Lets start the old "planet" argument again ...

And why not? hopefully it'll increase the SNR G. Crossposted to
sci.geo.geology for obvious reasons - it's about planet formation.

In article . 201, Paul
Lawler wrote:
Indeed... but I posit to you (most courteously, of course), that if Pluto
were discovered today it would not be classified as a "planet."

If Pluto were discovered today, I'd classify it as a planet.
It's got sufficient gravity to have collapsed into a sphere; it's got
a satellite; it's satellite to primary size ratio strongly suggests that,
however it formed, it formed by the same processes as did Earth-Moon. OK,
it's compositionally different, probably, but that's a question of the
availability of material in the region it formed more than it's process of
formation.
What are your reasons for classifying it as a non-planet? Size?
Composition? Location?

Incidentally, I'd classify the big 4 of the asteroid belt (Ceres,
Vesta, Pallas, Juno) and possibly a few others as being planets too - I like
the "strong enough gravity to be spherical" criterion because it's simple
and uniformly applicable. Your target smear has no noticeable variation in
it's reflected-light curve - it's close enough to spherical to be considered
a planet. It varies - it's a minor planet. (It wasn't until about 1978 that
the light curve of Pluto was well enough observed to establish that it
varied - and that's work that's still going on to deconvolve the signal from
Charon.)
Over to you.

--
Aidan Karley,
Aberdeen, Scotland,
Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233

Indeed... but I posit to you (most courteously, of course), that if Pluto
were discovered today it would not be classified as a "planet."

If only I (or anyone) knew what a 'planet' was, it would help me
decide whether Pluto was one.

IMHO, arguing that Pluto isn't a planet without a definition of what a
planet is is silly.

It's got sufficient gravity to have collapsed into
a sphere; it's got a satellite; it's satellite to
primary size ratio strongly suggests that,
however it formed, it formed by the same
processes as did Earth-Moon.

I don't think so. I think Pluto is maybe one of a thousand
icy objects that formed in the vicinity of Neptune.
Most of them were swept up by the planet, or
ejected from the system. Pluto was lucky enough
to find a happy home in the 3:2 resonance with
Neptune, but in the process it passed too close to
the big planet and was broken into two pieces.

If Pluto were discovered today, I'd classify it as a
planet.

It's a free country, call it what you please. But it's
really just a Kuiper-Belt planetesimal, and not even
the largest - that distinction belongs to Triton, which
found a happy home in a highly eccentric, backwards
orbit around Neptune.

Ben

In message , BHZellner
writes

It's got sufficient gravity to have collapsed into
a sphere; it's got a satellite; it's satellite to
primary size ratio strongly suggests that,
however it formed, it formed by the same
processes as did Earth-Moon.

I don't think so. I think Pluto is maybe one of a thousand
icy objects that formed in the vicinity of Neptune.
Most of them were swept up by the planet, or
ejected from the system. Pluto was lucky enough
to find a happy home in the 3:2 resonance with
Neptune, but in the process it passed too close to
the big planet and was broken into two pieces.


If you can cite a reference for that I'll be fascinated, but surely
Pluto never comes near Neptune? There's also the point that Pluto and
Charon have widely different properties. And finally, ISTR that people
are finding quite a few doublets in the Kuiper belt, which is a bit of a
puzzle.
--
What have they got to hide? Release the ESA Beagle 2 report.
Remove spam and invalid from address to reply.

In article , BHZellner
wrote:
Pluto was lucky enough
to find a happy home in the 3:2 resonance with
Neptune, but in the process it passed too close to
the big planet and was broken into two pieces.

The orbital dynamics of the Pluto-Charon-Neptune system are such
that Pluto and Charon could never have come close to Neptune. That's
what orbital resonance means.
If Pluto-Charon had ever come close enough to Neptune to have
been tidally disrupted, then at some later date (but not a lot later)
they must have encountered some other substantial mass concentration to
alter their orbit to one in resonance with Neptune. If that didn't
happen quickly (a few orbits, say a millennium) then Pluto-Charon would
have had another encounter with Neptune. See Shoemaker-Levy 9 for the
consequences of that sort of encounter.

--
Aidan Karley,
Aberdeen, Scotland,
Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233

Aidan Karley wrote:


Incidentally, I'd classify the big 4 of the asteroid
belt (Ceres, Vesta, Pallas, Juno)

Nitpick: While Juno was the third asteroid discovered, it's not
the third largest. It's more like the 15th largest. The biggest
are Ceres, Pallas, Vesta, 10 Hygeia, and 31 Euphrosyne.

--
Dan Tilque

In article , Dan Tilque wrote:
While Juno was the third asteroid discovered, it's not
the third largest.

