Earth-sized planets may be widespread in the Milky Way, since they don’t need metal-rich parent stars to form, suggests new analysis of data from NASA’s planet-hunting Kepler space telescope.

Planets up to about four times the diameter of Earth form under a broader range of environmental conditions than gas giant planets, the analysis shows.

Scientists looked at 152 stars hosting planets or suspected planets that are Neptune-sized  or smaller. They found that small planets, unlike gas giants, don’t need metal-rich parent stars to form.

“Our analysis based on the Kepler planet candidates indicates that terrestrial planets can form at a wide range of metallicities, including metallicities almost four times lower than that of the sun,” wrote lead author Lars Buchhave, with the Niels Bohr Institute at the University of Copenhagen.

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The study, published this week in Nature, has several interesting implications. For one, since so-called metal-poor stars — those made  of just hydrogen and helium — developed before metal-rich ones, planets around these stars (which formed from the same raw materials as their parent stars)  didn’t have the right stuff for life, at least life as we know it.

“The elements from which planets and our bodies are made did not exist,” Yale University astronomer Debra Fischer noted in a related Nature paper.

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But knowing that the formation of rocky planets can occur in lower-metal environments than those of gas giants lowers the bar for what it takes to create an Earth-like world, since they can form around a bigger range of stars, so it may have been easier for earths and Earth-like life to form.

“There could be some places in the universe where rocky planets and life got an earlier start than did Earthlings,”  Fischer wrote.

Image: This is 0.2 percent of the Kepler space telescope's field of view. The observatory is studying stars for signs of planets passing by. Credit: NASA/Ames/JPL-Caltech