[meteorite-list] Planet definition

From: Matson, Robert <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:50 2004
Message-ID: <AF564D2B9D91D411B9FE00508BF1C86904EE5A8F_at_US-Torrance.mail.saic.com>

Hi Rosemary and List,

> I have always considered if it had an orbit around a star it was
> called a planet.

While your definition is succinct, it fails in four cases:
asteroids, comets, dust and man-made satellites. (I won't muddy
things further with multiple-star systems). All of these objects
orbit our star, so you see size has to be part of the definition.

If it weren't for all the other plutinos that have been discovered,
I would be happy to call Pluto a planet. Likewise, if Ceres were
the only asteroid, I would promote it to planetary status. Instead,
Ceres and Pluto are simply the largest examples of their respective

By choosing the requirement that a planet have a greater mass than
the sum of all other bodies in a similar orbit, you avoid ambiguity
over the minor planets, and in particular the Trojan asteroids of
Mars and Jupiter). The definition correctly handles all the planetary
satellites as well, without the necessity of addressing the bodies
they orbit.

The only problem case I can imagine is if two nearly equally sized
bodies were orbiting the sun in nearly the same orbit. But for
the most part I think this is a non-problem, since two moderately-
sized planetoids cannot ordinarily coexist in a similar orbit,
unless they are orbiting each other (double-planet). Not sure
what happens if you put another earth 60 degrees ahead of or behind
earth in the same orbit (Lagrange points). There's probably a
mass limit on the second earth, beyond which L4 and L5 are no
longer stable. --Rob
Received on Wed 17 Mar 2004 12:03:08 AM PST

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