On Tuesday I mentioned the work of Lars A. Buchhave, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA), in connection with the Kepler-10c discovery. The latter is the so-called ‘mega-Earth’ now found to be seventeen times as massive as our own planet, with a diameter of about 29,000 kilometers. A larger population of solid planets with masses above 10 times that of Earth was suggested in the Kepler-10c paper (see Introducing the ‘Mega-Earth’ for more on this), with reference to Buchhave’s ongoing work.

Let’s take a closer look at what Buchhave is doing, because the intriguing fact is that planets four times the size of Earth and smaller comprise about three-quarters of the planets found by the Kepler mission. How large a role does the metallicity of the host star play in planet formation?

At the ongoing meeting of the American Astronomical Society in Boston, Buchhave explained his research methods, which involve measuring ‘metals’ — in astronomical parlance the elements heavier than hydrogen and helium — in stars hosting exoplanets. His team analyzed more than 2000 high-resolution spectra of Kepler Objects of Interest, yielding the metallicities and other parameters of 405 stars orbited by 600 exoplanet candidates.

A statistical examination of the results followed, with this outcome: Planet-hosting stars fall into three groups that can be defined by their compositions. Buchhave’s team found two dividing lines, one at 1.7 times Earth’s radius, the other at 3.9 times the radius of Earth. The inference is that planets smaller than 1.7 Earth radius are completely rocky, while those above 3.9 Earth radius are most likely gas giants. [Addendum: See kzb’s comment below re a mistake I made in the initial version of this post.]

That interesting region between 1.7 and 3.9 times the size of Earth is where we find the so-called ‘gas dwarfs,’ planets whose cores accreted gas from the protoplanetary disk but failed to grow into gas giants of Jupiter-class or larger. Says Buchhave:

“It seems that there is a ‘sweet spot’ of metallicity to get Earth-size planets, and it’s about the same as the Sun. That makes sense because at lower metallicities you have fewer of the building blocks for planets, and at higher metallicities you tend to make gas giants instead.”

From the paper:

…the observed peak in the metallicity–radius plane at 1.7R? suggests that the final
mass and composition of a small exoplanet is controlled by the amount of solid material available in the protoplanetary disk. A higher-metallicity environment promotes a more rapid and effective accretion process, thereby allowing the cores to amass a gaseous envelope before dissipation of the gas. In contrast, lower-metallicity environments may result in the assembly of rocky cores of several Earth masses on timescales greater than that inferred for gas dispersal in protoplanetary disks (<10 Myr), yielding cores without gaseous hydrogen–helium atmospheres.

To produce small, terrestrial worlds, then, stars with metallicities similar to the Sun are favored, while stars with gas dwarfs are likely to be those that are slightly more metal-rich. The stars most likely to produce gas giants contain the most metals, generally fifty percent more than the Sun. The finding is intriguing but comes with the caveat that Kepler is best at finding planets relatively close to their star, and the metallicity thesis needs to be tested over a wider range of orbits.


Image: Three kinds of planetary outcome are suggested by the metallicity of the host star, according to new work by Lars A. Buchhave and team. Credit: David Aguilar, CfA.

Whether or not Buchhave’s work can explain a Kepler-10c will depend upon gathering a larger sample of such worlds to work with. But so far his analysis implies that there is no evident cutoff in size for rocky worlds, the data showing that the mass and radius indicating the transition from rocky to gaseous planets should increase with orbital period:

Although additional data are required to confirm this relationship, the fit is apparently consistent with a critical core mass that increases with orbital period and an atmospheric fraction of 5%… If correct, this predicts the existence of more massive rocky exoplanets at longer orbital periods.

Thus the farther a planet is from its star, the larger it can grow before the accretion of a thick atmosphere turns it into a gas dwarf. When we have the observational tools to examine a wide range of planets in outer-system orbits, we may be able to confirm or refute Buchhave’s suggestion that truly massive ‘super-Earths’ like Kepler-10c may not be uncommon.

The paper is Buchhave et al., “Three regimes of extrasolar planets inferred from host star metallicities,” Nature 509 (29 May 2014), pp. 593-595 (abstract / preprint).