[omegaman]

http://www.wired.com/wiredscience/20...t-double-star/

[Saquist]

It's not actual "pressure" but shearing forces more commonly called tidal forces. Two or multiple stars will often orbit at different distances from each other and not necessarily close. Thus when the star are formed from an interstellar cloud the larger masses of the stars orbiting each other (protostars) will gravitationally brush normal formation locations like the Lagrange points that normally exist around solitary stars but in the presence of a second star or tertiary stars it was predicted there was no safe location for these planets to form.

Likely even in trinary systerms there might be an oscillation of stability for isolated planetary formation. Not as many planet would form though.