  One of the first stages of planet formation is the growth of small
planetesimals. This early stage occurs much before the dispersal of most of the
gas from the protoplanetary disk. Due to their different aerodynamic
properties, planetesimals of different sizes and shapes experience different
drag forces from the gas during this time. Such differential forces produce a
wind-shearing (WISH) effect between close by, different size planetesimals. For
any two planetesimals, a WISH radius can be considered, at which the
differential acceleration due to the wind becomes greater than the mutual
gravitational pull between the planetesimals. We find that the WISH radius
could be much smaller than the Hill radius, i.e. WISH could play a more
important role than tidal perturbations by the star. Here we study the WISH
radii for planetesimal pairs of different sizes and compare the effects of wind
and gravitational shearing (drag force vs. gravitational tidal force). We then
discuss the role of WISH for the stability and survival of binary
planetesimals. Binaries are sheared apart by the wind if they are wider than
their WISH radius. WISH-stable binaries can inspiral and possibly coalesce due
to gas drag. Here, we calculate the WISH radius and the gas drag-induced merger
timescale, providing stability and survival criteria for gas-embedded binary
planetesimals. Our results suggest that even WISH-stable binaries may merge in
times shorter than the lifetime of the gaseous disk. This may constrain
currently observed binary planetesimals to have formed far from the star or at
a late stage after the dispersal of most of the disk gas. We note that the WISH
radius may also be important for other processes such as planetesimal erosion
and planetesimal encounters and collisions in a gaseous environment.
