Planet occurence depends on Iron in stars.
Monday, JULY 21
Presentation at the International Astronomical Union Meeting in Sydney by D.A.Fischer and J.A. Valenti
Sydney, Australia - A comparison of 754 nearby stars like our sun - some with planets and some without - shows definitively that the more iron and other metals there are in a star, the greater the chance it has a companion planet.
From Exoplanets.org.