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

Dai, Fei; Schlaufman, Kevin C.; Reggiani, Henrique; Bouma, Luke; Howard, Andrew W.; Chontos, Ashley; Pidhorodetska, Daria; Van Zandt, Judah; Murphy, Joseph Akana M.; Rubenzahl, Ryan A.; Polanski, Alex S.; Lubin, Jack; Beard, Corey; Giacalone, Steven; Holcomb, Rae; Batalha, Natalie M.; Crossfield, Ian; Dressing, Courtney; Fulton, Benjamin; Huber, Daniel; Isaacson, Howard; Kane, Stephen R.; Petigura, Erik A.; Robertson, Paul; Weiss, Lauren M.; Belinski, Alexander A.; Boyle, Andrew W.; Burke, Christopher J.; Castro-Gonzalez, Amadeo; Ciardi, David R.; Daylan, Tansu; Fukui, Akihiko; Gill, Holden; Guerrero, Natalia M.; Hellier, Coel; Howell, Steve B.; Lillo-Box, Jorge; Murgas, Felipe; Narita, Norio; Palle, Enric; Rodriguez, David R.; Savel, Arjun B.; Shporer, Avi; Stassun, Keivan G.; Striegel, Stephanie; Caldwell, Douglas A.; Jenkins, Jon M.; Ricker, George R.; Seager, Sara; Vanderspek, Roland; Winn, Joshua N.
2023
ASTRONOMICAL JOURNAL
DOI
10.3847/1538-3881/acdee8
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.