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Yusuf B Gursey wrote:
>> I happen to think the definition proposed is a good start, especially in
>> that it doesn't as yet resort to arbitrary continuous (as opposed to
>> discrete) properties. I'd suggest that something were added to the
>> roundness criterion, namely signoficant internal layering - which I
>> suppose would throw out some smaller bodies, but I don't know ebough
>> geology for that, other possibilities are to consider diversity of
>> chemical elements present - and, in the case of binary systems, to
>> consider only the largest as a planet (even if that would have us throw
>> out large bodies, after all the Moon is just a moon - as the system's
>> gravity centre is inside Earth - even if it is larger than other
>> candidate planets).
>>
>> In short, to be a 'planet' (. kept, + added, - deleted),
>>
>> 1 . having a roughly round shape by virtue of its own gravity
>> 2 . not being a star
>> 3 . orbiting a star in a roughly round orbit
>> 4 + having a core, mantle, etc, or similar geological / chemical criterion
>>
>> In binary systems, instead of
>> 5 - the gravity centre being outside any of the bodies
>> 5 + the planet is the largest body even if the gravity centre is outside
>> of it, and the other(s) is/are moons
>>
>> 'Moon' would the be any body that meets at least 1, 2 and possibly 4,
>> but fails one or more of the others.
>>
>> I see that this doesn't take rogue planets into account, but certainly
>> (3) is rewritable.
>
> some size criterion shoudl also be included IMHO, which I think there
> is.
But there you go, then you have an arbitrary limit on a continuous
value. However, (1) offers a convenient workaround for that, and I
suppose (4) might also.
--
am
laurus : rhodophyta : brezoneg : smalltalk : stargate
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