[meteorite-list] 2003 UB313 Reignites a Planet-Sized Debate
From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Tue Feb 7 04:30:49 2006
Planet, from the Greek, "planetes," i.e., the wanderers.
So, from the initial usage, "planet" is operationally defined
as "a non-sidereal object visible from the surface of the
Earth using only the naked eye for detection."
Once we get up to the Copernican concept (which
dates all the way back to the Greeks also), the definition
becomes "a Solar System object visible from the surface of
the Earth using only the naked eye for detection."
With the discovery of Uranus, the definition is altered to:
"a Solar System object visible from the surface of the Earth
using the naked eye or using the telescope for detection."
At this point it is worth noting that the "major" minor planets
(Ceres, Vesta, etc.) and the planet Uranus are naked eye
objects. Uranus had been seen by many millions of human
beings as a dim bluish star for millennia, but it was only when
Herschel turned a telescope on it that he saw that it was a
disc, not a point (1781). Likewise the minor planets (1801).
With discovery of Neptune (1843) we enter an era when
the definition changes to "a Solar System object using
the telescope for detection," since Neptune never has and
never will be seen by the naked eye and cannot be called
"visible." By this time, the question of size enters the
equation. It becomes clear at this time that the multitude
of minor planets are all small, smaller than the first object
discovered, Ceres. This is the first criteria based on the
physical characteristics of the object to be applied to the
definition of planet. The definition ceases to be "operational"
and focuses instead on the "nature" of a "planet."
With Pluto (1930), it becomes "a Solar System object
using light-sensing technology for detection," since without
photography, Pluto could never have been found or identified
as a planet. The level of technology required to "see" 2003
UB313 makes discovering Pluto seem as primitive as the
Greeks watching the sky with an astrolabe in one hand...
As for the current quarrel over defining a planet, many of
the proposals contain archaic elements: a limit of 2000 miles
for the minimum diameter of anything called a planet is like
saying only individuals over five feet tall can be considered
human beings. It's arbitrary and meaningless.
On the other hand, the requirement of gravitationally
produced sphericity is functional and rational. But it's not
so simple. Just how spherical does a body have to be?
No axis less than 10% shorter than the major axis? 20%?
What about the KBO that's big but shaped like a pancake?
(And has satellites!) As far as that goes, Jupiter (and to a
lesser degree, Saturn) are not exactly ROUND; they're
squished. And if we had a really big gas giant instead of
a midget like Jupiter, it would very "non-spherical"!
Likewise, the "dominance" requirement seems reasonable,
but how far does the "dominant" region have to extend? Pluto
In? Pluto Out? If this criteria cannot be applied clearly and
obviously, it's not a good one. Further, it is based on the
particular characteristics of "our" Solar System.
How would you define "dominance" if the Solar System
had two Mars-sized worlds where the Asteroid Belt is, in
close but non-resonance orbits (hence stable)? You couldn't.
What about a "Solar System" that has a double planet?
Suppose the Moon was 4200 miles across and we and it
orbited about each other's barycenter? Which would be
"dominant"? The concept would be meaningless. "Double"
planets, even small double systems like the Earth/Moon and
Pluto/Charon, act as a "single" body gravitationally until you're
close enough to see their freckles...
The four sub-categories that were considered, "based on
their location and composition... Terrestrial planets, such as
Earth and Mars; Jovian planets, gas giants such as Jupiter;
Cisjovian planets, large asteroids such as Ceres; and Trans-
Neptunian planets, such as Pluto..." is a mixture of rational
and irrational features that is hard to untangle.
Location is a basically irrational criteria. "Cisjovian" as a
category to encompass large spherical asteroids means that
no object outside the orbit of Jupiter could be included. If
Ceres orbited between Jupiter and Saturn, how would it be
a different world?
As we have seen from the 170+ extra-solar planets
recently discovered, you can have gas giants bigger than
Jupiter closer to their star that Mercury is to ours! No doubt
there is, out there, a mix of sizes, types, and locations of
planets that are not represented in our Solar System. And,
after decades of detecting, analyzing, and possibly imaging
planets of other systems, we will need a universal, rational,
and comprehensive system of defining and categorizing
planets, say by 2050 AD. And, a system derived entirely by
observing what kind of planets just happen to exist in our
Solar System is a haphazard and short-sighted way to
derive a system.
