A Chondritic Solar Neighborhood

TL;DR

The study addresses whether exoplanetary rocks share the Solar System's compositional building blocks by analyzing bulk rock abundances polluting oxygen-bearing white dwarfs and comparing them to CI chondrite values, while also benchmarking against nearby stars from the Hypatia catalog. Using Monte Carlo uncertainty propagation and a reduced $\chi^2$ test, the authors find that the majority of exoplanetary rocks in the solar neighborhood are consistent with chondritic compositions, with no evidence for crust-like material in the samples. Galactic chemical evolution is shown to imprint trends in lithophile and siderophile elements, but the local WD and Hypatia samples are young/close enough that GCE does not dominate their rock-formation element ratios; nonetheless, older, metal-poor populations may have smaller metal cores on average. The work concludes that exotic compositions are not required to explain most rocks around polluted WDs, though Earth-like core mass fractions may not be universal, particularly for early Galaxy environments.

Abstract

A persistent question in exoplanet demographics is whether exoplanetary systems form from similar compositional building blocks to our own. Polluted white dwarf stars offer a unique way to address this question as they provide measurements of the bulk compositions of exoplanetary material. We present a statistical analysis of the rocks polluting oxygen-bearing white dwarfs and compare their compositions to rocks in the Solar System. We find that the majority of the extrasolar rocks are consistent with the composition of typical chondrites. Measurement uncertainties prevent distinguishing between chondrites and bulk Earth, but do permit detecting the differences between chondritic compositions and basaltic or continental crust. We find no evidence of crust amongst the polluted white dwarfs. We show that the chondritic nature of extrasolar rocks is also supported by the compositions of local stars. While galactic chemical evolution results in variations in the relative abundances of rock-forming elements spatially and temporally on galaxy-wide scales, the current sample of polluted white dwarfs are sufficiently young and close to Earth that they are not affected by this process. We conclude that exotic compositions are not required to explain the majority of observed rock types around polluted white dwarfs, and that variations between exoplanetary compositions in the stellar neighborhood are generally not due to significant differences in the initial composition of protoplanetary disks. Nonetheless, there is evidence from stellar observations that planets formed in the first several billion years in the Galaxy have lower metal core fractions compared with Earth on average.

Figures (12)

• Figure 1: Compositions of Solar System rocks compared to CI chondrite, for Bulk Earth (BE), Bulk Silicate Earth (BSE), Mid-Ocean Ridge Basalt (MORB), Continental Crust (CC), Bulk Silicate Mars (BSM), and E chondrites (EH). Error bars for each element correspond to the mean uncertainty in the WD abundances for that element. The reduced $\chi^2$ value for each correlation with CI chondrite is indicated in the plots at the upper left. Where $\chi^2_\nu$$\lesssim 3$ to 4, the data are taken as evidence for chondritic rocky parent bodies or planets.
• Figure 2: Comparison of WD pollution compositions to the CI chondrite composition, for the raw data (top) and steady-state adjusted abundances (bottom). The order of elements in each plot, from left to right, is Cr, Ni, Ca, Al, Fe, Si, all ratioed to Mg. The $\chi^2_\nu$ parameter appears in the upper left of each panel. WDs with white backgrounds are consistent with having accreted a chondritic rock composition. WDs with dark grey backgrounds are not considered a good fit to chondrite, using an $\alpha$ parameter of 0.05 for goodness of fit. WDs with light grey backgrounds have an outlier element that allows the WDs to pass as chondritic when the outlier is removed (outliers highlighted in orange).
• Figure 3: $\chi^2_\nu$ for each WD, relative to CI chondrite, for the raw abundances (circles) and steady-state adjusted values (triangles). The points are colored and grouped by $n$, the number of elements used to calculate $\chi^2_\nu$. The horizontal line shows a typical critical $\chi^2_\nu$ value ($\chi^2_\nu \sim 3$ to 4, based on the number of observed elements). In most cases, the steady-state values provide a better fit.
• Figure 4: Ternary diagram for all of the WD samples, for raw abundances. The white points indicate the OLV, OPX, and CPX quantities derived from the median values of Mg, Ca, and Si for each WD. We also demonstrate the spread in OLV, OPX, and CPX that is due to the uncertainties in the WD abundances by showing the spread from 100 random draws of Mg, Si, and Ca for WD Ton345. The total spread in points is truncated for visibility.
• Figure 5: Distribution of the distances of Hypatia catalog stars from the Sun (classified by stellar type). M dwarfs in the sample tend to be much closer to the Sun than other stars in the catalog. K stars also appear to have a bimodal distribution in distance.