Up on arxiv.org today is a paper, "Title: The perihelion precession of Saturn, planet X/Nemesis and MOND", by Lorenzo Iorio.

The perihelion is the point of the planets orbit where it is closest to the Sun. Over the millennia, this point moves, or precesses, under the gravitational pull of other objects. This is a tiny effect, spotted because we have accurate measurements for Saturn thanks to measurements with the Cassini probe. (We have no such measurements further out, for Neptune and Uranus; but we have a more famous case: the Pioneer Anomaly, unexplained changes in the path of the Pioneer probes). Iorio comes up with measurements for a potential planet X to explain this; an Earth-sized planet around 80-150 AU out, up to a Jupiter-sized object at 1000 AU. (1000 times the Earth-Sun distance).

I've written before about this: such a planet is remotely feasible to detect by ground-based systems (or perhaps Herschel). It would certainly help explain the architecture of the outer solar system. Perhaps we should start seriously looking for such an object ?

The Brown Dwarf possibility is also interesting; Centauri Dreams has an article on the Wide-field Infrared Survey Explorer (WISE) mission, due for November launch, thats expected to be capable of finding many cold brown dwarfs nearby.

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