Phantom wolf says

"Nothing I hve seen yet shows how such a planet could indeed enter the system without causing serious gravitational effects on planetary orbits, especially if it is supposed to have such serious effects on the Earth as some claim. "

What about Van Flanderns collisional theory of Planet X causing the asteroid belt?

"I would be serious surprised if a planet 20-50 times Earth's size (which by the way doesn't come near the size of a Brown Dwarf ..."

the idea, was that it was a brown dwarf with
**20-50 earth **masses, not size
.Approximately the size of Jupiter , this would be actually, though Matese infers a brown dwarf of only 2-3 earth masses.

"If a planet did follow such an orbit, the leaving orbit would be parabolic in shape and that would fire such a planet out of the system after one pass. "

Well then explain Sednas orbit, why does it not just,
"parabolically fire out of the system in one pass?'
What about all the comets ?.....

"That will result in any Planet of size at the most perhaps coming close to the outer planets, but nowhere near the inner ones."

still again, what about van Flanderns asteroid belt formational events ? Sednas orbit works in your description, as an outer solar system influence, but Sednas existence alone has shown that radical elliptical orbits that were never before thought possible of planets are now very real.
Marsden...
"How it got there in such an eccentric orbit that comes as close as 76 astronomical units
to our sun and goes all the way out to nearly 1000 astronomical units away is a complete mystery! There might still be something else out there causing this object´s peculiar orbit." *

Then take into account huge comets like NEATV1,
the cometary head almost twice the size of Jupiter,
and all the controversy over its core size.
Phil and Co. maintain the premise that NEATV1 had an inner core of very small proportions inside of the dirty snowball,
McCanney and his crowd maintain it was the size of Mercury .
I believe this as well,
that NEATV1 was a planetary sized object in cometary form and motion.

NEATV1 coming in from BEHIND the sun
http://soho.nascom.nasa.gov/data/rea...18_0754_c3.gif