Hmmm, I should have checked on that. I just grabbed the top 4
from the MPC orbits database. Just poking around in
http://arnold.usno.navy.mil/murison/...lElements.html to
get a listing that includes the diameters. Clickity click. Drawing an
entirely arbitrary line in the sand - 200km - which may or may not be
of any physical relevance ...
No Name Perhel Aphel Diameter
1 Ceres 2.55 2.99 848.4
2 Pallas 2.13 3.41 498.1
4 Vesta 2.15 2.57 468.3
10 Hygiea 2.76 3.51 407.1
511 Davida 2.58 3.76 326.1
704 Interamnia 2.61 3.52 316.6
52 Europa 2.78 3.41 302.5
87 Sylvia 3.21 3.77 260.9
31 Euphrosyne 2.44 3.86 255.9
15 Eunomia 2.15 3.14 255.3
16 Psyche 2.51 3.33 253.2
65 Cybele 3.08 3.79 237.3
3 Juno 1.98 3.36 233.9
324 Bamberga 1.78 3.59 229.4
451 Patientia 2.82 3.3 225
107 Camilla 3.2 3.75 222.6
532 Herculina 2.28 3.26 222.2
48 Doris 2.87 3.34 221.8
45 Eugenia 2.49 2.95 214.6
29 Amphitrite 2.37 2.74 212.2
121 Hermione 2.94 3.94 209
423 Diotima 2.94 3.19 208.8
13 Egeria 2.36 2.79 207.6
94 Aurora 2.9 3.43 204.9
88 Thisbe 2.31 3.22 200.6
Hmmm, #5 is a surprise - over 100 years after the first asteroid
and big ones are still being found.

Actually, returning to the point, I find from
http://www.spaceref.com/news/viewpr.html?pid=12473 and
http://alamoana.keck.hawaii.edu/news.../asteroid.html that Davida
is only roughly spherical. So that would suggest that from the "big
enough to have to be spherical" criterion, Davida is right on the edge.
I can't find images of Hygiea anywhere, while Vesta is known to be
subspherical (and apparently with a layered composition, which is
strongly indicative of a history qualitatively similar to the Earth's).
That would put a bottom limit for something being a "planet" at around
400km. (And of course, it's in a solar orbit.)

--
Aidan Karley,
Aberdeen, Scotland,
Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233

Aidan Karley wrote in message
dated:

And why not? hopefully it'll increase the SNR G. Crossposted
to
sci.geo.geology for obvious reasons - it's about planet formation.

In article . 201,
Paul Lawler wrote:
Indeed... but I posit to you (most courteously, of course), that if
Pluto were discovered today it would not be classified as a "planet."

If Pluto were discovered today, I'd classify it as a planet.
It's got sufficient gravity to have collapsed into a sphere;

Do we have good photos of Ceres and the like? Are they spherical?
And Jupiter is a lot 'less spherical' than Earth, Mars and the other
planets: it's visibly 'flattened' because of fast rotation.

it's got
a satellite;

Venus and Mercury have no satellites. And AFAIK there are several asteroids
(irregular shaped rocks, smaller than 100 km in every direction) with
satellites, some even with multiple 'moons'.

it's satellite to primary size ratio strongly suggests
that, however it formed, it formed by the same processes as did
Earth-Moon.

The satellites of Mars are suspected to be objects from the asteroid belt
captured by its gravity.

OK, it's compositionally different, probably, but that's a
question of the availability of material in the region it formed more
than it's process of formation.
What are your reasons for classifying it as a non-planet? Size?
Composition? Location?

I would say a nearly spherical OR ELLIPSOID shape that is caused by the
object's own gravity (so not if a smaller object is spherical 'by
accident').


Incidentally, I'd classify the big 4 of the asteroid belt
(Ceres,
Vesta, Pallas, Juno) and possibly a few others as being planets too -
I like the "strong enough gravity to be spherical" criterion because
it's simple and uniformly applicable. Your target smear has no
noticeable variation in it's reflected-light curve - it's close enough
to spherical to be considered a planet. It varies - it's a minor
planet. (It wasn't until about 1978 that the light curve of Pluto was
well enough observed to establish that it varied - and that's work
that's still going on to deconvolve the signal from Charon.)
Over to you.

In article , Mr. 4X wrote:
Do we have good photos of Ceres and the like? Are they spherical?
And Jupiter is a lot 'less spherical' than Earth, Mars and the other
planets: it's visibly 'flattened' because of fast rotation.

See my message
Message-ID: d
We certainly have imaging of over a half-dozen asteroids. Ones above
approximately 400km in diameter are sub-spherical to spherical; below that
their shapes rapidly become variable and irregular.

it's satellite to primary size ratio strongly suggests
that, however it formed, it formed by the same processes as did
Earth-Moon.

The satellites of Mars are suspected to be objects from the asteroid belt
captured by its gravity.

Which is not the method of formation which is considered most likely
for the Earth-Moon system or for the Pluto-Charon system. The "Giant
Impactor" process has been suggested as a plausible explanation for the very
high density of Mercury (GI - loss of much of the silicate layer of the
proto-Mercurian mantle) and the retrograde rotation of Venus.

I would say a nearly spherical OR ELLIPSOID shape that is caused by the
object's own gravity (so not if a smaller object is spherical 'by
accident').

A sphere is an ellipsoid with zero eccentricity. All planets above a
certain size (which from the figures I quoted in my message cited above
appears to be around 400km) are essentially fluid drops (or were before they
froze, in the case of the smaller objects) whose ellipsoidal form is
controlled by the interplay of gravity and rotation, with a small
contribution from viscosity in the interior.

--
Aidan Karley,
Aberdeen, Scotland,
Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233