QUESTION:
Is pluto considered a planet since it is smaller than the moon?

Answer from Marc W. Buie, Lowell Observatory on March 18, 1996:
There have been numerous press reports generated in March of 1996 on the eve of
the release of new HST images of Pluto.  Most of these reports seem to be
focusing on the question of whether Pluto is a proper planet or not.  That
is really too bad.  Everyone seems now to be wondering about this question
instead of getting excited by our first direct images of the surface of this
distant world.

So, let me add my voice to the fray and try to explain why we shouldn't be
re-writing our textbooks on this account.

At present, there are four basic categories of objects that inhabit our solar
system.  Here's a list of definitions for these categories that will set
the stage for deciding whether Pluto has been mis-classified.

Comet

     These objects are the sometimes spectacular visitors to the inner solar
     system but usually spend most of their time in the furthest reaches of
     the solar system.  They are very small bodies, not usually any bigger
     than 10 km or so.  Despite their very small size, they create huge
     spectacles when they get close enough to the Sun to release water from
     their surface.  This water escapes immediately into space and usually
     carries some dust or dirt along with it.  This material leaves huge
     tails and streamers in the sky.

Asteroid

     These objects are found and known in great abundance in our solar
     system and most of them inhabit the "asteroid belt" found between Mars
     and Jupiter.  Some of the known asteroids wander in as far as Venus or
     Mercury and a new batch has recently been recognized out well beyond
     the orbit of Pluto.  Asteroids are small, irregularly shaped objects,
     that have inert and airless surfaces.

Satellite

     This class of object is perhaps the simplest.  If the object orbits
     another larger object, then it is considered a satellite.  These bodies
     can, and do, span a great range of sizes and properties including those
     shared by all the other categories.

Planet

     A planet is a special term applied to the larger members of our solar
     system.  Rule #1, a planet must orbit the Sun.  Rule #2, it must be
     large enough that it's own gravity is strong enough to maintain a
     spherical shape.  There are certain properties we expect to see in a
     planet such as an atmosphere and  dynamic and active surfaces at some
     stage in their history.

In the end, these definitions are useful only if they provide useful labels
that collect objects of similar properties together.  Trying to stuff Pluto
into the Comet bin would clearly be wrong.  Pluto is far too big to behave
anything like the tiny chunks of ice we call comets.  Could Pluto be considered
an asteroid?  Well, you might be tempted because Pluto is so small.  But
consider the fact that Pluto has enough gravity to be spherical and retain
a significant atmosphere.  It also probably has an active surface and
very pronounced seasons.  These characteristic sure do make it sound more
like the other planets that the asteroids.

So why all the controversy?  Well, I think that we are seeing an outgrowth
of our increasing knowledge about and awareness of our solar system.
Astronomers are continuing to discover new and wonderous things orbiting
our sun and our view of the solar system is getting more complex.  This
is very natural.  I think what we're finding is that the traditional
groupings of the members of the solar system are now requiring a more
complex set of definitions.

For instance, there is an asteroid, known as (2060) Chiron, that looked
and acted just like an asteroid should for nearly 10 years after it was
discovered.  It's only distinguishing feature was that it had a very distant
and elliptical orbit compared to other asteroids.  In the last 10 years
we had a bit of a surprise.  All of a sudden, Chiron got much brighter
and there have now been images of dust and gas streaming away from its
surface, just like a comet.  The new behavior is so much like a comet that
many people now refer to this object as Comet Chiron, instead of asteroid
Chiron.  Is it a comet?  Well, it has some actions similar to comets but
it is much, much bigger than any comet we've ever seen.  Also, we know the
activity we see is not due to water sublimating from the surface, as is
the case for comets.

Perhaps we're now becoming aware of a new sub-class of object.  You can
easily break up the planets into three bins, (1) terrestrial planets, Mercury,
Venus, Earth, and Mars; (2) Jovian planets, Jupiter, Saturn, Uranus, and
Neptune; (3) Ice(?) planets, Pluto.  I don't have a catchy name for the third
bin but the idea is to encompass those planets that are less rocky and formed
in the outer regions of the solar system.  Pluto is, at present, the only
known member of this third class but there is good reason to believe there
are more out there, just much further away and harder to spot.  Maybe Chiron
could be considered for membership in the Ice planets to keep Pluto company.

Until all this gets settled, know this: Pluto was labeled a planet at the
time of its discovery by the International Astronomical Union who has
authority over such matters.  For Pluto's official status to change, that
same international group would have to rule on a change.  It certainly beyond
the authority of a single person to make these changes for us.  The latest
batch of naysayers putting Pluto down are not acting with any authority
and are really doing us all a disservice in the process.  It may make for
a flashier news story but it is based on misguided concerns.  Why not
concentrate on what a neat place Pluto is and think ahead to what we'll
all learn by continuing our studies of this unique world in our midst.