  We report the discovery and Doppler mass measurement of a 7.4-day
2.3-$R_\oplus$ mini-Neptune around a metal-poor K dwarf BD+29 2654 (TOI-2018).
Based on a high-resolution Keck/HIRES spectrum, the Gaia parallax, and
multi-wavelength photometry from the ultraviolet to the mid-infrared, we found
that the host star has $T_{\text{eff}}=4174^{+34}_{-42}$ K,
$\log{g}=4.62^{+0.02}_{-0.03}$, $[\text{Fe/H}]=-0.58\pm0.18$,
$M_{\ast}=0.57\pm0.02~M_{\odot}$, and $R_{\ast}=0.62\pm0.01~R_{\odot}$. Precise
Doppler measurements with Keck/HIRES revealed a planetary mass of
$M_{\text{p}}=9.2\pm2.1~M_{\oplus}$ for TOI-2018 b. TOI-2018 b has a mass and
radius that are consistent with an Earth-like core with a $\sim1\%$-by-mass
hydrogen/helium envelope, or an ice-rock mixture. The mass of TOI-2018 b is
close to the threshold for run-away accretion and hence giant planet formation.
Such a threshold is predicted to be around 10$M_\oplus$ or lower for a
low-metallicity (low-opacity) environment. If TOI-2018 b is a planetary core
that failed to undergo run-away accretion, it may underline the reason why
giant planets are rare around low-metallicity host stars (one possibility is
their shorter disk lifetimes). With a K-band magnitude of 7.1, TOI-2018 b may
be a suitable target for transmission spectroscopy with the James Webb Space
Telescope. The system is also amenable to metastable Helium observation; the
detection of a Helium exosphere would help distinguish between a H/He enveloped
planet and a water world.
