"On Thu Jun 7 14:20:32, DA wrote:
> Marcy, who with his colleagues has found more extrasolar
> planets than anyone, said detecting planets around a
> brown dwarf, even one that's a light year away, would be
> "an enormous challenge."

Could Marcy have forgotten the "old" astrometric methods of planet search? A close by dwarf is, as he said, close by. Combine that with its 100x less mass than the sun and you then have a body whose IR light image will wobble per the period of the orbiting planet.
It wobbles 100x more because of the mass difference plus another 10x or so because of the closeness. Bottom line, 1000x easier to detect the planets via astrometry, given 2001 state of the art IR sensitive CCD detectors."

Could the reason be that even though these dwarves could be much closer their light is much dimmer- requiring either a larger telescope or longer exposure. A longer exposure could blur the spectrum in which you are looking the slight "redshift".
Another factor could be that most of the light from a dwarf is in the lower spectrum of radiation (infrared). This could limit ground observation. And the longer wave length of infrared may have less "resolution" than the "normal" light of stars.
-gb