Paul Schlyter wrote, in part:
Ken Crosswell's "correct" answer to this question is:

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C. Pluto would be one of the brightest objects in the sky, outshining every star
except the Sun.
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But if Pluto were closer to us--as close as Mars, for example--it would be
incredibly bright, ancient people would have recognized it as a planet long ago,
and modern people probably wouldn't be debating its planethood!
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Ken Crosswell needs to learn some physics !!!

Superficially, he's right: Pluto's absolute magnitude is -1.0 and
Mars' absolute magnitude is -1.51. Therefore, if Pluto was as close
as Mars, it would shine 0.5 magnitudes fainter than Mars. When
closest, Mars shines at mag -2.8 which would mean Pluto would shine at
mag -2.3 if it was as close as Mars.

And that, of course, comes as a surprise. After all, Pluto is much
*smaller* than Mars, or even Mercury. Hence, we would have expected it
to be a dim planet.

However, the albedo of Pluto is very high - some 95%. This is because
Pluto is convered with deep frozen ices which are very bright. If we
put Pluto at Mars' distance, these ices would most likely melt away,
and Pluto's albedo would become much lower. Since Pluto is very
small, it wouldn't be able to hold any atmosphere which could have
clouds making the albedo higher (such as for Venus and, to a lesser
extent, the Earth). What the albedo of Pluto then would be is of
course hard to tell - perhaps 15% as for Mars? Or 7% as for the Moon
and Mercury? Let's say 10%.

So if Pluto would be as close as Mars, and its albedo would have been
lowered to 10% after the Sun had melted all those ices away, then
Pluto would shine at around magnitude 0 when as close to the Earth as
Mars is when closest - it would then shone as bright as Vega or
Arcturus. Pluto wouldn't be that close all the time though. Most of
the time, Mars shines approximately around magnitude +1 - if Pluto
followed Mars orbit, then Pluto would shine around magnitude +3.5 most
of the time. That would make Pluto a naked-eye object, although one
would need to know the sky to distinguish Pluto from many stars with a
similar brightness.

If Ceres or Vesta followed Mars' orbit, they would shine around
magnitude 4 to 5 most of the time, brightening to magnitude +1 when
closest to the Earth.

The planethood debate about (134340) Pluto is only temporary - just
like the planethood debate of (1) Ceres, (2) Pallas, (3) Juno and (4)
Vesta was temporary some 150 years ago.

Speaking about Ceres, Pallas, Juno, and Vesta... which one of them is
not like the others?

The answer is Juno: Ceres is the largest asteroid, and Pallas and Vesta
are the next two largest... but Juno is smaller than several other
asteroids besides those three. However, it happens to be unusually
bright for its size.

I really don't think that *albedo* is a very good characteristic to
include in the definition of a planet. Yet, the fact that objects more
readily seen are also more likely to be accepted as important does mean
that he *has* raised an interesting, and unusual, argument in the
Pluto-as-planet debate.

John Savard