From astrobites: “Can we constrain planetary mineralogy of the closest stars?”

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astrobites

Dec 23, 2017
Leonardo dos Santos

Article: The Star-Planet Connection I: Using Stellar Composition to Observationally Constrain Planetary Mineralogy for the Ten Closest StarsAuthors: Natalie Hinkel and Cayman Unterborn
First author’s institution: Department of Physics & Astronomy, Vanderbilt University

Status: Submitted to AAS Journals

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Phase diagram of the simulated interior of an Earth-like planet. It shows the different compounds that make up the mantle for varying depths. No image credit.

We get excited when hearing the news of a newly discovered planet inside the habitability zone of a star. But caution is advised: being actually habitable is much more involved than simply having the right amount of irradiation. Other aspects such as the chemical composition of a planet’s atmosphere and crust are key for life as we know it to flourish. The challenging nature of precisely measuring the chemistry of stars is daunting but, according to today’s paper, we may actually be capable of inferring about planetary mineralogy for our closest extrasolar neighbors.

Another dimension to habitability

Since directly measuring the composition of the solid innards of an exoplanet is far beyond our current capabilities, we often rely on indirect methods to infer about their bulk composition. For instance, if we measure the radius of a planet using the transit method and then its mass using radial velocities, then we can estimate its density.

Planet transit. NASA/Ames

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Las Cumbres Observatory

But this relation doesn’t provide enough fine details to allow an inference about, say, the size and composition of the core and mantle of the planet — i.e., its mineralogy. However, our current hypotheses on how planetary systems form suggest that the chemical composition of a star and the planets that orbit around it should be strongly connected. This means that, in principle, we could infer about the mineralogy of a planet if know the composition of the host star well enough.

Relative chemical abundances are key to habitability, such as the molar (number of atoms) ratio between iron and magnesium (Fe/Mg), which affects the size of a planet’s core and consequently the heat transfer to the surface. Another example is the presence of volatile elements (those that like to stay in gaseous phase) in the crust, which influence tectonic movement and geochemical cycles. The major controls in planetary mineralogy are the ratios of the elements magnesium, aluminum, silicon, calcium, and iron. Such ratios are routinely measured in atmospheres of stars, although with varying degrees of precision.

The overall idea of today’s paper is to assess if we can measure these abundances in stars in the solar neighborhood precisely enough to make any inferences about the mineralogy of putative rocky planets around them. In order to do that, the authors used the Hypatia Catalog of stellar measurements: they selected 10 nearby well-known stars with reliable chemical abundance estimates and started digging (see Fig. 1).

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Figure 1. Molar ratios (fraction of the number of atoms) of silicon and iron over magnesium for the entire Hypatia Catalog (red symbols) and the selected 10-star sample plus the Sun (black dots). These stars were selected for their low uncertainties in molar ratios.

See the full article here .

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