A Mini-Neptune Orbiting the Metal-poor K Dwarf BD+29 2654
Hellier, Coel; Batalha, Natalie M.; Kane, Stephen R.; Stassun, Keivan G.; Huber, Daniel; Howell, Steve B.; Howard, Andrew W.; Petigura, Erik A.; Chontos, Ashley; Rubenzahl, Ryan A.; Weiss, Lauren M.; Ricker, George R.; Vanderspek, Roland; Seager, Sara; Winn, Joshua N.; Jenkins, Jon M.; Caldwell, Douglas A.; Shporer, Avi; Ciardi, David R.; Dai, Fei; Fukui, Akihiko; Narita, Norio; Daylan, Tansu; Crossfield, Ian; Giacalone, Steven; Reggiani, Henrique; Murgas, Felipe; Fulton, Benjamin; Schlaufman, Kevin C.; Robertson, Paul; Striegel, Stephanie; Lillo-Box, Jorge; Burke, Christopher J.; Polanski, Alex S.; Isaacson, Howard; Beard, Corey; Holcomb, Rae; Lubin, Jack; Akana Murphy, Joseph M.; Dressing, Courtney; Van Zandt, Judah; Pallé, Enric; Savel, Arjun B.; Belinski, Alexander A.; Guerrero, Natalia M.; Boyle, Andrew W.; Pidhorodetska, Daria; Gill, Holden; Rodriguez, David R.; Bouma, Luke; Castro-González, Amadeo
United States, Australia, Russia, Spain, Japan, United Kingdom
Abstract
We report the discovery and Doppler mass measurement of a 7.4 days 2.3 R ⊕ 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 ${T}_{\mathrm{eff}}={4174}_{-42}^{+34}$ K, $\mathrm{log}g={4.62}_{-0.03}^{+0.02}$ , [Fe/H] = - 0.58 ± 0.18, M * = 0.57 ± 0.02 M ⊙, 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 ~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 ⊕ 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.