[meteorite-list] Mike Brown: The Eight Planets

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Aug 29 13:04:35 2006
Message-ID: <200608291620.JAA23055_at_zagami.jpl.nasa.gov>

http://www.gps.caltech.edu/~mbrown/eightplanets/
 
The Eight Planets

Mike Brown

On August 24th 2006 the word "planet" was given its first-ever
scientific definition by a vote of the International Astronomical Union.
With the raising of a few yellow cards in Prague Pluto was demoted from
full-fledged planet to "dwarf planet." The object 2003 UB313, sometimes
called Xena, sometimes called the "10th planet," which in many ways
precipitated this final debate, becomes the largest known dwarf planet.
Unless astronomers revisit this issue at some point in the future, it
is unlikely that there will ever be more than eight planets.

What was the problem with nine (or ten) planets?

Pluto and 2003 UB313 are significantly smaller than the other planets.
If you were to start to classify things in the solar system from
scratch, with no preconceived notions about which things belong in which
categories, you would likely come to only one conclusion. The four giant
planets -- Jupiter, Saturn, Uranus, Neptune -- belong in one category,
the four terrestrial planets -- Mercury, Venus, Earth, Mars -- belong in
one category, and everything else belongs in one or maybe more
categories. You wouldn't lump the largest asteroid -- Ceres -- in with
the planets, you would group it with the other asteroids. Likewise you
wouldn't group the largest object in the vast swarm of objects beyond
Neptune (the "Kuiper belt") with anything other than the Kuiper belt.
The previous nine (or ten) "planets" encompassed the group of giant
planets and the group of terrestrial planets and then awkwardly ventured
out into the Kuiper belt to take in one or two of the largest of those
objects. Using the word in this way makes no scientific sense whatsoever.

Two solutions to the problem of Pluto and 2003 UB313

Leave no iceball behind

Astronomers were faced with two options for a scientific definition
of the word planet. One option was to say that what makes a planet a
planet is simply the fact that it is large, round, and orbits a star.
Ignore everything else that you know and concentrate on that. Why round?
If you place a boulder in space it will just stay whatever irregular
shape it is. If you add more boulders to it you can still have an
irregular pile. But if you add enough boulders to the pile they will
eventually pull themselves into a round shape. This transition from
irregularly shaped to round objects is important in the solar system,
and, in some ways, marks the transition from an object without and with
interesting geological and planetary processes occuring (there are many
many other transitions that are equally important, however, a fact that
tends to be ignored in these discussions).

Of course, to truly talk only about the object in isolation and to
ignore everything else you know you should also ignore the fact that the
object is in orbit around another planet. It is hard to make a
consistent argument that a 400-km iceball should count as a planet
because it might have interesting geology, while a 5000-km satellite
with a massive atmosphere, methane lakes, and dramatic storms (Titan)
shouldn't be put into the same category, whatever you call it. For most
people, considering round satellites (including our Moon) "planets"
violates the idea of what a planet is.

The other difficulty with this definition is that it instantly makes 50
planets in the solar system with the likelihood of hundreds coming soon.
Such a huge change in the number of planets is a big hint that this
definition is a huge change in what we commonly think the word "planet"
means. While most astronomers would agree that round vs. non-round is an
important distinction in the solar system, most appear to feel that
forcing the word "planet" to be the word that describes this transition
is incorrect. This radical re-definition was initially proposed by the
IAU but was met by heated opposition and had to be dropped.

Consider the circumstances

The other scientific definition that makes sense is to acknowledge that
by any classification scheme that considers circumstances -- where the
object is, what else is in orbit near by, whether an object is a
satellite -- the first eight planets are clearly in a class of their
own. There are many ways to attempt to state this definition: a planet
is by far the dominant mass in its region of space, a planet has cleared
its neighborhood of all other significant masses, a planet is the
accumulation of most of the material in its orbital vicinity. While none
of these definitions can be stated with rigorous precision, they are
precise enough for the case of the Solar System where the division
between objects with have and have not mostly cleared their regions of
space is enormous.

One of the best ways to view this definition is to consider some of the
distinct regions of space. The asteroid belt, for example, is a
collection of small rocky bodies between Mars and Jupiter with many
millions of members. The largest asteroid Ceres is not nearly massive
enough to have accumulated all of the other asteroids, nor is it massive
enough to shove the asteroids out of the solar system. It is not a
dominant mass within the asteroid belt. The exact same could be said of
Pluto and 2003 UB313 (which are essentially the same size and both in
the Kuiper belt along with millions of other bodies). Every one of the
eight planets easily passes this test though. The eight planets were
created from an accumulation of most of the material that remained in
their vicinity. They are the dominant bodies in their regions of space.

