In astronomy, the word ‘metals’ refers to anything heavier than hydrogen and helium. Stars fuse hydrogen into helium and from there work their way into the higher elements until hitting iron, at which point the end quickly comes, with ‘star stuff,’ as Carl Sagan liked to put it, being flung out into the universe. Through stellar generations we can trace a higher concentration of the heavier elements as stars are born from the materials of their predecessors. And we’ve learned that those metal-rich stars are the most likely to produce gas giants like Jupiter and Saturn.

What’s intriguing is the issue of smaller planets and the conditions for their formation. After all, the content of the disk from which planets are formed parallels the metallicity of the host star. I’m looking at new research from Lars A. Buchhave (Niels Bohr Institute/University of Copenhagen) into planet formation, using data from the Kepler telescope. In Buchhave’s words:

“We have analysed the spectroscopic elemental composition of the stars for 226 exoplanets. Most of the planets are small, i.e. planets corresponding to the solid planets in our solar system or up to four times the Earth’s radius. What we have discovered is that, unlike the gas giants, the occurrence of smaller planets is not strongly dependent on stars with a high content of heavy elements. Planets that are up to four times the size of Earth can form around very different stars – also stars that are poorer in heavy elements.”

Buchhave and team focused on whether small, Earth-like planets needed the same kind of metal-rich environment demanded by the gas giants, at least those with short orbital periods. Given that planets like the Earth are made up of heavier elements — iron, silicon, oxygen, magnesium — you would assume that small planet formation would be much more efficient around metal-rich stars. The new paper, which has been published in Nature, argues that the idea is wrong, and that opens up a lot of territory. Without special requirements for heavy elements in their stars, Earth-like planets could indeed be widespread in the galaxy.

Image: This artist’s conception shows a newly formed star surrounded by a swirling protoplanetary disk of dust and gas. Debris coalesces to create rocky ‘planetesimals’ that collide and grow to eventually form planets. The results of this study show that small planets form around stars with a wide range of heavy element content, suggesting that their existence might be widespread in the galaxy. Credit: University of Copenhagen/Lars Buchhave.

The work also implies that small planets could form earlier in galactic history than has previously been thought. Buchhave’s work examines this through the study of the spectroscopic metallicities of the host stars of the 226 Kepler candidates chosen. The average metallicity for planets smaller than four Earth radii turns out to be close to that of the Sun, but Buchhave says in this NASA news release that stars with just 25 percent of the Sun’s metallicity can also form small planets. Meanwhile the data continue to support the preferential formation of gas giants around higher metallicity stars.

From Natalie Batalha (NASA Ames), a member of the Kepler science team:

“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.”

Indeed. We are learning that it doesn’t take that many generations of stars to start producing rocky worlds. The work was presented yesterday at the 220th meeting of the American Astronomical Society. “Giant planets prefer metal-rich stars. Little ones don’t,” says David Latham (Harvard-Smithsonian Center for Astrophysics). The CfA’s own news release says the work supports the core accretion view of planet formation, in which steadily accumulating planetesimals combine to form planets, with the largest quickly gathering hydrogen. Higher metallicities make quick formation of large cores more likely, which explains the connection between heavier metals and gas giants.

Are there SETI implications here as well? This from Jill Tarter (SETI Institute):

“The idea that very old stars could also sport habitable planets is encouraging for our searches. In particular, intelligent life has taken a long time to evolve here on Earth. Consequently, it’s reasonable to suppose that older planetary systems are more likely to have technological societies – the kind we might detect with our radio telescopes.”

And that reminds me to note that the SETI Institute is hosting SETIcon II in Mountain View, California from June 22-24, where those in attendance can rub elbows with the likes of Geoff Marcy and Debra Fischer. I see that tickets are still available to the public.

The paper is Buchhave et al., “An abundance of small exoplanets around stars with a wide range of metallicities,” published online in Nature 13 June 2012 (abstract).