An Illustration of a compact, multi-planet system. Researchers have found that such systems are more likely to form around stars with lower amounts of heavy elements than our own Sun.
Some planetary systems form in the absence of high amounts of heavy metal - space science news

Small compact planetary systems are more likely to form around stars that have lower amounts of heavy metals than our own Sun. This new insight, based on the observation of 700 stars, is very different from results of earlier research that focused on stars with higher levels of heavy metal. Low-metallicity stars are older than the ones with high heavy metal content, and the first planets have therefore likely formed around these. Thus, the formation of planets does not always follow the current model, which is based on the development of gas giant planets such as Jupiter around higher metallicity stars.

Read the full story: Yale University
Scientific publication: Astrophysical Journal Letters