New solar object

[Karlos]

IIRC, the pluto that was discovered was a fluke whilst looking for a larger body that would have sufficient mass that would account for the pertubations observed in Neptune's orbit.

I think the first time it was photographed, it fell on a flaw in the plate and was missed :-)

Supposedly, then, the real 'pluto' remains to be discovered. Given the non-linear increase in orbital distance of the planets, I expect it would have to be quite large to still exert an effect on neptune at such a distance.

I don't think xena qualifies...

[Karlos]

I don;t think sedna qualifies either. At the distances we are talking about, you'd need an neptune class planet or larger to exert the effects seen on neptune.

Or something very small and dense...

[Karlos]

I think a black hole would be a bit obvious if it were in such proximity. There'd be x-ray bursts everytime it scavenged any material unfortunate enough to get too close.

I'm only musing, of course; astronomy is a side interest of mine. I'm sure Blobrana can come up with far more interesting and accurate possibilities :-)