Orbital semi-major axes as a function of the mass of the host star for the known exoplanet population. Red circles show hot Jupiters (M> 1 M_J, period <10 days), black circles represent the rest of the sample. The size of the symbols scales with the planet mass. We also show the estimated stellar mass radii of main-sequence stars (Siess et al. 2000) and the stellar radii of the host stars. A clear gap of hot Jupiters is seen for M_∗> 1.5 M_⊙.