1Anton Pannekoek Institute for Astronomy, University of Amsterdam, Amsterdam, The Netherlands.
2Department of Earth Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands.
Astrobiology. 2019 Jul;19(7):867-884. doi: 10.1089/ast.2018.1930. Epub 2019 Apr 26.
Carbon-enriched rocky exoplanets have been proposed to occur around dwarf stars as well as binary stars, white dwarfs, and pulsars. However, the mineralogical make up of such planets is poorly constrained. We performed high-pressure high-temperature laboratory experiments ( = 1-2 GPa, = 1523-1823 K) on chemical mixtures representative of C-enriched rocky exoplanets based on calculations of protoplanetary disk compositions. These conditions correspond to the deep interiors of Pluto- to Mars-sized planets and the upper mantles of larger planets. Our results show that these exoplanets, when fully differentiated, comprise a metallic core, a silicate mantle, and a graphite layer on top of the silicate mantle. Graphite is the dominant carbon-bearing phase at the conditions of our experiments with no traces of silicon carbide or carbonates. The silicate mineralogy comprises olivine, orthopyroxene, clinopyroxene, and spinel, which is similar to the mineralogy of the mantles of carbon-poor planets such as the Earth and largely unaffected by the amount of carbon. Metals are either two immiscible iron-rich alloys (S-rich and S-poor) or a single iron-rich alloy in the Fe-C-S system with immiscibility depending on the S/Fe ratio and core pressure. We show that, for our C-enriched compositions, the minimum carbon abundance needed for C-saturation is 0.05-0.7 wt% (molar C/O ∼0.002-0.03). Fully differentiated rocky exoplanets with C/O ratios more than that needed for C-saturation would contain graphite as an additional layer on top of the silicate mantle. For a thick enough graphite layer, diamonds would form at the bottom of this layer due to high pressures. We model the interior structure of Kepler-37b and show that a mere 10 wt% graphite layer would decrease its derived mass by 7%, which suggests that future space missions that determine both radius and mass of rocky exoplanets with insignificant gaseous envelopes could provide quantitative limits on their carbon content. Future observations of rocky exoplanets with graphite-rich surfaces would show low albedos due to the low reflectance of graphite. The absence of life-bearing elements other than carbon on the surface likely makes them uninhabitable.
富碳岩石系外行星被认为存在于矮星、双星、白矮星和脉冲星周围。然而，这些行星的矿物组成还没有得到很好的约束。我们基于原行星盘成分的计算，对富碳岩石系外行星的化学混合物进行了高压高温实验室实验（ = 1-2 GPa， = 1523-1823 K）。这些条件对应于冥王星到火星大小的行星的深部内部和更大行星的上地幔。我们的结果表明，这些系外行星在完全分化后，由金属核、硅酸盐地幔和硅酸盐地幔顶部的石墨层组成。在我们实验的条件下，石墨是主要的含碳相，没有碳化硅或碳酸盐的痕迹。硅酸盐矿物学包括橄榄石、斜方辉石、单斜辉石和尖晶石，这与地球等贫碳行星的地幔矿物学相似，并且受碳含量的影响很小。金属要么是两种不混溶的富铁合金（富硫和贫硫），要么是铁-碳-硫体系中的单一富铁合金，其不混溶性取决于 S/Fe 比和核心压力。我们表明，对于我们的富碳成分，C 饱和所需的最小碳丰度为 0.05-0.7 wt%（摩尔 C/O ∼0.002-0.03）。C/O 比大于 C 饱和所需的完全分化的富碳岩石系外行星将在硅酸盐地幔顶部包含石墨作为额外的层。对于足够厚的石墨层，由于高压，钻石将在该层的底部形成。我们模拟了开普勒-37b 的内部结构，并表明仅 10 wt%的石墨层就会使其衍生质量减少 7%，这表明未来的太空任务确定具有无足轻重的气态包层的岩石系外行星的半径和质量，可能会对其碳含量提供定量限制。具有富含石墨表面的岩石系外行星的未来观测结果将由于石墨的低反射率而显示出低反照率。由于表面上除碳以外没有生命所需的元素，它们可能无法居住。
