Re: The Astronomers Giveth and the Astronomers Taketh Away

On Tue, 2 Aug 2005, in rec.arts.sf.written,
Gene Ward Smith <gwsmith@xxxxxxxxx> said:

>Del Cotter wrote:
>> I take the opposite view. If Pluto is no bigger than hundreds of outer
>> system swarming bodies, it clearly has no more claim to be a major
>> planet than Ceres. I'm with Isaac Asimov, we should:
>
>> - Grandfather in Mercury as a major planet (it's one of the ancient
>> originals, after all)
>
>And if one turns up larger than Mercury, what then?

Then it's easily a planet. What's your problem with that? Mercury is a
perfectly respectable planet. Consider the nine most massive bodies,
after Mars, in the "inner" solar system:

normalised to normalised to
Mercury Pluto

Name Mass Radius Mass Radius
---- ---- ------ ---- ------
Mercury 100% 100% 2558% 212%
Ganymede 45% 108% 1147% 230%
Titan 41% 106% 1047% 224%
Callisto 33% 99% 837% 210%
Io 27% 75% 689% 158%
Moon 22% 71% 570% 151%
Europa 15% 64% 371% 136%
Triton 6% 55% 166% 117%
Pluto 4% 47% 100% 100%

Only two bodies exceed Mercury in radius, neither by as much as 10%
(although Callisto comes within 1% of Mercury's radius). *No* body
comes within even a factor of two of Mercury's mass.

Compare with Pluto which is exceeded in mass by seven non-planetary
bodies within 30AU of the Sun, all but one of them exceeding its mass by
a factor of more than two, and all exceeding its radius by more than
15%. As a planet, Pluto is and always has been an embarrassment.

It's true I'd be slightly more comfortable with Mars as the lower
boundary, but only slightly, and in any case there are almost certainly
a large number of Mars-sized bodies orbiting within 1000 AU of the Sun,
and they have to be accounted planets. If there was a gas giant out
there it would be a planet. The object is not to rule out any planets
beyond Neptune, the object is to avoid any more charity-case "planets"
like Pluto.

--
Del Cotter
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