The IAU has proposed a definition which would add hundreds of new
planets to our solar system!
From the time of the announcement of the discovery of 2003 UB313 in
late July 2005, the "planetary" status of 2003 UB313 and of Pluto have
been in limbo. The International Astronomical Union
(IAU), the group charged with classifying objects in space, has just
released a proposed definition and will hold a vote on this proposal on
August 24th. If you want to read about the IAU proposal, immediately
jump to 4 below!
Why is there a problem with Pluto?
What are the possible solutions to the Pluto
1. Demote Pluto (8 planets)
2. Keep the status quo (9 planets)
3. Let in the newcomer (10
4. Leave no ice ball
(53 planets) <-- this is the IAU recommendation!
5. Do nothing (how many
The IAU proposal officially recognizes only 12
planets; where does the number 53 come from?
many planets are there in the Kuiper
What should the public think about 53 planets?
How am I going to vote on the proposal?
A little background: Why is there a
problem with Pluto (or 2003
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 current word "planet"
encompasses the group of giant planets and the group of terrestrial
planets and the awkwardly ventures 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, hence, the issue with Pluto.
What are the possible solutions to the
Pluto (and 2003 UB313)
I. Demote Pluto and 2003 UB313 (8
The simplest solution is the one that makes the most people cringe:
admit that we made a mistake in 1930 by calling Pluto a planet. We have
eight planets, and many thousands of asteroids and many thousands more
Kuiper belt objects. Pluto is simply the second largest of the known
Kuiper belt objects (2003 UB313 is the largest). Ceres is the largest
asteroid. Case closed.
But we can't demote Pluto, can we? Well, of course we can. We did the
same thing to the asteroid Ceres more than 150 years ago. When it was
first discovered in 1801 it was declared to be a planet (the 8th,
actually). Then another asteroid was found. Another planet! Then
another. Planet again! This got old quickly and soon these tiny bodies
realized to be part of a vast population of rocks orbiting between Mars
and Jupiter. They were wisely grouped into a single category, the word
"asteroid" was born, and Neptune was the new 8th planet.
It takes guts to demote a planet that many people claim to love.
But if the IAU had made this decision and stuck to it it would only
generation for everyone to accept the idea. People would even learn
that science is capable of correcting itself when it makes errors,
is a useful lesson to see in action.
As the discoverer of 2003 UB313, would I be upset by this decision? No.
Scientific decisions should be based on science, not sentiment. It
would be an excellent choice. I'd be sad to miss the chance to have
discovered the 10th planet, but I'd get over it.
Why didn't the IAU propose this definition? I think that astronomers
are as sentimental as the rest of the world and couldn't stomach
removing Pluto. Probably they also couldn't stomach the criticism that
II. Keep the status quo (9 planets)
We could always just say "OK, the word planet is an historical word, not
a scientific word, so let's just leave it at nine and ignore anything
else that comes in." I don't actually think anyone ever seriously
considered this idea, though it is sometimes floated around. This
definition would have the unintended effect of sending the signal that
discovery in the solar system is complete. It doesn't matter what else
we might find, we already know all the planets there are to know. This
signal would be a very very bad signal. To my knowledge no one has
seriously proposed this idea.
III. Let in the newcomer (10 planets)
A very simple solution to the Pluto/UB313 problem is to just define
Pluto to be a planet (much like in II. above) and say that anything
larger (currently only 2003 UB313) is also a planet.
Why do this? It makes no scientific sense at all, yet it appears to be
what most people on the street think that the definition of "planet"
should be. What are astronomers to do in a case where the public
clearly thinks one thing and scientists another?
One interesting question to ask is "who is most affected by this
decision?" Will it affect astronomers? Not at all. If there are
officially 8 or 9 or 10 or 53 planets astronomers will continue the
business of science by studying these objects to figure out what they
are made of, how they were formed, and what they can tell us about the
history of the solar system. In scientific discussions the word planet
is hardly ever used.
Will it affect the public? Much more, it seems. The only reason that
astronomers are spending so much time and effort on this question is
because of the effect that it appears that it will have on the public.
Given that it is the public, rather than astronomers, who care and will
be affected, one suggestion is that the astronomers simply get over it.
To most people the word "planet" is more cultural than scientific. It
is part of the mental landscape that we use to organize our ideas of
the universe around us. The best analogy I can come up with is with the
word "continent." The word sound like it should
have some scientific definition, but clearly there is no way to
construct a definition that somehow gets the 7 things we call
continents to be singled out. Why is Europe called a separate
continent? Only because of culture. You will never hear geologists
engaged in a debate about the meaning of the word "continent" though.
