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Posted by Paul Schlyter on August 17, 2005, 2:14 pm
Please log in for more thread options >It seems to me that they will have to either go back to the original
>five planets or redefine what a planet is.
>
>The original five planets are sphero=EFds - due to their own gravity -
>in orbit around their primary star.
>
>According to these characteristics any object that is sphero=EFd due to
>its own gravitational pull and in orbit around its primary star is a
>planet. Any other object in orbit around its primary star that does not
>have the gravitational pull to force its mass into a sphero=EFd shape
>should be called an astero=EFd.
That definition of yours will make Ceres and a few other large asteroids
"planets".
Also: no planet is an exact spheriod. Which means you'd have to set a
tolerance limit of the maximum allowed deviation from exact spheriod shape
for the body to still be called a "planet". Where would you like to set
that limit?
>A moon is defined as a body that is in orbit around another body that
>is not a star - with the common centre of mass within the volume of the
>primary. It stands to reason to distinguish astero=EFd moons and
>planeto=EFd moons, which respectively could not and did gravitationally
>pull their own mass into a sphero=EFd shape.
>
>When the common centre of mass is not within the primary non-star body
>one ought to speak of double planets or double astero=EFds when neither
>is face locked to the other, but of a planet and its moon when the
>latter is face locked to the former.
>
>Any non-star object not orbiting a star should be called a free
>astero=EFd or a free planeto=EFd.
>
>
>Dan Tilque wrote:
>
>> [1] I see a couple problems with the self-gravitating sphere
>> definition, but won't go into them unless there's some interest.
>
>I am interested.
>
--
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Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
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