No Evidence Yet For Continental Crust From Polluted White Dwarfs, But Exotic Compositions Are Possible

Recent studies have inferred the presence of continental crust on extra-solar planets due to high levels of Ca, Al or alkali metals in polluted white dwarf atmospheres. Some estimates of crust mass fractions exceed 75 wt. %. Such findings, if valid, would be momentous, as continental crust distinguishes Earth from its planetary neighbors. Our solar system would also then appear to be anomalous, as only Earth has a considerable fraction of such crust, which weighs in at paltry 0.4 wt. % of its total planetary mass. As a test of these inferences, we examine 23 pWDs where Mg, Si, Ca, and Fe are reported. The inclusion of Si and Mg are critical, as high-Si is the signature chemical attribute of “granitic” or continental crust, and because such rocks are also uniformly low Mg. In contrast, Ca, Al and the alkali metals can be highly enriched in rocks that are very un-representative of Earth’s continents, such as kimberlites, or some flood basalts. In addition, Mg, Si, Ca and Fe comprise >90% of the cations of rocky planets, and so we can compute mineral assemblages and rock types, which is not possible if only Ca, Al, Fe and alkali metals are known. In the event, we find that pWDs contain far too little Si and are too enriched in Mg for any to represent evolved crust, let alone the granitic rocks that characterize Earth’s continents; crust fractions estimated using projections from Ca-Mg-Fe space, are thus not reproducible in projections from Si-Mg-Fe. This result might not be surprising. If the parent stars of white dwarfs were close to Solar in composition, none would likely have silicate compositions capable of generating very large fractions of continental crust. And to the extent that such former planets were Earth-like, continental crust would exist at fractions too low to be detected, at least when only a few elemental abundances are known. But crust compositions inferred from polluted white dwarfs are not all like the basaltic rocks that cover Earth’s ocean basins, the Moon or Mars. Some polluted white dwarfs have quartz- or periclase-bearing mantle mineral assemblages; a few have large crust fractions whose lithologic analogs are rare on Earth—volcanic rocks erupted as so-called “flood basalts”, from the Emeishan Province of China, and the Siberian Traps. If these unusual crustal types do happen to dominate some polluted white dwarfs, they may point to planetary evolution paths that are exotic to our solar system.