[meteorite-list] WHAT IS A PLANET?

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Sun Jul 31 04:17:07 2005
Message-ID: <42EC88D7.1478FEC_at_bhil.com>

Hi,


    These recent discoveries of new "planets" is going to heat up the
on-going quarrel about what is and isn't a planet, with its increasingly
long definitions and conditions statements designed to trim reality in
the mold of the arguer's mind.

    Frankly, I used to belong to the no-more-planets school of opinion,
but 2003 UB313 has flipped me over like a
pancake on a griddle. It may do the same thing to others, or not. But
whatever the definition is, it shouldn't be too complex or long or
involved. It should be basic.

    The original definition of planet (from the Greek word "Planetes,"
or "wanderer") was the five lights in the sky that did not move with the
stars: Mercury, Venus, Mars, Jupiter, and Saturn. Observations and
calculations of their motions goes back 4000 years, possibly 6000.

    This original classification from the beginning contained two
radically different kinds of worlds: terrestrials and gas giants, a fact
that was not clear until the XVIIth century. Comets, once it was
understood that they were solar system objects and not weather, were
excluded.

    Ceres, the biggest asteroid, was first proposed for planethood
("between Mars and Jupiter, I place a planet") when discovered, but
rejected. Sorry, not big enough, and none of your small relatives
either, Minor Planets is all you get to be.

    Pluto got to join the Planet Club right away, It was initially
assumed to a small gas giant about 10,000 to 12,000 miles in diameter.
Then, Kuiper discovered it couldn't be more than 6500 miles in diameter.
Still bigger than Mars, but we're disappointed in you, I must say...

    Now we know that Pluto is not a big dog, but a runt (well, nice
moon, anyway), and sure enough, there's a faction on the Membership
Committee that feels we should ease this embarrassment out of The Club,
discreetly to be sure, but it simply doesn't belong, you know...

    2003 UB313 is going to stir all this up because that faction of the
Membership Committee unwisely chose to discriminate against newer
candidates on the grounds that they were not even as big as the
"unsuitable" Pluto, and I'm willing to bet that the same people will now
start tut-tut'ing over "unsuitable" inclinations.

    In the case of both 2003 UB313 and 2003 EL61, we see that these
bodies are surprisingly bright and the first question that comes to mind
is "Why did nobody discover them before now?" They weren't looking in
the right place, and those who were confident that there were bodies out
there bigger than Pluto have been justified in spades, redoubled.

    The question that starts nagging at me is why there isn't there just
as great a likelihood that there is a high inclination body four or five
times the size of Pluto (the size of our own little world) out at 140 or
150 AU? It would actually be dimmer than 2003 UB313, even at that size!

    The real reason that the existence of high inclination objects were
dismissed is that "current theory" about solar system formation demanded
that the "planets" be formed pretty much in the same plane. Well,
theories are fine, as long as reality doesn't walk in and stomp all over
them, but when it does, it behooves us to open our eyes a little bit
wider.

    And, in case you didn't notice, reality just did that very thing.

    As soon as 2003 UB313 flipped me over like a
pancake on a griddle on the planet question, a definition popped into my
mind at the same moment. It's irrational to insist that a new planet be
bigger than Pluto (because that was just an excuse to drag Pluto down,
anyway). The proper comparison is to the most analogous excluded class,
namely the 250,000 known "minor planets."

    So, here goes: if it goes around the Sun and is demonstrably (the
entire range of error bars above the Ceres diameter) larger than Ceres,
it's a planet. Welcome to The Club!

    That keeps asteroidal riff-raff out of The Club, likewise really big
comets (Charon?), and other oddities. And of course, we'll need to learn
all those new names. That's a problem. For a start, I've learned to
spell QUAOAR and even pronounce it ("Kwa-Oar"). So there.

    The people who discover these things really should be a bit more
aware about the names that they pick. Naming a planet-sized body after
an Eskimo Seal Goddess of Plenty is very PC, I'm sure, and I like SEDNA
myself, but there are lots of unused names in the traditional lexicon of
mythology, especially for figures of the nether regions. Persephone
("Per-SEF-fon-ee"), the Queen of Hades, is the most obvious example. But
save Her for that Earth-sized KBO, all right? She is The Queen, after
all...

    Let the sniping begin!


Sterling K. Webb
Received on Sun 31 Jul 2005 04:16:23 AM PDT