Make the Earth's Moon a Planet

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.

--
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