This view is the one officially adopted by the International
Astronomical Union. Because of the relatively chaotic process that
occured before reaching this very rational decision the actual wording
of the definition is not as precise as it might have been, giving people
room to quibble and to say that the definition is unclear. The important
point to remember, however, is that the difference between the eight
planets and everything else known in the solar system is so huge that
even a definition with a lot of wiggle room will not make any
difference. If you are trying to define the difference between North
America and Europe, for example, the exact position of the line that you
draw in the middle of the Atlantic Ocean does not matter much. The
precise definition in the IAU resolution may be a tad unclear, but the
concept is absolutely rock solid with absolutely no room for doubt about
which objects do and do not belong.

Here are some of the issues that have come up:

What about Pluto crossing Neptune's orbit?

Partly this issue has come up from an incorrect statement in an AP
wire story which says that Pluto is autmatically disqualified because it
crosses the orbit of Neptune. Untrue. Pluto is disqualified because it
is in the Kuiper belt but has not cleared out the Kuiper belt nor
accumulated most of the mass in the asteroid belt, nor does it dominante
the Kuiper belt. Pluto is part of a vast population and is rightly
classified with that population where it belongs.

But surely this means Neptune has not cleared out Pluto and thus is
not a planet, right? No. The problem here is simply with the hasty way
in which the final definition was drafted, not with the concept, which
is quite solid. And the concept is more important than a lawyerly
reading of the definition. Neptune has a mass more than 8000 times
greater than that of Pluto, and, in fact, totally dominates Pluto's
region of the Kuiper belt. Much of the material in the Kuiper belt has
indeed been tossed aside or accumulated by Neptune, but a very special
region ("the Plutinos") have actually been captured by Neptune instead.
We now know that Neptune formed much closer to the sun than where it was
today, and, as Neptune moved out, it pushed these Plutinos out with it
while forcing them into a peculariar orbit where they orbit the sun
precisely twice for every three orbits of Neptune. Pluto is the largest
of the Plutinos, and it and the others only exist where they do because
of the dominance of Neptune. While a lawyer could make a case that Pluto
has not been cleared by Neptune, the concept and intent of the
definition is sound, and Neptune's total domination of Pluto's dynamics
is actually an excellent demonstration of precisely the concept the
definition is meant to convey.

What about Jupiter and the Trojan asteroids?

Jupiter (and now also Neptune) is known to have asteroids in orbits
that are almost identical except 60 degrees ahead or 60 degrees behind
the planet, in what are known as the Lagrange points. Jupiter exceeds
the mass of these Trojan asteroids by a factor of many millions. Like
the case of Pluto and the Plutinos above, the Trojan asteroids are in
fact captured by Jupiter and only exist where they do because Jupiter
totally dominates their dynamics. Jupiter is so totally dominant in this
region that it even prevented the asteroid belt from accumulating into a
planet.

What about near-earth asteroids?

There are asteroids and comets strewn throughout the solar system that
don't fit nicely into the asteroid belt or into the Kuiper belt. Some of
these are, for example, the near-earth asteroids. Again, a lawyerly
reading of the definition might try to argue that there are therefore no
planets in the solar system whatsoever! The concept of clearing and
dominance is still sound. All of these extra bodies flying around the
solar system are on unstable orbits and will eventually get ejected from
the solar system or collide with a planet. This process, in fact, is
precisely what astronomers refer to when they talk about "clearing."
Much like at home, the process never actually ends. But the planets have
mostly cleared their regions, even if that process always continues.

What about the moon?

The earth hasn't cleared the moon, so why is the earth a planet? Like
the arguments about Pluto and Trojan asteroids above, the Earth totally
dominates the orbit of the Moon. And the pair totally dominate
everything else around. Ergo, by the concept, planets.

What about the astronomers who say this is a poor definition?
Astronomers might quibble with the definition, but no rational
astronomer is going to disagree with the concept. The precise wording of
the definition might need to be fixed still, but the hugely important
astronomical concept of what now separates planets from non-planets
should be clear to all.

Disclaimer: I didn't make the definition or even participate in the
vote, not being a member of the IAU and all. I'm just trying to explain
it. While I think it is the best possible scientific definition we could
have had, I am still in mourning.
Received on Tue 29 Aug 2006 12:20:24 PM PDT