P.S. space.com article 16 March 2004 concerning Sedna's odd elliptical orbit. Just below here are my words, including some quotes:
Two main astronomers willing to accept and the possibility of an earth or larger-sized planet affecting Sedna's elliptical orbit and rotation:
"Brown astronomer at California Institute of Technology who led the discovery of Sedna," and
"Marsden, who heads the Minor Planet Center in Cambridge, Mass., where newfound solar system bodies are catalogued."
Just to clarify: in the following article, in this latter portion of Brown's statements, he is saying his team is [considering searching an unexplored area of the sky, for an earth -sized planet.] Below this section on Brown, see Marsden.
"Brown, astronomer at California Institute of Technology who led the discovery of Sedna, said there is one unexplored region of space left, amounting to about 20 percent of the sky, that hasn't been searched for an Earth-sized object that would be orbiting at 70 AU and presumably in the main plane of the solar system. It is the region toward the bright galactic center, which is harder to search.
Brown said his team is considering making that search now."

"Marsden, who heads the Minor Planet Center in Cambridge, Mass., where newfound solar system bodies are catalogued.
Marsden favors an object closer in, a "planetary object," he told SPACE.com , perhaps at between 400 and 1,000 AU."

"Perhaps there's more than one planet out there," Marsden said. "Who knows? But let's suppose it is something of an Earth mass, maybe even a few Earth masses. A close approach could throw this object [Sedna] from something more circular into something more eccentric."

http://www.space.com/scienceastronom...th_040316.html

from planetbarb, most of the space.com article is copied below:

copyrighted material deleted by The Bad Astronomer

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Well, it's a big solar system, so there could definitely be some large planetoids hiding out there. And almost certainly 1,000s (100s of 1,000s?) of smaller objects in the Oort cloud.

So, what effect would they have on us? None. Nada. Zip. They're simply too far away, by several orders of magnitude. Will they get closer? Nope. Any large-ish planet that orbits close enough to the sun to be destructive (or even visible, really) would have been detected long ago either through direct observation or (more likely) by how it affected the other planets we do know about.

No unexplained glitches in the orbits of the inner planets = no unexplained planets that have ever visited the inner solar system.

Now, the far outer solar system is a different question (70+AU). There could be (and probably are) all kinds of exotic and weird objects out there just waiting to be discovered. I suspect that Sedna is just the first one of hundreds we'll eventually hear about as observation capabilities increase. And that doesn't include the 1,000s upon untold 1,000s that are simply too small or dim to see at that distance.

Phil Plait OK will not post articles just links
planetbarb


First, welcome to the board.

Second, long excerpts from copyrighted material is forbidden in the FAQ here. The link you posted is sufficient. Please read the FAQ.
_________________
Phil Plait
The Bad Astronomer
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badastro@badastronomy.com