Thanks to the Kepler planet-hunting mission and other surveys, the number of potential exoplanets—planets orbiting other stars—has grown. Of the 1,235 possible planets in the 2011 Kepler catalog, over half are smaller in size than Neptune. This indicates at least some may be rocky, similar in some respects to the terrestrial planets in the Solar System. While confirming and characterizing the individual planets is ongoing work, enough candidates are available that astronomers can perform statistical analyses on the worlds and their host star systems.

In particular, a new study published in Nature examined the relationship between the host star's chemistry and the size of the planets in orbit. The presence of chemical elements heavier than helium—which astronomers perversely refer to as metals—in a star's spectrum is a measure of the environment of planet formation. As described by authors Lars A. Buchhave et al., no strong correlation exists between the metal content of the host star and the presence of low-mass planets. This is in stark contrast to higher-mass planets (comparable to Jupiter), which preferentially orbit high-metal stars. In other words, terrestrial planets may orbit a higher fraction of stars in the galaxy, since they don't require a metal-rich environment for formation.

The widely accepted model of planet formation involves a disk of gas and dust surrounding the newborn star, known as the protoplanetary disk. As the name suggests, the protoplanetary disk gives rise to planets, but it also deposits material onto the star. The chemical composition of the material becomes part of the star's spectrum, which can be measured. The relative abundance of metals (compared to the base amount of hydrogen, which comprises most of any star's makeup) is known as the metallicity.

While the fraction of metals in any star is less than 1 percent by mass, modern astronomical methods are able to distinguish small variations from star to star. Additionally, the researchers in the current study developed a new statistical tool that allowed them to characterize metallicities even when the data was noisy, as with the Kepler catalog. (Noise in this case refers to excess photons from other sources; follow-up observations can reduce this noise by looking at the target star for longer times). Their technique involved matching real stellar spectra to simulated spectra with comparable noise levels.

Kepler identifies exoplanet candidates when they pass in front of their host stars, relative to observers on Earth. The amount of light blocked during the transit yields the planet's size (similar to how astronomers measured the size of the Solar System during the transits of Venus in the 18th and 19th centuries). Initial observations identify potential exoplanets, and follow-up observations are required to confirm whether they are actually planets. The advantage of this type of observation is that it can ideally locate planets that are smaller and less massive than what is found in other methods.

The authors examined 226 exoplanet candidates orbiting 152 stars. Of these, 175 potential planets were smaller than Neptune, which itself is about four times the diameter of Earth. Due to the limitations inherent in all planet-hunting methods, most of these candidates orbited very close to their host stars (about the distance of Mercury's orbit or closer). Some of the candidates were comparable to the smallest exoplanets yet discovered, though the errors preclude clear identification of Earth-sized planets at this time. The researchers compared the metallicity of the host stars to the radius of the exoplanet candidates.

While larger planets (those with radii bigger than four times Earth's) were strongly correlated with high-metallicity stars, the researchers found no clear relationship for small planets. Worlds smaller than Neptune were as likely to be found orbiting low-metallicity stars as high, meaning the protoplanetary environment giving them birth was far more diverse than for larger planets.

According to a widely accepted theory of planet formation, the gas in protoplanetary disks dissipates relatively quickly. As the authors point out, if this theory is correct, then a higher metallicity would increase the mass of protoplanetary cores, since the raw materials would contain a higher fraction of heavier chemical elements. Heavier grains of dust in high metallicity systems slowed the dissipation rate, in other words, meaning high-mass planets would have time to form. Low-mass planets, on the other hand, don't have to race the dissipation clock. These can form even when the metallicity is relatively low.

While there is likely a lower limit to the metallicity possible for planet formation, the current study suggests it's much lower than other models previously concluded. This means small, terrestrial planets could be very common in the Milky Way, since they need far less restrictive conditions for their formation.

Nature, 2012. DOI: 10.1038/nature11121 (About DOIs).