  Gravitational-wave detectors have opened a new window through which we can
observe black holes (BHs) and neutron stars (NSs). Analyzing the 11 detections
from LIGO/Virgo's first gravitational-wave catalog, GWTC-1, we investigate
whether the power-law fit to the BH mass spectrum can also accommodate the
binary neutron star (BNS) event GW170817, or whether we require an additional
feature, such as a mass gap, in between the NS and BH populations. We find that
with respect to the power-law fit to binary black hole (BBH) masses, GW170817
is an outlier at the 0.13\% level, suggesting a distinction between NS and BH
masses. A single power-law fit across the entire mass range is in mild tension
with: (a) the detection of one source in the BNS mass range ($\sim 1$--$2.5
\,M_\odot$), (b) the absence of detections in the "mass-gap" range ($\sim
2.5$--$5 \,M_\odot$), and (c) the detection of 10 sources in the BBH mass range
($\gtrsim 5 \,M_\odot$). Instead, the data favor models with a feature between
NS and BH masses, including a mass gap (Bayes factor of 4.6) and a break in the
power law, with a steeper slope at NS masses compared to BH masses (91\%
credibility). We estimate the merger rates of compact binaries based on our fit
to the global mass distribution, finding $\mathcal{R}_\mathrm{BNS} =
871^{+3015}_{-805} \ \mathrm{Gpc}^{-3} \ \mathrm{yr}^{-1}$ and
$\mathcal{R}_\mathrm{BBH} = 47.5^{+57.9}_{-28.8} \ \mathrm{Gpc}^{-3} \
\mathrm{yr}^{-1}$. We conclude that, even in the absence of any prior knowledge
of the difference between NSs and BHs, the gravitational-wave data alone
already suggest two distinct populations of compact objects.
