Kavli Affiliate: Christopher Burke
| Summary:
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 $T_texteff=4174^+34_-42$ K, $logg=4.62^+0.02_-0.03$, $[textFe/H]=-0.58pm0.18$, $M_ast=0.57pm0.02~M_odot$, and $R_ast=0.62pm0.01~R_odot$. Precise Doppler measurements with Keck/HIRES revealed a planetary mass of $M_textp=9.2pm2.1~M_oplus$ 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.
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