Polluted White Dwarfs Reveal Exotic Mantle Rock Types on Exoplanets in our Solar Neighborhood


Bulk silicate planet (BSP) compositions of polluted white dwarfs (PWDs) are compared to bulk silicate compositions of the inner planets20-22,24-27 and the BSPs inferred from Hypatia stars17, as well as (a) mantle rocks from Earth21 (peridotites and pyroxenites), and (b) crustal rocks from Earth31, Moon32 and Mars. MgO + SiO2 + CaO + FeO are normalized to 100%. BSPs are PWDs when metallic cores are removed from the bulk PWDs of Figure 1, to allow comparison to silicate compositions of the inner planets. “WD Unc.” indicates the propagated average uncertainty of PWD compositions. (b) shows that Earth’s continental rocks are a poor match for PWD silicate fractions. (a) and (b) show that rocks from Earth’s mantle are a good match for PWDs. But only one PWD matches bulk silicate Earth; most PWDs match rock types that are not dominant on any inner planet. Sol = Solar; MORB = Mid-Ocean Ridge Basalt.

Prior studies have hypothesized that some polluted white dwarfs record continent-like granitic crust--which is abundant on Earth and perhaps uniquely indicative of plate tectonics. But these inferences derive from only a few elements, none of which define rock type.

We thus present the first estimates of rock types on exoplanets that once orbited polluted white dwarfs--stars whose atmospheric compositions record the infall of formerly orbiting planetary objects--examining cases where Mg, Si, Ca and Fe are measured with precision. We find no evidence for continental crust, or other crust types, even after correcting for core formation. However, the silicate mantles of such exoplanets are discernible: one case is Earth like, but most are exotic in composition and mineralogy.

Because these exoplanets exceed the compositional spread of >4,000 nearby main sequence stars, their unique silicate compositions are unlikely to reflect variations in parent star compositions. Instead, polluted white dwarfs reveal greater planetary variety in our solar neighborhood than currently appreciated, with consequently unique planetary accretion and differentiation paths that have no direct counterparts in our Solar System.

These require new rock classification schemes, for quartz + orthopyroxene and periclase + olivine assemblages, which are proposed here.

Keith D. Putirka, Siyi Xu

Comments: 26 pages, 3 figures, published in Nature Communications
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
DOI: 10.1038/s41467-021-26403-8
Report number: Putirka, K. D., Xu, S., Nat Commun 12, 6168 (2021)
Cite as: arXiv:2111.03124 [astro-ph.EP] (or arXiv:2111.03124v1 [astro-ph.EP] for this version)
Submission history
From: Siyi Xu
[v1] Thu, 4 Nov 2021 19:46:03 UTC (3,497 KB)
https://arxiv.org/abs/2111.03124
Astrobiology, Astrogeology

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