Make the Earth's Moon a Planet

The IAU Draft proposal makes Charon a planet, part of a double planet,
because:

"For two or more objects comprising a multiple object
system, the primary object is designated a planet if it independently
satisfies the conditions above. A secondary object satisfying these
conditions is also designated a planet if the system barycentre resides
outside the primary. Secondary objects not satisfying these criteria
are "satellites". Under this definition, Pluto's companion Charon is a
planet, making Pluto-Charon a double planet."

The Earth's moon misses because the Earth-Moon barycenter is some 1600
km
below Earth's surface.

But this is an accident of history. In some billions of years Earth's
Moon will
spiral outward due to tidal forces and the barycenter will "reside
outside the
primary".

To avoid this arbitrariness I suggest that the barycenter test be
applied at a distance
equal to the gravitational radius of influence of the primary.
[Rt~Rp sqrt(Mp/Ms) where Rp is distance to sun, Mp and Ms mass of
primary and sun]

Then if RbRt(Ms/(Mp+Ms)) [Ms mass of secondary]
the secondary is a planet.

Thanks

Tom Clarke

"pt" == polymath tlc writes:

pt The IAU Draft proposal makes Charon a planet, part of a double
pt planet, because:
[...]

Note that this is not from the version of the resolution that was adopted.

pt The Earth's moon misses because the Earth-Moon barycenter is some
pt 1600 km below Earth's surface.

pt But this is an accident of history. In some billions of years
pt Earth's Moon will spiral outward due to tidal forces and the
pt barycenter will "reside outside the primary".

One would have to double-check the numbers on this. It is true that
the Moon is moving farther away from the Earth. I estimate that it
will have to move to a distance of about 520,000 km for the Earth-Moon
barycenter to lie outside of the Earth, or an increase of about 40%.
Will the Moon reach this distance before the Sun leaves the main
sequence, at which point, the Sun will begin to swell and solar tides
will play an increasing important role?

More practically, I believe that a similar issue came up at the IAU
(with respect to the Solar System in the past and whether Neptune
would have been considered a planet some 4 billion years ago). As the
chair of the committee said, The IAU General Assembly at the time can
deal with the issue.

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In article ,
Joseph Lazio wrote:
"pt" == polymath tlc writes:

pt The IAU Draft proposal makes Charon a planet, part of a double
pt planet, because:
[...]

Note that this is not from the version of the resolution that was adopted.

pt The Earth's moon misses because the Earth-Moon barycenter is some
pt 1600 km below Earth's surface.

pt But this is an accident of history. In some billions of years
pt Earth's Moon will spiral outward due to tidal forces and the
pt barycenter will "reside outside the primary".

One would have to double-check the numbers on this. It is true that
the Moon is moving farther away from the Earth. I estimate that it
will have to move to a distance of about 520,000 km for the Earth-Moon
barycenter to lie outside of the Earth, or an increase of about 40%.
Will the Moon reach this distance before the Sun leaves the main
sequence, at which point, the Sun will begin to swell and solar tides
will play an increasing important role?

A swelling Sun will not cause greater solar tides, as long as it
remains spherically symmetrical. Remember that the gravity from a sphere
where the density varies only with the distance from the center is equal to
the gravity of a point mass in the center of the sphere with the same
total mass - if you're outside the sphere. If you're inside the sphere,
it's only the mass closer to the center of the sphere which contributes
to the gravity - the mass outside of you does not contribute at all.

So if we assume that the mass of the Sun doesn't change much, and that the
Sun remains spherically symmetrical in its mass distribution, the solar
tides will remain unchanged as the Sun swells, as long as the Sun does
not "swallow" the Earth. If the Sun "swallows" the Earth, the solar
tides till decrease, not increase - of course in such a situation,
other forces will be much more important, such as the friction against
the gas in the interior of the Sun. And of course also the heat
of the Sun, which may vaporize the Earth.

More practically, I believe that a similar issue came up at the IAU
(with respect to the Solar System in the past and whether Neptune
would have been considered a planet some 4 billion years ago). As the
chair of the committee said, The IAU General Assembly at the time can
deal with the issue.

--
Lt. Lazio, HTML police | e-mail:
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html


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