There are some rational criteria:
a) gravitational primary, i.e., has to orbit the Sun, not be
the satellite of another "cold" body (with an exception for
"double" planets with diameters equal within 25%?)
b) Big enough to be self-sphericizing, i.e., round (within
10%? 20%?), unless dynamically distorted (gas giants,
that one odd KBO, and other freaks to be discovered
c) Compositionally derived from the major components
of the Solar nebula (where else would they come from?).
Composition and size are the key criteria, not composition
and location. The solar nebula (any stellar nebula) contained:
a) primordial, "extreme" volatiles, H2, He
b) moderate volatiles, CO2, CH4, NH3
c) mild volatiles, H2O, and then
a) with "weak" oxides, Fe, and many more,
usually with high melting points,
b) with "strong" oxides, rock, in other words,
usually with more moderate melting points.
That's it, all she wrote. From the quantities involved,
we end up with 1a and 1b "gas," 1c "ice," and 2a and
2b "rock" as the principal ingredients of worlds.
From those ingredients, we get planets that are
mostly "gas" (but with ice and/or rock cores). Because
it takes a lot of mass to gravitationally gather gas, they
need the cores to start the process, and because they
are strong gravitationally, they tend to get big, hence,
"gas giants," or Jovian planets. There are no small
"gas giants," obviously.
Next, smaller warmer worlds are composed largely without
1a and 1b volatiles; they are 2a and 2b worlds: made of
"rock." They may attract some 1c volatiles, and if they are
large enough or cold enough, some 1b volatiles, but
they're basically rock balls, or Terrestrial planets. They're
probably formed nearer to their star than other kinds of
Last, same-sized but colder worlds tend to be composed
of more equal amounts of 1c and 1b volatiles and
2a and 2b "rock," possibly with very little 2a "rock."
They're basically rock/ice balls, or Plutonian planets.
They're probably formed further from their star. The
large (some bigger than Terrestrial planets) rock/ice
satellites of "gas giants" are probably captured
Here our ignorance becomes a factor; we knew less
about such worlds than those worlds that we have studied
longer. There are probably variants. There could be
very large mostly-ice planets. We don't know. There
could be largely carbonaceous/rock/ice planets. We
don't know. As is usual in the case of ignorance, we
tend to despise things we don't know much about.
"Ice" in planet-sized quantities is a highly complex
material with many mineral crystalline polymorphs which
undoubtedly produce a geology as intricate and involved
as "rock" geology. The difference is that we are almost
entirely ignorant of this "geology" because of the physical
difficulty of studying it in Earth conditions.
Many of the very small bodies in the part of the Solar
System most visible to us may be examples of the junk
from planets were made. We know there are asteroids
that are entirely 2a metals. We know there are asteroids
that are both 2a and 2b metals, some differentiated and
some not. There are probably rock/ice asteroids and
ice/rock asteroids which we would call comets if we knew
what their home life was really like... We don't know.
There may be entirely ice asteroids, but we would call
them comets, a largely artificial distinction, but a useful
But the three basic planetary varieties are more than
convenient descriptions derived from observation. They
are essentially the inevitable consequence of forming
planets from the same interstellar goop as the stars are
made from. Elementary, my dear Watson.
So, as it happens, that is what planets will end up
being made from. But, if a planet formed from a odd
compositional variation, what difference would that
make in deciding whether it was "really" a planet or
not? If it's a 6000-mile sphere of iron, it's a planet.
If it's a 6000-mile sphere of ice, it's a planet. If it's a
6000-mile sphere of elemental carbon, it's a planet.
If it's a 6000-mile sphere of chopped liver, it's a planet.
But... I think those last couple of kinds are pretty rare.
It's probably a useful thing to distinguish "smaller"
planets from "larger" planets. Ceres was announced as
a planet in 1801. Ceres was accepted as a planet, even
after the discovery of Neptune, until well into the 1850's,
until the disappointment of its small size set in. Yet Ceres
may well be big enough and, when we learn more about it,
interesting enough to be considered a "planet," however
"minor." You can tell; I feel bad about Ceres...
