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

August 2023 • 2023AJ....166...49D

Authors • Dai, Fei • Schlaufman, Kevin C. • Reggiani, Henrique • Bouma, Luke • Howard, Andrew W. • Chontos, Ashley • Pidhorodetska, Daria • Van Zandt, Judah • Akana Murphy, Joseph 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-González, 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 • Pallé, 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.

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.


IPAC Authors


David Ciardi

Senior Scientist


Benjamin Fulton

Assistant Scientist