Heavy Metal Rock

A new study reveals that small, rocky Earth-like planets can develop in orbit around stars that vary widely in heavy element content.

Artist’s conception shows a newly formed star surrounded by a swirling disk of dust and gas that will eventually form planets.

University of Copenhagen/Lars Buchhave

There is good news in the continuing hunt for Earth-like planets. Astronomers have long known that large “gas giant” planets such as Jupiter are more likely to form in environments rich in “metals,” or elements heavier than hydrogen and helium. A new study has revealed, however, that small, rocky planets can develop in
orbit around stars that vary widely in heavy element content.

Astrophysicist Lars A. Buchhave of the Niels Bohr Institute and the Centre for Star and Planet Formation, University of Copenhagen, and twenty-eight colleagues used data from the Kepler spacecraft and ground-based observatories to examine the elemental composition of 152 stars harboring 226 planets, most of which are smaller than Neptune. A star and its planets form from the same cloud of gas and dust, so their elemental contents match closely. Stars with more heavy elements are known to favor the rise of Jupiter-like gas giant planets, which have large cores of rocky material. It had previously been thought that smaller rocky planets of terrestrial size might also need metal-rich parent stars: after all, heavy stuff, including silicon and iron, largely makes up our own Earth.

As it turns out, “terrestrial” planets are not so picky about where they form. In the study’s sample, such planets formed near stars with anywhere from as much as three times the Sun’s ”metallicity” to as little as one-fourth.

At the low end, having fewer heavy elements on hand might slow the growth of the massive cores needed for gas giants. The results suggest, according to Buchhave, that “small planets could be very, very common.” (Nature)