When we settle the System, Ceres will be an important
place, kind of like Chicago... OK, OK, so it's not New York.
So what? The Ceres Chamber of Commerce will be
forever struggling with a sense of inferiority. We are
NOT an asteroid, damn it!
(Note: besides 1 Ceres (the largest), only 4 Vesta
(2nd largest) and 10 Hygeia (4th largest) are reasonably
spherical; all the other asteroids are irregular. Ceres
appears to be a differentiated rock/ice body, and its axis
is roughly upright, like a planet, 5 to 10 degrees. Ceres'
surface is warmer than a bare body would be and it seems
to have both a thin atmosphere and frost. So, only three
traditional asteroids are in the running for planet-hood.
Ceres is a Plutonian planet; Vesta is a Terrestrial planet;
Hygeia, we don't know about.)
Let's face it: politics is involved. The French are proud
of LeVerrier for Neptune; let's forget that Vulcan idiocy.
Is the US going to give up good old Clyde, the only
American discoverer of a planet. My guess is NO!
The discoverer of Ceres, Giuseppe Piazzi, thought
his discovery was a comet! After months, he lost
it behind the Sun. He sent his observations to other
astronomers and gave up, not interested in a lousy
comet. The next year, the great mathematician Gauss
tested his new method of calculating an orbit on these odd
observations, and he sent his calculation to two German
astronomers, von Zach and Olbers (he of the paradox)
who re-discovered Ceres AND announced it as a PLANET,
which Piazzi never did. Thenafter, Ceres triggered the
formulation of the Bode Law (which isn't really a law,
it turns out, or is it?).
It is interesting that Ceres was a planet for more than
50 years before it was dumped from the roster and left
on the bench. It was just over 50 years from discovery
that folks began to whisper about Pluto not really being
"our kind of planet."
Most of the many discussions on internet astronomy
boards about the meaning of "planet" are, I discover,
fairly irrational. (The stupidest reason I found to be
given for demoting Pluto from planet status, by the way,
was that Pluto was boring...)
I found therein numerous suggestions that ANY
body composed largely of ice (40% or 50%) cannot be
a planet, regardless of size, a view that oddly enough,
seems to be echoed by many professionals in the field,
a truly odd view, considering the large number of planet-
sized bodies which ARE.
Which brings us to that odd KBO, the big one that ISN'T
round... 2003 EL61. It is not an ice body; it is not even a
rock/ice body. It is a ROCK body, solid rock, like the Earth
or Mercury or Venus or Mars. It has two moons (that we
know of). If it were our neighbor, we would call it a Terrestrial
planet without a second thought... except for one little
You see, it's as big as Pluto... one way. But the other way,
it's only 1/4 th as big as Pluto! That is, it has an equatorial
axis that is four times the length of the polar axis, and another
equatorial axis that is 2-3 times the length of the polar axis.
That can only be described as Truly Weird. I know what
we should name it: PANCAKE WORLD!
How can it be called a planet if it's a damned pancake?
Well, it's dynamically distorted. It had to be formed hot, molten
just like the rest of the Terrestrial planets, but it was spinning
so fast that it cooled and froze into the pancake shape. The
puzzling thing is not the shape, but the question of how a
hot molten Terrestrial body could have formed in the near-
absolute-zero environment of the far outer Solar System?
Perhaps it formed in the inner system like the Earth but got
thrown out. Could the rapid revolution (a four-hour "day") be
the result of an immense orbit-changing collision? Maybe
it's our long lost brother world...
Another problem is that the term "minor planet" has been
used for a century for the 100,000 asteroids! This pretty
much renders it useless for the job of distinguishing big
and small planets from each other, which we would prefer
was a gentle distinction. As usual, the history of a
terminology is completely entangled in the problem,
to the extent that simple direct terms can no longer be
used. Minor vs. Major? Planetoids vs. Planets? Planetinos
Because of sensitivity about terminology, the attempt
to avoid saying what you meant creates a tangled spate
of utterly silly and ridiculous terms, carefully disguised
as highly technical and inoffensive language: KBO's,
TNO's, not to mention Cubebinos, Plutinos, Two-tinos,
Argh! Stop! Stop! How is that making things better?
