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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Abstract

We show that the retrograde perihelion precession of Saturn \Delta\dot\varpi, recently estimated by different teams of astronomers by processing ranging data from the Cassini spacecraft and amounting to some milliarcseconds per century, can be explained in terms of a localized, distant body X, not yet directly discovered. From the determination of its tidal parameter K = GM_X/r_X^3 as a function of its ecliptic longitude \lambda_X and latitude \beta_X, we calculate the distance at which X may exist for different values of its mass, ranging from the size of Mars to that of the Sun. The minimum distance would occur for X located perpendicularly to the ecliptic, while the maximum distance is for X lying in the ecliptic. We find for rock-ice planets of the size of Mars and the Earth that they would be at about 80-150 au, respectively, while a Jupiter-sized gaseous giant would be at approximately 1 kau. A typical brown dwarf would be located at about 4 kau, while an object with the mass of the Sun would be at approximately 10 kau, so that it could not be Nemesis for which a solar mass and a heliocentric distance of about 88 kau are predicted. If X was directed towards a specific direction, i.e. that of the Galactic Center, it would mimick the action of a recently proposed form of the External Field Effect (EFE) in the framework of the MOdified Newtonian Dynamics (MOND).