DW> Criterion. (We are being picky about language.)

That's right.  Lest any lurkers wonder, this is a discussion about
language, not science.


Now, back to that discussion about science ...

 DW> I thought a bit more about the conditions for a satellite
 DW> having an orbit that is always concave toward the sun. Of
 DW> course, it boils down to the strengths of the gravitational
 DW> pulls of the planet and the sun on the satellite. If the sun
 DW> attracts it more strongly than the planet, then the
 DW> satellite's path will bend toward the sun, and away from the
 DW> planet, when the satellite passes between the planet and the
 DW> sun. So the condition is:

 DW> M / (R^2) > m / (r^2)

 DW> where M is the mass of the sun, m is the mass of the planet,
 DW> R is the radius of the planet's orbit around the sun, and r
 DW> is the radius of the satellite's orbit around the planet.

I believe it was Arthur C. Clarke who promoted this very same argument.  A
moon has a path that is concave towards the sun if and only if it is
attracted to the sun with a force that is stronger than the force to which
it is attracted to its planet.

Such a condition exists only for Earth's moon, or so I thought.  It may
have been that at the time Clarke made his argument, those outer moons of
Jupiter had not yet been discovered?

 DW> The Earth-Moon system system satisfies this condition. So do
 DW> some of the outermost satellites of Jupiter. But the
 DW> Pluto-Charon system does not.

No, you're right.  It's not even close for Pluto-Charon.

One argument promoting the status of double planet for the Pluto-Charon
system is the following.  The ratio of the mass of Charon to the mass of
Pluto (about 0.1) is larger than for any other moon-planet pair.  The next
largest is the Earth-Luna system (about 0.01).  It may very well have been
that at the time Clarke made his argument to promote the Earth-Luna system
as a double planet, its mass ratio was the largest known.

If Pluto's status as a planet is revoked, Earth-Luna's status as the
highest mass ratio for any planet-moon system would be restored!

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