Since we’ve just been looking at stellar metallicity and planet formation, news from the European Southern Observatory catches my attention. A new paper from ESO astronomers discusses the question of planetary debris falling onto the surface of stars, and its effects on what we observe. Evidence has been accumulating that planets tend to be found around stars that are enriched in iron. On average, stars with planets are almost twice as rich in metals as stars with no known planetary system.
But what exactly does this result mean? On the one hand, it’s possible that stars that are rich in metals naturally enhance planet formation. But the reverse is also possible: It could be that debris from the planetary system could have polluted the star itself, so that the metals we see aren’t intrinsic to the star. Bear in mind that a stellar spectrum shows only the star’s outer layers, so we can’t be sure what’s at the core. And in-falling planetary debris would stay in the star’s outer regions.
In other words, observed metallicity could actually be caused by the planetary system itself, and not by the star. The ESO team, led by Luca Pasquini, approached the question by studying red giant stars that have exhausted hydrogen in their core. And in the fourteen planet-hosting red giants under investigation, a clear difference appeared between these and normal planet-hosting stars. “We find that evolved stars are not enriched in metals, even when hosting planets,” says Pasquini. “Thus, the anomalies found in planet-hosting stars seem to disappear when they get older and puff up!”
Now it gets interesting, because we’re gaining insights that could affect the evolution of planet formation theories. The ESO astronomers think the difference between red giants and stars like our own in terms of metallicity studies is the size of the convective zone (see image and caption below). In a star like the Sun, this region is about two percent of the star’s mass, but the convective zone in red giants is 35 times larger. Any polluting metals would thus be 35 times more diluted in a red giant, and correspondingly that much more difficult to observe.
Image (click to enlarge): Artist’s impression of the structure of a solar-like star and a red giant. The two images are not to scale – the scale is given in the lower right corner. It is common to divide the Sun’s (and solar-like stars’) interior into three distinct zones. Here, note the uppermost, called the Convective Zone. It extends downwards from the bottom of the photosphere to a depth of about 15% of the radius of the Sun. The energy in the Convective Zone is mainly transported upwards by (convection) streams of gas. In red giants, the convection zone is much larger, encompassing more than 35 times more mass than in the Sun. Credit: ESO.
Artie Hatzes (Thüringer Landessternwarte Tautenburg) puts it starkly: “Although the interpretation of the data is not straightforward, the simplest explanation is that solar-like stars appear metal-rich because of the pollution of their atmospheres.” A co-author of the paper, Hatzes illustrates the tricky nature of metallicity studies. We may be seeing metal excesses resulting from heavy elements falling onto the star from its proto-planetary disk, making the case that how metals function in planet formation is something our theories are a long way from explaining.
And note this: The core accretion model of planet formation seems to be challenged by this finding. Core accretion assumes that planets ‘grow’ as protoplanetary materials bang together and accumulate until, gaining enough mass and forming a solid core, they are able to capture a gas atmosphere. The model depends on dust content to function, and implies that the host stars should show high metallicity down to their core.
The gravitational instability model is different. Let me quote from the Pasquini paper on this mechanism: “…a gravitationally unstable region in a protoplanetary disk forms self-gravitating clumps of gas and dust within which the dust grains coagulate and sediment to form a central core.” Alan Boss (Carnegie Institution of Washington), the major proponent of this theory, has argued that gravitational instability has little dependence on metallicity. The current ESO work seems to be a point in gravitational instability’s favor. The situation is still in flux, however, with more results being gathered from sub-giant star planet searches in other venues. The metallicity findings of these surveys should provide valuable new clues.
The paper is Pasquini et al., “Evolved stars hint to an external origin of enhanced metallicity in planet-hosting stars,” to be published in Astronomy & Astrophysics, with preprint available.