Julia,
Trying to bolster the Elenin/Niburu/Planet X fantasy, you took three words out of context which were 'can't be seen' suggesting there is a Jupiter-sized planet that could not be seen here inside our solar system.  You ommitted some other facts in your title, such as the statement in the first paragraph that it was referring to planet-sized objects that are 'away from the light of a star' or the other statement that says they average 10,000 to 20,000 LIGHTYEARS in distance.
 
If Planet X were coursing its way through our solar system, currently at comet Elenin's coordinates of 1.7 AU in distance, the light would take only about 15 light-minutes to bounce the light from the sun off of 'Planet X's surface and back to an earth-bound pair of eyes.  Yes it is true that we cannot see planets 10,000 to 20,000 LIGHTYEARS away.
 

In Christ,
Kevin Heckle