Re: OT Poll: Does our solar system have 8 or 9 or 10 known planets?


Robin Levett wrote:
jet wrote:


Klaus wrote:
jet wrote:
Joshua Zelinsky wrote:

rev.goetz wrote:

8

I would go with at least 10, possibly more. If we want a consistent
defintion of planet, I would suggest any body that has sufficient mass
to force itself to be spherical and orbits the sun as its primary orbit
(that is, to a close approximation, it has the sun at one focus of its
elipse). This would most likely throw in a few other bodies in the
Kuiper belt, but hey, it's consistent.

Josh Zelinsky


That would include the Moon and exclude Titan and the other large
satellites.


Last I checked, the Moon was orbiting Earth.
Klaus

Sort of. If you just look at the Earth-Moon system then you have a foci
about 1600 miles beneath the surface of the Earth, not even close to
the center. But when you include the sun it is apparent that the
primary orbit is around the Sun. Unlike the other moons the moon
doesn't loop and the orbit is always concave towards the sun. The
Earth-Moon is a binary planetary system. Wikipedia has a nice graphic:
http://en.wikipedia.org/wiki/Image:Moon_trajectory.jpg T

There is no sensible distinction to be drawn between the Moon and the other
planetary satellites. They are all orbiting the centre of mass of the
system they form with their parent body, which itself orbits the centre of
mass of that system taken with the Sun (modulo the effect of the other
planets). The fact that the Moon's orbit is *now* at a stage in its
evolution that it never goes "backwards", whereas other moons have not yet
reached that stage, is completely irrelevant to that point.

The Wikipedia article from which that graphic is drawn is a tad contentious.

--
Robin Levett
rlevett@xxxxxxxxxxxxxxxxxxxxxx (unmunge by removing big blue - don't yahoo)

Isn't that true of all Wikipedia articles :-)

Classifying the Earth-Moon system as a double planet is a bit
problematic as there is no consensus of what defines such a system. One
definition holds that the barycenter must be outside the both bodies.
This would include Pluto-Charon but exclude Earth-Moon. The other
definition is that the primary gravitational influence is the Sun and
not the larger body. This would include Earth-Moon and exclude
Pluto-Charon. If the moon weighed in 37% larger at 1.0067E+23 kg then
it would meet both criteria.

.

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