The Eight Planets
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,"
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
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)
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
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
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
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
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
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
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
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.
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