[meteorite-list] 2003 UB313 Reignites a Planet-Sized Debate

From: Matson, Robert <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Tue Feb 7 14:39:37 2006
Message-ID: <A8044CCD89B24B458AE36254DCA2BD070B178A_at_0005-its-exmp01.us.saic.com>

Hi Sterling and List,

The definition of a planet that I've encountered that I like
best is pretty scientifically concise and simple:

Any natural body orbiting a star that has a mass greater than the
sum of the masses of all other objects in a similar orbit.

The only fuzziness in the definition has to do with interpretation
of the words "similar orbit". Clearly there is a lot of variation
in orbital parameters within the main asteroid belt, and among
trans-Neptunian objects. But assuming "similar" isn't overly
precise, Ceres would probably not be considered a planet by this
definition. While Ceres is the largest main belt minor planet,
it's mass is not greater than the sum of the masses of all
other main belt asteroids.

Pluto is a little trickier since we only know the sizes and masses
of a few of the thousands of plutinos. The four largest plutinos
known are Orcus, Ixion, Rhadamanthus and Huya. (Pluto itself
isn't a "plutino" since plutino literally means little Pluto.)
The combined masses of these four are only a small fraction of
that of Pluto; however, there are estimated to be ~1400 plutinos
with diameters greater than 100 km. Is Pluto heavier than all of
these combined? Possibly. But if we open up the orbit similarity
restriction from plutino to Kuiper Belt Object, then Pluto definitely
loses its planetary status by the above definition.

Received on Tue 07 Feb 2006 02:39:14 PM PST

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