A Mini-Neptune Orbiting the Metal-poor K Dwarf BD+29 2654

Abstract

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 $T_{\text{eff}}=4174_{-42}^{+34}$ K, $\log g=4.62_{-0.03}^{+0.02}$, $[\text{Fe/H}]=-0.58\pm 0.18$, $M_{\ast }=0.57\pm 0.02M_{\odot }$, and $R_{\ast }=0.62\pm 0.01R_{\odot }$. Precise Doppler measurements with Keck/HIRES revealed a planetary mass of $M_{\text{p}}=9.2\pm 2.1M_{\oplus }$ for TOI-2018 b. TOI-2018 b has a mass and radius that are consistent with an Earth-like core with a $\sim 1\%$-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.

10 figures, 5 tables, accepted to AAS Journals