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Small planets do not need metal-rich stars

photo of Lars Buchhave

amount of elements heavier than hydrogen or helium in a star is called the "metallicity" of the star. In astronomical parlance, anything heavier than hydrogen or helium is a "metal." We know that planets are created in disks of gas and dust around new stars and that planets like Earth are composed mostly of the elements iron, oxygen, silicon and magnesium. It would be a reasonable assumption that the metallicity of a star mirrors the metal content of the planet-forming disk and that large quantities of heavy elements in the disk would lead to more efficient planet formation. That seems to be true for giant planets with short orbital periods--they tend to be associated with metal-rich stars.

But a research team led by Lars A. Buchhave, an astrophysicist at the Niels Bohr Institute and the Centre for Star and Planet Formation at the University of Copenhagen, studied the elemental composition of more than 150 Kepler stars harboring 226 Kepler planet candidates smaller than Neptune, including some comparable in size to the terrestrial planets in our Solar System. They find that, contrary to gas giants, the occurrence of small planets does not depend as strongly on the metallicity of the host star. This observation suggests that terrestrial-like planets may be widespread in the disk of our Galaxy, with no special requirement of higher metallicity stars for their formation.

"I wanted to investigate whether small planets needed a special environment in order to form, like the giant gas planets, which we know preferentially develop in environments with a high content of heavy elements," said Buchhave. "This study shows that small planets do not discriminate and form around stars with a wide range of heavy metal content, including stars with only 25 percent of the sun's metallicity."

Natalie Batalha, Kepler mission scientist at NASA's Ames Research Center at Moffett Field, Calif., commented that "Kepler has identified thousands of planet candidates, making it possible to study big-picture questions like the one posed by Lars. Does nature require special environments to form Earth-size planets? The data suggest that small planets may form around stars with a wide range of metallicities -- that nature is opportunistic and prolific, finding pathways we might otherwise have thought difficult."

The ground-based spectroscopic observations for this study were made at the Nordic Optical Telescope on La Palma in the Canary Islands; Fred Lawrence Whipple Observatory on Mt. Hopkins in Ariz.; McDonald Observatory at the University of Texas at Austin; and W.M. Keck Observatory atop Mauna Kea in Hawaii.

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