We all know what IAU will do.
Sterling K. Webb
----- Original Message -----
From: "Ron Baalke" <baalke_at_zagami.jpl.nasa.gov>
To: "Meteorite Mailing List" <meteorite-list_at_meteoritecentral.com>
Sent: Monday, February 06, 2006 1:25 PM
Subject: [meteorite-list] 2003 UB313 Reignites a Planet-Sized Debate
> Xena reignites a planet-sized debate
> Maggie McKee
> New Scientist
> 06 February 2006
> The heated debate over what constitutes a planet has reignited following
> last week's confirmation that the most distant planet-like object object
> ever seen in the solar system is larger than Pluto. But astronomers
> tasked with settling the issue say the argument could drag on for years.
> The International Astronomical Union (IAU), responsible for resolving
> such issues, assembled a special working group to decide on the
> definition two years ago, when a large new body called Sedna was found
> in the outer solar system.
> But since then, several other large worlds have been discovered,
> including 2003 UB313, unofficially dubbed Xena. This body became widely
> known as the "tenth planet" as it appeared to be larger than Pluto,
> which is about 2300 kilometres across. Now, new results from an
> independent team appear to confirm this, finding Xena is about 30% wider
> than Pluto.
> However, astronomers are bitterly divided over what constitutes a
> planet. And when the IAU's working group was forced to issue its verdict
> in October 2005, it failed to find a definition all 19 members could
> agree on. So it simply reported on the relative popularity of three
> different proposals - each group member was allowed to vote for more
> than one proposal.
> Keeping Pluto
> A narrow majority of 11 members favoured deeming anything larger than
> 2000 kilometres a planet. Under this scheme, Pluto would remain a planet
> and it would be joined by several newly discovered worlds, including
> Xena. But some group members argued such a size cut-off was arbitrary,
> set only so Pluto could retain the title of ninth planet.
> Another option attempted to come up with a scientific justification for
> a size cut-off. In this plan, planets would have to be massive enough
> for their gravity to hold them in a stable shape - a requirement that
> could be met by objects as small as 600 kilometres across.
> "It complicates matters because we get some dozens of new planets, but
> on the other hand, there's some scientific justification" for the size
> cut-off, says group member Brian Marsden of the Harvard-Smithsonian
> Center for Astrophysics in Cambridge, Massachusetts, US, who had the idea.
> In this scheme, which eight group members voted for, all planets would
> be sorted into four sub-categories based on their location and
> composition, those being:
> o Terrestrial planets, such as Earth and Mars
> o Jovian planets - gas giants such as Jupiter
> o Cisjovian planets - large asteroids such as Ceres
> o Trans-Neptunian planets, such as Pluto
> Another proposal argued that a planet is the dominant body in its
> immediate neighbourhood. This would demote Pluto, as it is one of
> several bodies of similar size in the Kuiper Belt - a ring of icy
> objects beyond Neptune. Six group members voted for this option, which
> would leave the solar system with eight official planets.
> Division and discord
> Because the group was so divided it simply issued a report on its
> discord, and not a resolution for the IAU's executive committee to put
> up for a wider vote. That vote would most likely occur at an IAU general
> assembly meeting, which occur every three years. The next will take
> place in Prague, Czech Republic, in August 2006.
> That meeting could see a vote if the executive committee, a
> representative from an IAU member nation, or one of the IAU's divisions
> puts forward a resolution on the definition of a planet. "But I don't
> think that is likely given the difficulty the experts had in coming up
> with a consensus," says Robert Williams, one of 10 members of the IAU's
> executive committee and an astronomer at the Space Telescope Science
> Institute in Baltimore, Maryland, US.
> Williams adds that waiting may also have scientific merit because
> astronomers have been focusing on defining planets in our solar system,
> while little is known about the formation and evolution of the 170 or so
> known planets around other stars.
> "We're trying to define things we don't fully understand," he says. "It
> may be a bit frustrating that we're not quite at the point where we can
> agree on what a planet is. But the more important thing is a fundamental
> understanding of what's going on - I would put that as a higher priority
> than the naming convention we adopt."
Received on Tue 07 Feb 2006 04:30:33 AM PST