Hello David,

->> One of the things that changed is that objects have been discovered
->> beyond Neptune's orbit that are not classied as planets. And recently
->> at least two them have been found that may be larger than Pluto.
 >
 > When it was originally discovered, Pluto was thought to be
 > similar in mass to Neptune and Uranus. Inaccurate measurements
 > of alleged perturbations caused by Pluto's gravity led to this
 > conclusion.

As I understand it, the discovery of Pluto was more luck than arithmetic.
It just happened to be very near the predicted spot, but the calculations
were way off.

 > The fact that it looked so small was a bit of a mystery. One
 > theory that was around when I was a kid was that Pluto's surface was
 > shiny, so it  was like a big metal ball. All that could be seen with
 > telescopes was the tiny reflection of the sun in this surface.
 > The rest of the planet was invisible.

Yes, I remember that theory.

 > This theory bit the dust when an occultation of a star by Pluto was
 > observed. The star's light was *not* blocked by the presumed
 > invisible part of Pluto. Pluto really was as small as it looked. At
 > about the same time, the error in the mass estimate was discovered.

Yep.

 > But the classification of Pluto as a planet, which was partly
 > based on its apparent large mass,

Well, Mercury is considered a planet and it isn't very much bigger than
Pluto. Pluto is about the same size as the moon. Some argue that our moon
is much to big too classify as a proper moon and that earth-moon system
should be classified as a double planet.

Plus that Pluto has a moon of its own, which is a pretty strong point for
classifying it as a planet I'd say.

 > had become ingrained in astronomical culture.

I guess nobody gave ir real thought at the time, it was just labelled a
planet. Period.

 > *Maybe*, because of the other large Kuiper Belt Objects that
 > have recently been discovered, this may change. Personally, I
 > hope that it does.

Why? Does it really matter?

Cheers, Michiel

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