Putirka Keith D, Xu Siyi
Department of Earth and Environmental Sciences, California State University, 2576 E. San Ramon Ave, MS/ST 24, Fresno, CA, 93740, USA.
Gemini Observatory/NSF's NOIR Lab, #314, 670N. A'ohoku Place, Hilo, HI, 96720, USA.
Nat Commun. 2021 Nov 2;12(1):6168. doi: 10.1038/s41467-021-26403-8.
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 discernable: 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.
先前的研究推测，一些受污染的白矮星记录了类似大陆的花岗岩地壳，这种地壳在地球上很丰富，可能是板块构造的独特标志。但这些推断仅基于少数几种元素，没有一种能确定岩石类型。因此，我们首次估计了曾经环绕受污染白矮星运行的系外行星上的岩石类型，这些白矮星的大气成分记录了先前环绕其运行的行星天体的陨落情况，我们研究了精确测量镁、硅、钙和铁的案例。即使在对核心形成进行校正之后，我们也没有发现大陆地壳或其他地壳类型的证据。然而，这些系外行星的硅酸盐地幔是可辨别的：一个案例类似地球，但大多数在成分和矿物学上都很奇特。由于这些系外行星的成分差异超过了附近4000多颗主序星的成分差异范围，它们独特的硅酸盐成分不太可能反映母恒星成分的变化。相反，受污染的白矮星揭示了我们太阳系附近比目前所认识到的更多样化的行星，因此有独特的行星吸积和分化路径，在我们的太阳系中没有直接对应的情况。这些需要新的岩石分类方案，用于石英 + 斜方辉石和方镁石 + 橄榄石组合，本文对此进行了提议。
