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

We report the discovery and Doppler mass measurement of a 7.4 days 2.3 R circle plus 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 multiwavelength photometry from the UV to the mid-infrared, we found that the host star has = T-eff 4174(-42)(+34) K, log g (4.62) (-0.03) = + 0.02, [Fe/H] = - 0.58 +/- 0.18, M-* = 0.57 +/- 0.02 Me, and R-* = 0.62 +/- 0.01 R-?. Precise Doppler measurements with Keck/HIRES revealed a planetary mass of M-p = 9.2 +/- 2.1 M-? for TOI-2018 b. TOI-2018 b has a mass and radius that are consistent with an Earthlike core, with a similar to 1%-by-mass hydrogen/helium envelope or an ice-rock mixture. The mass of TOI-2018 b is close to the threshold for runaway accretion and hence giant planet formation. Such a threshold is predicted to be around 10M(circle plus) or lower for a low-metallicity (low-opacity) environment. If TOI-2018 b is a planetary core that failed to undergo runaway 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.

Automated summary

We report the discovery and Doppler mass measurement of a 7.4 days 2.3 R circle plus mini-Neptune around a metal-poor K dwarf BD+29 2654 (TOI-2018). The host star has a temperature of 4174 K, log g of 4.62, [Fe/H] of -0.58, M* of 0.57 Me, and R* of 0.62 R⊙. TOI-2018 b has a planetary mass of 9.2 M⊕ and its mass and radius are consistent with an Earthlike core with a hydrogen/helium envelope or an ice-rock mixture. The system is a potential target for transmission spectroscopy and metastable Helium observation.