[meteorite-list] Double Planets

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Fri, 18 Mar 2011 21:11:23 -0500
Message-ID: <94B1B9AD1013463393E0005543F9F1D8_at_ATARIENGINE2>

Hi, Walter, List,

> The dividing line between planet-moon and dual-planet seems to be
> whether the center of gravity (barycenter) of the bodies is either in
> space or beneath the surface of one of the bodies...

You're not wrong, Walter.

What would have made Pluto-Charon a true
"double planet system" under those original IAU
definitions (the ones that were shot down in favor
of dwarfism) was the fact that the barycenter of
the Pluto-Charon system was well OUTSIDE both
bodies. This means that they truly orbit each other,
dancing around a point in space between them.

If Charon was much smaller or much further away
from Pluto, the barycenter would move closer to
the center of mass of Pluto until it was inside the
planet. The barycenter of the Earth-Moon system
lies inside the Earth, about 1710 km down in the
mantle, wandering up and down a bit with the
eccentricity and tilted, of course.

All the figures for Earth-Moon and Pluto-Charon
and the formulas for all that barycenter stuff are
nicely accumulated in this:
http://en.wikipedia.org/wiki/Barycenter#Barycenter_in_astronomy

Interestingly, the path of the barycenter of the Sun
and all planetary bodies taken together passes through
the body of the Sun much of the time, even through the
fusion core of the Sun, and yet part of the time that
barycenter is outside the Sun. (There's a diagram.) It's
a 179-year cycle. I wonder what that does to the surface
and if it affects the sunspot cycle? Big arguments about that:
http://www.google.com/webhp#hl=en&sugexp=ldymls&xhr=t&q=barycenter+jose&fp=a0e1d04ac32ef934

Playing with the numbers... if the Moon were 40%
heavier than it is, the barycenter would be just ABOVE
the surface of the Earth, outside the planet, and we
would meet the definition of "double planet."

OR, if the Moon was the same weight but 335,000 miles
away instead of 240,000, then too the barycenter of the
system would be just ABOVE the surface of the Earth.

Of course in a billion years or so, the Moon WILL be
that far away, so relax... We'll get there.

Be sure and scroll down to the animations of a number
of double systems, including Earth-Moon and Pluto-Charon.
It's like watching drunken mice waltz...


Sterling K. Webb
------------------------------------------------------------------------------
Dear Cap'n: I changed the subject line... I've reformed.
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----- Original Message -----
From: "Walter Branch" <waltbranch at bellsouth.net>
To: <fallingfusion at wi.rr.com>; <meteorite-list at meteoritecentral.com>
Sent: Friday, March 18, 2011 6:48 PM
Subject: Re: [meteorite-list] The Term "Planetary"


Hello Ryan,

The Earth's moon is very large, relative to the planet it orbits. In
the
astronomical literature, the earth-moon system is sometimes refereed to
as a
dual planetary system. The dividing line between planet-moon and
dual-planet seems to be whether the center of gravity (barycenter) of
the
bodies is either in space or beneath the surface of one of the bodies
(don't
quote me, though I may be wrong).

I have seen Pluto-Charon sometimes referred to as a dual-planetary
system
(though now I guess it would be a dual dwarf planetary system.

-Walter

----- Original Message -----
From: <fallingfusion at wi.rr.com>
To: <meteorite-list at meteoritecentral.com>
Sent: Friday, March 18, 2011 2:44 PM
Subject: [meteorite-list] The Term "Planetary"


To the list,

I was sitting here reading some emails, and just thought...

Who in the world ever came up with the term "Planetary" in reference to
meteorites.

First of all, our Moon isn't a planet.. and secondly, to my knowledge,
the
only "Planetary" meteorites in current existence have an origin of Mars.
Hence, "Martian" meteorites. Did I miss the big announcement of those
from
Venus and Mercury?

Regards,

Ryan

Sent on the Sprint? Now Network from my BlackBerry?
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Received on Fri 18 Mar 2011 10:11:23 PM PDT