When geologists talk about the earth and its land masses they define
precisely what they are talking about; they say "continental crust" or
"continental drift" or "continental plates" but almost never
Astronomers might be wise to learn from the geologists. Let culture
define "planet" and let astronomers get back to the more important
business of actually doing science.
IV. Leave no ice ball behind (53
planets, and counting)
definition is the one
IAU chose to propose
There is a fourth solution, which is to fix the problem of Pluto being
too small by making many many more planets so that Pluto is no longer
even close to the limit. Even better, make the new definition have a
Such a solution would be to declare that
everything that is large enough to be round due to its own self-gravity
is a planet. As long as it is not a moon.
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. By this proposed
definition, you would then have a planet.
All of the ten current planets -- including Pluto and 2003 UB313 -- are
round, so this means we don't have to demote anybody. What else is
large enough to be round? The asteroid Ceres -- that one that was once
a planet and got demoted -- is, in fact, round, so more than a century
after its demotion it would be back to being a planet.
While the IAU is only officially willing to call 12 objects in the
solar system round, we know with very little doubt that the Kuiper belt is home to perhaps a
hundred or more round objects. We
don't know the precise number because we don't know exactly how big an
icy object in the Kuiper belt has to be to be round, but if we look at
the icy satellites of the giant planets we see that everything larger
than 400 km (250 miles) across is round while things smaller than 200
km (125 miles) across are not round. So somewhere in between is the
transition. In the Kuiper belt we currently know of about 44
objects (including Pluto and 2003 UB313) that are larger than 400 km,
so, at a minimum, we have 44+8+1=53 planets, by this scheme. We are not
through searching the Kuiper belt, but when we are we are likely to
have about 100 planets.
Most people might think that a proposal to suddenly go from 9 to 53
planets would have no chance of passing, but I give this one good odds
of passing the IAU vote. Why? It sounds scientific, it saves Pluto, and
suddenly makes many more people discovers of planets. Of course,
it does even greater damage to the popular concept of the word planet
by suddenly adding 44 new ones, all of which are so small that they
could easily fit all together inside the earth's moon (which, of
course, doesn't count as a planet) with plenty of room to spare, but
perhaps that's a small
price to finally have a definition after all of this time.
V. Make no decision
While the IAU has proposed a definition, the membership may vote no.
What happens then? We are left in the same state we started, with no
scientific definition, but with most people thinking there are 9 or 10
planets. I think this result is essentially similar to II. above. I
give it a 30% chance.
The IAU proposal officially recognizes only
12 planets; where does the number 53 come from?
By the proposed IAU definition, anything large enough to be pulled by
its own gravity into the shape of a sphere and which is in orbit around
a star is a planet. The proposal officially recognizes 12 planets (the
nine previously recognized plus Ceres and Pluto's moon Charon plus 2003
UB313) creates a complex committee procedure for an object to become
officially recognized. This part of the proposal is perhaps the
weakest. In no other area of astronomy is there a definition for a
class of objects and then a committee that has to decide if an object
fits the definition. There are simply definitions. If an object fits
the definition it is part of the class. If the IAU proposal is accepted
then scientifically all of the spherical objects out there are
indeed classified as planets, regardless of how long it takes for
a committee to officiailly declare them to be so.
A relatively simple analysis show
that there are currently 53 known objects in the solar system which are
likely round. Another few hundred will likely be discovered in the
relatively near future. Regardless of what the official count is from
the IAU proposal these object all fit the scientific definition of the
word planet and if the scientific definition is to have any credibility
they should all generally be considered planets.
What should the public think about 53
Most people, when first confronted with a proposal to make 44 new
planets in the solar system, seem to react by looking blankly for a
second, then shaking their heads and muttering something about
astronomers being crazy. Astronomers are not actually crazy, at least
most of them. Astronomers have needed a good scientific definition of
the word "planet" for many years now and this one works well for
scientists. It doesn't, however, work terribly well for the rest of the
world. The solution is the one that should have happened long ago: a
divorce of the scientific term "planet" for the cultural term "planet."
No one expects school children to name the 53 planets (most, in fact,
don't even have names). If I were a school teacher I would teach 8, or
9, or perhaps 10 planets and then say "scientists consider many more
things to be planets too" and use that opportunity to talk about how
much more there is in the solar system. But at the end of the day I
would talk about 8 or 9 or 10. Not 53.
Culture and science have always meant something different when they use
the word planet, and with this new scientific definition so clearly far
removed from what the rest of the world things a planet is there will
no longer be any need to confuse the scientific word with the cultural
How am I going to vote on the IAU resolution?
This one is easy to answer. I am not an IAU member, I took no part in
drafting the resolution, and I get no vote. If I were to vote, however,
I would have to decide that while the definition itself is viable the
extra non-scientific beauracratic barrage attached to the resolution
would doom it for me.