  Multimessenger observations of the neutron star merger GW170817 and its
kilonova proved that neutron star mergers can synthesize large quantities of
r-process elements. If neutron star mergers in fact dominate all r-process
element production, then the distribution of kilonova ejecta compositions
should match the distribution of r-process abundance patterns observed in
stars. The lanthanide fraction (XLa) is a measurable quantity in both kilonovae
and metal-poor stars, but it has not previously been explicitly calculated for
stars. Here, we compute the lanthanide fraction distribution of metal-poor
stars ([Fe/H] < -2.5) to enable comparison to current and future kilonovae. The
full distribution peaks at log XLa ~ -1.8, but r-process-enhanced stars
([Eu/Fe] > 0.7) have distinctly higher lanthanide fractions; log XLa > -1.5. We
review observations of GW170817 and find general consensus that the total log
XLa = -2.2 +/- 0.5, somewhat lower than the typical metal-poor star and
inconsistent with the most highly r-enhanced stars. For neutron star mergers to
remain viable as the dominant r-process site, future kilonova observations
should be preferentially lanthanide-rich (including a population of ~10% with
log XLa > -1.5). These high-XLa kilonovae may be fainter and more rapidly
evolving than GW170817, posing a challenge for discovery and follow-up
observations. Both optical and (mid-)infrared observations will be required to
robustly constrain kilonova lanthanide fractions. If such high-XLa kilonovae
are not found in the next few years, that likely implies that the stars with
the highest r-process enhancements have a different origin for their r-process
elements.
