[meteorite-list] 53 planets, soon to be 80

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Wed Aug 16 18:58:28 2006
Message-ID: <qh87e250nsfriv29vdvhr198agulo6a0k4_at_4ax.com>

By the "if it is round, and not orbiting another planet, it's a planet"
definition, our solar system now has 53 planets, with the number soon to jump to
80. I feel a great disturbance in the Force, as if millions of voices of
schoolchildren suddenly crying out in terror.

(see the site to see the charts)

http://www.gps.caltech.edu/~mbrown/whatsaplanet/howmanplanets.html

How many planets are there?
While most people would answer that there are 9 or perhaps 10 planets, a
proposal by the International Astronomical Union that will be voted on soon
would significantly increase the number of objects that astronomers call
planets. The proposal is to call any object that is large enough to make gravity
cause it to become round a planet.

How many planets would this make? The nine planets that everyone knows are all
round, so they are clearly planets. Ceres, the largest asteroid, is also round
and would become a planet (the fifth). The big question, then, is how many new
planets are there in the Kuiper belt, a region of rocky/icy bodies beyond
Neptune, and the home of Pluto and 2003 UB313 ("the 10th planet").

While we can't see most of the objects in the Kuiper belt well enough to
determine whether they are round or not, we can estimate how big an object has
to be before it becomes round and therefore how many objects in the Kuiper belt
are likely round. In the asteroid belt Ceres, with a diameter of 900 km, is the
only object large enough to be round, so somewhere around 900 km is a good
cutoff for rocky bodies like asteroids. Kuiper belt objects have a lot of ice in
their interiors, though. Ice is not as hard as rock, so it less easily
withstands the force of gravity, and it takes less force to make an ice ball
round. The best estimate for how big an icy body needs to be to become round
comes from looking at icy satellites of the giant planets. The smallest body
that is generally round is Saturn's satellite Mimas, which has a diameter of
about 400 km. Several satellites which have diameters around 200 km are not
round. So somewhere between 200 and 400 km an icy body becomes round. Objects
with more ice will become round at smaller sizes while those with less rock
might be bigger. We will take 400 km as a reasonable lower limit and assume that
anything larger than 400 km in the Kuiper belt is round, and thus a planet.

How many objects larger than 400 km are there in the Kuiper belt? We can't
answer this question precisely, because we don't know the sizes of more than a
handful of Kuiper belt objects (for an explanation why, see the discussion on
the size of 2003 UB313), but, again, we can make a reasonable guess. If we
assume that the typical small Kuiper belt object reflects 10% of the sunlight
that hits its surface we know how bright a 400 km object would be in the Kuiper
belt. As of late August 2006, 44 objects this size or larger in the Kuiper belt
(including, of course, 2003 UB313 and Pluto), and one (Sedna) in the region
beyond the Kuiper belt. In addition our large ongoing Palomar survey has
detected approximately 30 more objects of this size which are currently
undergoing detailed study.

We have not yet completed our survey of the Kuiper belt. Our best estimate is
that a complete survey of the Kuiper belt would more than triple this number.

For now, the number of known objects in the solar system which are likely to be
round is 53, with the number jumping to 80 when the objects from our survey are
announced, and to more than 200 when the Kuiper belt is fully surveyed.

The large number of new planets in the solar system are very different from the
previous 9 planets. Most are so small that they are smaller across than the
distance from Los Angeles to San Francisco. They are so small that about 30,000
of them could fit inside the earth.
Received on Wed 16 Aug 2006 06:59:30 PM PDT