Xena has a very high aldebo. A brown dwarf would have a lower aldebo than Xena, so it would need to be even closer.<br /><br />But brightness isn't the only important thing in making something easy to discover. Keep in mind that Xena is the 4th brightest KBO. Why were over 500 dimmer ones discovered before it? It's because Xena, at its distance, has a very slow proper motion. Earlier surveys missed it because its speed was below a certain threshold. Only when Sedna inspired Mike Brown et al to lower the threshold, was Xena discovered.<br /><br />Something at 600 AU will be even slower, and hence difficult to distinguish from a background star.<br /><br />Regarding Nemesis stirring up the Oort Cloud on a periodic basis and sending comets crashing into Earth, here's something I don't understand:<br /><br />Oort Cloud comets are believed to be at about 60,000 AU. They should have an orbital speed of ~120 m/s. For them to drop to the inner solar system, they would need to have this orbital speed reduced to less than ~1.5 m/s. Comets whose velocity is changed to ~2m/s won't drop much closer to the Sun than Jupiter. ~5 m/s and above, and they don't even make it as close to the Sun as the Kuiper Belt. So the inner solar system is a very small target.<br /><br />And those that do get sent to the inner solar system have another lottery they must win. Earth is a very small target in the inner solar system. To get hit, the comet must be in the ecliptic when it crosses the 1AU mark. And even if it is, The Earth, with its 12000 km diameter and 1 billion km orbital track around the Sun, only occupies 1/75000 of its orbit at a time.<br /><br />So it would take millions of comets falling to the inner solar system for every hit. Would Nemesis be able to alter the velocities of millions of comets such that their new orbital velocities are in the small window of 0-1.5 m/s?