[meteorite-list] 2003 UB313 Reignites a Planet-Sized Debate
From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Tue Feb 7 20:16:21 2006
Actually, Ceres contains almost 40% of
the mass of the "Belt." We used to think
that Ceres constituted less of the mass,
but it turns out that the Belt is deficient in
the smallest sizes of asteroids predicted
by the so-clled "power law."
Ceres is at that "magic" distance of
2.8 AU predicted by the Titus-Bode "law,"
but the wholly pragmatic definition of the
"Belt" that we use encompasses everything
from outside Mars to inside Jupiter.
That vast expanse covering 3.5 AU is
hardly a viable definition of "similar" orbits.
For "similar," I favor that zone adjacent
to a body's orbit, inside of which there is
no orbit in which a similar body would be
dynamically stable for the long (n x 10^9
In the case of asteroids, given the
breakup time scale of various "families,"
it's hard to call the central region where
Ceres is long-term stable, I suspect
largely due to Ceres' presence.
The main mass of the Asteroid Belt is
concentrated right where Ceres is (no
coincidence). Ceres' mean distance is
414,000,000 km with Pallas at 415,000,000 km,
Juno at 400,000,000 km, Vesta at 353,000,000 km,
and Eunomia at 396,000,000 km, all big
muthas, which total about 70% of Ceres'
mass! If Ceres misses being 50% of the
mass in Belt #1, Zone II, it doesn't miss
it by much.
As for some rocks, remember, it takes
1,000,000,000 (a billion) 1-km asteroids
to mass up to Ceres, or a million 10-km
asteroids. Size matters. For asteroids,
Pluto is actually a clearer case, mass-wise.
It would take, geometrically, 11,648 100-km
plutinos to equal Pluto mass. But, there's also
a de-compensation of density to throw into
the equation. I didn't do it (too lazy) but it
would probably take 12,500 100-km plutinos
to make a Pluto. 1400 is, like, no problemo...
But the truth is that we Earthlings live in a
neighborhood where rocks and rubble, asteroids
and comets, satellites and smaller planets, are
few, far between, or entirely absent. We regard
it as "normal" to look out over our broad expanses
of planetary space empty of any interesting features
in the local landscape, like a vast green well-mowed
lawn with no insects, no birds, no rabbits nor deer,
no trees nor flower beds, no statues or birdbaths,
and definitely no other people... Well, you get the
idea. We are horrified at a few thousand NEA's and
get all fluttery when a comet wanders by every
decade or so. Provincial...
It ISN'T normal. Ok, the Asteroid Belt is more
cluttered and so is the Kuiper Belt than we're
used to -- so what? ALL the outer system is
cluttered; it's the inner system that's bare. In a
solar system that's 80 AU across (at least), we're
in the inner 3 AU, which has been swept abnormally
clean and bare. Personally, I think it's because of
that big STAR right in the middle of the place...
It has warped our thinking. It SEEMS perfectly
normal to demand that the 99% of the solar system
that isn't our neighborhood should be exactly LIKE
our neighborhood, but we've all met people like
that, who object to every place things aren't
just like they are back home.
I checked into the 50 years that Ceres was
accepted without question as a planet. Its large
rivals were all discovered within the first ten years,
but by 1850, only a total of 10 asteroids are known.
By 1868, it was over 100, and by 1891 (when the
first photographic discovery was made) there were
332 known, all discovered visually by comparing
the sky with the few star charts available. In fact,
this search was the chief reason for the development
of comprehensive and reliable star charts.
But, no asteroid BIGGER than Ceres was
ever found. The first TBO was discovered in
1992, 62 years after the discovery of Pluto,
and the numbers have been ballooning more
dramatically than the number of asteroids did.
The discovery of an object not barely but
substantially bigger than Pluto, however, was
the trigger for a lot of mis-informed furror.
I call it "mis-informed" because we just don't
know enough yet. The IAU doesn't need to
wait another year; it needs to wait another
A body as big as the Earth at 150 AU would
be harder to detect than 2003 UB313 was,
as would a Neptune sized body at 600 AU.
The conventional notion is that the Kuiper
Belt "ends" at 100 or 200 AU, whatever
"ends" means, is ridiculous. We know of asteroids
with orbits that go out to 1000 AU. I see no
reason why Kuiper Town should "end" short
of the border with Oortville.
I will remind everyone that the discovery of
Pluto, which was thought to resolve the issue
of Neptune's orbital residuals, did not, because
the mass ain't there. That's one indication of
more mass in the outer system than we think.
The Pioneer anomaly is another. Not enough
to use in the traditional methods of orbital
mechanics, but real enough. Of course, it may
be only the residual of many undiscovered
bodies, but the Universe could still have
a surprise or two up its sleeves.
There's LOTS of elbow room out there.
Sterling K. Webb
----- Original Message -----
From: "Matson, Robert" <ROBERT.D.MATSON_at_saic.com>
To: "Sterling K. Webb" <sterling_k_webb_at_sbcglobal.net>; "Meteorite Mailing
Sent: Tuesday, February 07, 2006 1:39 PM
Subject: RE: [meteorite-list] 2003 UB313 Reignites a Planet-Sized Debate
> Hi Sterling and List,
> The definition of a planet that I've encountered that I like
> best is pretty scientifically concise and simple:
> Any natural body orbiting a star that has a mass greater than the
> sum of the masses of all other objects in a similar orbit.
> The only fuzziness in the definition has to do with interpretation
> of the words "similar orbit". Clearly there is a lot of variation
> in orbital parameters within the main asteroid belt, and among
> trans-Neptunian objects. But assuming "similar" isn't overly
> precise, Ceres would probably not be considered a planet by this
> definition. While Ceres is the largest main belt minor planet,
> it's mass is not greater than the sum of the masses of all
> other main belt asteroids.
> Pluto is a little trickier since we only know the sizes and masses
> of a few of the thousands of plutinos. The four largest plutinos
> known are Orcus, Ixion, Rhadamanthus and Huya. (Pluto itself
> isn't a "plutino" since plutino literally means little Pluto.)
> The combined masses of these four are only a small fraction of
> that of Pluto; however, there are estimated to be ~1400 plutinos
> with diameters greater than 100 km. Is Pluto heavier than all of
> these combined? Possibly. But if we open up the orbit similarity
> restriction from plutino to Kuiper Belt Object, then Pluto definitely
> loses its planetary status by the above definition.
Received on Tue 07 Feb 2006 08:16:16 PM PST