Testing Their Metal: Small Exoplanets Abound in Diverse Stellar Environments

Unlike gas giants (think: Jupiter), small, rocky exoplanets seem not to favor one flavor of host star
Kepler 20-f


ANCHORAGE—In planet formation, slow, steady and small wins the race.

Astronomers using NASA's Kepler spacecraft have found that small planets such as Earth can form around all manner of stars, whereas massive gas giant planets like Jupiter tend to take shape around stars with large concentrations of heavy elements such as iron and oxygen.

The researchers published their findings online in Nature on June 13 and announced the results at the semiannual meeting of the American Astronomical Society being held here this week. (Scientific American is part of Nature Publishing Group.)

In the early years of exoplanetary science, beginning with the first discovery of a planet orbiting a sunlike star in 1995, the majority of known worlds outside of the solar system were giants much like Jupiter—and sometimes much more massive. Those heavyweights have the largest effects on their planetary systems and as such are the easiest to detect. Researchers noticed that the kinds of stars hosting giant planets tended to contain relatively high levels of so-called metals (an astronomer's term for any element heavier than hydrogen or helium.)

The chemical fingerprints of those stars point back to the makeup of the ancient disks of dust and gas from which the planets congealed, hinting that, at least for large worlds, having lots of metals around encourages planets to form. "If there's a lot of stuff in the disk, then we have a higher chance of finding these hot Jupiter planets," said lead study author Lars Buchhave of the Niels Bohr Institute at the University of Copenhagen.

The question was, do small planets—the Earth and Neptune analogues of the galaxy—follow the same trend? With the advent of modern planet-hunting instruments such as Kepler, a space telescope built to seek out Earth-size bodies, astronomers have finally gotten a peek at the smaller denizens of the planetary zoo.

Buchhave and his colleagues took spectral measurements of 152 stars that Kepler has inspected and where the spacecraft has projected the presence of 226 planets in total, most of them smaller in diameter than Neptune and some as small as Earth. (The mission has identified more than 2,000 probable planets altogether, but only dozens have been confirmed with follow-up observations.) They found that the host stars of those diminutive worlds are a diverse bunch, spanning a wide range of metallicities. On average, the small planets orbit stars roughly as metal-rich as the sun, a star of fairly ordinary composition, whereas giant exoplanets tend to inhabit planetary systems more enriched in metals.

That should not come as a total surprise, notes astronomer Andrew Howard of the University of California, Berkeley. After all, according to prevailing theoretical models, a giant planet acquires a solid core and then gathers gases and ices around that core to swell up to a Jupiter-like diameter. So that core must take shape before the gaseous disk dissipates under the intense radiation of the newly formed star. "To form a Jupiter, it's a race against the clock," Howard says. A metal-rich environment speeds the growth of the core, helping a gas giant take shape before it is too late. A smaller, rockier planet, on the other hand, is not as dependent on that ephemeral reservoir of gas; it can grow more gradually, even after the gas in the protoplanetary disk has evaporated.

"In my opinion, it points unambiguously to the fact that the formation of gas giant planets is quite a constrained process," says astronomer Debra Fischer of Yale University. An interesting question to pursue now, she says, is how low stellar metallicity can go before planet formation shuts off entirely.

The finding could bode well for Kepler's attempts and those of other exoplanet-finding campaigns, as small planets such as ours seem not to be picky about where they pop up. "Small planets could be widespread in our galaxy, simply because they don't need a special environment in which to form," Buchhave said.

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