"Erland Gadde" wrote in message
om...
Recently, A 10:th planet was discovered in the Solar system: Sedna.

Sedna has not been given planet status.


But I also learned that it would probably not be classified as a
planet, and also that Pluto should lose it's planetary status.

The question is then, how should "planet" be defined? If Pluto and
Sedna wouldn't qualify as planets, wouldn't it be equally wrong to
lump together the small inner planets (Mercury, Venus, Earth, Mars)
and the giant gaseous outer planets (Jupiter, Saturn, Uranus, Neptune)
as one group of celestial objects, planets?

This issue will never be settled to everyone's satisfaction.
Philosophical inertia leaves Pluto as a planet, so far.


Another question: When Pluto was discovered in 1930, wasn't its
existence and position predicted to explain the disturbances of
Neptune's orbit, just as Neptune's existence and position was
predicted in 1846, to explain the disturbances of Uranus' orbit?

But, as I understand it today, it was a coincidence that Pluto was
discovered near the predicted position, for Pluto is too small to
cause the disturbances of Neptune's orbit (as is Sedna). We also see
that over the decades, the estimates of Pluto's size decreased. 30-40
years ago, Pluto was believed to be about the same size, or even
somewhat bigger, than Mars, but nowadays, it is believed to be even
smaller than Mercury! (Sedna is even smaller.) Was it the erroneus
belief that Pluto caused of the disturbances in Neptune's orbit, that
led to these overestimations of Pluto's size in the past?

It had more to do with the poor knowledge of the albedo of
Pluto. The size estimate of a reflecting object depends
strongly on its albedo.


But what is then the cause of the disturbances in Neptune's orbit?

Mass estimates for the outer planets were slightly off, and
corrected by the information gathered by Voyager probes. When
the new masses were inserted into the gravitational models,
the "disturbances" vanished.

About 1986, I heard that a 10:th planet was discovered, a very big,
but cold, object, that was detected by some space telescope. This
object was big enough to cause the disturbances of Neptune's orbit.
But what happened to this big, cold object? I never heard about it
after that. Were the telescope data discovered to be misinterpreted,
or what?

It was a wisp of interstellar cirrus imaged in the infra red.
It was never a solid body. The lousy bit of sensationalitic
reporting that appeared in the popular press provided a shopping
list of things it could have been. This list included a giant
planet at great distance, amongst other things. The woo-woos
latched onto this speculation and pretended it was fact. They've
been doing so ever since, despite repeated corrections and
information to the contrary in the literature; they simply choose
to ignore all publications on the topic that appeared after the
original speculative one.


All information about these issues would be appreciated.


Regards,

Erland Gadde