This diagram is to show that stars have a very low metallicity only gas giant planets as (because heavy elements to form smaller planets are rare).
It follows that come in the search for Earth-like exoplanets only star with a metallicity in question, which is at least as high as that of the sun.
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Looking at the chart thus shows that there is no exoplanets with less than 0.1 Jupiter masses at a metallicity of less than -0.75 (date June 2012).