It is well known that the current census of extrasolar planets shows
an interesting correlation with their host stars: An extrasolar
planet is more likely to be found around a high metallicity star than
around a low metallicity star. (Here "metallicity" is the standard
astronomiccal usage meaning any element heavier than helium.)

I ran across the following interesting comment in an abstract by
Sozzetti et al., "A Keck/HIRES Doppler Search for Planets Orbiting
Metal-Poor Dwarfs":

We present results from our ongoing spectroscopic search for giant
planets within 1 AU around a well-defined sample of metal-poor stars
[...]. However, about 7% of the stars in our sample exhibits velocity
trends indicative of the existence of companions. [...]

In other words, be careful of selection effects!

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