The 10:th planet

Recently, A 10:th planet was discovered in the Solar system: Sedna.

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?

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?

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

All information about these issues would be appreciated.


Regards,

Erland Gadde