hydrogen body. This is due to the effects of gravitational compression.

Interesting way of looking at them big round(ish) chaps! Now, if you can just get Bode's Law in there as well...

Bob Shaw

[SFJCody]

Spitzer observed 2005 FY9 on 22 December:

http://ssc.spitzer.caltech.edu/approvdprog...lan/week109.txt

and (90482) Orcus on the 15th

http://ssc.spitzer.caltech.edu/approvdprog...lan/week108.txt

[SFJCody]

My attempt at a categorization system based on mass

A solar system inventory

Stars: 1 (Sol, Type G2V)

Brown dwarfs: 0

Gas giants/ice giants: 4
Jupiter
Saturn
Neptune
Uranus

Tier Zero worlds: 0

Tier One worlds: 9
Earth
Venus
Mars
Mercury
Ganymede
Titan
Callisto
Io
Luna

Tier Two worlds: 13+
Europa
Triton
Pluto
Titania
Oberon
Rhea
Charon
Iapetus
Ariel
Umbriel
Dione
(1) Ceres
Tethys
+other TNOs (2003 EL61, 2005 FY9, 2003 UB313...)

Tier Three worlds: 3+
Enceladus
Miranda
Mimas
+TNOs

Tier Three irregulars: 21+
(4) Vesta
(2) Pallas
(10) Hygiea
Proteus
(511) Davida
(704) Interamnia
Nereid
(3) Juno
(16) Psyche
(6) Hebe
(624) Hektor
(87) Sylvia
Hyperion
(7) Iris
(324) Bamberga
Elara
(15) Eunomia
Amalthea
Phoebe
Himalia
(45) Eugenia
+TNOs

[alan]

Your groups remind me of something I read years ago. A plot of the surface gravity of the solar system's largest bodies shows steps similar to Bode's Law. Jupiter's (at the cloud tops) is about 2.5 times Earth's. Venus, Saturn, Uranus and Neptune are close to Earth's. Mars and Mercury at 0.38 are about 1 / 2.5 . Divided by 2.5 again is 0.16 which is the moon's surface gravity. Io, Europa, Ganymede, Titan, and Callisto are similar ranging from 0.18 to 0.13.

[ugordan]

SFJCody: either I'm looking at the graph the wrong way or you have got Enceladus' density wrong. Your graph suggests slightly above 1 g/cm^3 while recent measurements by Cassini show this to be more around 1.6 g/cm^3. It's much denser than similarly sized Mimas.

[SFJCody]

SFJCody: either I'm looking at the graph the wrong way or you have got Enceladus' density wrong.

Yep, well spotted. Correction made.

[SFJCody]

http://www.dfw.com/mld/dfw/news/nation/13515853.htm

The International Astronomical Union, a worldwide alliance of astronomers, has been struggling for almost two years to agree on a definition for planets. Three proposed definitions were circulated last month, but a decision isn't likely until spring, according to Robert Williams, an astronomer at the Space Telescope Science Institute in Baltimore and a member of the International Astronomical Union group working on the planet problem.

Depending on what definition is adopted, Pluto may be demoted from its status as the solar system's smallest planet to what astronomers call a "trans-Neptunian object." That's a fancy name for any world beyond Neptune, which is 2.8 billion miles from the sun, 30 times farther than Earth.

Pluto would remain a planet if the International Astronomical Union accepts a definition that would declare a planet to be any round object larger than 1,000 kilometers -- 625 miles -- across that orbits the sun.

So far, nine such mini-worlds, including Pluto (diameter 1,430 miles), are known to dwell in the frigid Kuiper Belt.

They are "a completely different type of object that predominates in the outer solar system beyond Neptune," said Richard Pogge, an astronomer at Ohio State University.

The largest and most distant of the ice dwarfs is nicknamed Xena after the television warrior princess. Discovered in 2003, it is 1,600 miles across and 20 percent bigger than Pluto. Xena has a moon of its own, named Gabrielle after the TV Xena's sidekick.

If Xena and Pluto are counted, our solar system has 10 planets; if they're not, it has eight. But if all the known objects larger than 625 miles across are included, there would be 17 planets.

[Guest_BruceMoomaw_*]

So, how's about my 1999 suggestion that the cutoff line be a diameter of 2000 km instead? That would allow Pluto to retain its long-established historical membership in the Planet Club, without loosening the membership requirements enough to let all sorts of small riffraff into it.

[edstrick]

Reiterating my arm-waving arguement, posted some months back...

Planetisimals, whether rocky or icy, grow by mechanically colliding with the dust and gravel and smaller bodies in their environment.

When the reach a certain size, so that their escape velocity gets to be some number like the average encounter speed with the small stuff they're eating, their "capture cross section area" starts to get to be significantly larger than their physical cross section. This is what I call the vaccuum cleaner effect, due to their gravity.

These protoplanets, no longer planetisimals, now can grow faster than the small stuff, and engage in runaway accretion, cut short by the dissipation of the local nebula, or by gravitational stirring (for example by Jupiter at the asteroid belt) increasing collissional velocities so things are broken up, rather than accreting, in colisions.

My arguement is that a PLANET is an object that had gotten into the gravitationally-enhanced accretion mode, regardless of the absolute diameter involved, for that radius from the sun in the presolar nebula. Pluto, and maybe a few of these other objects really do seem to be off the trend in size-frequency plots in the Kuiper belt, but I'd love to see a best current plot of that, using (1) magnitudes as a standin for real diameters, and (2) diameters, for those bodies that have real numbers rather than albedo dependent estimates.

[Bob Shaw]

So, how's about my 1999 suggestion that the cutoff line be a diameter of 2000 km instead?  That would allow Pluto to retain its long-established historical membership in the Planet Club, without loosening the membership requirements enough to let all sorts of small riffraff into it.

Bruce:

Anything which makes the Moon a planet is OK by me!

Me, I think a witch is a wooden duck (unless she floats).

Bob Shaw

[edstrick]

quoting Elphaba: "I'M MELTING... I'm melting... i'm m......"

Beware of Quantum Ducks: QUARK QUARK!

[SFJCody]

http://kencroswell.com/TenthPlanetFirstAnniversary.html

Recently, a nineteen-member committee of the International Astronomical Union (IAU) attempted to define "planet" and clarify whether Pluto and the tenth planet are really planets. However, the committee failed to reach a decision.

Committee members voted on three proposals. Each member was free to vote for more than one proposal.

The first proposal is the one children learn in school: the solar system has nine planets from Mercury to Pluto. This definition might imply that the tenth planet is also a planet, since it is larger than Pluto.

This proposal received eleven votes from the nineteen-member committee.

A second proposal suggested classifying planets according to type--for example, calling some of them terrestrial planets and others giant planets. Under this scheme, Pluto and the tenth planet would be considered trans-Neptunian planets, but so would several members of the Edgeworth-Kuiper belt that are smaller than Pluto.

This proposal received eight votes from the nineteen-member committee.

The third and final proposal said the solar system has only eight planets, those from Mercury to Neptune. Under this proposal, neither Pluto nor the tenth planet would qualify as planets.

This proposal received just six votes from the committee.