[kazagistar]

You can't see a doppler shift if the pole of the orbit is facing sol, right? So there are still systems with invisible planets.

[roywiggins]

You can measure the position of the star in the sky and use that to detect systems that are "face on" to us (with the axis pointing at us) but it's harder.

http://www.planetary.org/explore/space-topics/exoplanets/ast...

[martinpw]

Sounds like 'harder' is an understatement - according to that article no planets have been discovered with this technique, and it is unlikely to yield any discoveries any time soon. (Technique is to directly observe the star wobbling in space rather than measure doppler shifts.)

[KodiakLabs]

You are correct. A component of the velocity of the wobble has to be in the line of sight. If not, it would be an edge case.