[meteorite-list] Double Planets

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Fri, 18 Mar 2011 22:28:43 -0500
Message-ID: <75E7F9ABC9944CD7878372CBD2422323_at_ATARIENGINE2>

Larry is right (who could doubt it?).

I got it backward. Charon would have to move
closer to Pluto to get their barycenter inside
Pluto. Presently, their center-to-center distance
is 19,600 km.

If you pushed'em to only 10,650 km apart, the
barycenter would be at Pluto's surface. The two
planetary surfaces would then be only 8900 km
apart! (They're 17,850 km apart now.) Then,
they'd have to be even closer for the barycenter
to be inside Pluto.

That would be quite a view! Either way.


Sterling K. Webb
-----------------------------------------------------------------------
----- Original Message -----
From: <lebofsky at lpl.arizona.edu>
To: "Sterling K. Webb" <sterling_k_webb at sbcglobal.net>
Cc: "Walter Branch" <waltbranch at bellsouth.net>;
<fallingfusion at wi.rr.com>; <meteorite-list at meteoritecentral.com>
Sent: Friday, March 18, 2011 9:29 PM
Subject: Re: [meteorite-list] Double Planets


> Hi Sterling:
>
> I don't like to disagree with you (YES!), but I wonder if you have
> ever
> been on a seesaw. If you move Charon away from Pluto, the center of
> mass
> moves away from Pluto, not toward it, you have a longer lever arm.
>
> You actually get it right when you talk about the Moon later on!
> Moving
> the Moon 50% farther away puts the barycenter outside the Earth.
>
> Larry
>
>> 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.
>> ------------------------------------------------------------------------------
>> ----- 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 11:28:43 PM PDT