  Radial velocity surveys find Jupiter mass planets with semi-major axes a less
than 0.1 AU around ~1% of solar-type stars; counting planets with $a$ as large
as 5 AU, the fraction of stars having planets reaches ~ 10% {Marcy,Butler}. An
examination of the distribution of semi-major axes shows that there is a clear
excess of planets with orbital periods around 3 or 4 days, corresponding to
a~0.03$ AU, with a sharp cutoff at shorter periods (see Figure 1). It is
believed that Jupiter mass planets form at large distances from their parent
stars; some fraction then migrate in to produce the short period objects. We
argue that a significant fraction of the `hot Jupiters' (a<0.1 AU) may arise in
binary star systems in which the orbit of the binary is highly inclined to the
orbit of the planet. Mutual torques between the two orbits drive down the
minimum separation or periapse r_p between the planet and its host star (the
Kozai mechanism). This periapse collapse is halted when tidal friction on the
planet circularizes the orbit faster than Kozai torque can excite it. The same
friction then circularizes the planet orbit, producing hot Jupiters with the
peak of the semimajor axis distribution lying around 3 days. For the observed
distributions of binary separation, eccentricity and mass ratio, roughly 2.5%
of planets with initial semimajor axis a_p ~ 5au will migrate to within 0.1au
of their parent star. Kozai migration could account for 10% or more of the
observed hot Jupiters.
